An Upper Limit on the Initial Temperature of the Radiation-Dominated Universe

Abstract

Gravitational waves (GWs) are produced by colliding particles through the gravitational analogue of electromagnetic bremsstrahlung. We calculate the contribution of free-free emission in the radiation-dominated Universe to the stochastic GW background. We find that the energy density of the resulting GW radiation is heavily dependent on the number of elementary particles, $N_{\mathrm{tot}}$, and the maximum initial temperature, $T_{\max }$. We rule out $N_{\mathrm{tot}}\gtrsim N_{\mathrm{SM}}$ for $T_{\max }\sim T_{\mathrm{Planck}}\approx 10^{19}$ GeV and $N_{\mathrm{tot}}\gtrsim 10^{13}\times N_{\mathrm{SM}}$ for $T_{\max }\sim 10^{16}$ GeV, where $N_{\mathrm{SM}}$ is the number of particles in the Standard Model. In the case of inflation, existing cosmological data constrain $T_{\max }\lesssim 10^{16}$ GeV. However, alternative models to inflation such as bouncing cosmologies allow for $T_{\max }$ near $T_{\mathrm{Planck}}$. At the energy scales we are considering, the extra number of particles arise naturally in models of extra dimensions.

Comments: 5 pages, 2 figures, submitted to PRL, comments welcome