Hazgrav

The recent evidence for a stochastic gravitational wave background (GWB) in the nanohertz band, announced by pulsar timing array (PTA) collaborations around the world, has been posited to be sourced by either a population of supermassive black holes binaries or perturbations of spacetime near the inflationary era, generated by a zoo of various new physical phenomena. Gravitational waves (GWs) from these latter models would be explained by extensions to the standard model of cosmology and possibly to the standard model of particle physics. While PTA datasets can be used to characterize the parameter spaces of these models, energy conservation and limits from the cosmic microwave background (CMB) can be used a priori to bound those parameter spaces. Here we demonstrate that taking a simple rule for energy conservation and using CMB bounds on the radiation energy density can set stringent limits on the parameters for these models.

As pulsar timing arrays (PTAs) transition into the detection era of the stochastic gravitational wave background (GWB), it is important for PTA collaborations to review, and possibly revise, their observing campaigns. The source of the GWB is unknown, and it may take years to pin down its nature. An astrophysical ensemble of supermassive binary black holes is one very likely source for the GWB. Evidence for such a background should come in the form of detectable anisotropies in the GWB and resolvable binary signals. A “single source” would be a boon for gravitational astrophysics, as such a source would emit gravitational waves for millions of years in the PTA frequency band. Earlier studies have shown that the observational strategies for finding single sources are somewhat different than for finding the statistical correlations needed for the detection of a stochastic background. Here we present generic methods for studying the effects of various observational strategies, taking advantage of detector sensitivity curves, i.e., noise-averaged, frequency-domain detection statistics. The statistical basis for these methods is presented along with myriad examples of how to tune a detector towards single, deterministic signals or a stochastic background. The importance of the uncorrelated half of the GWB, i.e. the pulsar-term, will be discussed as one of the most important sources of noise in the observational era of PTAs.