Download A timeline of the universe

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0
∞
THE BIG BANG marks the birth of the universe,
when space, time, and matter came into
existence. ALL ILLUSTRATIONS BY ASTRONOMY: ROEN KELLY
The first
billion years
After the release of
the cosmic background
radiation, darkness fell over
the cosmos. ⁄ ⁄ ⁄ BY Adam Frank
2
© 2013 Kalmbach Publishing Co. This material may not be reproduced in any form
without permission from the publisher. www.Astronomy.com
200,000
Age of universe (years)
300,000
380,000
Redshift (z)
600
THE CMB formed approximately
380,000 years after the Big Bang.
The different colors denote tiny density
enhancements that later condensed
into the first structures. WMAP
▼
Frequency (gigahertz)
200
400
1,000
THE SPECTRUM of the cosmic microwave
background (CMB) matches that of a black body
with a temperature of 2.73 kelvins. This shows
nearly all of the universe’s radiant energy was
released within a year of the Big Bang.
▲
Intensity
Temperature
= 2.73 K
0.5
0.1
Wavelength (centimeters)
0.05
B
ig Bang theory tells us the
entire universe — all space,
time, matter, and dimension —
emerged from a single titanic
explosion that set the cosmos
in motion. Light brilliant beyond descrip­
tion flooded the infant universe.
There’s a second part to the scientific
story, however, that many people have not
heard: Darkness soon returned with a ven­
geance. The cosmic Dark Ages began less
than 1 million years after the Big Bang
and lasted for a billion or so years.
For astronomers, the story is still com­
ing into focus as more powerful telescopes
and faster computers illuminate the full
scope of cosmic history. Using these tools,
astronomers are taking their first steps
toward understanding the strange uni­
verse of the Dark Ages, a cosmos devoid
of galaxies that started forming the ear­
liest structures, with starlight shining
for the first time.
Adam Frank is an astrophysicist at the University
of Rochester in New York and a member of Astron­
omy’s Editorial Advisory Board.
Beginning at the beginning
To understand the Dark Ages, we must
briefly touch on the first epoch of light:
the Big Bang. After this violent birth, the
universe was a smooth, hot, dense soup
of exotic high-energy particles. Matter
and radiation were mixed so closely that
they shared a common temperature. In
the parlance of physics, matter and radi­
ation were strongly coupled. A photon
could cross only a tiny fraction of the
­universe before being absorbed and then
re-emitted by some particle of matter.
Then, as the universe expanded, it
cooled, taking the particle-radiation mix
through a series of dramatic transitions.
They included the creation of protons
and neutrons, building blocks of all
atomic nuclei, about 1 second after the
Big Bang. A scant 3 minutes later, the
­universe created the lightest nuclei: helium
(2 neutrons, 2 protons) and a little lithium
(3 protons, 3 neutrons).
During all these changes, matter and
radiation remained strongly linked. After
about 380,000 years, the universe had
expanded and cooled to the point where
electrons and protons could catch each
other and bind into atomic hydrogen. This
was the great parting of ways between
matter and light. The birth of atomic
hydrogen, the most abundant element in
the universe, meant the end of one cosmic
era and the beginning of the Dark Ages.
Astronomers often call the era when
neutral hydrogen formed “recombination”
because electrons and protons combined
to form atomic hydrogen. It’s a misnomer,
however — this is the first time these
atoms formed. In most stories of the Big
Bang, the emphasis on this era lies with
the sudden transparency of the universe to
photons and the birth of the cosmic micro­
wave background (CMB). When recombi­
nation occurred, photons previously linked
to naked electrons and protons found
themselves with nothing to interact with.
They were free to expand unimpeded with
the universe, with their wavelength
stretching along with cosmic expansion.
We see them today as relics of the Big
Bang. If the still-smooth universe became
transparent to photons and thus light, why
does recombination signal the beginning
THE GOODS SURVEY uses advanced telescopes, including Hubble, Spitzer, and
Chandra, to target a fairly empty region
of space . The goal is to get a census of
the universe and to probe back toward
the beginning of cosmic structure. NASA/ESA
3
380,000
Age of universe (years)
200 million
1,000
400 million
Redshift (z)
▼ THE CMB FORMED when the universe
had cooled enough for electrons to join
with protons, allowing CMB photons to
travel unimpeded through the cosmos.
Proton
Electron
Photon
T H E
D A R K
A
▲ THE DARK AGES reigned for about a billion
years. During this era, neutral hydrogen absorbed
most visible light, so the universe appeared dark.
However, gravity was starting to pull matter into
the structures we see today. NASA/ESA/STSCI
of the Dark Ages? The answer lies with
visual light, the kind our eyes respond to.
While the neutral hydrogen gas could
not absorb cosmic background photons, it
efficiently absorbed visual and ultraviolet
(UV) light. As soon as neutral hydrogen
dominated the universe, visual and UV
photons became trapped close to any source
producing them. Most of the hydrogen gas
in today’s universe is “ionized,” meaning it
consists primarily of bare hydrogen nuclei
and free electrons. Stellar UV light maintains this ionization. These photons pack
enough punch to tear electrons off any
­neutral hydrogen atoms that form by
recombination. The current dominance of
ionized hydrogen is one reason we can see
so far with optical telescopes. The Dark
Ages were the epoch of cosmic history
between the initial formation of neutral
hydrogen and its eventual destruction.
What was it, however, that illuminated
the universe with the glow we now recognize as starlight? How did the cosmos go
from a smooth, dark sea of neutral gas
emerging from the recombination era to
the multitudes of stars and galaxies that
dominate today? The Dark Ages mark an
era of transitions, not only from blackness
to light, but also from formlessness to form.
The universe at recombination was
extraordinarily smooth. From detailed
studies of the CMB, astronomers know that
any ripples, bumps, and lumps in the density of cosmic plasma were 1⁄10,000 as small as
matter’s average density. The universe we
live in now bears little resemblance. Today’s
lumps and blobs — stars and galaxies — are
far denser than an average volume of space,
which is pretty close to a vacuum. The
DID STRUCTURE FORM
from the top down, or
from the bottom up?
journey through the Dark Ages takes
astronomers through the epoch when
tiny, initial bumps, or perturbations, in
the cosmic stew first began exerting their
gravitational influence, feeding on surrounding gas in an effort to grow into
the structures we see today.
“There is tension in the early universe
between expansion and collapse,” explains
Greg Bryan of Columbia University. Work-
ing with his former thesis advisor, Mike
Norman of the University of California at
San Diego, and Tom Abel of Stanford University, Bryan has pioneered using advanced
computer simulations to study the formation of the universe’s first structures.
In the absence of cosmic expansion, a
lump of matter denser than its surroundings
will draw in material at an ever-increasing
rate. The denser the growing lump gets, the
stronger its gravity becomes, and the faster
it draws new material inward — a runaway
collapse. The expansion of the universe
changes this process. Just as gravity tries
to draw some over-dense lump together,
the universe’s expansion pulls it apart.
“During the earlier years of cosmolog­
ical study, this struggle between gravity
growing structures and expansion diluting
them caused astronomers a lot of consternation,” says Bryan. Originally, astronomers
had hoped that any perturbation, even one
as small as an atom, could grow into a cluster of galaxies. The push-pull of expansion
and gravity dashed that hope.
“There needs to be a range of perturbations that already exists at recombination,”
explains Bryan, “and they need to be large
enough to allow gravity to do its work so
the structures we see now can emerge.”
Volker Springel (Max Planck Institute for
Astrophysics), et al.
210 million years
THE MILLENNIUM RUN simulation tracked 10 billion dark-matter particles to see how cosmic structure formed. This sequence shows the
universe at an age of 210 million years (z = 18.3), well within the Dark Ages. Each image left to right zooms in by a factor of 4.
4
600 million
800 million
1 billion
5.7
▼
G E S
THE DARK AGES came to an end a
billion years after the Big Bang, once early
stars and galaxies like this one reionized
most of the universe. NASA/ESA/STSCI
▼
STARS with hundreds of times the Sun’s
mass began to form in the later stages of
the Dark Ages. However, darkness still
reigned because most of the universe still
consisted of neutral hydrogen.
▼
GALAXIES also started to make their
appearance before the end of the Dark
Ages. This one existed just 900 million
years after the Big Bang. NASA/ESA/STSCI
Astronomers now understand that while
the perturbations seen in the microwave
background may be tiny, those bumps —
imprinted at the Big Bang’s earliest stages
— are large enough to give gravity the head
start it needed once the Dark Ages began.
Perturbations grew slowly as the Dark
Ages progressed. Eventually, the density
contrast between a growing lump and
its surroundings became obvious, and
­gravity halted expansion in that region.
Winners and losers
When the cosmic plasma entered the Dark
Ages, perturbations imprinted on it from
the beginning ranged in size. Some were
planet-sized wiggles, and some were vast
undulations that stretched across galactic
distances. For many years, astronomers
did not know which of these wiggles grew
fastest. Did structure form from the top
down, or from the bottom up?
“You must remember that most of the
matter in the universe is dark matter,”
explains Bryan. Given dark matter’s preponderance — it makes up more than 80
percent of the universe’s mass — its properties determined how structure formed.
“For many years, people fought between
hot-dark-matter and cold-dark-matter
models,” says Bryan. “Hot” and “cold” in
this context mean fast and slow moving.
The difference is important because fastmoving dark matter tends to stream out of
any small clumps trying to grow. It’s a bit
like flicking marbles across a flat surface
and trying to catch them in shallow depressions. If the marbles move quickly, they
barely know the depressions exist. If they
roll slowly, they have a better chance of
­getting trapped.
Hot-dark-matter models need big collections of stuff to form before gravity can
make the structure collapse. With hot dark
matter, the largest wiggles grew fastest.
Smaller clumps formed once the big ones
were done — top-down structure formation.
Cold dark matter does the opposite. Little wiggles grew fastest, so structure formed
from the bottom up. Once the clumps
­collapsed, they merged with other small
clumps to form ever-larger structures. By the
early 1990s, astronomers had ample evidence that only cold-dark-matter models
could produce the kinds of structures we
see in the current universe. “The cold-darkmatter models won because they compared
better with observations,” says Bryan.
According to Bryan, the first objects to
become gravitationally bound may have
been Earth-sized clumps of dark matter.
But while the clump might have been about
Earth’s size, it would have been tenuous —
at best a dark-matter ghost of a planet.
Dark clump to bright star
In today’s universe, a giant halo of dark
matter surrounds the visible part of every
galaxy. Astronomers also use the term halos
for the first dark-matter structures to form.
At a redshift around z = 60, the dark matter
halos had grown to contain almost 1,000
times the Sun’s mass. Ordinary matter had
not yet joined the party. The 1,000 solar
masses were too little to cause ordinary
matter, mostly hydrogen gas, to clump.
While dark matter began to form significant structures, ordinary gas was still too
hot and moving too fast to be contained in
the shallow gravity wells of nascent halos.
Only when the dark matter halos grew
to 10,000 times the Sun’s mass could the
hydrogen clump, too. The first clumping
of ordinary matter — our kind of stuff —
marked a critical moment in cosmic history.
Unlike dark matter, normal (or “baryonic”)
matter can dissipate its energy. In other
words, it can cool and slow down, which
allowed hydrogen and helium to condense
at the center of a growing dark matter halo.
Volker Springel (Max Planck Institute for
Astrophysics), et al.
1 billion years
THE DARK AGES CONCLUDED about 1 billion years after the Big Bang (z = 5.7). At this time, the first stars — massive beasts weighing
more than 100 solar masses drawn together by cold dark matter — already had formed. Galaxies were just starting to form.
5
1 billion
5.7
Age of universe (years)
5.0
3 billion
Redshift (z)
7 billion
5 billion
2.5
1.0
▲
THE GALAXY UDF 5225 existed about 1.2
billion years after the Big Bang. Its reddish
color is typical of such distant galaxies, whose
light has been shifted far to the red. NASA/ESA/STSCI
▲
GALAXY UDF 2881 lies at a redshift of 4.6,
so we see it about 1.4 billion years after the Big
Bang. Most such distant galaxies look ragged
because they’re still forming. NASA/ESA/STSCI
This was the moment when the universe’s
first star was poised to form.
To make a star, matter must collapse
into the bottom of a gravitational well. The
more gas that falls into the well, the higher
the density. Eventually, nuclear fusion
occurs, and the star turns on.
Astronomers understand how stars form
in the current cosmos and know enough to
paint the outline for forming the first gener­
ation of stars. Life in the Dark Ages was not
that simple, however. In astronomers’ quest
to comprehend the births of the first stars,
they have been thrown many curveballs.
“The absence of heavy elements is the
most important difference between modern
star formation and the creation of the first
stars,” explains Bryan. Astronomers call
every element more massive than helium
a metal. And metals are, for the most part,
created inside stars. Metals in an astrophysical gas can shed heat far more efficiently
than hydrogen or helium atoms. The photons emitted by metals stream out of the
gas, taking energy with them. In this way,
even traces of metals can act as highly
effective refrigerants.
