A description of dark energy. Talk for a general audience

1. Dark Energy
Friday, February 13, 2009

2. Friday, February 13, 2009

3. Theory Expansion Supernovae Acceleration
Friday, February 13, 2009

4. Theory Expansion Supernovae Acceleration
Friday, February 13, 2009

5. Friday, February 13, 2009

6. Einstein’s equation says mass/
energy causes space-time curvature
• Curvature = Energy Density
Friday, February 13, 2009

7. What about empty space?
Friday, February 13, 2009

8. What is so special about
empty space?
Friday, February 13, 2009

9. What is so special about
empty space?
Can have curvature even with no matter
Friday, February 13, 2009

10. What is so special about
empty space?
Can have curvature even with no matter
Can have energy in empty space
Friday, February 13, 2009

11. Pressure is related to change in
energy in expanding volume
Positive pressure does work as it expands --
loses energy and cools
Friday, February 13, 2009

12. Pressure is related to change in
energy in expanding volume
Positive pressure does work as it expands --
loses energy and cools
Friday, February 13, 2009

13. Positive energy density has
negative pressure Λ
Λ
Λ
Λ
Λ
Λ
Λ
Volume increased
Internal energy increased
Pressure is negative
Friday, February 13, 2009

14. In General relativity,
pressure causes gravity
• Relativity: Space ~ Time
• Energy ~ Momentum
• Energy density ~ Pressure
• T is a mix of pressure and energy density
μν
• Positive vacuum energy causes anti-gravity
Friday, February 13, 2009

15. Einstein introduced the cosmological
constant to allow a static universe
“In order to arrive at this consistent view, we admittedly had to
introduce an extension of the field equations of gravitation
which is not justified by our actual knowledge of gravitation. ...
That term is necessary only for the purpose of making possible a
quasi-static distribution of matter....”
The antigravity from the cosmological
constant balances the gravity from matter
Disproved by the discovery of the expansion of the universe
“My Greatest Blunder”
Friday, February 13, 2009

16. Recap
Einstein introduced cosmological constant negative
pressure antigravity to allow static universe
Friday, February 13, 2009

17. Theory Expansion Supernovae Acceleration
Friday, February 13, 2009

18. Measure distance in Astronomy by
comparing brightness of object to known
reference
Apparent Magnitude
20
15
Color
Friday, February 13, 2009

19. Measure distance in Astronomy by
comparing brightness of object to known
reference
Du
st
Apparent Magnitude
20
Distance Modulus
15
Color
Bigger apparent magnitude = fainter = farther away
Friday, February 13, 2009

20. Cepheid Variable are a
good reference
Friday, February 13, 2009

21. Henrietta Leavitt discovered
Cephied Period Luminosity relation
Friday, February 13, 2009

22. Original plates used to
discover Cepheids
Friday, February 13, 2009

23. Take negative of one
plate
Friday, February 13, 2009

24. Friday, February 13, 2009

25. Overlap eliminates all
but variable star
Friday, February 13, 2009

26. Friday, February 13, 2009

27. Friday, February 13, 2009

28. Friday, February 13, 2009

29. Friday, February 13, 2009

30. Friday, February 13, 2009

31. Friday, February 13, 2009

32. Distance
Time
Today
Friday, February 13, 2009

33. Distance
Time
Today
Friday, February 13, 2009

34. Distance
Time
Today
Friday, February 13, 2009

35. Distance
Time
Age of the Universe
12 Gya Today
Friday, February 13, 2009

36. Friday, February 13, 2009

37. HST Hubble Constant Key Project
Freedman et al. 2001
Friday, February 13, 2009

38. Distance
Time
12 Gya Today
Friday, February 13, 2009

39. Distance
Time
12 Gya Today
Friday, February 13, 2009

40. Distance
Time
Age of the Universe
12 Gya 10 Gya Today
Friday, February 13, 2009

41. M55 Globular Cluster
Friday, February 13, 2009

42. Distance
Time
12 Gya 10 Gya Today
Friday, February 13, 2009

43. Formation of
Globular Clusters
Distance
13 Gya Time
Age of the Universe
12 Gya 10 Gya Today
Friday, February 13, 2009

44. Recap
Einstein introduced cosmological constant negative
pressure antigravity to allow static universe
Cepheid variables used to measure distance
(bright, identifiable, known brightness) also used to
show expansion of the universe
Friday, February 13, 2009

45. Theory Expansion Supernovae Acceleration
Friday, February 13, 2009

46. Looking for a brighter
standard
Cepheids are the brightest stars
Supernovae are much brighter than cepheids
Friday, February 13, 2009

47. A star is in balance between
gravity and pressure
Hydrostatic equilibrium
Friday, February 13, 2009

48. When stars are hot, pressure
comes from gas pressure
PV=NRT
Friday, February 13, 2009

49. When stars are cold, pressure
comes from quantum mechanics
Cannot put
more than two
electrons in
the same state
Electrons must
be moving
quickly even if
cold
Friday, February 13, 2009

50. When stars are cold, pressure
comes from quantum mechanics
Cannot put
more than two
electrons in
the same state
Electrons must
be moving
quickly even if
cold
Friday, February 13, 2009

51. Finite speed of light places a
limit on maximum size of core
Friday, February 13, 2009

52. Finite speed of light places a
limit on maximum size of core
• As core gets heavier, electrons have to move
faster to hold it up
• Eventually, the speed of light prevents them
from moving faster
• Core starts to shrink
• Gravity gets stronger
• Core shrinks faster
• Gravity gets stronger
Friday, February 13, 2009

