Lecture Slides
CHAPTER 16: The Evolution of the Universe
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will,
San Diego City College
Copyright © 2015, W. W. Norton & Company
The Evolution of the Universe
 Explain the evidence
for the Big Bang.
 Describe how the
conditions of the early
universe influenced our
modern universe.
 Predict the fate of the
universe.
Big Bang
 Because light travels
at a finite speed, it
takes time for the light
from a distant object
to reach us.
=> Look-back time
 Distant galaxies have
a large look-back
time.
Class Question
We have introduced the concept of look-back time.
Which of the following statements is correct?
A. We see distant galaxies as they are now.
B. We see distant galaxies as they were in the past.
C. We see distant galaxies as they will be in the
future.
Big Bang: Hubble’s Law
 Previously, we
introduced the Hubble
Law, which pointed to
an expanding universe.
 Galaxies were closer
together in the past.
 Galaxies will be farther
apart in the future.
Big Bang: Hubble Time
 Recall that the Hubble
Constant can be used
to determine the age
of the universe.
 Hubble time: time
when separation
between galaxies was
zero. This is equal to
1/Hubble Constant.
=>Estimate of age of
the universe.
Big Bang: Age of the Universe
 The Hubble time was 13.8 billion years, which
means that the universe started 13.8 billion years
ago. => Big Bang.
 Galaxies are not flying away from each other.
 Space itself is stretching or expanding.
Class Question
The value of H0 is about 70 km/sec/Mpc. Suppose it
were twice as big. Compared to our current
estimates, what would the age of the universe be?
A. The same as now.
B. Younger.
C. Older.
Big Bang: Expansion
 The expansion does not affect atoms, stars, or
anything else, including laws of physics.
 Everything in the universe was once in a tiny volume!
The Big Bang happened everywhere.
Big Bang: Redshift
 As light comes to us
from distant galaxies,
the space the light
travels through
expands, and the light
is also stretched out.
=> Cosmological
redshift
 Greater distance
traveled => greater
redshift.
Big Bang
 If all matter is in a
small volume, it
means conditions
were very hot.
 Due to expansion,
light redshifted,
and temperatures
dropped.
Big Bang: Prediction
 Prediction in the late
40’s by cosmologists
(Gamow and Alpher): a
blackbody spectrum
uniformly redshifted by
the expansion of the
universe to a
temperature of about
5–10 Kelvin.
Big Bang: Cosmic Microwave Background (CMB)
 A blackbody spectrum with
a temperature of about 3 K
was discovered in 1965 by
Penzias and Wilson. They
discovered that no matter
how hard they tried to get
rid of “noise” it was always
there. This led to their Nobel
Prize in 1979.
 The sky faintly glows in
microwaves.
= Cosmic microwave
background (CMB)
radiation.
Big Bang: Microwaves
 The cosmic microwave
background is from the
time when the universe
was young, hot, and
ionized.
 At several hundred
thousand years, the
temperature cooled so
protons and electrons
could form neutral H
atoms.
=> Recombination
Big Bang: Microwaves (Cont.)
Big Bang: Microwaves (Cont.)
Amazing data from the Cosmic Background
Explorer (COBE)
 At this point, light was
no longer blocked from
its travel by all of the
matter.
 The light could travel
freely and cooled
from a temperature of
about 3000K to about
2.7 K, as confirmed by
satellite data. The
cosmic background is
now observed at
microwave wavelengths
(~1mm)
Big Bang: CMB Variations
 Slight variations
in CMB give
clues to the rise
of structure in the
early universe.
 Discovery and
follow-up
observations
awarded Nobel
Prizes in Physics
(for COBE in
2006.
Big Bang: CMB Variations (Cont.)
Big Bang: CMB Variations (Cont.) COBE
data, removing Sun and Galaxy’s motion
Big Bang: CMB Variations (WMAP data)
Big Bang: CMB Variations (Planck data)
Fate of the Universe
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The fate of the universe depends on how much
matter there is in the universe.
Gravity from ordinary and dark matter slows the
expansion.
The faster the universe is expanding, the harder it is
to stop it and make it contract.
The more mass there is, the more gravity there is to
halt the expansion.
Is there enough gravity to halt the expansion?
Fate of the Universe: Critical Density
 Express this amount needed
as a ratio to the critical
density: mass = ρ/ρc
 If mass = 1, density equal
the critical density.
 Universe slows to a stop at
an infinite time.
Fate of the Universe: Ratio the Actual Density
 If mass > 1, the
expansion will
stop and the
universe will
contract.
