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Neil F. Comins • William J. Kaufmann III
Discovering the Universe
Ninth Edition
CHAPTER 18
Cosmology
WHAT DO YOU THINK?
1.
2.
3.
4.
5.
What is the universe?
Did the universe have a beginning?
Into what is the universe expanding?
How strong is gravity compared to the
other forces in nature?
Will the universe last forever?
In this chapter you will discover…
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cosmology, which seeks to explain how the universe
began, how it evolves, and its fate
the best theory that we have for the evolution of the
universe—the Big Bang
how astronomers trace the emergence of matter and the
formation of galaxies
how astronomers explain the overall structure of the
universe
our understanding of the fate of the universe
Cosmological Redshift
Just as the waves drawn on this rubber band are stretched along with
the rubber band, so too are the wavelengths of photons stretched as
the universe expands.
Bell Labs Horn Antenna
This Bell Laboratories horn antenna at Holmdel, New Jersey,
was used by Arno Penzias (right) and Robert Wilson in 1965
to detect the cosmic microwave background.
In Search of Primordial Photons
The Wilkinson Microwave Anisotropy Probe (WMAP) satellite,
launched in 2001, improved upon the measurements of the
spectrum and angular distribution of the cosmic microwave
background taken by the COBE satellite.
In Search of Primordial Photons
The balloon-carried telescope BOOMERANG orbited above
Antarctica for 10 days, collecting data used to resolve the
cosmic microwave background, with 10 times higher resolution
than that of COBE. All of these experiments found local
temperature variations across the sky but no overall deviation
from a blackbody spectrum.
Spectrum of the Cosmic Microwave Background
The little squares on this graph are COBE’s measurements of the
brightness of the cosmic microwave background plotted against
wavelength. To a remarkably high degree of accuracy, the data
fall along a blackbody curve for 2.73 K. The peak of the curve is
at a wavelength of 1.1 mm, in accordance with Wien’s law.
Microwave Sky
This map of the microwave sky
was produced from data taken by
instruments on board COBE. The
galactic center is in the middle of
the map, and the plane of the
Milky Way runs horizontally
across the map. Color indicates
Doppler shift and temperature:
Blue is where the microwave
background is blueshifted and
warmer, while red is where it is
redshifted and appears cooler.
This Doppler shift across the sky
is caused by Earth’s motion
through the microwave
background. The resulting
variation in background
temperature is quite small, only
0.0033 K above the average
radiation temperature of 2.726 K.
Our Motion Through the Microwave Background
Because of the Doppler effect, we detect shorter wavelengths in the
microwave background and a higher temperature of radiation in that part
of the sky toward which we are moving. This part of the sky is the area
shown in blue in the previous figure. In the opposite part of the sky,
shown in red, the microwave radiation has longer wavelengths and a
cooler temperature.
Unification of the Four Forces
The four physical forces were initially a single force. This symmetry
was broken as the universe expanded and cooled. This figure shows
the time and temperature of the universe when the forces separated
from each other.
Early History of the Universe
Current theory holds that as the universe cooled, the four forces separated
from their initial unified state. The inflationary epoch lasted from 10–36 s to
10–32 s after the Big Bang. Quarks became confined together, thereby
creating neutrons and protons 10–6 s after the Big Bang. The universe
became transparent to light (that is, photons decoupled from matter) when
the universe was about 1.2 x 1013 s (380,000 years) old. The physics of the
Planck era is presently unknown.
Cause of Inflation
(a) The universe formed in an unstable energy state that
(b) began to transition to a stable configuration. (c) This
transition provided the energy that caused inflation.
Observable Universe Before and After Inflation
Shortly after the Big Bang, the universe expanded by a factor of about
1050 due to inflation. This growth in the size of the presently observable
universe occurred in a very brief time (shaded interval).
Observable
Universe
This diagram shows why we only see part of the entire universe. As time passes,
this volume grows, meaning that light from more distant galaxies reaches us. The
farthest galaxies we see (inset) as they were within a few hundred million years
after the Big Bang. These galaxies, formed at the same time as the Milky Way,
appear young because the light from their beginnings is just now reaching us.
While the light from the most distant galaxies we see was traveling toward us, the
universe has been growing. Therefore, objects that appear 13 billion ly away from
us today are actually about 3 times farther away today. (Inset) This image shows
some of the most distant galaxies we have seen.
Pair Production and Annihilation
(a) A particle and an antiparticle can be created when a high-energy
photon collides with a nucleus. (b) Conversely, a particle and an
antiparticle can annihilate each other and emit energy in the form of
gamma rays. (The processes are more complex and are only
summarized in these drawings.)
Evolution of Density
For approximately 30,000 years
after the Big Bang, the
gravitational effects from
photons (ρrad, shown in red )
exceeded the effects of all the
matter in the universe (ρm,
shown in blue). This early period
is said to have been radiation
dominated. Later, however,
continued expansion of the
universe caused ρrad to become
less than ρm, at which time the
universe became matter
dominated.
