1. The Cosmic Perspective
of Cosmology
NOTES: Cosmological Terminology:
Cosmology: the study of the large scale structure
and evolution of the universe.
Homogeneity: the claim the universe has the same density
in all locations at the largest scale.
Isotropy: the claim the universe looks the same in all directions.

2. Cosmology is NOT cosmetology…
though a cosmologist makes up the face of the universe.

3. Cosmology is the study of the large scale structure
and evolution of the universe: in space & time.

4. Homogeneity: the claim the universe has the same density in all locations at the largest scale. (Center for Cosmological Physics at the University of Chicago)

5. Cosmological redshift
z = redshift) = Δλ/λ = v/c (for small v = expansion speed)

6. Isotropy: the claim the universe looks the same in all directions.
Counts of distant galaxies and radio-sources appear
to provide evidence that the universe is isotropic.

7. Pre 1998 Cosmology --ignoring dark energy in space:
In 1998, using Supernova Ia’s,
universe was found to be ruled
by dark energy,
making it accelerate.
We ignore this at first.

8. = density of universe/critical density
Ω (Omega)
= density of universe/critical density
critical density = grams per cubic centimeter

9. Alexander Friedmann (Russian) 1920
(utilizing Einstein’s General Theory):
gave us three possible expanding universes.

10. Destiny = barely stop expanding, it reaches a maximum speed but
With no dark energy:
Flat Universe:
Curvature = 0
Omega = 1
Destiny = barely stop expanding,
it reaches a maximum speed but
with no collapse
Closed Universe:
Curvature > 0
Omega > 1
Destiny = collapse
Open Universe:
Curvature < 0
Omega < 1
Destiny = endless
decelerating expansion

11. Graph of Friedmann’s three expansions:

12. Galaxies are not flying apart through
space, they are being carried away
by the expansion of space itself
(like small marks in the surface
on an inflating balloon).
The Big Bang was not like an
explosion in a pre-existing space,
but rather an explosion of space itself.

13. However, there were problems with Friedmann’s universe
which resulted in the Inflationary Universe of
Andre Linde (Russia1979)
and Alan Guth (US 1980).
Guth got the Nobel Prize for a theory that didn’t work.
Later, Linde adapted his own theory to one that worked.

14. Three problems led to
a theory of early rapid
inflation:
1. Horizon Problem  The isotropy of the microwave background
indicates that regions A and B in the universe were very
similar to each other when the radiation we observe left them,
but there has not been enough time since the Big Bang for
them ever to have interacted physically with one another.
Why then should they look the same?

15. 2. Flatness Problem If the universe deviates even
slightly from critical density, that deviation grows rapidly in time.
For the universe to be as close to critical as it is today,
it must have differed from the critical density in the past
by only a tiny amount.

16. 3. Magnetic monopole problem:
Universe is believed to produces North magnetic poles
without a South, but we don’t observe any.
Friedmann’s universe didn’t expand enough to
make them far apart.

17. Inflation: Scale vs time (sec):

18. Just as ice takes up more volume than water,
space was believed to have undergone a expanded
phase change related to the Grand Unified Theory (GUT)
of elementary particles. (Guth’s Theory). However, this
meant that protons should decay and they don’t.

19. Chaotic Inflation: Among many parallel universes,
one expands like ours with a bubble that bursts and explains
the lumpy structure of the universe necessary to produce clusters,
galaxies, and stars. This is called Scale-free lumpiness.
Andrei Linde (now at Stanford) showed this.

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

21. 22.1 Unseen Influences in the Cosmos
Our goals for learning
What do we mean by dark matter and dark energy?

22. What do we mean by dark matter and dark energy?

23. 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

24. Contents of Universe “Normal” Matter: ~ 4.4% Dark Matter: ~ 25%
Normal Matter inside stars: ~ 0.6%
Normal Matter outside stars: ~ 3.8%
Dark Matter: ~ 25%
Dark Energy ~ 71%

25. What have we learned? What do we mean by dark matter and dark energy?
“Dark matter” is the name given to the unseen mass whose gravity governs the observed motions of stars and gas clouds
“Dark energy” is the name given to whatever might be causing the expansion of the universe to accelerate

26. 22.2 Evidence for Dark Matter
Our goals for learning
What is the evidence for dark matter in galaxies?
What is the evidence for dark matter in clusters of galaxies?
Does dark matter really exist?
What might dark matter be made of?

27. What is the evidence for dark matter in galaxies?

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

29. Rotation curve
A plot of orbital velocity versus orbital radius
Solar system’s rotation curve declines because Sun has almost all the mass

30. Who has the largest orbital velocity?
A, B, or C?

31. Who has the largest orbital velocity?
A, B, or C?
Answer: C

32. Rotation curve of merry-go-round rises with radius

33. Rotation curve of Milky Way stays flat with distance
Mass must be more spread out than in solar system

34. Mass in Milky Way is spread out over a larger region than the stars
Most of the Milky Way’s mass seems to be dark matter!

