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 2008. 46-385

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”!

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 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.

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

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Millenium simulation (1010 particles)(Springel, White, Jenkins et al.)

• FIG 8

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?

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=w0 + wa(1-a):

= 0.70±0.15, w0 ≈ -1.0 ±0. 2, wa = 0±1.

MAJOR QUESTIONS NOW

• Is dark energy something else than vacuum energy?

• Does GR selfconsistently describe cosmic acceleration and structure formation?