Download Big Bang

The Beginning of Time
The Big Bang
Our goals for learning:
• What were conditions like in the early
universe?
• What is the history of the universe
according to the Big Bang theory?
To what extent can we apply the Scientific Method?
Test theory against observation and experiment.
THE BIG BANG
 We observe the universe to be expanding.
So, it must have started out much smaller.
 Run the Hubble Expansion backwards.
 If you go all the way back to when the universe was an
infinitesimal object – that moment was the beginning of
space and time, the Big Bang. [Remember “space” is expanding
between galaxies.]
 The universe must have been much smaller, hotter and
denser earlier in time. How much hotter?
• Note: 1/H0 is the “age” of the universe for a constant rate of expansion
First, the Big Bang Theory
Temperature versus Time
1 second old
The Early
Universe must
have been
extremely hot
and dense, with
emphasis on
the word
“extreme”.
What happens
under such
Now
conditions?
We know how the laws of physics work up to about 1015 K or 10-10s
PARTICLE CREATION
Photons converted into
particle-antiparticle
pairs and vice-versa
E = mc2
in reverse
The early universe was
full of particles, antiparticles and radiation
because of its very high
temperature. Look at a
SPACE-TIME diagram.
The Planck Era
Before the Planck
time (~10-43 sec)
we don’t know
what happened
because we have
no theory of
quantum gravity.
At extremely small
sizes (~10-33 cm),
Einstein’s Theory of
Gravity and the
Quantum Theory are
not in agreement.
Because space is so compressed there are huge energy fluctuations from point to point. Energy and mass are equivalent,
which implies huge random gravitational fields that warp space and time, the opposite from Einstein ’s smooth
The Forces of Nature
Strong Force: binds nuclei – like a restraining spring
that prevents the (3) quarks that make up protons and
neutrons from escaping
Electromagnetism: binds atoms
Weak Force: responsible for radioactive decay
Gravity: holds planets together
Why FOUR forces?
Do these forces perhaps unify at high temperatures?
Four known forces
in universe:
Strong Force
Electromagnetism
Weak Force
Gravity
Observations at giant particle
accelerators have proved that
these force unite at high energy.
Do the forces unify at even high temperatures?
Four known forces
in universe:
Strong Force
Electromagnetism
Weak Force
Gravity
We are currently looking for
evidence that the first three forces
29 K (10-38s).
merge
around
10
GUT = Grand Unified Theory
What about the temperature of the Big Bang?
Four known forces
in universe:
Strong Force
Electromagnetism
Weak Force
Gravity
Superstring theory is based on the idea that everything is made from
tiny ribbons of energy. Strings are much smaller than we can detect.
Major Events since the Big Bang
GUT Era
Lasts from Planck
time (~10-43 sec) to
end of GUT force
(~10-38 sec).
Perhaps the
simplest
elementary
particles form.
But, a critical
transition may cause
enormous expansion.
When the forces separate it is like a change of phase from liquid to a solid.
Energy was released that drove an ultra rapid expansion of space.
Electroweak Era
Lasts from end of
GUT force (~10-38
sec) to end of
electroweak force
(~10-10 sec)
Intense radiation fills all of space.
Particles create and annihilate.
The universe is expanding and
cooling. Eventually it cools to 100
million times the core of the Sun.
Suddenly weak & electromagnetic
forces separate. From this time/
temperature onwards we have
direct experimental evidence.
Particle Era
Photons convert into all
sorts of matter.
Amounts of matter and
antimatter nearly equal;
ends ~1 milliseconds.
(Roughly 1 extra proton
for every 109 protonantiproton pairs!)
We don’t know why there was such
an excess, but it is the reason we
are all here now.
Era of
Nucleosynthesis
Begins when matter
annihilates
remaining
antimatter at
~ 0.001 sec
Nuclei begin to
fuse; temperature
is now a few billion
degrees.
Era of Nuclei
Helium nuclei
form at age ~ 3
minutes
Universe has
become too cool to
blast helium apart
Expansion and cooling continue
75% H and 25% He
But it is still a hot, opaque plasma
Era of Atoms
Neutral atoms form at
age of ~380,000 years.
At 3,000 K the mix
becomes transparent to
radiation, instead of being
an opaque plasma.
