Dark Energy Bengt Gustafsson: Current problems in Astrophysics Lecture 3 Ångström Laboratory, Spring 2010

Reading: Frieman, Turner, Huterer: Dark Energy and the Accelerating Universe, Ann Rev. Astron. & Astrophys

Friedmann equations: H 2 = (a’/a) 2 = 8  G  /3 - k/a  +  /3 a”/a = -4  G/3 (  + 3p) +  /3 Einstein (1917). Zeldowich (1968): w i = p i /  i w v = -1  = 8  G  v = - 8  Gp v Estimated value gets very big. ”Cosmological constant problem”!

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

Discovery 1998: SN Ia studies Possible due to demonstration that SNe Ia are useful as ”standard candles” (Phillips 1993) -> Chandrasekhar mass 1.4 M sun. -> 0.6 M sun of 56 Ni, 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!

Riess et al., Perlmutter et al.

Critique: Peak luminosity metallicity (age) dependent? Dust content or properties varying with metallicity? Selection bias in low-z sample?

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)

Millenium simulation Millenium simulation (10 10 particles) (Springel, White, Jenkins et al.)

FIG 8

WHAT IS IT? Cosmological constant? No further insight Vacuum energy?  v ~ ∑ fields g i k max 4  Cutoff at Planck scale (10 19 GeV) => x too much! Even electron mass scale => U absurd!  SUSY: g i = +1 for bosons, -1 for fermions  No perfect symmetry though, if broken at ~ 1TeV => x10 60  If vacuum energy scale randomly varying and somewhere very low, we would live there (Weinberg 1987). Anthropic explanation?

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??

Near (?) future: More observations! Supernovae Galaxy clusters Baryonic Acoustic Oscillations Weak gravitational lensing From Frieman et al. (2008)

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=w 0 + w a (1-a):   = 0.70±0.15, w 0 ≈ -1.0 ±0. 2, w a = 0±1.

MAJOR QUESTIONS NOW Is dark energy something else than vacuum energy? Does GR selfconsistently describe cosmic acceleration and structure formation?