bbn: constraints from cmb experiments

BBN: Constraints from CMB

experimentsJoanna Dunkley

University of Oxford

IAUS Geneva, Nov 9 2009

Universe starts out hot, dense and filled with radiation.

As the universe expands, it cools. During the first minutes, light elements form After 400,000 years, atoms form After ~100,000,000 years, stars start to form After ~1 Billion years, galaxies and quasars

CMB as probe of fluctuations

Linear theoryBasic elements have been

understood for 30 years (Peebles, Sunyaev & Zeldovich)

Numerical codes agree to better than 0.1% (Seljak et al 2003)

Constraining baryon density

10 100 1000

Competition between gravity and pressure in acoustic oscillations. Odd peaks are compressions, even are

rarefactions. More baryons: enhance the compressions. At small scales, less Silk damping.

We measure one number:

Ωbh2

WMAP 5-yr data

Hinshaw et al 2009

Hinshaw et al 2008

Actual multi-frequency maps

(23-94 GHz)

TextCosmic variance limited to l=530

Hinshaw et al 2009

WMAP power

spectrum

ΛCDM Cosmological Model

• Flat universe filled with baryons, CDM, cosmological constant, neutrinos, photons.• Gaussian, adiabatic, nearly scale-invariant fluctuations• Baryon density is 0.0227 ± 0.0006

Dunkley et al 2009

Relating to familiar quantities

• Baryon-to-photon ratioη10 (WMAP) = 6.23 ± 0.17

• For given ratio, can predict the element abundances (or for given abundance measurements can infer the ratio)

Element Inferred ratio η10

Deuterium

6.0 ± 0.4

3Helium 5.6+2.2-1.44Helium 2.7 +1.2-0.9

Lithium 3 ± 0.6Steigman 2007 Not most up to date: see Gary

Steigman’s talk

Primordial helium fraction

• Number density of electrons before recombination

ne = nb (1 − Y ) • We usually assume Y = 0.24 for CMB analysis.

• But smaller ne (larger Y) increases mean free path of Compton scattering, giving more Silk damping on small scales

• Current CMB limits: Y< 0.45

(95% CL, WMAP5, Dunkley et al 2009)

In the end, Y and baryon density should be consistent in SBBN. Too early to tell from CMB.

Trotta & Hansen 2004

Prospects for Planck

(Planck Blue Book)

(Mortonson & Hu 2008)

From Planck:Sigma(Ωbh2) = 0.0002 (three times better)Sigma(Y) = 0.012 for Planck (>10 times better)

Measuring relativistic speciesRelativistic species, e.g.

neutrinos, that don’t couple to photons/baryons, affect expansion rate and acoustic oscillations. From CMB Neff = 4.4 +-1.5.Change the abundance prediction.

Prospects for Planck and ACT/SPT

(Dunkley et al 2009)

• Want to make sure the CMB-inferred Neff is consistent with BBN-inferred measure.

•Errors on Neff=0.24 from Planck. ACT/SPT limits of about 0.4.

• Both small scale temperature and polarization help.

•Discrepancy could be sign of non-standard behavior

Summary• The CMB power spectrum provides a constraint on baryon density at time of recombination. Data from WMAP tells us Ωbh2 = 0.0227± 0.0006, or η10=6.23 ±0.17.

• This is consistent with Deuterium (and 3He) measures, but tension with lithium and somewhat 4He.

• We can also try to measure helium fraction directly from CMB to check SBBN, but currently weak constraints (Y<0.45).

• Planck (and ACT/SPT) will improve both these numbers with small-scale measurements.

• Also measure Neff, number of relativistic species with CMB. Errors are now 1.5, consistent with BBN. Errors should soon be 0.2 from Planck. Can compare to the inferred number from BBN to check consistency.