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BBN: Constraints from CMB experiments

BBN: Constraints from CMB experiments. Joanna 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

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BBN: Constraints from CMB experiments

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  1. BBN: Constraints from CMB experiments Joanna Dunkley University of Oxford IAUS Geneva, Nov 9 2009

  2. 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

  3. CMB as probe of fluctuations Linear theory Basic elements have been understood for 30 years (Peebles, Sunyaev & Zeldovich) Numerical codes agree to better than 0.1% (Seljak et al 2003)

  4. Constraining baryon density We measure one number: Ωbh2 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.

  5. WMAP 5-yr data Hinshaw et al 2009

  6. Actual multi-frequency maps (23-94 GHz) Hinshaw et al 2008

  7. WMAP power spectrum Cosmic variance limited to l=530 Text Hinshaw et al 2009

  8. 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

  9. Relating to familiar quantities • Baryon-to-photon ratio • η10 (WMAP) = 6.23 ± 0.17 • For given ratio, can predict the element abundances (or for given abundance measurements can infer the ratio) Steigman 2007 Not most up to date: see Gary Steigman’s talk

  10. 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

  11. Prospects for Planck (Planck Blue Book) • From Planck: • Sigma(Ωbh2) = 0.0002 (three times better) • Sigma(Y) = 0.012 for Planck (>10 times better) (Mortonson & Hu 2008)

  12. Measuring relativistic species Relativistic 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.

  13. Prospects for Planck and ACT/SPT • 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 (Dunkley et al 2009)

  14. 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.

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