Era of Recombination
(a) Before recombination, the
energies of photons in the cosmic
background were high enough to
prevent protons and electrons from
forming hydrogen atoms.
(b) As soon as the energy of the
background radiation became too
low to ionize hydrogen, neutral
atoms came into existence.
Structure of the Early Universe
This microwave map of the entire sky, produced from data taken by the
Wilkinson Microwave Anisotropy Probe (WMAP), shows temperature variations
in the cosmic microwave background. Red regions are about 0.00003 K
warmer than the average temperature of 2.73 K; blue regions are about
0.00003 K cooler than the average. (Inset) These tiny temperature fluctuations,
observed by BOOMERANG, are related to the large-scale structure of the
universe today, indicating where superclusters and voids grew. The radiation
detected to make this map is from a time 380,000 years after the Big Bang.
Structure of the Early Universe
Acoustic peaks show the sizes of the hot spots on the inset map in the
previous figure, along with overtones that provide information about the
kinds of matter in the universe.
Galaxies Forming by Combining Smaller Units
This painting indicates how astronomers visualize the burst of star
formation that occurred within a few hundred million years after the Big
Bang. The arcs and irregular circles represent interstellar gas
illuminated by supernovae.
Galaxies Forming by Combining Smaller Units
Using the Hubble and Keck telescopes, astronomers discovered two
groups of stars (arrows) 13.4 billion ly away that are believed to be
protogalaxies from which bigger galaxies grew. These protogalaxies
were discovered because they were enlarged by the gravitational
lensing of an intervening cluster of galaxies.
Galaxies Forming by Combining Smaller Units
The Chandra X-ray Observatory imaged gravitationally bound gas
around the distant galaxy 3C 294. The X-ray emission from this gas is
the signature of an extremely massive cluster of galaxies, in this case, at
a distance of about 11.2 billion ly from us.
Stellar Birth Rates
This figure shows that star formation started quickly in the
life of the universe and has been tapering off ever since.
Stellar Birth Rates
Most of the stars in an elliptical galaxy are created in a brief burst
of star formation when the galaxy is very young. In spiral galaxies,
stars form at a more leisurely pace that extends over billions of
years.
Creation of Spiral and Elliptical Galaxies
A galaxy begins as a huge cloud of primordial gas that collapses
gravitationally. (a) If the rate of star birth is low, then much of the gas
collapses to form a disk, and a spiral galaxy is created. (b) If the rate
of star birth is high, then the gas is converted into stars before a disk
can form, resulting in an elliptical galaxy.
Mapping Dark Matter
(Top) The Hubble Space
Telescope observed that galaxies
in the same direction, but at
different distances from Earth,
undergo different amounts of
gravitational lensing. (Bottom)
Much of this effect is due to dark
matter. By subtracting out the
lensing effects of intervening
galaxies, the distorted shapes of
the galaxies at various distances
enable astronomers to determine
the distribution of dark matter.
Possible Geometries of the Universe
The shape of space (represented here
as two-dimensional for ease of
visualization) is determined by the
matter and energy contained in the
universe. The curvature is either (a)
positive, (b) zero, or (c) negative,
depending on whether the average
matter and energy density throughout
space is greater than, equal to, or less
than a critical value. The lines on each
curve are initially parallel. They
converge, remain parallel, or diverge,
depending on the curvature of space.
Cosmic Microwave Background and the Curvature of Space
Temperature variations in the early universe appear as “hot spots” in the cosmic
microwave background. The apparent sizes of these spots depend on the
curvature of space. (a) In a closed universe with positive curvature, light rays
from opposite sides of a hot spot bend toward each other. Hence, the hot spot
appears larger than it actually is (dashed lines). (b) The light rays do not bend in
a flat universe. (c) In an open universe, light rays bend apart. The dashed lines
show that a hot spot would appear smaller than its actual size.
Dimmer Distant Supernova
SN 1997ff , more than 10 billion
ly away, was dimmer than
expected, indicating that the
distance to it is greater than the
distance it would have if the
universe had been continually
slowing down since the Big
Bang. This supports the notion
that an outward (cosmological)
force is acting over vast
distances in the universe. The
arrow on the first inset shows
the galaxy in which the
supernova was discovered. The
bright spot on the right inset
shows the supernova by
subtracting the constant light
emitted by all the other nearby
objects.
Dimmer Distant Supernova
The distances and brightnesses of many very distant supernovae are plotted
on this diagram. The locations of the most distant supernovae in the upper
region strongly indicate that the universe has been accelerating outward for
the past 6 billion years.
Big Picture of the Evolution of the Universe
This figure shows our current thinking about the evolution of star and galaxy
formation in the early universe, as well as the present-day acceleration of
the universe’s expansion.