The first stars, however, formed from
gas lacking metals. This primordial gas had
almost no way to cool. “The cooling is very
slow until hydrogen atoms combine to form
hydrogen molecules,” says Bryan. He also
points out that molecular hydrogen in the
modern universe forms on dust grains,
which are made of metals. Without dust,
molecular hydrogen formed so slowly that
dark matter halos grew to a million solar
masses before ordinary matter could really
begin to cool. “These dark matter halos are
literally microgalaxies, one-millionth the
THE UNIVERSE’S FIRST
stars may all have been
hundreds of solar masses.
mass of a present-day galaxy, by the time
the first star can begin to form,” says Bryan.
The wait leads to disappointment on a
cosmic scale. According to Bryan’s calculations, the type of star that finally contracts
from the primordial gas is a kind of monster rarely seen in the current universe.
In today’s cosmos, any star larger than
8 solar masses is considered massive and
has a harder time forming than its smaller
▲ BEAUTIFUL SPIRAL STRUCTURE defines UDF
423, a galaxy that existed 6 billion years after
the Big Bang (43 percent of the current cosmic
age), at a redshift of 1. NASA/ESA/STSCI
cousins. Stars more than 100 times the
Sun’s mass are so rare that we know of
only a handful in the entire Milky Way.
A giant’s birth and death
According to Bryan, the universe’s first
stars may all have been this big. Over the
past several years, Bryan, Norman, and
their collaborators have used advanced
computer simulations to track the evolution of cosmic structure from the era of
recombination to the formation of the first
star. Unlike traditional simulation methods,
Bryan’s code simultaneously tracks a huge
range of scales, everything from vast arcs of
still-forming dark matter halos millions of
light-years across down to the outlines of
the first star with a radius just a few times
that of Earth’s orbit.
After running the codes continuously
for years, the researchers found that each
microgalaxy halo produces exactly one
massive star. Each star weighs a few hundred solar masses. But the story doesn’t end
there. Massive stars burn hot and die fast.
In their brief lives, they dramatically affect
their environment.
The massive stars in Bryan’s calculations
produce a torrent of UV photons that ionize every hydrogen atom out to well beyond
Volker Springel (Max Planck Institute for
Astrophysics), et al.
4.7 billion years
GALAXIES AND CLUSTERS appear prominent by the time the cosmos is 4.7 billion years old (z = 1.4). On the biggest scales (left), the
­ niverse still looks fairly smooth, but closer up, individual galaxies and clusters start to dominate.
u
6
9 billion
11 billion
13 billion
0.3
13.7 billion
0.02
0
▼
INTERACTING GALAXIES NGC 6872 and
IC 4970 lie “just” 300 million light-years
distant (z = 0.02), close enough that Earthbased telescopes can show great detail. ESO
▼
ELLIPTICAL and spiral galaxies intermingle
in the cluster CL 0053–37, located 2 billion
light-years from Earth, so we see it as it was
11.7 billion years after the Big Bang. ESO
▼
LIGHT still dominates the universe today
(epitomized by our neighboring galaxy, the
Large Magellanic Cloud) because hot stars
ionize most of the cosmos. NOAO/AURA/NSF
1,000 light-years. “The ionization produced
by the big stars heats the surrounding gas
and, in the process, can completely unbind
all the baryons from the still-forming dark
matter halo,” explains Bryan. Even worse,
when a star this size runs out of nuclear
fuel — after only a million years — it ends
its life in a titanic explosion that blows
away the halo gas.
Only a star bigger than 300 times the
Sun’s mass can escape this fate. Such hypergiant stars may implode as a black hole
without ever exploding.
From darkness to darkness
Although the massive stars that form in
Bryan’s simulations ionize lots of gas, they
don’t pack enough punch to alter the balance of neutral-to-ionized hydrogen atoms
completely. The universe remains dark.
“We really don’t know what happens next,”
declares Bryan. “Does the next generation
of stars form in blast waves from first­generation supernovae?” Bryan and other
astronomers now are focusing their efforts
on the era when the Dark Ages ended —
sometime between redshift 17 and approx­
imately redshift 6.
“We know that the growing dark matter
halos create objects that probably look like
today’s dwarf galaxies,” says Bryan. On the
baryon side of the equation, successive stellar generations eventually created enough
metals to bring the star-formation process
in line with what we see today. At that
point, stars of all masses can form.
Still, astronomers have yet to fill in the
details, and many of our current ideas may
be wrong. For example, no one knows what
role black holes played in re-ionizing the
universe. Any black hole that formed during this epoch would have drawn in gas at a
prodigious rate. As the gas fell toward the
black hole, it emitted ionizing UV photons.
“People used to think that re-ionization
and ‘first light’ occurred quickly, at a redshift of around 6 or 7,” says Bryan. “These
days, that is not so clear.” Did black hole
accretion effectively contribute to filling
the universe with visible light? How did
the galaxy-formation process continue to
the point of building the great spirals and
ellipticals we see today? Answers to these
questions remain unknown.
Seeing first light
While questions concerning the first billion
years abound, hope for answers does as
well. An array of new instruments, scheduled to come online during the next decade
or two, holds the promise of directly probing the Dark Ages and its end in the era
of re-ionization.
The highest profile instrument will be
Hubble’s successor: the James Webb Space
Telescope (JWST). Ultraviolet and visible
light emitted during the era of re-ionization
now has been redshifted to the infrared.
With a 6.5-meter mirror and high sensitivity in the infrared, JWST will explore the
nature of the objects that lit up the universe
at the end of the first billion years.
In addition, astronomers hope the
Square Kilometer Array will directly probe
radio waves emitted by clumps of atomic
hydrogen gas during the Dark Ages. The
ability to map ripples in gas density at such
early times and far-flung distances could
prove to be a watershed. Astronomers
would get their first clear view of the early
universe’s initial structures.
From light to darkness to light again
— this describes the unveiling of our universe’s early years. By illuminating the Dark
Ages, the age of first structures and massive
primeval stars, astronomers are exploring a
critical gap in cosmic history. With only the
outlines filled and much still to be written,
there can be no doubt we live in our own
privileged epoch.
Volker Springel (Max Planck Institute for
Astrophysics), et al.
13.7 billion years
IN TODAY’S UNIVERSE, rich galaxy clusters follow filaments of dark matter, with large voids separating the denser regions. Ordinary
­ atter now largely follows the lead established by dark matter, and dark matter halos surround individual galaxies.
m
7
The discovery of the cosmic microwave
background (CMB) confirmed — in the
eyes of science — the Big Bang theory.
The CMB’s clumpiness gives astronomers
evidence for theories ranging from what
our universe’s contents are to how modern structure formed. Astronomy: Roen Kelly
8
© 2013 Kalmbach Publishing Co. This material may not be reproduced in any form
without permission from the publisher. www.Astronomy.com
s p e c ia l CO S M OLO G Y iss u e
The accident
that saved the
Big Bang
While adjusting an antenna, two astronomers
uncovered cosmic static, leading to one of the greatest
discoveries of all time. ⁄⁄⁄ BY JAMES TREFIL
S
ometimes the most profound insights come from the most mundane experiences.
So it is with the discovery of the cosmic microwave background (CMB), which
manifested itself as an engineering nuisance.
You can learn a lot about the history of the universe and the CMB sitting around a
campfire. Early in the evening, when the fire is hot, the coals are bright yellow, maybe
even white. Later in the evening, before you roll out your sleeping bag, they glow a dull
red. The next morning, they don’t glow at all, yet they feel warm when you hold your
hand over them. The CMB has undergone a similar process, although spread over billions of years instead of hours.
This illustrates one of the basic laws of physics: Every object at a temperature above
absolute zero radiates into its environment, with the type of radiation depending on the
object’s temperature. Your roaring campfire, for example, gives off visible light, with a
lot of yellow — and a small amount of green and blue — wavelengths.
As the coals cool, the radiation slides to lower-energy (and longer-wavelength) red
light. By morning, the cooling takes the radiation down below the visible range and into
the infrared. This is the heat you still feel.
During the past 14 billion years or so, the universe itself has gone through a process similar to those coals in your fire. It started out bathed in high-energy gamma
rays. As it expanded and cooled, the type of radiation slid to longer wavelengths —
through visible light, infrared, and finally, down to microwave. The discovery of this
9
X RAYS
GAMMA RAYS
–16
10
m
–14
10
m
The electromagnetic spectrum
shows the difference between highenergy gamma rays and low-energy
microwaves. The universe’s expansion
stretched gamma-ray wavelengths from
10–12 meters to long, 0.2-centimeter microwaves in the CMB. Astronomy: Roen Kelly
so-called cosmic microwave background
and a full understanding of its significance
is one of the most fascinating tales in the
history of cosmology.
Cosmic models
It may be hard to believe with today’s scientific evidence, but not so long ago, scientists had serious doubts about the Big
Bang theory. Everyone knew the universe
is expanding, of course — Edwin Hubble
had settled that in the 1930s. But a group
of British astrophysicists suggested a picture of the universe quite different from
the Big Bang.
Called the steady state model, the theory
agreed with the observation of galaxies
moving away from each other, but it said
ULTRAVIOLET
–12
10
m
new matter (and eventually new galaxies) is
created in the spaces left behind.
In the 1960s, this model was proposed
as a serious alternative to the Big Bang,
although few believed in it. A problem in
deciding which theory was correct was that
both the Big Bang and steady state models
explained the one piece of observational
evidence we had about the universe — galaxies are moving away from each other.
Astronomers needed a new piece of data —
a new observation — that would establish
which theory was correct.
Signal in static
The missing puzzle piece came from an
unexpected source. The early 1960s was
when trans-Atlantic television transmissions started. The system — unbelievably
primitive by modern standards — bounced
TV signals off an orbiting mylar balloon,
about 100 feet (30 meters) across.
Getting these signals through was such
a technological feat at the time that a legend “Live from Europe” would appear at
–10
10
10– 8 m
m
the bottom of the TV screen when they
were shown.
Because scientists were pointing receivers
at the sky to get a signal, they needed to know
exactly what else was coming in from other
sources — sources that interfered with the
weak beam from the satellite. Two scientists at
Bell Labs in New Jersey — Arno Penzias and
Robert Wilson — worked to answer this
question. Because TV signals are in the
microwave part of the spectrum, the two took
an old microwave antenna, pointed it at the
sky, and began to survey what was out there.
They quickly ran into a problem. No
matter which way they pointed their
antenna, they found a faint whisper of
microwaves raining down.
In the field of electronics, detecting a
signal coming from every direction usually means there is a problem with the
equipment circuitry. To Penzias and Wilson, the faint hiss was a red flag, an indication that they needed to check their
apparatus for flaws.
They evaluated one circuit after another.
They even noted that pigeons had roosted
in their antenna, coating the interior with a
“white dielectric substance,” which they
removed. But the hiss persisted.
Then, a colleague mentioned a group of
astrophysicists at Princeton University who
were examining the consequences of the Big
Bang. In fact, those astrophysicists had
determined the cooling universe should be
bathed in microwave radiation, which could
be thought of as an echo of the Big Bang.
Observation becomes proof
The best way to understand this prediction
is to refer back to the campfire example.
This time, imagine you are watching it cool
WHILE SURFING THE RADIO, Robert Wilson
(left) and Arno Penzias unexpectedly discovered the CMB with this horn-type antenna. This
picture was taken in 1978, after they received
their Nobel Prize. Astronomical Society of the Pacific
10
VISIBLE
MICROWAVES
INFRARED
m
from inside the coals. At the beginning, you
would see visible light coming at you from
all directions. As the campfire cools, the
radiation slides to red, then to infrared, but
it is always coming from every direction.
In the same way, the theorists argued,
observers on Earth should see radiation
raining down from all directions. As the
universe cooled and expanded, the radiation
dropped to long-wavelength microwaves.
The faint hiss the observers couldn’t get rid
of was precisely the signal that should have
been there. What had begun as a routine
survey became a dazzling revelation about
the cosmos. Penzias and Wilson shared the
1978 Nobel Prize in physics for their work.
The CMB discovery removed all doubts
about the Big Bang and sent the steady state
model to the dusty back room. During the
next couple of decades, whatever controversy existed about the CMB centered on
whether it is the kind of radiation we
should expect from an object that has been
cooling for 14 billion years.
–1
10 m
⁄
10 m
B l a c k B o d y R a d iati o n
The Universe’s Temperature
Any object with a temperature above absolute zero radiates a range of energy. The
energy radiated follows a spectral distribution curve — known as a blackbody curve. The
curve is typically shown as intensity versus wavelength, and it peaks at a characteristic
wavelength, which corresponds to a particular temperature.
The spectral distribution curve eventually led to the origins of quantum theory. When
deriving the calculation, physicist Max Planck determined the energy emitted or absorbed
must be in discrete
packets — quanta, or
Frequency (gigahertz)
photons. The blackbody
60 100
300
400
500
200
600
curve is also known as
400
the Planck radiation
curve. — Liz Kruesi
Temperature = 2.725 K
300
200
100
A perfect fit
The laws of physics require that every
object above absolute zero gives off radiation, and the lower the temperature, the
longer the radiation wavelength. The laws
say precisely how much of each type of
radiation a body at a given temperature
should emit. They also say an object radiates a range of wavelengths: a small amount
of long- and short-wavelength radiation,
with most of its energy radiated at a particular wavelength that depends on the
object’s temperature.
