Cosmological Constraints on Late-time Entropy Production

Abstract

We investigate cosmological effects concerning the late-time entropy production due to the decay of non-relativistic massive particles. The thermalization process of neutrinos after the entropy production is properly solved by using the Boltzmann equation. If a large entropy production takes place at late time t$\simeq$ 1 sec, it is found that a large fraction of neutrinos cannot be thermalized. This fact loosens the tight constraint on the reheating temperature T_R from the big bang nucleosynthesis and T_R could be as low as 0.5 MeV. The influence on the large scale structure formation and cosmic microwave background anisotropies is also discussed.

Figures (5)

• Figure 1: (a) Evolution of the the energy density of $\nu_{e}$ (solid curve) and $\nu_{\mu}$ (dashed curve) for $T_{R}=2$ MeV. (b) Distribution of $\nu_{e}$ (solid curve) and $\nu_{\mu}$ (dashed curve) for $T_{R}=2$ MeV. The dotted curve is the thermal equilibrium Fermi-Dirac distribution.
• Figure 2: $N_{\nu}^{\rm eff}$ as a function of $T_{R}$.
• Figure 3: Contours of the confidence levels in ($\eta,T_R$) plane for (a) observational value of Low $^4$He and (b) High $^4$He. The inner (outer) curve is 68$\%$ (95$\%$) C.L..
• Figure 4: Contours of $\Gamma_{\rm s} = 0.2$ (bold), $0.3, 0.4, 0.5$ and $0.6$ on the ($\Omega_0, N_\nu^{\rm eff}$) plane for $h=0.7$.
• Figure 5: Power spectra of CMB anisotropies (left top panel) and polarization (right top panel) of models with $N_{\rm eff}^{\rm eff}=3, 2$ and $0.5$. Bottom two panels show $(C_\ell(N_{\rm eff})- C_\ell(3))/C_\ell(3)$ with $N_{\rm eff}^{\rm eff}=2.9, 2.5$ and $2$ for CMB anisotropies (left bottom) and polarization (right bottom).