Cosmology of a brane radiating gravitons into the extra dimension

TL;DR

This paper investigates how graviton emission into extra dimensions influences brane cosmology, revealing a transition from constant to rapidly growing Weyl radiation at high energies, with implications for nucleosynthesis constraints.

Contribution

It provides a self-consistent analysis of bulk graviton effects on brane cosmology, especially the evolution of Weyl radiation across energy regimes, which was not previously understood.

Findings

Weyl parameter is constant at low energies, affecting Friedmann equations.

At high energies, Weyl parameter grows rapidly as a^4.

Estimated Weyl radiation complies with nucleosynthesis bounds.

Abstract

We study in a self-consistent way the impact of the emission of bulk gravitons on the (homogeneous) cosmology of a three-brane embedded in a five-dimensional spacetime. In the low energy regime, we recover the well known result that the bulk affects the Friedmann equation only via a radiation-like term $\C/a^4$, called dark or Weyl radiation. By contrast, in the high energy regime, we find that the Weyl parameter $\C$ is no longer constant but instead grows very rapidly as $\C\propto a^4$. As a consequence, the value of $\C$ today is not a free parameter as usually considered but is a fixed number, which, generically, depends only on the number of relativistic degrees of freedom at the high/low energy transition. Our estimated amount of Weyl radiation satisfies the present nucleosynthesis bounds.