The Sun, for example, has an outer surface
at about 5,800 kelvins, so it emits most of its
radiation in the form of visible light. It also
gives off small amounts of both radio and
ultraviolet — the latter is obvious every time
you get a sunburn. The curve that shows the
amount of radiation at each frequency for a
given object is called a blackbody curve (see
“The universe’s temperature,” to right).
1m
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0.5
0.2
0.1
0.07
0.05
Wavelength (centimeters)
NASA’S COBE measured the CMB’s spectral distribution and
found it has a perfect
blackbody curve — the
best example found in
nature. Its maximumintensity peak, at
roughly 0.2 centimeter,
corresponds to a temperature of 2.725 K.
Intensity is given as
energy flux, jansky, per
solid angle, steradian.
A roaring campfire gives off visible white light. As it
cools, the emitted radiation slides to red, and eventually infrared, which is the heat you feel the following day. This is an example of a
blackbody: an object that radiates
energy, with the most prominent wavelength corresponding to a certain
temperature.
Astronomy: Roen Kelly
10
–2
Astronomy: Roen Kelly, after NASA/COBE
10 m
RADIO WAVES
–4
Intensity (megajansky per steradian)
–6
11
Cosmic
Background
Explorer (COBE)
Wilkinsin
Microwave
Anisotropy Probe
(WMAP)
THE ANGULAR RESOLUTION difference between COBE and WMAP is staggering.
COBE was launched in 1989; its data was released in 1992. The scientific community was
in awe at the sky maps COBE took, limited as they were by the spacecraft’s low angular
resolution (7°). WMAP followed in 2001, with its first data release in 2003. Its sky maps
show far greater detail than COBE because the probe has an angular resolution of 0.3°,
or 18'. NASA/WMAP Science team
Earth’s atmosphere absorbs some microwave wavelengths, so parts of the expected
spectral curve were not visible to groundbased antennae. The search to fill in the
gaps of the blackbody curve was postponed
until the technology was ready, in 1989,
when NASA’s Cosmic Background Explorer
(COBE) satellite was launched.
Sitting above Earth’s obscuring atmosphere, COBE measured the radiation with
unparalleled accuracy. When University of
California at Berkeley astronomer George
Smoot first presented the COBE data at a
scientific meeting in 1992, the audience of
sober, tweed-clad scientists burst into wild
James Trefil, a member of Astronomy’s editorial board, is a professor of physics at George
Mason University in Fairfax, Virginia.
applause when the perfect blackbody curve
was flashed on the screen.
Answers raise more questions
The microwave radiation was exactly what
it should have been for a universe that had
cooled to an average temperature of 2.725
K above absolute zero. But there was
another problem.
Although the microwaves fall on Earth
from all directions, as expected, the radiation is the same no matter which direction
we look. This is true to an accuracy of
about one part in 100,000, which means
that all parts of the universe, whether we
look up, down, or sideways, are at the
same temperature.
To understand why this poses a problem,
think about another everyday experience. If
you’re in your bathtub and you feel the
water cool off, you turn on the hot water. At
first, the water is warmer near the tap than
at the far end of the tub, but after a while, all
the water comes to the same temperature. In
the language of physics, the water in the tub
eventually reaches thermal equilibrium.
The CMB tells us that the universe, like
the water in the bathtub, is in thermal
equilibrium. It has the same average temperature everywhere.
The problem is introduced when you
trace the universe’s expansion backward.
There wasn’t enough time for the universe
to establish thermal equilibrium before the
expansion moved the universe’s contents
away from each other. It would be as if you
turned on the hot water in your bathtub
and the temperature of the water rose
instantly, even in the most distant parts.
This was called the horizon problem,
because the universe’s expansion would
have moved matter from one part of the
universe so much that, as seen by other
parts, that matter would appear to have
moved over the horizon. Different sections could not communicate with each
other to establish thermal equilibrium.
This problem — along with others —
The CMB has given us
new insights into both
the beginning and the
end of the universe.
was resolved in the 1980s with the development of inflation theory (see “Seeing
the dawn of time,” Astronomy, August
2005, for more information).
According to inflation, the universe in
its earliest stages was much smaller than
you would expect if you do a backward
extension of the expansion; the universe
was smaller than a sub-atomic particle. In
this small state, thermal equilibrium was
easy to establish. Then there was a period
12
of very rapid expansion — called inflation
— driven by forces that can be calculated
from the theory of elementary particles. In
this expansion, the universe grew to the
size of a softball, and eventually to the universe we see today.
⁄
C o n ti n u i n g t h e Q u e st
Next up for the Challenge
So the regularity of the background taught
us one lesson about the early evolution of
the universe, and the small differences —
the ones that amount to one part in 100,000
— taught us another.
For the first 400,000 years or so, the universe was so hot that no atoms could form.
If an electron attached to a nucleus, the
next collision would knock it off.
Matter consisted of what physicists call a
plasma — gas that is so hot it is ionized and
has loose electrons and nuclei knocking
around. (The Sun is mostly plasma, and
there is a partial plasma in every operating
fluorescent lightbulb.) Plasmas absorb radiation, which means light and other forms of
radiation get trapped in them and are constantly absorbed and re-emitted. Astrophysicists say the universe was “opaque”
during this time.
Once the universe was about 400,000
years old, it had cooled to the point that neutral atoms could form — electrons and
nuclei combined. This moment is called
“recombination” (even though there wasn’t a
previous “combination”). Atoms are largely
transparent to radiation — which is why
light can travel such long distances through
Earth’s atmosphere. After recombination, the
radiation that was initially trapped in the
universe’s plasma was free to escape.
The microwave background is the highenergy photons that were released when
atoms formed, and the photons have spent
the last roughly 14 billion minus 400,000
years being stretched out into microwaves
as the universe expanded.
This is why the seemingly insignificant
differences (scientists call them “anisotropies”) in the CMB are so important. Places
where matter was starting to clump
together would have been at a slightly
ESA/Medialab
What those spots tell us
THE THIRD-GENERATION CMB PROBE,
Planck Surveyor, will provide astronomers with even more detail about the
composition and fate of the universe.
higher temperature than the surroundings,
and we should be able to see that difference
by looking at temperature differences in
the CMB’s spots.
The early concentrations of matter
served as the points where matter — and
eventually galaxies — condensed. Because
of this, they often are called “seeds,”
although I prefer the more poetic “ripples
at the beginning of time.”
Ripples were first seen in data from the
COBE satellite. So important is this information that in 2001 another satellite — the
Wilkinson Microwave Anisotropy Probe
(WMAP) — was launched with the specific
mission of measuring the CMB’s spots to
unparalleled accuracy. The comparison
between COBE and WMAP is equivalent
to putting on glasses to read the fine print
in a document.
Forty years later
The detailed measurements of the CMB
provided by many sources, including
In February 2007, the European Space Agency
(ESA) will launch the third-generation CMB
probe: Planck Surveyor. Compared to WMAP,
Planck has roughly twice the angular resolution — 10' versus WMAP’s 18'.
Planck Surveyor involves crucial science
because it will observe a wider frequency
range than COBE or WMAP. Whereas
WMAP covered the frequencies from 23 to
94 GHz, Planck will reach far past that
range, to cover 30 to 857 GHz. This will
allow Planck to improve on WMAP’s measurements by separating foreground signals from CMB signals and by seeing 10
times the frequency range WMAP saw.
With the increased sensitivity, Planck
will provide information on the universe’s
fundamental parameters — including spatial curvature and expansion — in addition
to measuring structure size within the CMB.
The first Planck data release may be in
late 2010. — L. K.
WMAP, have given astronomers precise
information about our universe. We can
now say with 99-percent certainty that the
universe is 13.7 billion years old, and that
the release of radiation from the plasma —
the moment of last scattering — took place
379,000 years after the Big Bang.
WMAP also provides input into our
current understanding that the universe is
made of about 4-percent ordinary matter,
23-percent dark matter, and 73-percent
dark energy. It was, in fact, corroborating
data from WMAP that made scientists so
willing to accept the notion that almost
three-quarters of the universe is in the form
of something called dark energy — something whose composition we still have to
discover, but which will ultimately govern
the fate of the universe.
The CMB has given us new insights into
both the beginning and the end of the universe — not bad for an observation that
started out as a calibration measurement
designed to improve TV reception.
13
Nuclear reactions
in the universe’s
first minutes
made the lightest
elements. How
it happened laid
the groundwork
for everything
that followed.
⁄ ⁄ ⁄ BY Adam Frank
How the
Big Bang
forged the
Neutron
Proton
Energy release
The birth of deuterium. The most fragile of
the light elements, deuterium (H2), formed in the
­universe’s first minutes when protons and neutrons
stuck together. All astrophysical processes destroy
this nucleus, so its abundance has been declining
since the Big Bang. Deuterium’s absorption feature
in the spectra of quasars helps astronomers pin
down its original abundance. The fusion reactions
illustrated here involve photon emission; other, faster
reactions also were present.
All illustrations: Astronomy: Roen Kelly
Three quarks constitute
neutrons and protons
Deuteron
14
© 2013 Kalmbach Publishing Co. This material may not be reproduced in any form
without permission from the publisher. www.Astronomy.com
first elements
M
oments after the Big Bang, as the universe
quickly expanded from an unimaginably
dense, impossibly hot state, something wonderful happened. Over the course of the first
3 minutes, the first elements were born.
Every instant of every day, evidence that the universe
began in a cosmic fireball stares us in the face. Proof that the
universe was once hot and dense resides in the very atoms
from which the stars, planets, and we ourselves are built.
Big Bang nucleosynthesis — BBN, for short — is the field
of astrophysics linking the observed abundances of the chemical elements to theoretical predictions based on the Big Bang.
Along with the universal redshift of galaxies and the cosmic
microwave background, BBN is one of the great pillars on
which modern cosmology stands.
BBN is a remarkable mix of precise astronomical observation and exacting physical theory. Using only the abundances
of the lightest elements, hydrogen and helium, BBN spins out
a detailed picture of our cosmic beginnings. It is a remarkable
tale and a grand triumph of science’s power and precision.
Most amazing of all, the events that drive this story, with consequences stretching across space and time, unfolded in little
more than the span of a typical TV commercial break.
Elemental origins
We recognize more than 116 distinct chemical elements today.
Each appears different to us — copper is metallic and shiny,
Adam Frank is an astrophysicist at the University of Rochester in New York
and a member of Astronomy’s Editorial Advisory Board.
while sulfur is yellow and powdery — because the atoms
­making each element differ. It’s hard to believe scientists
were still vigorously debating the reality of atoms even 100
years ago. But once researchers confirmed the reality of the
atom in the early decades of the previous century, they began
probing its internal structure.
Every atom, they found, contains a central nucleus composed of one or more protons, which carry a positive electric
charge. Hydrogen, the simplest and most abundant element,
has a single proton in its nucleus. It’s the number of protons
in a nucleus that distinguishes one element from another.
The nucleus also may contain another particle, called a
neutron. It’s slightly heavier than the proton and lacks an
­electrical charge. The number of neutrons in a nucleus is
what distinguishes one variation of a single element — called
an isotope — from another.
A third kind of particle, the negatively charged electron,
orbits each nucleus at a great distance. Compared to protons
and neutrons, electrons weigh next to nothing. The discovery
of atomic, and then nuclear, structure answered questions
about the nature of matter that had haunted philosophers
and scientists for 2,000 years.
Until the 1930s, physicists could not explain elemental
abundances. Why is it so much easier to find hydrogen atoms
than, say, iron atoms? And good luck finding a lutetium atom.
Hydrogen is vastly more abundant than iron, which is vastly
more abundant than lutetium. Why?
In 1937, German-American physicist Hans Bethe (1906–
2005) was returning by train to his Ithaca, New York, home
following a nuclear physics conference in Washington. It just
15
Stellar nucleosynthesis predicted a cosmos with too little
might have been the most productive train ride in history: By using
helium. Observations show that helium makes up about 24 percent
the time to explore equations for the newly developing science of
of the universe’s normal matter. Everything heavier accounts for
nuclear physics, Bethe discovered the secrets of stellar fusion. Takless than 2 percent of the total, and all the rest is hydrogen. For
ing into account the high temperatures and densities inferred by
years, astronomers were left scratching their heads at this glaring
astronomers to exist at the centers of stars, Bethe showed how
failure in the midst of a spectacular success.
­simple elements can be squeezed together to form more complex
In fact, the answer had already been found and forgotones, a process that releases energy.
ten. Hiding in their journals was a paper that could
In a single stroke, Bethe showed how the fusion
solve the light-element puzzle. But accepting the
of elements fuels the stars, that stellar cores are
solution it offered meant opening a door to
alchemical furnaces transmuting one kind of
Hiding in
the dawn of time.
matter into another. Bethe’s success convinced physicists and astronomers that the
astronomers’ journals
handiwork of stars could explain all the
Beyond steady state
elements and their abundances.