Radiation streams to every
part of the cosmos,
establishing a kind of
background glow – we
detect this today as the
Cosmic Microwave
Background.
Era of Galaxies
Galaxies form at age
~1 billion years.
In principle, we could
“see” this happen using
the next generation of
big telescopes in space
and on the ground.
And here we are, ~14
billion years later,
thinking about all of this!
What have we learned?
• What were conditions like in the early universe?
– The early universe was so hot and so dense that
radiation was constantly producing particleantiparticle pairs and vice versa
• What is the history of the universe according to
the Big Bang theory?
– As the universe cooled, particle production stopped,
leaving matter instead of antimatter
– Fusion turned remaining neutrons into helium (first 3
minutes)
– Radiation traveled freely after formation of atoms (no
more electron scattering); age = 380,000 years
Evidence for the Big Bang
Our goals for learning:
• How do we observe the radiation left over
from the Big Bang?
• How do the abundances of elements support
the Big Bang theory?
Primary Evidence
(in addition to Hubble expansion)
1) We have detected the leftover radiation
from the Big Bang – the Cosmic
Microwave Background.
2) The Big Bang theory correctly predicts the
abundance of helium and other light
elements (Deuterium, Lithium).
The cosmic
microwave
background – the
radiation left over
from the Big Bang
– was detected by
Penzias & Wilson
in 1965.
(Both engineers,
working at Bell
Labs in Murray
Hill, New Jersey.)
No free electrons
means the gas is
transparent
Scattering by
electrons makes
the plasma opaque
Background radiation from Big Bang has been freely
streaming across universe since atoms formed at a
temperature ~3,000 K. Thermal Spectrum: Visible/IR
photons with wavelength of 1 millionth of 1 meter.
Background has perfect
The young universe
thermal radiation
had
a THERMAL
spectrum
at temperature
2.73 K SPECTRUM
Peak wavelength = 1 micrometer
But …
Expansion of the universe has redshifted thermal radiation
from that time to ~1000 times longer wavelengths: the
photons now have mm wavelengths. These are microwaves.
Patterns of structure observed by WMAP tell us “genetic
code” of universe. WMAP is the Wilkinson Microwave
Anisotropy Probe satellite which has been mapping the
cosmic microwave background.
REMEMBER THIS REACTION IN THE SUN’S CORE?
Protons and neutrons combined to make long-lasting helium
nuclei when universe was ~ 3 minutes old – because the
(rapidly dropping) temperature was just right.
Big Bang theory prediction: 75% H, 25% He (by mass)
No free neutrons around, all bound in atoms
Matches observations of nearly primordial gases
More detailed evidence – the deuterium
(heavy hydrogen) abundance was
established with the Keck Observatory by
UCSD researchers.
Abundances of
other light
elements agree
with Big Bang
model having
4.4% normal
matter – more
evidence for
WIMPS!
What have we learned?
• How do we observe the radiation left over from
the Big Bang?
– Radiation left over from the Big Bang (when the
plasma became neutral atoms at 3,000 K) is now in
the form of microwaves (due to expansion)—the
Cosmic Microwave Background—which we can
observe with a radio telescope.
• How do the abundances of elements support the
Big Bang theory?
– Observations of helium and other light elements
agree with the predictions for fusion in the Big Bang
theory; we started with 25% He, stars add some more
The Big Bang and Inflation
Our goals for learning:
• What aspects of the universe were
originally unexplained with the Big Bang
theory?
• How does inflation explain these features of
the universe?
• How can we test the idea of inflation?
Mysteries Needing Explanation
1) Where does structure come from, that eventually
leads to galaxy formation?
 The structure begins with tiny, tiny fluctuations in
energy at the quantum level
2) Why is the overall distribution of matter so
uniform?
 How can distant parts of the universe be almost
identical in temperature?
3) Why is the density of the universe so close to the
critical density? Or why does space look FLAT?
Start with quantum ripples in spacetime INFLATION is a
process that can
make all the
structure by
stretching tiny
quantum ripples to
enormous size.
Spacetime expands
much faster than the
speed of light, which
is OK because it is
NOT matter.
These ripples in
density then become
the seeds for all
structures.
Expansion occurs in about 10-36 s
The ripples in density then become the seeds for all structures
Spacetime
Diagram
How can microwave temperature be nearly identical on
opposite sides of the sky (smooth to 1 part in 100,000)?
