CHAPTER 11
COSMOLOGY IN THE 21ST CENTURY
Topic Summaries
cosmology The study of the nature,
11 . 1 Introduction to the Universe
origin, and evolution of the universe.
Does the universe have an edge and a center?
•
Astronomers conclude that it is impossible for the universe to have an edge
because it introduces logical inconsistencies. If the universe has no edge, then it
cannot have a center.
The darkness of the night sky leads to the conclusion that the universe is not
infinitely old. If it were infinite in extent and age, then every spot on the sky
would glow as brightly as the surface of a star. This problem, commonly known
as Olbers's paradox, leads to the conclusion that the universe had a beginning.
Edwin Hubble's 1929 discovery that the redshift of a galaxy is proportional to its
distance is known as the Hubble law and indicates that the universe is expanding. Tracing this expansion backward in time brings you to an initial highdensity, high-temperature state commonly called the big bang.
The galaxies do not recede from a single point. They recede from each other as
space expands between them.
The CMB is blackbo y radiation with ate peratu eof abp ut 2 (3 K uniforrT).IY
spread over the entire sky. It is the light fr m the lliig bang re e,_qsed m Ill~ er
at the time of recombination and now redshifted by a factor of 1,1 00.
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During the first 3 minutes of the big bang, nuclear fusion converte some o
the hydrogen into helium but was unable to make many other heavy atoms
because no stable nuclei exist with atomic weights of 5 or 8. Today hydrogen
and helium are common in the universe, but heavier atoms are rare.
11.3 Space and Time, Matter and Energy
How does the universe expand?
In its major features, the universe is isotropic and homogeneous. It looks the
same in all directions and in all locations.
Isotropy and homogeneity together lead to the cosmological principle, the
idea that there are no special places in the universe. Except for local differences,
every place is the same.
•
observable universe The part
of the universe that you can see
from your location in space and
in time.
temperature state from wh ich the
expanding universe of galaxies began.
How do you know that the universe began with a big bang?
•
tween theory and evidence regarding
the darkness of the night sky.
big bang The high-density, high-
11 . 2 The Big Bang Theory
The background radiation is clear evidence tha the
big bang.
Olbers's paradox The conflict be-
General relativity explains that cosmic red shifts are caused by the stretching of
photon wavelengths as they travel through expanding space-time.
Model universes with flat (uncurved) space-time are infinite and can expand
forever. A flat universe would have an average density equal to what is called
the critical density. Modern observations indicate that the universe is flat.
The amounts of deuterium and lithium-? show that normal baryonic matter
can make up only about 4 percent of the critical density. Dark matter must be
nonbaryonic and makes up less than 30 percent of the critical density. For the
universe to be flat, there must be another major component aside from baryonic matter and dark matter.
Hubble time The age of the
universe, equivalent to 1 divided by
the Hubble constant. The Hubble
time is the age of the universe if it
has expanded since the big bang at a
'Constant rate.
cosdhc microwave background
radia 'on (CMB) Radiation from the
hot matter of the universe soon after
1
the big bang. The large redshift makes
it appear to come from a blackbody
with a temperature of 2.7 K.
antimatter Matter composed of
antiparticles, which upon colliding
with a matching particle of normal
matter annihilate and convert the
mass of both particles into energy.
The antiproton is the antiparticle of
the proton, and the positron is the
antiparticle of the electron.
recombination The stage, within
400,000 years of the big bang, when
the gas became transparent to
radiation.
dark age The period oftime after
the glow of the big bang faded into
the infrared and before the birth
of the first stars, during which the
universe expanded in darkness.
reionization The stage in the
early history of the universe when
ultraviolet photons from the first stars
ionized the gas filling space.
Key Terrns
Topic Surnrnaries
isotropy The observation that, in its
11 . 4
general properties, the universe looks
the same in every direction.
homogeneity The observation that,
on the large scale, matter is unifo rmly
spread through the universe.
cosmological principle The
assumption that any observer in any
galaxy sees the same g eneral features
of the universe.
open universe A model of the
universe in which space-time is
curved in such a w ay that the universe
is infinite.
closed universe A model of the
universe in which space-time is
curved to meet itself and the universe
is finite.
Modern Cosmology
How has the universe evolved, and what will be its fate?
The inflationary theory pro poses t hat the universe expanded dramatically a
tiny fraction of a seco nd after the big bang.
Inflation solves the flatness probl em because the sudden inflation forced the
universe to beco me flat, just as a spot o n an inflating balloon beco mes flatter
as the balloon inflates.
Inflation solves the horizon pro blem because the part of the universe that is
now o bservable w as so small before inflat ion that energy could move and
equalize the temperature everywhere.
Observations of type Ia supernovae reveal that the expansion of t he universe
is speeding up. Thi s is t houg ht to be due to "d ark energy:•
The natu re of d ark energy is un known. It may be described by Einstein's cosmological constant, or it may change w ith ti me, in w hich case astronomers
refer to it as quint essence.
• The observed value of the Hubble constant implies that th e universe is 13.7
billion years o ld. The future fate of the universe depends on the nature of
d ark energy. If dark energy increases in strength wit h time, t he universe
could end in a big rip.
flat universe A model of the
universe in which space-time is not
curved.
critical density The average
density of the universe needeel to
make its curvature flat.
nonbaryonic matter Proposed
d ark matter made up of particles
other than protons and neutrons
(baryons).
cold dark matter Dark matter that
is made of slow-moving particles.
flatness problem In cosmology,
the peculiar circumstance that the
earl y universe must have contained
almost exactly the right amount of
1. How can Earth be located at
the center of the observa ble
universe if you accept the
Copernican principle?
2. Why couldn't atomic nuclei exist
w hen the universe w as less than
2 minutes o ld?
3. Why must the universe have
been very uniform during its first
50,000 years?
4. What evidence show s that the
expansion of the universe is
accelerating?
matter to make space-time flat.
horizon problem In cosmology,
the circumstance that the primordial
cosmological constant A
supercluster A cluster of galaxy
background radiation seems much
constant in Einstein's equations of
clusters.
more isotropic than can be explained
space and time that represents a
by the standard big bang theory.
large-scale structure The
force of repul sio n.
distribution of clusters and
inflationary big bang A version
quintessence A possible form
superclusters of galaxies in filaments
of the big bang theory, derived
of dark energy that can change in
and walls enclosing voids.
from grand unified theories of
strength as the universe ages.
particle physics, that includes a rapid
big rip The fate of the universe
expansion when the universe w as
very young.
if dark energy increases with time
and galaxies, stars, and even atoms
are eventually ripped apart by
the accelerating expansion of the
universe.