In 1948, Ralph Alpher (1921–2007),
was a paper that could
They were both right and wrong.
a wiry, young graduate from George
solve the puzzle, but its
In 1957, British astronomers Geoffrey
Washington University, wrote a doctoral
and Margaret Burbidge, American
thesis that began, for the first time, at the
solution meant opening
astronomer Willy Fowler, and British
beginning. Under the tutelage of George
a door to the dawn
astrophysicist Fred Hoyle published a
Gamow (1904–1968), a Russian-refugee
monumental work that put the theory of
physicist
known as much for his heavy
of time.
stellar nucleosynthesis on firm ground. Often
drinking as for his genius, Alpher set out
known as B2FH, the paper refined earlier studies
to describe nuclear physics in the realm of an
infant expanding universe.
into a single coherent picture that accounted for the
It’s difficult to imagine now how bold, how radical
observed abundances of elements — almost.
this endeavor was. In 1948, few scientists were thinking about
While the astronomers could nail down elements like carbon,
­cosmology, and those who were had locked themselves into the
oxygen, and iron, their model couldn’t get the simplest elements
so-called steady-state model. Steady-state cosmology held that,
right. The theory predicted
even with expansion, the universe never changed its appearance
hydrogen and helium
or its condition. The cosmos had always looked — and always
proportions that were
completely different would look — just as it does now.
Gamow and Alpher were beyond the leading edge. The origin
from what astronof the elements had always been a cherished problem to Gamow.
omers observe.
Deuteron
He asked Alpher to imagine what might happen in a
universe that started out small, hot, and dense
and expanded to its present enlarged, cold,
tenuous state. Specifically, Gamow asked
Alpher to work out the nuclear reactions that might occur during the hot,
Triton (H3)
dense period. Within the space of a
year, Alpher had worked out many
Neutron
Energy release
Helium genesis, part 1. Primordial
fusion also created tritium (H3), an
­unstable, radioactive form of hydrogen.
Some tritium nuclei captured a proton to make normal helium (He4).
Stars also make helium, so this
element is ever more common in
the universe. Ionized hydrogen
gas clouds in other galaxies clue
astronomers into helium’s abundance before stars shone.
Proton
Energy release
Normal helium (He4)
16
Nuclear speak
GEORGE GAMOW, a RussianAmerican scientist and a
­pioneer in nuclear physics,
suggested the universe
­originated from a hot sea
of radiation and particles.
AIP Emilio SegrÈ Visual Archives
AIP Emilio SegrÈ Visual Archives
Big Bang
The event that spawned
space, time, and the
expanding universe.
RALPH ALPHER and Gamow
found that observed abundances of light elements,
like hydrogen and helium,
are a consequence of a hot,
expanding early universe.
of the crucial aspects with meticulous attention to mathematical
detail. It was a triumphant physics tour de force — but one the
­scientific establishment promptly forgot. While Alpher’s first cal­
culations contained some missteps, they got the fundamentals of
Big Bang nucleosynthesis correct.
Alpher, along with collaborator Robert Herman, spent the next
few years refining his models and examining the implications of a
cooling and expanding universe. The team even predicted the presence and temperature of a cosmic microwave background from the
redshifted light released when the universe had cooled enough that
electrons could combine with nuclei to form atoms.
Alpher said he and Herman expended “a hell of a lot of energy”
giving talks to convince astronomers that the results deserved a
serious look. But their work received little attention, and, in a tragedy of cosmic proportions, the two physicists ultimately gave up in
frustration. Alpher left academia to work for General Electric while
Herman moved on to General Motors Research Laboratories.
In the mid-1960s, the weight of new data finally forced the
acceptance of Big Bang cosmology. But, even then, Alpher’s
immense contribution was largely ignored, with credit given
almost solely to Gamow.
In the years since, others have refined the picture Alpher and
Gamow first glimpsed. Its predictions of simple-element abundances prove that we understand something about cosmic
­origins. The secret of BBN, the secret Alpher, Gamow, and
Herman knew first, occurs just after the cosmos began.
Fusion and the Big Bang
Astronomers see galaxies rushing away from one
another in today’s expanding universe. But if we
could run cosmic evolution backward, everything
would draw together. The cosmos would become
denser and hotter. As the clock runs backward
toward the Big Bang, structures like galaxies melt
into a thickening soup of primordial gas. Run the
clock back further, and the gas also breaks down into
a smooth, ultrahot sea of protons, neutrons, and other
Deuteron
A hydrogen nucleus (proton) bound to a neutron;
a nucleus of deuterium.
Fusion
The merger of protons and
neutrons to form atomic
nuclei, accompanied by
a characteristic energy
release. The fusion of
hydrogen into helium
powers the Sun.
Nucleon
A proton or neutron.
Nucleosynthesis
Processes in stars and the
early universe that create
new atomic nuclei from
existing protons and
­neutrons.
Triton
A hydrogen nucleus
­(proton) bound to two
neutrons; a nucleus of
­tritium.
s­ ubatomic particles. At this point, the universe has a temperature
of about 100 billion kelvins. A teaspoon of cosmic matter weighs
more than 100,000 tons.
This is where BBN begins. By going back only to about 0.01
second after the beginning, physicists limit themselves to a temperature and density domain they can work with comfortably.
More than 60 years of particle accelerator experiments validate
their understanding. Running the clock forward from 0.01 second,
BBN describes the universe’s next 3 minutes in astonishing detail.
From the chaos of those first moments, fusion physics leaves an
unalterable imprint on the universe. To choreograph this dance,
BBN requires two critical components — an understanding of
fusion processes and the physical conditions in the young cosmos.
A hydrogen nucleus (denoted H) is a single proton. Helium
nuclei (denoted He4) have two protons and two neutrons. Fusing
hydrogen into helium is a battle between electromagnetism and the
strong nuclear force, two of the four forces that govern the cosmos.
While it’s easy to push neutral neutrons together, every proton
carries a positive electric charge. Like charges repel via the elec­
tromagnetic force, which gets stronger as the particles get closer.
(It’s like trying to force the same poles of two magnets together.)
To fuse into more complex nuclei, protons must overcome this
electromagnetic barrier.
The strong nuclear force is more powerful than electromagnetism. But it has the odd property of kicking in only when protons
and neutrons get really close to each other.
At a high enough density and temperature, protons whiz
around fast enough that some collisions have the energy to push
them past the electromagnetic barrier and trigger fusion. But
because the universe is expanding and cooling, Big Bang nucleosynthesis becomes a race against time.
Beat the clock
The universe’s rapid expansion and cooling leaves only a brief window for nuclear fusion to occur. Einstein’s theory of relativity specifies the expansion rate; nuclear physics specifies the temperature
and density at each moment in cosmic history. But as the young
17
Neutron
Proton
Deuteron
Normal
helium (He4)
Energy release
“Light” helium (He3)
Energy release
Helium genesis, part 2. “Light” helium (He3) also formed
in the Big Bang’s opening act. Stars convert deuterium to
He3 , but, beyond this, little is known. Some argue the
actions of stellar furnaces result in little net He3 production
or destruction. If this is true, the total amount of deuterium
and He3 remains approximately constant.
universe ages, each temperature
and density regime allows only certain particles to exist and certain kinds
of reactions between those particles.
Fusion can’t start until protons and neutrons —
collectively, nucleons — form. A millionth of a second after the Big grinds to a halt. More complex elements must await the first stars
Bang, when the temperature is a mere 2 trillion K, the universe has
— several hundred million years in the future.
cooled enough that quarks can coalesce into protons and neutrons.
Scientists must follow all possible reactions, their pace,
About 1 second after the Big Bang, the ratio of neutrons to
and all their products. Most importantly, physicists
­protons becomes fixed, and fusion reactions can begin. But this
must perform these calculations in a cosmic backwindow of opportunity lasts only 3 minutes. After this time, the
ground of continually changing temperature and
cosmos will have expanded and cooled so much, it won’t support
density. It is a tremendous task. But when the
fusion reactions at all.
smoke clears, BBN predicts exactly how much
As BBN begins, protons outnumber neutrons 7 to 1. The
hydrogen, helium, deuterium, and other
difference emerges because neutrons are slightly
light elements exist in the cosmos.
heavier than protons, and this mass difference
allows a neutron to decay spontaneously into a
From H to us
proton, electron, and a ghostly particle called
While stellar nucleosyntheFusion
a neutrino. Left to its own devices, a lone
sis could not match the
neutron will, on average, decay into a proobservation that helium
reactions begin
ton and an electron in just 15 minutes.
makes up one-quarter of
the cosmos’ mass, Big Bang
about 1 second
nucleosynthesis nails it right out
Save the neutron
after
the
Big
Bang
of the gate. BBN’s main prediction is
Fusion saved the neutrons. They collided
the
copious early production of He4. This
with the abundant protons and fused
and last only
together as a deuteron — the simplest
result ends up being remarkably insensi3 minutes.
compound nucleus. A deuteron, a nucleus
tive to details in the calculation. Barring
of deuterium (denoted H2), is a second stable
major changes to the basic scenario, BBN
always leads to helium production close to the
isotope of hydrogen.
observed amount. Fundamentally, all that really
Deuteron formation can’t start up in earnest
matters is that a Big Bang occurred.
until about 100 seconds after the Big Bang. Once it
Helium abundance isn’t all that sensitive to conditions in the
does, it triggers a cascade of reactions that leads to nuclei with 2
early universe, but deuterium is another story. The denser the early
protons and 2 neutrons — helium. For example, a deuteron may
universe was at the beginning of the fusion era, the more likely it is
collide with a neutron to make tritium (H3), which then collides
that all the deuterium would end up in helium nuclei. That some
with a proton to make normal helium (He4). Or the deuteron
deuterium remains — even 0.01 percent relative to hydrogen —
could collide with a proton to make a nucleus of light helium
tells physicists something about the young universe.
(He3), which then collides with a neutron to make He4.
This and other trace elements let physicists determine the
Other reactions create a small amount of lithium and beryllium.
­universe’s baryonic density — a measure of matter like protons
But that’s as far down the periodic table as we can go before fusion
18
The early universe’s chemical content
Time after Big Bang
1 second
Greater
1 minute 5 minutes
1 hour
Protons (H)
Tritium (H3)
Lesser
(Li 7)
Berylli
um (Be7)
Normal
helium (He4)
Lithium (Li6)
Lithium
(He 4
)
Deuterium (H2)
He
liu
m
Fraction of total mass
Neutrons
Helium (He3 )
100 billion K
10 billion K
1 billion K
The amount of deuterium peaks about 100
seconds after the Big
Bang, but much of it
becomes swept into
helium nuclei. Fusion
with these helium nuclei
then builds lithium and
beryllium. But Be7 isn’t
stable, and the nucleus
decays to Li7 with a halflife of 53 days. Tritium
also decays, with a 12year half-life, to He3. None
of the beryllium or tritium
formed during BBN survives today.
100 million K
Temperature (kelvins)
Triton
Energy release
Lithium nucleus (Li7)
Constructing lithium. The heftiest survivor of Big
Bang nucleosynthesis is lithium (Li7). Some stars produce the element, others destroy it. Astronomers study
its abundance in the atmospheres of stars in our galaxy’s halo to infer its original value. Observations and
stellar models suggest these stars contain about half of
the lithium available before stars began to shine.
and neutrons — with high accuracy. Using precise measurements
of light-element abundances in regions as diverse as stars and
intergalactic clouds, astrophysicists now can claim that the density of normal matter in the cosmos is only around 2 percent
of the value needed to halt the universe’s expansion in the future.
Most astronomers and cosmologists believe the universe’s total
density (the sum of all kinds of matter and energy) exactly equals
this critical density.
Because BBN predicts such a tiny fraction for stuff like us, the
rest of the universe must be composed of dark matter and dark
energy — “dark” in the sense that astronomers don’t yet under-
stand what they are. In this way, BBN not only has provided proof
that a Big Bang must have occurred, but it also gives us strong evidence that we have much to learn about our universe.
Much has changed from Alpher and Gamow’s first calculations
60 years ago to the current era of precision cosmology. Now, a
wealth of high-quality data lets scientists test competing cos­
mological models. But while astronomers have firm reasons for
believing in the reality of the Big Bang, they don’t need to rely on
cutting-edge physics to do so. Big Bang nucleosynthesis shows us
that a brief period of well-understood physics has consequences
that trickle down 13.7 billion years to the universe we observe.
19
on
d
se
c
45
se
c
on
d
10 –
se
co
nd
10 –
40
10–36 to 10–32 second
— Inflation occurs
nd
10 –
15
se
co
nd
10 –
20
se
co
nd
10 –
25
se
co
10–36 second —
Strong nuclear
force splits off
10 –
30
se
co
nd
10 –
35
10–43 second —
Gravity splits off
10–32 to 10–5 second
— Sea of quarks
and antiquarks
IN ITS FIRST
SECOND, the universe witnessed an
explosive rate of expansion known as inflation, the
birth of the four fundamental
forces, and the creation of a
sea of relic neutrinos that is still
with us. Astronomy: Roen Kelly
© 2013 Kalmbach Publishing Co. This material may not be reproduced in any form
without permission from the publisher. www.Astronomy.com
10–5 second —
Protons and
neutrons form
20
The Big
Bang
1
second
The birth of the universe
released a torrent of
neutrinos. Where are they?
