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Dark Energy
Bengt Gustafsson: Current problems in Astrophysics
Lecture 3 Ångström Laboratory, Spring 2010
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Reading:
Frieman, Turner, Huterer: Dark Energy and the Accelerating Universe, Ann Rev. Astron. &
Astrophys 2008. 46-385
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Friedmann equations:
• H2 = (a’/a)2 = 8G/3 - k/a + /3• a”/a = -4G/3 ( + 3p) + /3Einstein (1917). Zeldowich (1968):
wi = pi/i
wv = -1
= 8Gv = - 8Gpv
Estimated value gets very big. ”Cosmological constant problem”!
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Among astronomers in the 80’s and 90’s: growing uneasiness with =
0.
• Globular clusters older than Hubble age• Large scale structure simulations• Statistics of lensed quasars
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Discovery 1998: SN Ia
studies• Possible due to demonstration that SNe Ia are useful as ”standard candles” (Phillips 1993)
• -> Chandrasekhar mass 1.4 Msun.
-> 0.6 Msun of 56Ni, radioactive
decacy determines peak luminosity.
AND due to mosaics of CCD:s in cameras
on 4m class telescopes. Searches for supernovae scheduled.To determine cosmic deceleration!
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Riess et al.,
Perlmutter et al.
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Critique: Peak luminosity
metallicity (age) dependent?
Dust content or properties varying with metallicity?
Selection bias in low-z sample?
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Futher evidence
- Cosmic Microwave Background- Large Scale Structure:
– Baryonic acoustic oscillations– Integrated Sachs-Wolfe Effect– Weak gravitational lensing
-X-ray clusters (requiring f = b/dm not to vary with z)
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Millenium simulation
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Millenium simulation (1010 particles)(Springel, White, Jenkins et al.)
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• FIG 8
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WHAT IS IT?
• Cosmological constant? No further insight• Vacuum energy? v ~ ∑fields gi kmax
4
Cutoff at Planck scale (1019 GeV) => x10120 too much! Even electron mass scale => U absurd!
SUSY: gi = +1 for bosons, -1 for fermions No perfect symmetry though, if broken at ~ 1TeV => x1060
If vacuum energy scale randomly varying and somewhere very low, we would live there (Weinberg 1987). Anthropic explanation?
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What is it?, cont.
Scalar fields? Opens up more possibilities. May clump?May relate to inflation? Does not solve v problem. w≠-1. May vary with a(z).
Modified gravity? Interesting ideas, no self-consistent theory in agreement with LSS and CMB results yet.
Inhomogeneities?Kolb et al. Enough??
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Near (?) future: More observations!
• Supernovae• Galaxy clusters• Baryonic Acoustic Oscillations• Weak gravitational lensing
From Frieman et al. (2008)
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WHAT WE KNOW NOW:
• Strong evidences for accelerated expansion
• Dark energy a probable cause• Vacuum energy problematic
= 0.76±0.02, w ≈ -1.0 ±0.1(stat) ±0.1 (syst) if w constant.
Assuming w=w0 + wa(1-a):
= 0.70±0.15, w0 ≈ -1.0 ±0. 2, wa = 0±1.
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MAJOR QUESTIONS NOW
• Is dark energy something else than vacuum energy?
• Does GR selfconsistently describe cosmic acceleration and structure formation?