Effective number of neutrinos and baryon asymmetry from BBN and WMAP

TL;DR

This work tests for additional relativistic energy density in the early universe by parameterizing it with $\Delta N_{ u}$ and combining CMB constraints from WMAP with primordial abundance measurements from BBN. The analysis uses D/H and $^4$He to jointly constrain the baryon-to-photon ratio $\eta$ and the expansion rate, finding results consistent with the standard model: $N_{ m eff}$ near 3 and $\eta$ around a few times $10^{-10}$. The joint constraints yield $N_{ m eff}=1.7$–$3.0$ and $\eta=(5.53$–$6.76)\times10^{-10}$ at 2σ, illustrating remarkable agreement between the epoch of BBN and recombination. Overall, the CBR and BBN data are highly complementary, supporting the standard cosmology while constraining possible beyond-Standard-Model contributions to the radiation density.

Abstract

We place constraints on the number of relativistic degrees of freedom and on the baryon asymmetry at the epoch of Big Bang Nucleosynthesis (BBN) and at recombination, using cosmic background radiation (CBR) data from the Wilkinson Microwave Anisotropy Probe (WMAP), complemented by the Hubble Space Telescope (HST) Key Project measurement of the Hubble constant, along with the latest compilation of deuterium abundances and measurements of the primordial helium abundance. The agreement between the derived values of these key cosmological and particle physics parameters at these widely separated (in time or redshift) epochs is remarkable. From the combination of CBR and BBN data, we find the 2σranges for the effective number of neutrinos and for the baryon asymmetry (baryon to photon number ratio η) to be 1.7-3.0 and 5.53-6.76 \times 10^{-10}, respectively.

Figures (10)

• Figure 1: The CBR degeneracy between $\omega_{\rm M}$ and $\Delta N_\nu\;$ is evident from the $1\sigma$ and $2\sigma$ contours from the WMAP data.
• Figure 2: The power spectrum for the best-fit ($N_{\nu}$$=2.75$) to the WMAP data is the solid line. With all other parameters and the overall normalization of the primordial spectrum fixed, the spectra for $N_{\nu}$$=1$, $N_{\nu}$$=5$ and $N_{\nu}$$=7$ are the dotted, dot-dashed and dashed lines, respectively. The data points represent the binned TT power spectrum from WMAP.
• Figure 3: The $1\sigma$ and $2\sigma$ contours in the $\eta_{10}$--$\Delta N_\nu\;$ plane from WMAP data. The solid (dotted) lines correspond to $t_0>11$ (12) Gyr. The cross marks the best-fit at $\omega_{\rm B}=0.023$ and $\Delta N_\nu\;$$=-0.25$.
• Figure 4: Isoabundance curves for D and $^4$He in the $\eta_{10}$ -- $\Delta N_\nu\;$ plane. The solid curves are for $^4$He (from top to bottom: Y = 0.25, 0.24, 0.23). The dashed curves are for D (from left to right: $10^{5}$(D/H) = 3.0, 2.5, 2.0. The data point with error bars corresponds to $y_{\rm D}$ = 2.6$\pm 0.4$ and Y = 0.238$\pm 0.005$; see the text for discussion of these abundance values.
• Figure 5: The band is the SBBN predicted relation between the primordial abundances of D and $^4$He, including the errors ($\pm 1\sigma$) in those predictions from the uncertainties in the nuclear and weak interaction rates. The point with error bars is for the relic abundances of D and $^4$He adopted here (see the text).