10–12 second —
Electromagnetic
and weak nuclear
forces split
co
nd
⁄ ⁄ ⁄ BY Steve Nadis
1s
ec
on
d
10 –
5
se
co
nd
10 –
10
se
A
1 second —
Cosmic neutrino
background
forms
stronomers who study the early universe divide into two
camps: theorists and observers. Theorists routinely
work on ideas like inflation, which began some 10
trillion-trillion-trillionths of a second after the Big
Bang. Observers, on the other hand, hit a roadblock if they try to look back further than 380,000 years after the
Big Bang — when photons of light were first set free and created
the cosmic microwave background (CMB).
Nonetheless, observers may have one path to take them back
further. The Big Bang created a flood of neutrinos — subatomic
particles that rarely interact with matter — that should fill the
­universe with an estimated 300 neutrinos per cubic centimeter,
producing a background like the CMB. And those primordial, or
“relic,” neutrinos date back to a mere second after the Big Bang.
No one has detected a relic neutrino, which by now would have
cooled to a frosty 1.95 kelvins, although scientists have pondered
the problem for decades. The chief difficulty is that neutrinos are
Contributing Editor Steve Nadis is a science writer living in Cambridge,
Massachusetts. He enjoys writing about cosmology and the early universe.
21
Neutrino primer
Z-burst creation
ANISOTROPY
A lack of uniformity in, say, the cosmic background, seen when looking
in different directions.
le
tic
ar
Low-energy
relic neutrino
p
Z-
COBE
The Cosmic Background Explorer, a
NASA satellite launched in October
1989 that made detailed measurements of the background radiation.
High-energy
cosmic-ray
neutrino
r
r
ed
am
be rth
rst Ea
bu rd
Z- wa
to
COSMIC MICROWAVE BACKGROUND
The cooling afterglow of cosmic genesis, released about 380,000 years
after the Big Bang at the time when
matter and radiation parted ways.
Z-particle decays in 3x10–25 second
into an average of about:
1 baryon-antibaryon pair
10 neutral pions Decay into 20
high-energy photons
17 charged pions Decay into
electron-positron and
neutrino-antineutrino pairs
COSMIC NEUTRINO BACKGROUND
The flood of neutrinos cosmologists
suspect was released 1 to 2 seconds
after the Big Bang.
COSMIC STRINGS
One-dimensional defects in the
structure of space-time.
DOMAIN WALLS
Two-dimensional defects in the
structure of space-time.
ULTRAHIGH-ENERGY NEUTRINOS in
cosmic rays rarely should interact with
the Big Bang’s relic neutrinos, creating
a Z-particle and a pronounced dip in
the cosmic-ray energy spectrum.
GRAND UNIFICATION THEORY (GUT)
A theory that combines electromagnetism, the strong nuclear force, and
the weak nuclear force.
INFLATION
A period of rapid expansion that took
place 10–36 second after the Big Bang.
MAGNETIC MONOPOLES
Hypothetical particles that contain
only one magnetic pole.
THE MOON is being targeted by GLUE — the Goldstone Lunar Ultra-high energy
neutrino Experiment. Scientists
hope to see flashes as neutrinos explode in the lunar soil.
T. A. Rector/I. P. Dell’Antonio/NOAO/AURA/NSF
THE 70-METER GOLDSTONE radio
telescope looks for neutrinos crashing
into the Moon. It hasn’t found any. NASA/JPL
elusive by nature, even under the best
­circumstances, and their interactivity
decreases with the square of their energy.
After cooling for nearly 14 billion years,
Big Bang neutrinos would pass through
virtually everything without a trace. In the
unlikely event that a cold neutrino did react
with one of the traps astronomers have laid,
the resulting signal would be “frustratingly
minuscule,” according to University of
Hawaii physicist John Learned.
Thomas Weiler of Vanderbilt University
realized in 1982 that astronomers needed a
different approach — something that went
beyond conventional neutrino searches.
Suppose some neutrinos are flying around
at ridiculously high energies, roughly a
­billion times greater than our best particle
accelerators can achieve. There is a special
energy at which the probability of this
hyperkinetic neutrino interacting with a
cold relic ­neutrino (or antineutrino) goes
way up. This occurs at the resonant energy
of the Z-particle, which is the product of
this neutrino-antineutrino collision.
22
Astronomy: Roen Kelly
Milky Way
THE ANITA-LITE EXPERIMENT flew over Antarctica (with Mount Erebus in the background) aboard a scientific balloon in 2003. The experiment looked for high-energy neutrino interactions with the ice. It came up dry. NASA
To make a Z-particle, the fast-moving
neutrino would need some 1022 to 1023 electron volts (eV) of energy before slamming
into its relatively inert counterpart. Anyone
lucky enough to witness lots of these events
would notice a dip at the Z resonance level
— the energy at which the Z-particle is
produced. Observing such a “Z-dip,” as
Weiler calls it, would confirm a cosmic
neutrino background that parallels the
CMB — and predates it by 380,000 years.
But the technique would yield another
important benefit: Physicists could calculate the neutrino mass because it is dictated
by the Z resonance energy (already firmly
established) and by the measured absorption energy. With this new data, physicists
could, in principle, calculate the masses
of the three known neutrino types — electron, muon, and tau — by identifying
three separate dips.
In this way, Weiler says, we could compute the neutrino spectrum, just as investigators determine atomic spectra by shining
a light on atoms and seeing dark lines at
wavelengths where electrons absorb energy.
Z-particles decay in a fraction of a second
to make new forms of cosmic rays — protons, neutrons, and pions — that decay, in
turn, to make gamma rays and neutrinos.
The hope is that Earth-based observatories
might pick up some of these “Z-bursts” if
they originate relatively nearby.
Weiler knew from the onset that two
incredible things had to happen for his
scheme to work. The first requirement,
that neutrinos have mass, has been borne
out in experiments since his original paper
was published. The second prerequisite,
that neutrinos are somehow accelerated
to tremendous energies, is contingent on
as-yet-unobserved physics. Researchers
have proposed two sources for ultrahighenergy neutrinos. The first is the decay of
so-called topological defects. These hypothetical entities — such as cosmic strings,
domain walls, and magnetic monopoles —
are like defects seen in ice and other crystals and pack lots of energy into a small
space. The second possibility is the decay
of heavy (and still theoretical) particles
­created when the universe was ultrahot.
The requirement for exotic sources,
whose existence has not been verified,
makes the proposition a long shot, Weiler
concedes. No cosmic rays have ever been
detected with energies much above 1020 eV,
THE FORTE (Fast On-orbit Recording
of Transient Events) satellite observed
Greenland in the late 1990s. It looked
for lightning bolts in the ice cap created
by the impact of a high-energy neutrino. It had no success. Los Alamos/Sandia/DOE
yet he’s speculating about particles accelerated to energies 100 to 1,000 times greater.
Scientists will need a new generation of
neutrino and cosmic-ray detectors to confirm this conjecture, along with lots of luck.
Yet the payoff would be great. The CMB
offers a glimpse of the universe when it was
380,000 years old. That’s when the Big Bang
photons decoupled from what had been
an ionized plasma and started streaming
freely. Neutrinos, which interact much
more weakly than photons, decoupled from
matter and radiation when the universe
was just a second or two old. “The neutrino
background would enable us to look much
farther back than the CMB, offering a
direct probe of the universe 1 second after
the Big Bang,” says Andreas Ringwald, a
physicist at DESY in Hamburg, Germany.
Fluctuations in the neutrino background, formed when the universe was
much smaller and denser, would be on
­correspondingly smaller scales than the
fluctuations observed in the CMB. This
would afford scientists new insights on
inflation and the initial conditions that led
to structure formation. Relic neutrinos are
also thought to have played vital roles in
nucleosynthesis and the universe’s general
evolution. “Learning about the physics of
the universe at 1 second,” says Princeton
University’s David Spergel, “could tell us
about the strength of gravity, for example,
while placing interesting constraints on the
number of particles created.”
The contribution to fundamental physics could be immense, claims MIT cosmologist Max Tegmark. “Information about
the number of neutrinos and their masses
is the final frontier of the standard model
of physics. It’s the big unknown — the one
thing that hasn’t been well-measured.”
“The cosmic neutrino background is
a profound prediction of Big Bang theory
and a very frustrating one,” adds University
of Hawaii physicist Peter Gorham. “It’s like
nature’s joke on us: Here’s something that
must exist, but you may never get to see
it.” On the other hand, Gorham admits,
“There’s no physical reason why there
shouldn’t be ultrahigh-energy particles,
so it makes sense to look.”
23
Searching for a neutrino signature
THE COSMIC MICROWAVE BACKGROUND (CMB) appears to show the subtle imprint of relic neutrinos. A simulation of the CMB with
neutrinos (left) matches what WMAP observed. The net effect of the neutrinos (right) is small. Oxford University
He has, in fact, looked for these particles
in what might seem to be the unlikeliest
places: the Moon, Greenland, and Antarctica. In 2003, Gorham and his colleagues
reported on the Goldstone Lunar Ultrahigh energy neutrino Experiment (GLUE).
The team trained the Goldstone radio telescope in California on the Moon to look for
explosions resulting from super-energetic
neutrinos (about 1021 eV) slamming into it.
Such a collision would release an enormous
amount of energy in the form of relativistic
electrons that would, in turn, create a flash
of light. “The burst would be so intense that
for 1 nanosecond, it would be brighter than
the brightest quasar,” explains UCLA phys­
icist David Saltzberg, the project’s lead
researcher. But nothing like this was seen.
Gorham and other researchers looked
for the effects of an ultrahigh-energy neutrino shower on the Greenland icecap,
drawing on data collected by the FORTE
(Fast On-orbit Recording of Transient
Events) satellite from 1997 to 1999. An
impact of this sort would have created a
10-meter-long lightning bolt inside the ice
and an attendant radio pulse. But again, the
team failed to identify such an event.
A 2004 balloon experiment over Antarctica called ANITA-LITE — a prototype of
the more ambitious ANITA experiment
that lifted off in December 2006 — also
came up dry.
The three experiments, Gorham contends, rule out supercharged neutrinos as
the main source of the most energetic cosmic rays. In the 1990s, Weiler had proposed
that collisions between neutrinos and antineutrinos in our galaxy’s halo would cause
Z-bursts that could unleash the highestenergy cosmic rays ever detected on Earth.
The origin of these cosmic rays remains
one of the great mysteries of science
because ordinary cosmic rays can’t travel
far without losing energy through inter­
actions with CMB photons.
Weiler proposed that neutrinos could
travel unfettered across the universe, creating these cosmic rays relatively nearby
through the Z-burst mechanism. “But our
results show there would not be a high
enough flux of ultrahigh-energy neutrinos
to make those cosmic rays,” Gorham says.
PLANCK, a spacecraft launched in May
2009, will examine the cosmic microwave background in unprecedented
detail. It seeks further evidence of the
neutrino background. ESA
“Of course, we have not ruled out the Zburst process itself, which might still be the
best way of directly observing the cosmic
neutrino background.”
“While Tom Weiler’s Z-burst idea is very
cool, we don’t know whether neutrinos get
accelerated to high enough energies to
make it happen,” comments FermiLab
astrophysicist Scott Dodelson. Fortunately,
he says, there are indirect ways of studying
relic neutrinos — an approach he compares
to “seeing how the needle disturbs the haystack, without seeing the needle itself.”
The idea is to look for the imprint of the
neutrino background on the CMB, on galaxy distribution (as seen, for instance, by
the Sloan Digital Sky Survey), and on the
more general distribution of mass revealed
by gravitational-lensing experiments. “The
CMB is like money in the bank,” Dodelson
says. “We’ve done these experiments for
more than a decade, and they’ve always
delivered more than expected. Those
­studies, combined with galaxy surveys
and lensing experiments, offer a powerful
suite of techniques and the surest way of
getting at the neutrino background.”
Drawing on WMAP findings released
in March 2006 and on large-scale structure
and supernova data, Steen Hannestad, a
physicist at the University of Aarhus in
Denmark, claims to have seen the imprint
of relic neutrinos on the CMB with better
than 99.99 percent confidence. The effect
is subtle, Hannestad explains, “But if the
[relic] neutrinos weren’t there, you’d see
a completely different temperature anisotropy pattern in the CMB.” Because neutrinos constituted about 40 percent of the
energy density in the early universe —
­putting them almost on a par with photons
— they contributed to the expansion of the
universe. However, they did not make a
comparable contribution to structure formation, which has a pronounced bearing
on the temperature pattern observed in the
microwave background.
In a 2005 paper published in Physical
Review Letters, Roberta Trotta of Imperial
College London and Alessandro Melchiorri
of the University of Rome carried the analysis a step further. “Not only can we detect
24
THE PERSEUS GALAXY CLUSTER is a rich collection of galaxies imaged by the Sloan Digital Sky Survey. Astronomers who map the distribution of galaxies in the universe expect to see the imprint of the cosmic neutrino background. Robert Lupton and the Sdss Consortium
the sea of [Big Bang] neutrinos, we can also
detect the fluctuations, or wiggles, on top
of that sea,” Trotta says. These neutrino
anisotropies have a gravitational effect on
the CMB, smoothing out the clumpiness of
the early universe on small scales by streaming out of dense regions at essentially the
speed of light. “The cosmic neutrino background affects the onset of gravitational
collapse,” he adds. “If there were no neutrinos, or fewer neutrinos, star formation and
galaxy formation would start earlier.”
