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Chapter 16
Dark Matter, Dark Energy, and
the Fate of the Universe
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Normal matter: The kind of matter we are familiar
with, which is made of atoms and responds to the
electromagnetic force and gravity.
Dark matter: An undetected form of mass that
emits little or no light but whose existence we
infer from its gravitational influence
Dark energy: An unknown form of energy that
seems to be the source of a repulsive force
causing the expansion of the universe to
accelerate
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Contents of Universe
• Normal matter: ~ 4.9%
• Dark matter: ~ 26.8%
• Dark energy: ~ 68.3%
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Rotation curve
A plot of orbital
speed versus
orbital radius
Solar system’s
rotation curve
declines because
Sun has almost
all the mass.
Rotation Curve of the Solar System
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The rotation
curve of the
Milky Way stays
flat with
distance.
Mass must be
more spread out
than in the solar
system.
Rotation Curve of a Spiral Galaxy
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The mass in the
Milky Way is
spread out over
a larger region
than the stars.
Most of the
Milky Way’s
mass seems to
be dark matter!
Encircled Mass as a Function of Distance for a Spiral Galaxy
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The visible
portion of a
galaxy lies
deep in the
heart of a
large halo of
dark matter.
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We can
measure
orbital
velocities in
other spiral
galaxies
using the
Doppler
shift of the
21-cm line
of atomic H.
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Spiral galaxies all tend to have orbital velocities that
remain constant at large radii, indicating large amounts
of dark matter.
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We can
measure the
velocities of
galaxies in a
cluster from
their Doppler
shifts.
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The mass we
find from
galaxy
motions in a
cluster is
about
50 times
larger than
the mass in
stars!
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Clusters contain
large amounts of
X ray–emitting hot
gas.
The temperature of
hot gas (particle
motions) tells us
cluster mass:
85% dark matter
13% hot gas
2% stars
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Gravitational lensing, the bending of light rays by gravity,
can also tell us a cluster’s mass.
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All three methods of measuring cluster mass indicate
similar amounts of dark matter.
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Does dark matter really exist?
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Our Options
1. Dark matter really exists, and we are observing the effects of its gravitational attraction.
2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter.
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Our Options
1. Dark matter really exists, and we are observing the effects of its gravitational attraction.
2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter.
Because gravity is so well tested, most
astronomers prefer option #1.
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What might dark matter be made of?
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How dark is it?
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How dark is it?
… not as bright as a star.
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Two Basic Options
• Ordinary Matter (MACHOs)
— Massive Compact Halo Objects:
dead or failed stars in halos of galaxies
• Exotic Particles (WIMPs)
— Weakly Interacting Massive Particles:
mysterious neutrino-like particles
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Compact starlike
objects occasionally
make other stars
appear brighter
through lensing…
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Compact starlike
objects occasionally
make other stars
appear brighter
through lensing…
… but there are not
enough lensing
events to explain all
the dark matter.
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Why WIMPs?
• There’s not enough ordinary matter.
• WIMPs could be left over from the Big Bang.
• Models involving WIMPs explain how galaxy
formation works.
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Will the universe continue expanding forever?
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Fate of
universe
depends on
the amount
of dark
matter.
Critical
density of
matter
Not enough
dark matter
Lots of
dark
matter
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Amount of matter is ~25% of
the critical density, suggesting
fate is eternal expansion.
Not enough
dark matter
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But expansion
appears to be
speeding up!
Dark energy?Not enough
dark matter
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Estimated age depends on both dark matter and dark energy.
old older oldest
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The brightness of distant white dwarf supernovae tells us
how much the universe has expanded since they exploded.
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An accelerating universe is the best fit to supernova data.
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Chapter 17
The Beginning of Time
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The universe
must have
been much
hotter and
denser early
in time.
Estimating the Age of the Universe
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The early
universe
must have
been
extremely
hot and
dense.
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Photons converted into
particle–antiparticle
pairs and vice versa.
E = mc2
The early universe was
full of particles and
radiation because of its
high temperature.
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Defining Eras of the Universe
• The earliest eras are defined by the kinds of forces present in the universe.
• Later eras are defined by the kinds of particles present in the universe.
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Four known forces
in universe:
Strong Force
Electromagnetism
Weak Force
Gravity
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Four known forces
in universe:
Strong Force
Electromagnetism
Weak Force
Gravity
Do forces unify at high temperatures?
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Four known forces
in universe:
Strong Force
Electromagnetism
Weak Force
Gravity
Do forces unify at high temperatures?
Yes!
(Electroweak)
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Four known forces
in universe:
Strong Force
Electromagnetism
Weak Force
Gravity
Do forces unify at high temperatures?
Maybe
(GUT)
Yes!
(Electroweak)
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Four known forces
in universe:
Strong Force
Electromagnetism
Weak Force
Gravity
Do forces unify at high temperatures?
Maybe
(GUT)
Yes!
(Electroweak)Who knows?
(String Theory)
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Planck Era
Time: < 10-43 s
Temp: > 1032 K
No theory of quantum
gravity
All forces may have been
unified
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GUT Era
Time: 10-43–10-38 s
Temp: 1032–1029 K
GUT era began when
gravity became distinct
from other forces.
GUT era ended when strong
force became distinct from
electroweak force.
This was accompanied by
inflation.
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Electroweak Era
Time: 10-38–10-10 s
Temp: 1029–1015 K
Strong force and
electroweak force now
separate
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Particle Era
Time: 10-10–0.001 s
Temp: 1015–1012 K
Amounts of matter and
antimatter are nearly equal.
(Roughly one extra proton
for every 109 proton–
antiproton pairs!)
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Era of Nucleosynthesis
Time: 0.001 s–5 min
Temp: 1012–109 K
Began when matter
annihilates remaining
antimatter at
~ 0.001 s.
Nuclei began to fuse.
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Era of Nuclei
Time: 5 min–380,000 yrs
Temp: 109–3000 K
Helium nuclei formed at age
~3 minutes.
The universe became too cool
to blast helium apart.
Radiation could not travel far
The universe was “opaque”
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Era of Atoms
Time: 380,000 years– 1Gy
Temp: 3000–20 K
Atoms formed at age
~380,000 years.
The universe became
“transparent”.
Cosmic background
radiation is released.
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Era of Galaxies
Time: ~1Gy–present
Temp: 20–3 K
The first stars and galaxies
formed by ~1 billion years
after the Big Bang.
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Primary Evidence for the Big Bang
1. We have detected the leftover radiation
from the Big Bang.
2. The Big Bang theory correctly predicts the
abundance of helium and other light
elements in the universe.
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The cosmic
microwave
background—
the radiation left
over from the
Big Bang— was
detected by
Penzias and
Wilson in 1965.
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Background radiation from the Big Bang has been
freely streaming across the universe since atoms
formed at temperature ~3000 K: visible/IR.
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Expansion of the universe has redshifted thermal
radiation from that time to ~1000 times longer
wavelength: microwaves.
Background has
perfect thermal
radiation spectrum
at temperature 2.73
K.
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Protons and neutrons combined to make long-lasting helium nuclei when the universe was ~5 minutes old.
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Big Bang theory prediction: 75% H, 25% He (by mass)
Matches observations of nearly primordial gases