Precision Early Universe Cosmology from Stochastic Gravitational Waves

TL;DR

This paper explores how stochastic gravitational wave measurements can probe early universe physics, including relativistic species, particle content, and fundamental constants, with high sensitivity.

Contribution

It demonstrates that future gravitational wave detectors like LISA and DECIGO can measure early universe parameters and test models beyond the Standard Model with unprecedented precision.

Findings

LISA can measure free streaming fraction down to 10^{-3}

Sensitivity to deviations in g_* and beta functions at 10^5 GeV

Future detectors can test models like split SUSY, WIMPs, and axions

Abstract

The causal tail of stochastic gravitational waves can be used to probe the energy density in free streaming relativistic species as well as measure $g_\star(T)$ and beta functions $\beta(T)$ as a function of temperature. In the event of the discovery of loud stochastic gravitational waves, we demonstrate that LISA can measure the free streaming fraction of the universe down to the the $10^{-3}$ level, 100 times more sensitive than current constraints. Additionally, it would be sensitive to $\mathcal{O}(1)$ deviations of $g_\star$ and the QCD $\beta$ function from their Standard Model value at temperatures $\sim 10^5$ GeV. In this case, many motivated models such as split SUSY and other solutions to the Electroweak Hierarchy problem would be tested. Future detectors, such as DECIGO, would be 100 times more sensitive than LISA to these effects and be capable of testing other motivated scenarios such as WIMPs and axions. The amazing prospect of using precision gravitational wave measurements to test such well motivated theories provides a benchmark to aim for when developing a precise understanding of the gravitational wave spectrum both experimentally and theoretically.