Early Universe Tomography with CMB and Gravitational Waves

TL;DR

This paper explores how combining future CMB and gravitational wave measurements can reconstruct the Universe's thermal history, including reheating and entropy production, by breaking degeneracies present in CMB data alone.

Contribution

It demonstrates that joint CMB and GW data can determine reheating temperature, entropy production, and the start of the radiation era, offering new insights into early Universe physics.

Findings

Future CMB and GW data can break degeneracies in cosmic expansion history.

Combined measurements can determine reheating temperature and entropy production.

The approach can probe scenarios with late-time entropy production.

Abstract

We discuss how one can reconstruct the thermal history of the Universe by combining cosmic microwave background (CMB) measurements and gravitational wave (GW) direct detection experiments. Assuming various expansion eras to take place after the inflationary reheating and before Big-Bang Nucleosynthesis (BBN), we show how measurements of the GW spectrum can be used to break the degeneracies associated with CMB data, the latter being sensitive to the total amount of cosmic expansion only. In this context, we argue that the expected constraints from future CMB and GW experiments can probe a scenario in which there exists late-time entropy production in addition to the standard reheating. We show that, for some cases, combining data from future CMB and GW direct detection experiments allows the determination of the reheating temperature, the amount of entropy produced and the temperature at which the standard radiation era started.