Testing alternative theories of gravity using low frequency gravitational waves

Abstract

General Relativity aptly describes current gravitational observations. However, there is great theoretical interest in its validity in untested regimes. Alternative theories of gravity attempt to relax some of the assumptions made, leaving distinct signatures that are absent in Einstein's theory, namely the presence of alternative polarizations of gravitational waves that manifest from the emission of gravitational scalar and vector dipole radiation in black hole binaries. To study this lower order multipole of radiation, it is desirable to work in a regime where the quadrupolar tensor radiation of general relativity is as quiet as possible. This motivates working with supermassive black hole binaries in their slowly evolving inspiral phase, when they are well separated from merger, emitting low frequency gravitational waves. Using a frequentist framework, we study the detectability of a stochastic background of each polarization using pulsar timing arrays, which is currently the most technically developed and viable method for studying low frequency gravitational waves, correlating the observed time delays of pulsars. We also find that astrometry, which measures transverse displacements of the apparent position of stars, turns out to have a very similar correlation structure as the time delays measured by pulsar timing arrays. We lastly study how effective using a pulsar timing array is at studying a loud, foreground binary with these alternative polarizations, using a Bayesian framework. Low frequency gravitational wave astronomy proves advantageous for studying these exotic signatures.