 If mass < 1,
expansion will
go on forever,
slowing due to
gravity, but
never stopping.
Fate of the Universe: Mass of Dark Matter
 Ordinary stars and galaxies:
only  = 0.02.
 Dark matter in and between
galaxies:  = 0.3.
Fate of the Universe
 At the end of the 20th
century, we thought we
understood the expansion.
 1990s: We were surprised
to discover the expansion
is speeding up!
The vertical axis is the
separation between between
2 points in space
Fate of the Universe: Expansion of the Universe
 If the expansion were
slowing, then the rate
of expansion at earlier
times should be larger than
now.
 We see evidence that the
expansion was instead
smaller in the past, and
larger now!
Fate of the Universe: Accelerating Expansion
 The expansion is
accelerating, not slowing
down.
 This indicates that there is
a type of energy in the
universe that pushes on
space, acting against
gravity.
Fate of the Universe: Dark Energy
 This energy is called dark
energy.
 This dark energy is
associated with the
cosmological constant, .
 Changes possible
outcomes since it is more
difficult for gravity to
reverse the expansion.
Fate of the Universe: Recent Estimates
 Recent estimates:
•  is about 0.7
• mass is about 0.3.
• Sum total is 1, which
means the universe is
“flat.”
 Universe is accelerating
and will expand forever.
Class Question
If the observed density of the universe is greater
than the critical density, what is the fate of the
universe?
A. The universe will exist forever as it is right now.
B. The universe will expand forever.
C. The universe will collapse back on itself.
Early Universe
 Pairs of particles and antiparticles
were created and annihilated in
the early universe.
 Particle type is dependent on
temperature and energy.
=> Higher energy allows for
greater particle mass.
Early Universe (Cont.)
Early Universe (Cont.)
Early Universe: Electron-Positron Pairs
 The universe originally was full of photons,
particles, and anti-particles.
 As the universe cooled, the particles and
antiparticles annihilated each other, somehow
leaving slightly more particles than
antiparticles.
Early Universe: The Standard Model
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There are four forces that govern the universe:
gravity, electromagnetic, weak nuclear, and strong
nuclear.
Each came into being at a different time very early
in the history of the universe.
QED describes the electromagnetic force.
Electroweak describes QED + weak.
Standard model describes the combination
of the ideas in QCD (strong) and electroweak.
Early Universe: Theory of Everything
 In the early universe,
strong force +
electroweak force =
grand unified theory
(GUT).
 Predicts proton decay.
 GUT + gravity = theory
of everything.
 Some GUTS include
extra “compact”
dimensions!
History of
the Universe
Early Universe: The Cooling Universe
 As the universe cooled
after the Big Bang:
• Established physical
forces.
• Photons, electrons,
positrons.
• Matter-antimatter
annihilation, leaving
many photons and a little
normal matter.
Early Universe: The Cooling Universe (Cont.)
 As the universe cooled
after the Big Bang:
• Matter and photons
cooled.
• Atomic nuclei formed
(Big Bang
nucleosynthesis).
• Recombination: atoms,
galaxies, stars.
Inflation: The Flatness and Horizon Problem

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Case for the Big Bang is very compelling, but there
are some puzzles to consider.
The flatness problem: The combination mass +
 is very close to 1. Why?
The horizon problem: The CMB is almost exactly
the same temperature in all directions although
these regions would have lost contact with each
other long ago.
Inflation: Early, Rapid Expansion
 One explanation:
inflation—a very rapid
expansion at extremely
early times.
 Smooths and flattens
the early universe.
 The inflationary Big
Bang is the current
most accepted model
for the origin of the
Universe.
Inflation: Multiverse
 There could be other universes
almost identical to ours.
=> multiverse.
 An infinite universe could contain
an infinite number of disjointed,
observable universes.
Chapter Summary
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The universe started with a Big Bang, which
occurred nearly 14 billion years ago, and has been
expanding ever since.
The universe has no center and no edge, although
the observable universe has limits.
Observations of the cosmic microwave background
support the Big Bang Theory.
Current estimates predict that the universe is both
expanding and accelerating, and will expand
forever.
Astronomy in Action
Observable vs. Actual Universe
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Astronomy in Action
Infinity and the Number Line
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Astronomy in Action
Expanding Balloon Universe
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Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will,
San Diego City College
This concludes the Lecture slides for
CHAPTER 16: The Evolution
of the Universe
wwnpag.es/uou2
Copyright © 2015, W. W. Norton & Company