Trotta and Melchiorri ran computer
simulations in the absence of neutrino
anisotropies and compared the results to
actual data from WMAP and the Sloan
Survey. Their analysis affirmed the presence of the neutrino anisotropies at the
95 percent confidence level. “We hope to
learn more about the neutrino wiggles —
not just confirming that the wiggles exist
but also determining their structure and
distribution,” Trotta says. “We’re really just
at the beginning.”
The latest WMAP findings (released
in March 2008) nearly cement the case for
the cosmic neutrino background. The data
confirm its existence to a confidence level
of better than 99.5 percent. The neutrinos
have such a big effect on the CMB that
s­ cientists claim they made up 10 percent
of the universe 380,000 years after the Big
Bang. This compares with the 12 percent
contribution from atoms, 15 percent from
photons, and 63 percent from dark matter.
Tegmark expects to see rapid progress
on this front. By way of comparison, he
says, people started looking for fluctuations
in the CMB soon after it was discovered in
1963. “But zero progress was made toward a
deeper understanding of theory until 1992,
when the COBE results were announced,”
Tegmark says. “It’s the same with the neutrino background: Now that the sensitivity
has reached the point where we can learn
interesting things about it, we’ll learn rapidly.”
Weiler is glad to see advances in indirect
measurements, but he still believes it’s
important to get direct observations “to
make sure what we’re seeing really is the
neutrino background and not the decay
products of relic neutrinos.” FermiLab’s
Dodelson, for instance, has proposed a
“neutrinoless universe” scenario in which
relic neutrinos annihilated themselves
some 10,000 years after the Big Bang, leaving behind a background of other particles.
The search for ultrahigh-energy neutrinos
now awaits experiments that rely on bigger
detector volumes like the full-scale ANITA.
Cosmic neutrino physics may eventually
reach the required level of precision, says
Ringwald. “It took decades to establish reliable sources of [1012 eV] gamma rays, but
now there are dozens of known sources. The
same could happen with cosmic neutrinos.”
Confirmation of Weiler’s Z-dip and
Z-burst hypothesis would have several
­profound ramifications. First would be
the clear-cut detection of the cosmic
­neutrino background. Second would be a
determination of the neutrino mass. Third
would be the observation of physics at
the grand unification theory (GUT) scale,
pointing to exotic GUT particles or even to
more exotic cosmic strings. Each of these
would be important, warranting — in the
opinion of Gorham and Dodelson — an
automatic Nobel Prize.
Rather than getting carried away by
dreams, Weiler is mindful of the harsh
­reality: Although the physics of Z-dips
and Z-bursts looks robust, nature still has
to cooperate by providing ultrahigh-energy
neutrinos from sources that have not yet
been identified — and might not even exist.
“My 1982 idea may still be the best idea we
have for directly detecting relic neutrinos,”
Weiler says. “But 25 years have passed, and
now we need an even better idea.”
25
Before
light
there was
Astronomers are poised to
explore the mysterious cosmic
Dark Ages. /// BY STEVE NADIS
The transition from the cosmic Dark
Ages to the first galaxies took hundreds of
millions of years. Neutral hydrogen may be
the key to what happened during this mysterious epoch. ASTRONOMY: CHUCK BRAASCH AND ROEN KELLY
26
12COSMOS
©
Kalmbach Publishing Co. This material may not be reproduced in any form
⁄ ⁄ ⁄ 2013
2006
without permission from the publisher. www.Astronomy.com
C
old dark matter models of Big
Bang cosmology predict the
emergence of structure during the “Dark Ages” — the
period after the Big Bang’s heat was
unleashed and before the first luminous
sources turned on — but direct evidence
is hard to come by.
Dark matter is invisible, by definition, and has eluded detection efforts.
However, the most abundant element of
“ordinary matter,” hydrogen, has a convenient observational signature. To
astronomers hoping to probe this cosmic dark era, neutral hydrogen’s 21centimeter line may be all they have.
A neutral hydrogen atom consists of
one proton and one electron. The electron’s direction of “spin” — spin is an
intrinsic property of particles — can be
either aligned with or opposed to that of
the proton. The atom has slightly greater
energy if the spins are aligned. Because
of this, when the electron flips its spin
from aligned to opposed — from a
higher to a lower energy state — it emits
energy in the form of a photon with a
wavelength of 21 centimeters [see “At
the atomic level,” page 14].
“That’s why everyone is so excited
about hydrogen’s 21-centimeter line,”
says Ger de Bruyn of ASTRON, the
Netherlands Foundation for Research in
Astronomy.
So where’s the signal?
Radio telescopes will be used to track
primordial hydrogen because any 21cm
radiation from early epochs has been
stretched to meters-long radio waves by
the universe’s expansion. Because of
this, the new telescopes will look at
wavelengths between 1.5 and 5.3 meters.
Snaring these long-wavelength signals,
says Harvard theorist Lars Hernquist,
“is the next big frontier in observational
cosmology” — a sentiment shared by the
people behind the telescopes.
Indeed, radio-astronomy activity
abounds these days, as researchers prepare to hunt for the “cosmic hydrogen
background.” Prototypes have already
been built for several radio surveys: the
LOw-Frequency ARray (LOFAR) in the
Netherlands; the PrimevAl Structure
Telescope (PAST) in China; and the
Mileura Widefield Array (MWA) in
Australia. “This is not just a few experiments,” notes Massachusetts Institute of
Technology (MIT) astrophysicist Miguel
Morales. “It’s the birth of a field.”
Jeff Peterson, a Carnegie Mellon
physicist coleading PAST with XiangPing Wu of China’s National Astronomical Observatory, is looking forward to
practicing astronomy at redshift 17,
some 200 million years after the Big
Bang. “We have quasar astronomy at
redshift 6 — a billion years after the Big
Bang — and cosmic microwave background [CMB] astronomy at redshift
1,000 — a few hundred thousand years
after the Big Bang — but nothing in
“This is not just a
few experiments. It’s
the birth of a field.”
— Miguel Morales
between,” he says. “Here’s a chance to
see 5 times farther than Hubble or Sloan
[Digital Sky Survey] — a chance to
advance astronomy fivefold.”
Astronomers hope to figure out
where neutral hydrogen has accumulated over time and when it reverted to
its ionized form. Space now contains
vast regions of ionized hydrogen, designated HII. After the Big Bang, all gas
was hot and ionized. The universe
cooled as it expanded, and hydrogen
became neutral, with its constituent protons and electrons joining together as
neutral hydrogen at the moment of
recombination. The CMB followed
shortly afterward.
Neutral gas in the intergalactic
medium reionized hundreds of millions
of years later — the exact time is still to
be determined — when it was heated by
radiation from early stars, galaxies, and
quasars. The universe went from ionized
to neutral and back to ionized.
The last phase, called reionization,
may be discernible as a bump in the
sky’s radio background. “Our best
chance of detecting the reionization signal is if the neutral gas disappeared
quickly,” explains Steve Furlanetto of
the California Institute of Technology. If
the change was gradual, it will be harder
to pick out amid the background noise
of the Milky Way and other galaxies.
If astronomers can determine the
frequency of the “bump” in the radio
spectrum when the neutral gas abruptly
vanished, they’ll immediately know the
redshift, which will tell them when
reionization happened. Next, they’ll
look for how it happened: In other
words, what sources lit up the universe?
Zooming in on reionization
The origin of the first stars has fascinated humans for centuries, but interest
in the reionization epoch is more recent.
The research gained momentum in
2003, when investigators on the Wilkinson Microwave Anisotropy Probe
(WMAP) team announced that reionization of the universe’s intergalactic gas
likely occurred roughly 200 million
years after the Big Bang, rather than 1
billion years after, as previously thought.
WMAP narrowed down the onset of
reionization to between 100 million
years (redshift 30) and 400 million years
(redshift 11) after the Big Bang, but it
could not follow the process over an
extended period of time. If reionization
occurred early, at redshift 30, it’s doubtful any neutral-hydrogen instruments
under development will have the sensitivity to detect it. Background emissions
from radio galaxies are noisier at low
frequencies because galaxies — including the Milky Way — give off more lowenergy, and hence low-frequency,
photons than high-energy photons.
An analysis of two distant quasars’
absorption spectra observed by the
Sloan Survey suggests more than 10 percent of the universe’s hydrogen was still
neutral 900 million years after the Big
Bang, according to Harvard’s Abraham
Loeb and University of Melbourne’s Stuart Wyithe. The findings were welcome
news to MIT radio astronomer Jacqueline Hewitt, who heads MWA’s reionization group. “That means it’s worth doing
this experiment because reionization is
still going on,” Hewitt says. Even with
27
At the atomic level
Ly
γ
Ly δ
s
rie
se et)
n
l
ma vio
Ly ltra
(U
β
Ly
Hδ
α
Ly
Hγ
Hβ
Balmer series
(Visible)
6
n=
n= 4
n=5
n=3
n=2
n=
1
Hα
3γ
3β
3α
Hydrogen’s spectrum is the
simplest of all elements. As an
electron falls from a higher to lower
energy level, the energy difference is
emitted as a photon. The most studied
lines in astronomy are hydrogen alpha
(Hα), the smallest fall of the visible Balmer
series; and Lyman alpha (Lyα), the smallest drop
of the ultraviolet Lyman series. The Balmer series
— Hα, Hβ, Hγ, and Hδ — gives rise to the lines commonly seen in a spectroscopic view of hydrogen gas.
Pa
(In sch
fra en
re se
d) rie
s
21cm photon
Spins aligned
Spins opposed
Electron
Proton
flips
Electron
Proton
Zooming in on the lowest energy level — neutral hydrogen’s ground state,
n=1 in the top diagram — the electron’s spin can be either aligned to the proton’s spin
(left) or opposed (right). As the electron’s spin flips from the higher energy state to the
lower energy state, it will emit a 21cm wavelength photon. astronomy: roen kelly
The main projects now underway —
LOFAR, PAST, and MWA — all rely on
multiple antennae with no moving parts,
but their search strategies differ. PAST will
point in just one direction — the North
Celestial Pole — gaining deep observations of a small field of view.
Of these projects, LOFAR has the longest baseline, a 62-mile-diameter (100
kilometers) array, which offers the greatest
sensitivity. MWA is just a mile (1.5 km)
across but has the largest field of view. By
surveying the sky for longer times, MWA
astronomers hope to match LOFAR’s sensitivity. Before long, says Morales, “We’ll
see who guessed right.”
Having different instruments with different designs is critical, says Peterson,
“because this is difficult astronomy, possibly more difficult than the CMB.” While
most challenges for CMB astronomy are
technical in nature, the main problems for
21cm astronomy lie in the sky.
The radio background’s temperature is
200 Kelvin — 75 percent of which is from
the Milky Way, 25 percent from other
radio galaxies and quasars. Whereas the
neutral-hydrogen signal is a mere 20 millikelvin — 10,000 times dimmer. “We can’t
identify individual radio galaxies and
remove their signals,” Peterson says.
Meanwhile, the ionosphere — a layer of
ionized particles between 50 and 200 miles
(80–320 km) above Earth’s surface —
intermittently reflects radio waves, creating havoc for radio astronomers. At higher
redshifts, the problem gets worse.
Radio-frequency interference from television and radio transmitters and airplanes further confounds matters. “It’s
definitely going to be hard,” admits Chris
Carilli of the National Radio Astronomy
Observatory. “If it were easy, we’d have
done it already.”
Resolving a history
present capabilities, she adds, “There
should be something to see.”
Experimental variety
Some technology used in these efforts is
not new. “A telescope like PAST could have
been built years ago, but no one made the
investment,” says Peterson. He and his
Steve Nadis, an Astronomy contributing editor, lives in Cambridge, Massachusetts.
fellow astronomers are deploying standard
TV and radio antennae — items that can
be purchased at a local electronics store.
The ingenuity lies in linking large
numbers of these devices in sophisticated
ways to make them perform like one large
dish. “It takes a lot of computing power to
bring the information together, but computing costs keep falling, unlike the cost of
steel,” notes Morales. “It’s an inexpensive
way of increasing your sensitivity.”
Astronomers have been spurred on by the
potential payoff. “21cm measurements
offer the richest data set on the universe’s
initial conditions that we can see on the
sky,” says Loeb. Calculations he did with
Harvard colleague Matias Zaldarriaga
show neutral-hydrogen anisotropies “contain an amount of information orders of
magnitude larger than any other cosmological probe,” says Loeb.
Whereas CMB measurements are taken
at a single redshift — one moment in time
28
X rays
21cm emission
Quasar
UV
Bubbly universe
ys
ra
The distribution of this neutral gas, in
turn, can reveal whether the ionizing
sources are stars or quasars. Each primordial light source carves out a bubble of ionized gas within an otherwise neutral sea.
Over time, the bubbles expand and overlap
until reionization is complete.
Two scenarios exist. Either stars and
galaxies caused ionization or quasars were
to blame. Each leads to a geometrically
distinct picture. “If quasars dominated the
process, there should be a small number of
large bubbles,” explains Loeb. “If normal
galaxies drove reionization, there’d be a
large number of small bubbles.”
From an observational standpoint,
bubbles around quasars are the easiest
places to start probing reionization
because they’re dramatic features whose
positions are well known. Lincoln Greenhill of the Harvard-Smithsonian Center
for Astrophysics intends to use the Very
Large Array (VLA) in New Mexico to
study these bubbles.
