Precision Early Universe Cosmology from Stochastic Gravitational Waves

Understand

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.