Constraining The Early-Universe Baryon Density And Expansion Rate

TL;DR

This paper constrains the early-Universe baryon density and expansion rate by comparing Big Bang Nucleosynthesis, CMB, and LSS data, confirming standard model consistency and highlighting lithium abundance discrepancies.

Contribution

It provides combined constraints on N_nu and eta_10 from multiple cosmological observations, refining early-Universe parameter limits.

Findings

BBN strongly constrains N_nu between 1.6 and 3.3.

CMB/LSS data tightly constrain baryon density, independent of N_nu.

Predicted lithium abundance exceeds observations by a factor of 3 or more.

Abstract

We explore constraints on extensions to the standard models of cosmology and particle physics which modify the early-Universe expansion rate S = H'/H (parametrized by the effective number of neutrinos N_nu). The constraints on N_nu and the baryon density parameter (eta_B = n_B/n_gamma = 10^(-10)*eta_10) from BBN at 20 minutes are compared with those from the CMB at 400 kyr and LSS at 14 Gyr. BBN provides the strongest constraint on N_nu (1.6 < N_nu < 3.3 at 95% confidence), but a weaker constraint on eta_B. The CMB/LSS best constrain the baryon density (5.9 < eta_10 < 6.4 at 95% confidence), independent of N_nu, but provide a relatively weak N_nu constraint, consistent with N_nu = 3. Using the best fit values and the allowed ranges of the CMB/LSS-derived parameters to calculate the BBN-predicted primordial abundances yields excellent agreement with the observationally inferred abundance of deuterium and good agreement with 4He, confirming the consistency between the BBN and CMB/LSS results. However, the BBN-predicted abundance of 7Li is high, by a factor of 3 or more. We comment on the value of N_nu and a possible anomaly in the matter power spectrum inferred from observations of the Ly-alpha forest. The good agreement between our BBN and CMB/LSS results permit us to constrain any post-BBN entropy production as well as to limit the production of any non-thermalized relativistic particles and, allow us to combine them finding 95% ranges, 1.8 < N_nu < 3.2 and 5.9 < eta_10 < 6.4.