In collaboration with Carilli and others, Greenhill wants to install a dipole
The quasar’s ultraviolet photons, which
the neutral medium absorbs strongly, ionize the gas and thereby cease emissions
[see “Making bubbles in space,” below].
Greenhill hopes to determine the size and
growth rate of the bubbles, while gathering
details about the surrounding medium.
After first tackling the most extreme
cases, radio astronomers will attempt to
observe ever-smaller fluctuations until the
neutral hydrogen background is fully
mapped. The ultimate goal, says Carilli, is
“3-D tomography” — the equivalent of a
CT scan of the universe. “Instruments like
LOFAR won’t have the sensitivity for this
experiment, but all hope is not lost,” he
says. “Unlike WMAP, the COsmic Background Explorer, did not map the CMB.
Instead, it made a statistical measurement,
antenna on each of VLA’s 27 dishes to
intercept low-frequency radio signals. The
initial plan is to examine the three most
distant quasars, all lying beyond redshift
6.2 in the reionization epoch.
When other far-flung quasars are discovered, they’ll be added to the list. By
taking advantage of VLA’s existing infrastructure, says Greenhill, “We can do this
fast and cheap.” With the Smithsonian
Astrophysical Observatory covering the
$100,000 equipment costs, they just need
to secure telescope time.
Greenhill and his colleagues will focus
on the thin “emission shell” lying at the
edge of the bubble — the most likely
source of 21cm radiation. X rays from the
quasar penetrate and heat the neutral gas,
giving rise to 21cm emissions.
21cm
emission shell
Ionized gas shell
making bubbles in space
Quasars emit both X-ray and ultraviolet
(UV) photons. Although both types of photons travel at the speed of light, higherenergy X rays interact less with the
surrounding media, and thus get out faster.
UV photons ionize most of the neutral
hydrogen because they are more abundant
and the gas absorbs these lower-energy
photons more easily. Energy from the UV
photons ionizes the hydrogen by stripping
away electrons from the hydrogen nuclei.
As ionization continues — primarily by UV
photons — a cavity of ionized gas forms
and grows around the quasar, just like a
steadily inflating bubble.
The X rays propagate through the gas
with less energy loss — and penetrate farther — because they are less likely to be
A 21cm EMISSION SHELL is always
radiating away just ahead of the shell
of ionized gas. By looking for these
shells, or bubbles, astronomers will be
able to map reionization.
absorbed. They heat the gas, therefore,
without ionizing it, giving rise to 21cm
radiation. As X rays propagate outward
from the quasar, a thin emission shell —
where 21cm emission takes place — forms
just beyond the ionized bubble. When the
UV photons reach this shell, the gas ionizes, and emission ends. At that point, the
X rays have penetrated farther into the
neutral medium to heat the outlying neutral hydrogen, causing additional 21cm
radiation. The emission shell moves outward radially, always one step ahead of the
UV-driven ionization front. — S. N.
29
astronomy: roen kelly
— neutral-hydrogen measurements can
look at numerous redshifts, and hence different times. The result should be the universe’s definitive history of hydrogen.
To search the Dark Ages, astronomers
have designed instruments to explore both
available spectra: absorption and emission.
In early epochs, when the universe’s gas
was colder than the microwave background, neutral hydrogen absorbed CMB
flux, leaving a telltale imprint on the lowfrequency (radio) end of the spectrum.
Astronomers can trace neutral hydrogen’s distribution in this era by finding
regions with fewer CMB photons. The
technique, according to Loeb and Zaldarriaga, could detect hydrogen clumps in the
nascent universe 300 light-years across —
a resolution roughly 1,000 times better
than WMAP’s.
Once neutral hydrogen is heated above
the CMB temperature by the first stars
(but before reionization), the gas will emit
21cm radiation. Says Carilli, “The 21cm
line is the only direct probe we have of the
neutral intergalactic medium.”
The absorption signal has a lower frequency, and therefore, longer wavelengths
than the emission signal, which occurred
in a different epoch.
A timeline of the universe
Each cosmic epoch blends into the next because direct
cutoffs are difficult to observe. Astronomers are working to
understand parts of each era gradually, to see the full picture eventually. Although many cosmological riddles are
unanswered, and probably will remain so for many years,
next-generation telescopes and detectors will allow astronomers to look back hundreds of millions of years to probe
the main question: How did structures form?
Year
s
after
the B
ig Ba
ng (l
og
1 arithmic
scale
)
10
100
and near-term instruments should be able
to do something comparable.”
De Bruyn agrees, saying researchers
will try a statistical approach first, looking
for fluctuations in 21cm emissions compared to the average background level. To
represent their data, astronomers will produce a “power spectrum” that shows how
fluctuations vary at different scales.
Furlanetto believes a power-spectrum
analysis of 21cm emissions can help estab-
lish the characteristic size of hydrogen
fluctuations throughout the universe’s history because “these variations will peak on
the scale of the bubble size.” Also, a different peak will occur at each redshift, and
each of these peaks should grow over time.
Therefore, Furlanetto says, “At each epoch
in the reionization history, there should be
a preferred scale for the size of bubbles.”
A computational model he devised with
Hernquist and Zaldarriaga suggests a pre-
iza
tio
np
ha
se
ferred scale of
about 10' (1/6 of a
degree — equivalent to
several millions of light-years
in length) for bubbles in the middle of reionization. Today’s instruments
are aiming for that same scale.
Good “seeing”
the era of light
The cosmic Dark Ages ended once the
universe’s neutral hydrogen was completely reionized by the first stars and galaxies. The luminous period that
followed holds almost as many
questions as the Dark Ages.
Because astronomers know
important steps in structure
formation occurred shortly
after the Dark Ages ended and the stars
ignited, they want to explore this era soon.
One of the instruments astronomers are
pinning their hopes on is the James Webb
Space Telescope. NASA plans to launch
that instrument in 2013.
With the formation of the first galaxies,
dust absorbed starlight and re-emitted
infrared radiation. Astronomers think light
from this process created a “cosmic infrared background.” By studying this light,
Ion
Scheduled for a 2013 launch,
the James Webb Space Telescope —
Hubble’s infrared successor — will look
back to early galaxy formation.
Northrop Grumman Space Technology
astronomers can observe the “structural
clumpiness” of the early universe.
The infrared radiation traces early protogalaxies, helping us understand how the
universe’s structure evolved. — Liz Kruesi
Initially, LOFAR will look for the characteristic sizes of ionized hydrogen bubbles;
its search begins around 2007. The array
will comprise 15,000 antennae deployed
in a five-arm spiral spread out over a 62mile-diameter (100 km) area in its initial
phase. Morales is skeptical about
LOFAR’s chance for success in such a
densely populated area. With competing
radio and TV signals, he says “It’s like
doing astronomy in an outdoor disco.”
De Bruyn is confident his team can
deal with those challenges, but he
acknowledges the site’s shortcomings. For
starters, they’ll have to curtail observations at redshift 11.5 to avoid interference
from FM radio broadcasts. But optical
astronomers also have to adapt to clouds
and inclement weather, he says. “Not
everyone can observe in Hawaii or Chile.
30
can point in any direction. “We’ll look for
holes in the galactic radio emissions,” says
Morales, “seeking out the quietest, darkest
places in the sky.” Test observations began
in March 2005. Current plans call for an
early 2007 start date, assuming the U.S.
team can contribute its share of the cost.
Just because one site is best doesn’t mean
other sites can’t support good science.”
In contrast to LOFAR’s crowded setting in the Netherlands, PAST sits in the
remote Ulastai Valley of western China,
shielded from interference by high mountains. “We can choose our frequencies on
the basis of cosmology,” says Peterson,
rather than the whims of FM broadcasting. With a goal of 10,000 antennae
installed upon completion of the project,
PAST will survey the regions that existed
200 million years after the Big Bang,
before they grew into the large-scale
structures we see today.
In Australia, MWA has a similar target, aiming for great big lumps of neutral
hydrogen that will eventually become galaxy clusters, rather than individual galaxies. Although PAST will have a larger
collecting area — 80,000 square
1,00
0
meters versus MWA’s
8,000 — MWA
10,0
00
Groundwork for the future
The neutral-hydrogen signal will have to
be confirmed by more than one instrument because of its faintness on the radio
sky. Verification has proved crucial in
CMB findings, notes Peterson, and it
underscores the value of having multiple
instruments and multiple approaches.
These early “pathfinder experiments”
can help astronomers confront challenges
posed by the ionosphere, radio galaxies,
and interference so the next-generation
radio telescope, the Square Kilometer
Array (SKA), can be optimized before it’s
built (in a location still to be determined)
says Carilli, who chairs SKA’s science
advisory team. Scheduled for operation in
2020, SKA will have roughly 10
times LOFAR’s collect1 mi
llion
ing area.
100,
000
10 m
illion
With the added sensitivity, says Carilli,
“We can go from imaging the most
extreme objects in the universe to imaging
normal objects.” The goal is to examine
small angular scales to chart the early
clumpiness of neutral hydrogen, which
should tell astronomers about structure
formation, inflation, dark matter, and
other cosmic riddles.
If things go well, and SKA lives up to
its billing, there’s the potential to extract a
wealth of information from the neutralhydrogen signal, says Zaldarriaga. But he
still considers the proposition dicey.
“Whether or not this can be done in practice remains to be seen.”
Ron Ekers, a radio astronomer based
at the Commonwealth Scientific and
Industrial Research Organization
(CSIRO) — Australia’s national scientific
research organization — is trying to pinpoint the timing of reionization. What
caught Ekers’ attention is the challenge.
“I’m attracted to difficult experiments,
which makes 21 centimeters a good area
to work in,” he says. When asked at a
recent conference about the highest redshift astronomers can measure, Ekers
replied, “There’s no cutoff. It’s difficult
now, and it just gets harder and harder.”
100
milli
on
1 bil
lion
10 b
illion
Da
rk A
Cosmic
ge
s
microwave
background
First
stars
Re
ion
iza
tio
np
Quick terms
ha
se
Cosmology: The study of the universe.
Photon: A “bullet” of energy; no mass; e.g., visible light, X rays.
Dark matter: 22 percent of the universe; invisible; as of now,
only detected from its gravitational effects.
Redshift: A measure of distance; as the universe expands, light’s
wavelength is stretched and its color shifts toward red.
Cosmic Microwave Background (CMB): The imprint from
when light separated from matter; shows beginnings of structure.
Protogalaxies
form
Galaxies
form
Astronomy: roen kelly/
WMaP science team
Present day
31
In search
of the
first
Detecting the earliest stars will help astronomers unlock secrets of the
infant universe. /// BY RAY JAYAWARDHANA
L
ooking up at the star-studded sky
on a clear night, it’s difficult to
imagine there was a time when the
universe contained no stars, no galaxies, no shining celestial bodies of
any kind. Yet, for a significant stretch of the
first roughly 100 million years after the Big
Bang, total darkness permeated the cosmos.
Then came the first generation of stars, the
brilliant and short-lived ancestors of our Sun
and its kind, which burnt through the dense
fog of primordial hydrogen and helium atoms.
Investigating how these first stars ushered in
the cosmic dawn and made possible all else
that followed — including human life on Earth
some 14 billion years later — is one of the hottest research frontiers in cosmology today.
Hiding in the first billion years
In the beginning, there were no atoms, only a
dense, hot soup of electrons, protons, and
other elementary particles moving around and
Ray Jayawardhana, a professor of astronomy and
astrophysics at the University of Toronto, studies the
formation of planets, brown dwarfs, and stars.
scattering light at all wavelengths. As the infant
universe expanded and cooled below a few
thousand Kelvin, protons could hold on to
electrons to make neutral atoms of hydrogen
and helium for the first time. The cosmic
microwave background — the relic glow of the
hot, early universe — carries the imprint of
cosmic structures at the time of this transition,
380,000 years after the Big Bang. That etching
is remarkably smooth, with density variations
of only 1 part in 100,000.
Cosmologists now use sophisticated computer simulations and deep observations with
giant telescopes to unravel the tale of how the
first stars and galaxies grew out of those tiny
ripples. It’s a story shrouded in darkness, literally. Between the time neutral atoms formed
and the moment the first stars lit up, the universe was in its so-called Dark Ages. Thus,
there’s little chance for us to see directly the
buildup of the first objects. Instead, we have to
rely on theory to guide us.
Using sufficiently powerful telescopes, we
can expect to see the effects of those first stars
on their environment and their immediate
descendants — and, in turn, test the theoretical
32
18COSMOS
© 2013 Kalmbach Publishing Co. This material may not be reproduced in any form
2006
without permission from the publisher. www.Astronomy.com
⁄⁄⁄
stars
THE FIRST STARS FORMED from the universe’s
primordial material: almost all hydrogen and
helium gas, with trace amounts of lithium gas.