Percentages of the Major
Components of the Universe
Summary of Key Ideas
The Big Bang
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Astronomers believe that the universe began as an
exceedingly dense cosmic singularity that expanded
explosively in an event called the Big Bang. The Hubble
law describes the ongoing expansion of the universe and
the rate at which superclusters of galaxies move apart.
The observable universe extends about 13.7 billion ly in
every direction from Earth to what is called the cosmic
light horizon. We cannot see any objects that may exist
beyond the cosmic light horizon because light from these
objects has not had enough time to reach us.
The Big Bang
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According to the theory of inflation, early in its existence,
the universe expanded very rapidly for a short period,
spreading matter that was originally far from our location
(and hence at different temperatures and densities)
throughout a volume of the universe so large that we
cannot yet observe it. The observable universe today is
thus a growing volume of space containing matter and
radiation that was in close contact with our matter and
radiation during the first instant after the Big Bang (and
hence at the same temperature, pressure, and density).
Inflation explains the isotropic and homogeneous
appearance of the universe.
A Brief History of Spacetime, Matter,
Energy, and Everything
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Four basic forces—gravity, electromagnetism, the strong
nuclear force, and the weak nuclear force—explain the
interactions observed in the universe.
According to current theory, all four forces were identical
just after the Big Bang. At the end of the Planck time
(about 10-43 s after the Big Bang), gravity became a
separate force. A short time later, the strong nuclear force
became a distinct force. A final separation created the
electromagnetic force and the weak nuclear force.
Before the Planck time, the universe was so dense that
known laws of physics do not describe the behavior of
spacetime, matter, and energy back then.
A Brief History of Spacetime, Matter,
Energy, and Everything
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In its first 30,000 years, the universe was radiation
dominated, during which time photons prevented matter
from forming clumps. Then it was matter dominated, during
which time superclusters and smaller clumps of matter
formed. Today it is dark-energy dominated. Dark energy of
some sort supplies a repulsive gravitational force that
causes superclusters to accelerate away from each other.
During the first 380,000 years of the universe, matter and
energy formed an opaque plasma, called the primordial
fireball. Cosmic microwave background radiation is the
greatly redshifted remnant of the universe as it existed
about 380,000 years after the Big Bang.
A Brief History of Spacetime, Matter,
Energy, and Everything
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About 380,000 years after the Big Bang, spacetime
expansion caused the temperature of the universe to fall
below 3000 K, allowing protons and electrons to
combine and thereby form neutral hydrogen atoms. This
event is called the era of recombination. The universe
became transparent during the era of recombination,
with the photons that existed back then still traveling
through space today. In other words, the microwave
background radiation is composed of the oldest photons
in the universe.
A Brief History of Spacetime, Matter,
Energy, and Everything
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Clusters of galaxies and individual galaxies formed from
pieces of enormous hydrogen and helium clouds, each
of which became a separate supercluster of galaxies.
All of the superclusters and some of the clusters of
galaxies within each supercluster are moving away from
one another.
Supermassive black holes appear to have “seeded” the
formation of most galaxies.
During the matter-dominated era, structure formed in the
universe. As the universe goes farther into the dark
energy–dominated era, the large-scale structure of
superclusters of galaxies will fade away.
The Fate of the Universe
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The average density of matter and dark energy in the
universe determines the curvature of space and the
ultimate fate of the universe.
Observations show that the universe is flat and that the
cosmic microwave background is almost perfectly isotropic,
resulting from a brief period of very rapid expansion (the
inflationary epoch) in the very early universe.
The universe is accelerating outward and it will expand
forever.
Key Terms
Big Bang
closed universe
confinement
cosmic light horizon
cosmic microwave background
cosmological constant
cosmological redshift
cosmology
dark ages
dark energy
decoupling
era of recombination
expanding universe
Grand Unified Theory (GUT)
homogeneity
inflation
inflationary epoch
isotropy
isotropy problem (horizon problem)
matter-dominated universe
open universe
pair production
Planck era
Planck time
primordial fireball
primordial nucleosynthesis
quark
quintessence
radiation-dominated universe
strong nuclear force
superstring theories
Theories of Everything
universe
weak nuclear force
WHAT DID YOU THINK?
What is the universe?
 It is all of the matter, energy, and
spacetime that will ever be detectable from
Earth or that will ever affect us.
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WHAT DID YOU THINK?
Did the universe have a beginning?
 Yes. It occurred about 13.7 billion years
ago, in an event called the Big Bang.
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WHAT DID YOU THINK?
Into what is the universe expanding?
 Nothing. The Big Bang created space and
time (spacetime), as well as all matter and
energy in the universe. Spacetime is
expanding to accommodate the expansion
of the universe.
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WHAT DID YOU THINK?
How strong is gravity compared to the
other forces in nature?
 Gravity is by far the weakest force.
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WHAT DID YOU THINK?
Will the universe last forever?
 Current observations support the belief
that the universe will last forever.
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