The
answer …
Regions now on opposite sides of the sky were once
VERY close before INFLATION pushed them far apart
Density =
Critical
Density >
Critical
Density <
Critical
The overall geometry
of the universe is
closely related to total
density of matter &
energy. (Einstein’s
General Theory – matter
tells the universe how to
curve and the curvature
tells matter how to move.)
Spacetime can have
curvature.
These pictures are only 3dimensional representations of
spacetime.
The real universe is bigger
than we can see – cosmic
horizon – distance light
can travel since time of
Big Bang
Inflation of the
universe flattens
the overall
geometry, like the
inflation of a
balloon, causing
overall density of
matter plus
energy to be very
close to critical
density, that is
the geometry of
spacetime is flat.
Patterns of structure observed by WMAP show us the
“seeds” of universe.
Observed patterns of structure in universe agree (so far)
with the “seeds” that inflation would produce.
“Seeds” Inferred from CMB
• Overall geometry is flat
– Total mass+energy has critical density
• Ordinary matter ~ 4.4% of total
• Total matter is ~ 27% of total
– Dark matter is ~ 23% of total
– Dark energy is ~ 73% of total
• Age of 13.7 billion years
In excellent agreement with observations of present-day universe
and models involving inflation and WIMPs!
What have we learned?
• What aspects of the universe were originally
unexplained with the Big Bang theory?
– The origin of structure, the smoothness of the
universe on large scales, the nearly critical
density of the universe
• How does inflation explain these features?
– Structure comes from inflated quantum ripples
– Observable universe became smooth before
inflation, when it was very tiny
– Inflation flattened the curvature of space,
bringing expansion rate into balance with the
overall density of mass-energy
What have we learned?
•
How can we test the idea of inflation?
–
–
•
We can compare the structures we see in detailed
observations of the microwave background with
predictions for the “seeds” that should have been
planted by inflation
So far, our observations of the universe agree very
well with models in which inflation planted the
“seeds” for future gravitational collapse and the
formation of galaxies and stars
How is the “flatness” problem of the universe explained?
•
•
•
•
By the Hubble expansion
By faster-than-light motion of the first particles in the early universe
By space experiencing enormous inflation in size over very short time
By accident. It just happens to be so.
Observing the Big Bang for Yourself
Our goals for learning:
• Why is the darkness of the night sky
evidence for the Big Bang?
Olbers’s Paradox
If universe were
1) infinite
2) unchanging
3) everywhere
the same
Then, stars would
cover the night sky
Olbers’s Paradox
Analogy is with a
forest of trees that
is so dense with
more or less
identical trees that
every line of sight
is blocked.
To solve a paradox,
question the
assumptions!
Night sky is
dark because
the universe
changes
with time!
As we look
out in space,
we can look
back to a
time when
there were
no stars!
The universe
started out
with no stars
and galaxies
13.7 billion
years ago.
It is not
infinite,
eternal and
unchanging!
This is
strong
evidence for
a Big Bang.
What have we learned?
• Why is the darkness of the night sky
evidence for the Big Bang?
– If the universe were eternal, unchanging, and
everywhere the same, the entire night sky
would be covered with stars
– The night sky is dark because we can see back
to a time when there were no stars
Science Fact to Science Fiction
• We know that the Sun is a thermonuclear furnace
• We know that the Sun will not go supernova, instead it will become a
Red Giant star and then a dense White Dwarf the size of Earth
• But heavier stars will explode and become ultra-compact spinning objects
made entirely of neutrons, or if even heavier, they will collapse to become
black holes
• There is evidence for black holes from a few solar masses to billions of
solar masses
• The universe began 13.7 billion years ago in a Big Bang event that
caused the enormous expansion of space we see today
• There are at a minimum of 4-80 billion roughly Earth-sized planets in
orbits around Sun-like stars at the right distance such that, if they have
surface water, it will be liquid and capable of supporting life.
• None of this is science fiction … any more
Final Review – Topics Covered
Since Midterm (2/3rds of final)
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Telescopes
Sun & Stars
Star Birth
Exoplanets
Death of High and Low Mass Stars
White Dwarfs, Neutron Stars and Black Holes
Our Galaxy
Other Galaxies and Active Galaxies
Dark Matter, Dark Energy and the Big Bang