Because these stars were massive, they lived
fast and died young, just 2 million years after
their births. Lynette Cook for Astronomy
33
Atomic billiards
Fusion
Deuterium
nucleus
Fusion
Helium-3
nucleus
Fusion
Fusion
Fusion
Helium-4
nucleus
Helium-3
nucleus
Deuterium
nucleus
Proton
Positron
Neutrino
Neutron
Gamma-ray
photon
STARS SHINE as a result of energy released during nuclear reactions. The simplest reaction
— nuclear fusion — combines, or fuses, hydrogen nuclei together to produce helium and
neutrinos. As a result, energy is released. Extremely high temperatures are required to
combine nuclei. The universe’s primordial material was mostly hydrogen and helium nuclei.
Once the gas collapsed enough because of gravity, nuclei were both hot enough and close
enough to fuse. And then there was light: The first star lit up. Astronomy: Roen Kelly
predictions. We have observed some galaxies and quasars shining brightly a billion
years after the Big Bang, so the first stars
must have formed sometime earlier.
Making the first stars
It may seem futile to try to figure out the
birth of the first stars in the distant, unobservable past, when our understanding of
present-day star formation is far from complete. But, in some ways, the recipe was
simpler back then. We don’t need to include
the complicating influences of magnetic
fields threading gas clouds or shocks from
nearby supernovae in the prescription for
the very first stars.
The only ingredients in the mix were
hydrogen, helium, and traces of deuterium
and lithium — plus “dark matter.” Dark
matter accounts for over 80 percent of matter in the universe, and its influence is felt
primarily through gravity. And one doesn’t
need to worry about the cooling effects of
heavier elements or dust produced by previous generations of stars.
In recent years, astronomers have set up
elaborate computer codes to model the formation of the first stars. Perhaps the most
detailed simulations are those done by Tom
Abel of Stanford University, Greg Bryan of
Columbia University, and Michael Norman
of the University of California, San Diego
(see “The universe’s first light” on page 22).
Another group, including Volker Bromm of
the University of Texas, Austin, and Yale
University’s Richard Larson and Paolo
Coppi, has done simulations using a simpler set of assumptions. However, this team
explored a wider range of possibilities.
All of these simulations show that the
minuscule density fluctuations — the
lumps in the primordial soup — in the
early universe acted as seeds for growing
gas clouds. These fluctuations became the
nodes of a network of filaments along
which more gas continued to flow in. These
clouds contracted under their own gravity
and heated up to over 1,000 Kelvin.
A small number of hydrogen atoms
paired up to become hydrogen molecules,
which helped cool the gas by emitting
infrared radiation. Once temperatures in
the densest regions dropped to a few hundred Kelvin, the cloud clumps could contract further. Cooling was crucial: It
allowed ordinary matter, which cooled by
emitting radiation and, therefore, continued contracting, to separate from dark matter. The dark matter didn’t cool and, thus,
remained scattered throughout the cloud.
Some of the densest gas clumps collapsed until they lit up. Because cooling by
small numbers of hydrogen molecules isn’t
very efficient, these star-forming clumps
were much warmer than their counterparts
today. (Dust grains and other molecules
cool present-day clouds more efficiently.)
The gas clumps also had to be more massive in order for gravity to overcome the
outward pressure from the hotter gas. This
means the first star-forming clumps were
probably several hundred times more massive than the Sun.
Did all of that mass go into a single star?
Even the most-detailed simulations do not
show any tendency for these clumps to
fragment as they contract. Thus, theorists
think the first stars were rather massive and
luminous: Estimates for the upper mass
limit range from about 300 to 1,000 solar
masses, and the luminosities could have
been millions of times that of the Sun.
The cosmic dawn
The birth of these first-generation stars
marked the cosmic dawn. These behemoths
were also extremely hot, with surface temperatures approaching 20 times the Sun’s.
They emitted primarily ultraviolet light.
Their energetic radiation would have
started to heat and ionize the neutral
hydrogen atoms in their vicinity, carving
out a growing bubble of ionized gas around
each one. As more and more stars formed
over hundreds of millions of years, these
The first stars’ ejecta
polluted surrounding gas
with ingredients for dust,
planets, and even life.
bubbles would have overlapped, until
nearly all the universe’s gas became ionized.
Most of the first stars probably ended
their brief, but brilliant, lives as exploding
supernovae within a few million years.
Theory predicts stars with masses between
140 and 260 times that of the Sun blow up
completely, expelling the heavier elements
they produce through nuclear fusion. The
first stars’ ejecta seeded surrounding gas
with ingredients for dust, planets, and even
life, and were eventually incorporated into
future generations of stars.
34
The first stars and their descendants
First star
Massive blue star
(100 solar masses)
Blue giant
ONE OF THE FIRST STARS would have been extremely
massive — 100 solar masses in this example — formed
mostly from hydrogen, helium, and a tiny amount of lithium gas. After just a few million years, the star burned its
fuel and ended in fantastic style: as a huge explosion.
The star’s material — including heavy elements — was
ejected. Either its core collapsed as the first black hole, or
the explosion was powerful enough to blow up completely and scatter the star’s material throughout space.
Brilliant
explosion
Sun
Black hole
Sun today
(1 solar mass)
Red giant
Protoplanetary nebula
FOR 10 BILLION YEARS, our Sun steadily burns, converting hydrogen to helium in its core. After that point, the
Sun will expand 100 times, and its outer layer will cool. It
becomes a red giant. The Sun will fuse helium into
heavier elements (carbon and oxygen), burning brighter
and brighter for tens of millions of years. Finally, the Sun
will shed its outer layers — initially as a protoplanetary
nebula — releasing elements crucial to life. What was
once the Sun’s core will contract into a white dwarf. Radiation from the white dwarf will cause the element-rich
material to glow as a planetary nebula. Astronomy: Roen Kelly
White dwarf
Planetary nebula
35Not to scale
www.astronomy.com 21
The universe’s
first light
THE FIRST STAR to form was massive — between 30 and 300 solar
masses — and sat inside the first
forming protogalaxy, as shown by
Tom Abel, Greg Bryan, and
Michael Norman in their computer
simulations. Hydrogen and helium
gas — the universe’s primordial
material — contracted to form the
earliest generation of stars. These
huge stars burned bright, lived
fast, and died young — after just
a 2-million-year life cycle.
Visualization: Ralf KÄhler and Tom Abel
Simulation: Tom Abel, Greg Bryan, Michael Norman
ASTROSPEAK
Dark Ages
The roughly 200,000 years after
the cosmic microwave background radiation was released,
but before the first luminous
sources (stars) turned on.
Deuterium
An isotope of hydrogen whose
nucleus is composed of one
neutron and one proton (compared to hydrogen, which has
no neutrons), nicknamed
“heavy hydrogen”; has twice the
mass of hydrogen.
Quasar
(Abbreviated from “quasistellar object”) the most luminous objects in the universe;
thought to be powered by
supermassive black holes; lie
extremely far away.
Redshift
As the universe expands, light’s
wavelength is stretched and its
apparent color becomes redder;
used as a measure of cosmological distance with the symbol z;
today is z = 0.
Supernova
Explosive death of a star at least
8 times the mass of our Sun;
expels elements into space that
eventually seed planets and life.
At 65 light-years (20 parsecs) across, this
shows cold gas in the center of one of the
first protogalaxies.
Follow the contours to zoom inside clumps
of different gas densities. One star will form
in the center, where the density is highest.
Both above and below this mass range,
dying massive stars are expected to collapse
into black holes without ejecting much of
their mass. These stars didn’t enrich their
progeny by much. But their remnant black
holes may have lit up as “mini-quasars” as
they accreted surrounding material, providing additional sources of light and ionizing
radiation that ended the Dark Ages.
Some of the stellar corpses may have
merged with one another to build up the
cores that seeded protogalaxies. This could
explain why supermassive black holes
appear to lurk in the centers of quasars and
many galaxies. Thus, the demise of the first
stars was perhaps even more important
than their birth to all that came afterwards.
There is little chance of catching the first
stars in their spectacular death throes, even
with today’s largest telescopes. But some of
them may have given rise to gamma-ray
bursts, which are much more luminous
than supernovae, at the edge of the observable universe. Detecting such a distant
gamma-ray burst is our best hope for probing the end of the Dark Ages directly.
million years after the Big Bang (at a redshift of 6.4), they found a telltale signature
of neutral gas: Essentially, all of the quasar’s
ultraviolet light had been absorbed by
hydrogen atoms in the line of sight.
Slightly closer quasars do not show such
complete absorption. These findings suggest the last patches of neutral hydrogen gas
were ionized around that time.
There has been a new twist. Preliminary
measurements of the degree of polarization
in the cosmic background by NASA’s
Wilkinson Microwave Anisotropy Probe
(WMAP) satellite indicate the universe was
reionized much earlier — sometime
between 200 and 500 million years after the
Big Bang (at a redshift of 10–20), rather
than at 900 million years, as implied by the
quasar observations. These findings may
show the two ends of the reionization
period: one the beginning, and one the end.
The apparent discrepancy between the
WMAP and quasar results remains “a genuine big puzzle, and no one knows the physical mechanism that can explain such an
extended reionization history,” says Zoltan
Haiman of Columbia University. The
answer may come from the European Space
Agency’s Planck satellite, to be launched in
2007, which will make more reliable measurements of polarization in the cosmic
microwave background.
From an ancient era
In the meantime, other ways to investigate
those early days exist: by looking for the
effects of those first stars on their surroundings. In 2001, a group of astronomers
led by Robert Becker of the University of
California, Davis, detected possible signs of
the final stages of cosmic reionization.
In the spectrum of one of the most distant quasars known, dating to about 900
Finding different methods
Astronomers would like to find other, perhaps even more distant, quasars and galaxies that could contain the second generation
36
The gas density rises rapidly as the primordial gas radiates and collapses
faster and faster.
At the center of the protogalaxy, where
the gas density is greatest, material continues to collapse into a star.
The white-hot first star — roughly 10
times our solar system’s width — burns
quickly because of its massive size.
of stars. Simulations predict these protogalaxies would be small and relatively faint.
Several surveys have been undertaken
recently, using the Japanese-built Subaru
Telescope in Hawaii and the European
Southern Observatory’s Very Large Telescope in Chile, to look for such objects. A
network of radio dishes now being built in
northern Chile — the Atacama Large Millimeter Array — will search for distant starforming galaxies at millimeter wavelengths.
The observers have come up empty so
far. Unless they get lucky, success may have
to await the launch of the James Webb
Space Telescope, the planned successor to
Hubble, or the construction of a 20- or 30meter optical telescope on the ground.
Directly detecting neutral hydrogen is
another way to probe the reionization era.
Hydrogen atoms emit photons at a characteristic wavelength of 21 centimeters, in the
radio. This radiation from the early universe
would be redshifted to longer wavelengths
— a few meters — by the time it reaches us.
Several new radio observatories are
planned for the near future to look for
hydrogen’s signature. The LOw-Frequency
ARray (LOFAR), with 15,000 antennae
spread over 62 miles (100 km), is under
construction in the Netherlands, and its
prototype is taking data. Ue-Li Pen of the
Canadian Institute for Theoretical Astrophysics and his collaborators in the United
States and China want to begin searching
sooner and at low cost: They are placing
thousands of commercially available TV
antennae on a remote plateau in China and
expect to begin searching for hydrogen
emission within the next year.
during the death throes of the first stars
and incorporated into the next generation.
Timothy Beers of Michigan State University
describes HMP stars as “scribes” of the
early universe. “Their atmospheres retain
the memory of the composition of the gas
from which they formed,” he explains.
To account for the abundance pattern in
these two stars, their progenitors must have
masses as low as 25 times that of the Sun,
according to a new theoretical model by
Nobuyuki Iwamoto and his colleagues in
Japan. To test this emerging scenario,
astronomers now are looking for other
examples of ancient, iron-poor stars. It isn’t
easy to find them because they are dim and
usually live in the outskirts of the galaxy.
The Sloan Digital Sky Survey’s recent
extension includes a search called the Sloan
Extension for Galactic Understanding and
Evolution (SEGUE). This study will collect
spectra of 250,000 stars in the Milky Way
through summer 2008, and it may reveal
other candidates for HMP stars.
“The SEGUE project will allow us, for
the first time to get a ‘big picture’ of the
structure of our Milky Way,” says Heidi
Newberg of Rensselaer Polytechnic Institute in New York.
Clearly, we haven’t unraveled the full
story of the cosmic dawn yet. Some missing
chapters may be deciphered in the spectra
of old stars in the Milky Way’s halo. Other
parts would be revealed in snapshots from
billions of light-years away. And plot twists
could emerge as astronomers put the pieces
together over the next decade.
in our own neighborhood
Other astronomers are searching for clues
closer to home. While the first stars lived fast
and died young, low-mass stars that formed
soon after their deaths may still lurk in the
Milky Way’s halo. Element abundances in
the atmospheres of these old and iron-poor
second-generation stars could tell us how
their massive progenitors lived and died.
tHe DeMise
of the first stars
was perhaps even
more important than
their birth.
Within the past few years, astronomers
have identified two such “hyper-metalpoor” (HMP) stars. HE 0107–5240, located
in the southern constellation Phoenix, and
HE 1327–2326, which lies in Coma Berenices, contain 1/200,000 and 1/300,000 of the
Sun’s iron abundance, respectively. But
these iron-deficient stars are enhanced in
other (lighter) elements, such as carbon.
This was a surprise. Previous theoretical
calculations suggested much iron, but only
small amounts of carbon, would be ejected
37
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