53. Two types of
supernovas
• Binary white dwarf (Type Ia)
• Massive star (Type Ib,c Type II)
Friday, February 13, 2009

54. Friday, February 13, 2009

55. Type 1a supernovas
should be similar
• Progenitors have same mass
• Formed slowly, history not important
• Should have same brightness
• Make a good distance estimator
Friday, February 13, 2009

56. Follow up image
Friday, February 13, 2009

57. Friday, February 13, 2009

58. Friday, February 13, 2009

59. •Supernovae seen long ago are moving more slowly than they should.
•Expansion of universe was less in the past than today
•Universe is accelerating
Friday, February 13, 2009

60. Recap
Einstein introduced cosmological constant negative
pressure antigravity to allow static universe
Cepheid variables used to measure distance
(bright, identifiable, known brightness) also used to
show expansion of the universe
Supernovae much brighter than cepheids, can show
expansion in distant galaxies.
Friday, February 13, 2009

61. Theory Expansion Supernovae Acceleration
Friday, February 13, 2009

62. Distance
Time
12 Gya 10 Gya Today
Friday, February 13, 2009

63. Formation of
Globular Clusters
Distance
13 Gya Time
Age of the Universe
12 Gya 10 Gya Today
Friday, February 13, 2009

64. Expansion History of the Universe
s
Perlmutter, Physics Today (2003) nd r
a
xp eve
er
0.0001
0.001
fo s
0.01
relative
0.1
1
se
lap
brightness
1.5 l
co
Relative to Today's Scale
Scale of the Universe
1.0 0
0.25
redshift
After inflation, 0.5
ed
the expansion either... 0.75
ted
rat
ra 1
0.5
ele
e le
cc
dec
1.5
a
en 2
th
,
ys
te d 2.5
a 3
r
lwa
e le
c
de 5
...or a
past today future
st 0.0
0.0
f ir
–20 –10 0 10
Billions Years from Today
Friday, February 13, 2009

65. Friday, February 13, 2009

66. Friday, February 13, 2009

67. Friday, February 13, 2009

68. Friday, February 13, 2009

69. Friday, February 13, 2009

70. Friday, February 13, 2009

71. Friday, February 13, 2009

72. Friday, February 13, 2009

73. Friday, February 13, 2009

74. Friday, February 13, 2009

75. Supernova Cosmology Project
3
Knop et al. (2003)
No Big Bang Spergel et al. (2003)
Allen et al. (2002)
2
Supernovae
1
ΩΛ
CMB
expands forever
ly
ollapses eventual
0
rec
Clusters clo
se
d
fla
-1
t
op
en
0 1 2 3
ΩM
Friday, February 13, 2009

76. The fine tuning problem
“Natural” value for energy density:
or
46,000,000,000,000,000,000,000,000,000,000
,000,000,000,000,000,000,000,000,000,000,0
00,000,000,000,000,000,000,000,000,000,000
,000,000,000,000,000 J/m3
Therefore, the cosmological constant was
believed to be zero until 1998
Friday, February 13, 2009

77. No one knows what makes
the cosmological constant
Is it really constant?
Need more distant supernovae
Friday, February 13, 2009

78. Friday, February 13, 2009

79. Friday, February 13, 2009

80. SNAP
DESTINY
ADEPT
Friday, February 13, 2009

81. Supernova Cosmology Project
ΩΜ , ΩΛ
Knop et al. (2003)
0.25,0.75
0.25, 0
1, 0
24
Supernova
22
Cosmology
Project
effective mB
20
18
Calan/Tololo
& CfA
16
14
0.0 0.2 0.4 0.6 0.8 1.0
redshift z
Friday, February 13, 2009

82. 0.0
Flat Universe network of cosmic strings
Constant w
–0.2 w = –1/3
–0.4
w = pu / quot;u
99%
range of
95%
Quintessence
–0.6 90%
models
68%
–0.8
cosmological constant
w = –1
–1.0
0.0 0.2 0.4 0.6 0.8 1.0
!M = 1 - !w
SNAP Satellite
Target Statistical Uncertainty
68%, 90%, 95%, and 99% confidence regions in the ΩM –w plane for an additional energy density component, Ωw ,
n equation-of-state w = p/ρ. (For Einstein’s cosmological constant, Λ, w = −1.) The fit is constrained to a flat
Ωw = 1). Also shown is the expected confidence region allowed by SNAP assuming w = −1 and ΩM = 0.28.
wavelength calibrated spectra extending to wavelength regions where “gray” dust is no longer gray will
hypothetical large-grain dust’s absorption properties. Armed with the extinction – color excess properties
Friday, February 13, 2009

83. Recap
Einstein introduced cosmological constant negative
pressure antigravity to allow static universe
Cepheid variables used to measure distance
(bright, identifiable, known brightness) also used to
show expansion of the universe
Supernovae much brighter than cepheids, can show
expansion in distant galaxies.
Expansion of the universe is accelerating, deep
challenge to explain why.
Friday, February 13, 2009

84. Recap
Einstein introduced cosmological constant negative
pressure antigravity to allow static universe
Cepheid variables used to measure distance
(bright, identifiable, known brightness) also used to
show expansion of the universe
Supernovae much brighter than cepheids, can show
expansion in distant galaxies.
Expansion of the universe is accelerating, deep
challenge to explain why.
Friday, February 13, 2009