What is the lowest possible reheating temperature?

TL;DR

This study derives a robust lower bound on the reheating temperature after inflation by solving a coupled Boltzmann system for a heavy decaying particle, neutrinos, and the electromagnetic plasma, and by performing a joint likelihood analysis with light-element abundances, CMB, and large-scale structure data. The authors examine both standard decay scenarios and a direct decay channel to neutrinos, finding that T_RH must be greater than about 4 MeV in most cases, with a narrow parameter region allowing around 1 MeV. The work also translates these cosmological bounds into constraints on large extra dimensions, obtaining M > 2000 TeV for two extra dimensions and M > 100 TeV for three, representing the strongest current limits. Overall, the paper highlights how precision cosmology tightly constrains nonstandard reheating and high-energy model scenarios relevant to baryogenesis and beyond-standard-model physics.

Abstract

We study models in which the universe exits reheating at temperatures in the MeV regime. By combining light element abundance measurements with cosmic microwave background and large scale structure data we find a fairly robust lower limit on the reheating temperature of T_RH > 4 MeV at 95% C.L. However, if the heavy particle whose decay reheats the universe has a direct decay mode to neutrinos, there are some small islands left in parameter space where a reheating temperature as low as 1 MeV is allowed. The derived lower bound on the reheating temperature also leads to very stringent bounds on models with $n$ large extra dimensions. For n=2 the bound on the compactification scale is M > 2000 TeV, and for n=3 it is 100 TeV. These are currently the strongest available bounds on such models.

Figures (7)

• Figure 1: The effective number of neutrino species as a function of $\Gamma_\phi$ when there is no direct decay into neutrinos, $b_\nu=0$.
• Figure 2: $T_\gamma$ and $\rho_\phi$ as functions of time for $\Gamma_\phi=6.4 \,\, {\rm s}^{-1}$, $b_\nu=0$ and two different initial times. The full line is for $t_i=8.8 \times 10^{-3}$ s, whereas the dashed is for $t_i=1.8 \times 10^{-3}$ s.
• Figure 3: Contour plot of $N_\nu$ for different $m_\phi$ and $\Gamma_\phi$. The top left plot is for $b_\nu=0.1$, the top right for $b_\nu=0.5$, the bottom left for $b_\nu=0.9$, and the bottom right for $b_\nu=1.0$.
• Figure 4: The distribution function for $\nu_e$ for different values of $T_\gamma$ when $\Gamma=6.4 \,\, {\rm s}^{-1}$, $b_\nu=1$, and $m_\phi=120$ MeV. The dotted line is for $T_\gamma=2.18$ MeV, the dashed for $T_\gamma=0.42$ MeV, the long-dashed for $T_\gamma=0.19$ MeV, and the full line for $T_\gamma=0.01$ MeV. The full grey (red) line is an equilibrium distribution with $T_\nu=T_\gamma$.
• Figure 5: The distribution function for $\nu_e$ for different values of $T_\gamma$ when $\Gamma=50 \,\, {\rm s}^{-1}$, $b_\nu=1$, and $m_\phi=120$ MeV. The dotted line is for $T_\gamma=7.7$ MeV, the dashed for $T_\gamma=0.93$ MeV, the long-dashed for $T_\gamma=0.23$ MeV, and the full line for $T_\gamma=0.01$ MeV. The full grey (red) line is an equilibrium distribution with $T_\nu=T_\gamma$.