35. Mass within Sun’s orbit:
1.0 x 1011 MSun
Total mass:
~1012 MSun

36. The visible portion of a galaxy lies deep in the heart of a large halo of dark matter

37. We can measure rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic H

38. Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter

39. Broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting
These galaxies also have dark matter

40. 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. It’s especially rich in dark matter
D. It’s just like the Milky Way

41. 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. It’s especially rich in dark matter
D. It’s just like the Milky Way

42. What is the evidence for dark matter in clusters of galaxies?

43. We can measure the velocities of galaxies in a cluster from their Doppler shifts

44. The mass we find from galaxy motions in a cluster is about
50 times larger than the mass in stars!

45. 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

46. Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass

48. All three methods of measuring cluster mass indicate similar amounts of dark matter

49. 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 cluster’s stars
C. Measure temperature of its hot gas
D. Measure distorted images of
background galaxies

50. Thought Question B. Measure total mass of cluster’s stars
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 cluster’s stars
C. Measure temperature of its hot gas
D. Measure distorted images of background galaxies

51. Does dark matter really exist?

52. 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

53. 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

54. What might dark matter be made of?

55. How dark is it?

56. How dark is it?
… not as bright as a star.

57. 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

58. 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

59. MACHOs occasionally make other stars appear brighter through lensing

60. MACHOs occasionally make other stars appear brighter through lensing
… but not enough lensing events to explain all the dark matter

61. Why Believe in WIMPs? There’s not enough ordinary matter
WIMPs could be left over from Big Bang
Models involving WIMPs explain how galaxy formation works

62. What have we learned?
What is the evidence for dark matter in galaxies?
Rotation curves of galaxies are flat, indicating that most of their matter lies outside their visible regions
What is the evidence for dark matter in clusters of galaxies?
Masses measured from galaxy motions, temperature of hot gas, and gravitational lensing all indicate that the vast majority of matter in clusters is dark

63. What have we learned? Does dark matter really exist?
Either dark matter exists or our understanding of our gravity must be revised
What might dark matter be made of?
There does not seem to be enough normal (baryonic) matter to account for all the dark matter, so most astronomers suspect that dark matter is made of (non-baryonic) particles that have not yet been discovered

64. 22.3 Structure Formation Our goals for learning
What is the role of dark matter in galaxy formation?
What are the largest structures in the universe?

65. What is the role of dark matter in galaxy formation?

66. 
Gravity of dark matter is what caused protogalactic clouds to contract early in time

67. WIMPs can’t contract to center because they don’t radiate away their orbital energy

68. Dark matter is still pulling things together
After correcting for Hubble’s Law, we can see that galaxies are flowing toward the densest regions of space

69. What are the largest structures in the universe?

70. Maps of galaxy positions reveal extremely large structures: superclusters and voids

71. Time in billions of years
0.5
2.2
5.9
8.6
13.7 13 35 70 93
140
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

72. Models show that gravity of dark matter pulls mass into denser regions – universe grows lumpier with time

73. Structures in galaxy maps look very similar to the ones found in models in which dark matter is WIMPs

74. What have we learned?
What is the role of dark matter in galaxy formation?
The gravity of dark matter seems to be what drew gas together into protogalactic clouds, initiating the process of galaxy formation
What are the largest structures in the universe?
Galaxies appear to be distributed in gigantic chains and sheets that surround great voids

75. 22.4 The Fate of the Universe
Our goals for learning
Will the universe continue expanding forever?
Is the expansion of the universe accelerating?

76. Will the universe continue expanding forever?

77. Does the universe have enough kinetic energy to escape its own gravitational pull?

78. Fate of universe depends on the amount of dark matter
Critical density of matter
Lots of dark matter
Not enough dark matter

79. Amount of dark matter is ~25% of the critical density suggesting fate is eternal expansion
Not enough dark matter

80. But expansion appears to be speeding up!
Dark
Energy?
Not enough dark matter

81. old
older
oldest
Estimated age depends on both dark matter and dark energy

82. 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

83. 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

84. Is the expansion of the universe accelerating?

85. Brightness of distant white-dwarf supernovae tells us how much universe has expanded since they exploded

86. Accelerating universe is best fit to supernova data

87. What have we learned? Will the universe continue expanding forever?
Current measurements indicate that there is not enough dark matter to prevent the universe from expanding forever
Is the expansion of the universe accelerating?
An accelerating universe is the best explanation for the distances we measure when using white dwarf supernovae as standard candles