Chapter 22 Dark Matter, Dark Energy, and the Fate of the Universe

1
Chapter 22Dark Matter, Dark Energy, and the
Fate of the Universe
2
What do we mean by dark matter and dark energy?
3
Unseen Influences

• 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

4
Contents of Universe

• Normal Matter 4.4
• Normal Matter inside stars 0.6
• Normal Matter outside stars 3.8
• Dark Matter 25
• Dark Energy 71

5
What is the evidence for dark matter in galaxies?
6

• We measure the mass of the solar system using the
  orbits of planets
• Orb. Period
• Avg. Distance
• Or for circles
• Orb. Velocity
• Orbital Radius

7
Rotation curve A plot of orbital velocity versus
orbital radius Solar systems rotation curve
declines because Sun has almost all the mass
8
Who has the largest orbital velocity? A, B, or C?
9
Who has the largest orbital velocity? A, B, or C?
Answer C
10
Rotation curve of merry-go-round rises with radius
11
Rotation curve of Milky Way stays flat with
distance Mass must be more spread out than in
solar system
12
Mass in Milky Way is spread out over a larger
region than the stars Most of the Milky Ways
mass seems to be dark matter!
13
Mass within Suns orbit 1.0 x 1011 MSun
Total mass 1012 MSun
14
The visible portion of a galaxy lies deep in the
heart of a large halo of dark matter
15
We can measure rotation curves of other spiral
galaxies using the Doppler shift of the 21-cm
line of atomic H
16
Spiral galaxies all tend to have flat rotation
curves indicating large amounts of dark matter
17
Broadening of spectral lines in elliptical
galaxies tells us how fast the stars are
orbiting These galaxies also have dark matter
18
Thought Question

• What would you conclude about a galaxy whose
  rotational velocity rises steadily with distance
  beyond the visible part of its disk?
• A. Its mass is concentrated at the center
• B. It rotates like the solar system
• C. Its especially rich in dark matter
• D. Its just like the Milky Way

19
Thought Question

• What would you conclude about a galaxy whose
  rotational velocity rises steadily with distance
  beyond the visible part of its disk?
• A. Its mass is concentrated at the center
• B. It rotates like the solar system
• C. Its especially rich in dark matter
• D. Its just like the Milky Way

20
What is the evidence for dark matter in clusters
of galaxies?
21
We can measure the velocities of galaxies in a
cluster from their Doppler shifts
22
The mass we find from galaxy motions in a cluster
is about 50 times larger than the mass in stars!
23
Clusters contain large amounts of X-ray emitting
hot gas Temperature of hot gas (particle
motions) tells us cluster mass 85 dark
matter 13 hot gas 2 stars
24
Gravitational lensing, the bending of light rays
by gravity, can also tell us a clusters mass
25
(No Transcript)
26
All three methods of measuring cluster mass
indicate similar amounts of dark matter
27
Thought Question

• What kind of measurement does not tell us the
  mass of a cluster of galaxies?
• A. Measure velocities of cluster galaxies
• B. Measure total mass of clusters stars
• C. Measure temperature of its hot gas
• D. Measure distorted images of
• background galaxies

28
Thought Question

• What kind of measurement does not tell us the
  mass of a cluster of galaxies?
• A. Measure velocities of cluster galaxies
• B. Measure total mass of clusters stars
• C. Measure temperature of its hot gas
• D. Measure distorted images of background
  galaxies

29
Does dark matter really exist?
30
Our Options

• Dark matter really exists, and we are observing
  the effects of its gravitational attraction
• Something is wrong with our understanding of
  gravity, causing us to mistakenly infer the
  existence of dark matter

31
Our Options

• Dark matter really exists, and we are observing
  the effects of its gravitational attraction
• 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

32
What might dark matter be made of?
33
How dark is it?
34
How dark is it?

• not as bright as a star.

35
Two Basic Options

• Ordinary Dark Matter (MACHOS)
• Massive Compact Halo Objects
• dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
• Weakly Interacting Massive Particles
• mysterious neutrino-like particles

36
Two Basic Options

• Ordinary Dark Matter (MACHOS)
• Massive Compact Halo Objects
• dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
• Weakly Interacting Massive Particles
• mysterious neutrino-like particles

The Best Bet
37
MACHOs occasionally make other stars appear
brighter through lensing
38
MACHOs occasionally make other stars appear
brighter through lensing but not enough
lensing events to explain all the dark matter
39
Why Believe in WIMPs?

• Theres not enough ordinary matter
• WIMPs could be left over from Big Bang
• Models involving WIMPs explain how galaxy
  formation works

40
What is the role of dark matter in galaxy
formation?
41
?
Gravity of dark matter is what caused
protogalactic clouds to contract early in time
42
WIMPs cant contract to center because they dont
radiate away their orbital energy
43
Dark matter is still pulling things
together After correcting for Hubbles Law, we
can see that galaxies are flowing toward the
densest regions of space
44
What are the largest structures in the universe?
45
Maps of galaxy positions reveal extremely large
structures superclusters and voids
46
Time in billions of years
2.2
5.9
8.6
13.7
0.5
35
70
93
140
13
Size of expanding box in millions of lt-yrs
Models show that gravity of dark matter pulls
mass into denser regions universe grows lumpier
with time
47
Models show that gravity of dark matter pulls
mass into denser regions universe grows lumpier
with time
48
Structures in galaxy maps look very similar to
the ones found in models in which dark matter is
WIMPs
49
Will the universe continue expanding forever?
50
Does the universe have enough kinetic energy to
escape its own gravitational pull?
51
Fate of universe depends on the amount of dark
matter
Critical density of matter
Lots of dark matter
Not enough dark matter
52
Amount of dark matter is 25 of the critical
density suggesting fate is eternal expansion
Not enough dark matter
53
But expansion appears to be speeding up!
Dark Energy?
Not enough dark matter
54
old
older
oldest
Estimated age depends on both dark matter and
dark energy
55
Thought Question

• Suppose that the universe has more dark matter
  than we think there is today how would that
  change the age we estimate from the expansion
  rate ?
• A. Estimated age would be larger
• B. Estimated age would be the same
• C. Estimated age would be smaller

56
Thought Question

• Suppose that the universe has more dark matter
  than we think there is today how would that
  change the age we estimate from the expansion
  rate ?
• A. Estimated age would be larger
• B. Estimated age would be the same
• C. Estimated age would be smaller

57
Is the expansion of the universe accelerating?
58
Brightness of distant white-dwarf supernovae
tells us how much universe has expanded since
they exploded
59
Accelerating universe is best fit to supernova
data