Probing Dynamical Spacetimes with Gravitational Waves

Abstract

This decade will see the first direct detections of gravitational waves by observatories such as Advanced LIGO and Virgo. Among the prime sources are coalescences of binary neutron stars and black holes, which are ideal probes of dynamical spacetime. This will herald a new era in the empirical study of gravitation. For the first time, we will have access to the genuinely strong-field dynamics, where low-energy imprints of quantum gravity may well show up. In addition, we will be able to search for effects which might only make their presence known at large distance scales, such as the ones that gravitational waves must traverse in going from source to observer. Finally, coalescing binaries can be used as cosmic distance markers, to study the large-scale structure and evolution of the Universe.With the advanced detector era fast approaching, concrete data analysis algorithms are being developed to look for deviations from general relativity in signals from coalescing binaries, taking into account the noisy detector output as well as the expectation that most sources will be near the threshold of detectability. Similarly, several practical methods have been proposed to use them for cosmology. We explain the state of the art, including the obstacles that still need to be overcome in order to make optimal use of the signals that will be detected. Although the emphasis will be on second-generation observatories, we will also discuss some of the science that could be done with future third-generation ground-based facilities such as Einstein Telescope, as well as with space-based detectors.

Propagation of gravitational waves in a universe with slowly-changing equation of state

“…While the diminution in gravitational wave amplitude we predict in subsection 3.2 is not great, it should be detectable by planned gravitational wave observatories. As predicted by [10], gravitational waves can be used as standard sirens for measuring the cosmological equation of state as long as they can be associated with electromagnetic components.…”

“…As we approach the beginnings of gravitational wave cosmology [10] it becomes necessary to explore the full range of potential models which might affect our initial observations. Riesse et al's [1] observation of acceleration greatly expanded the parameter space of our models for cosmology but the paucity of observational data coupled with the unknown character of the "dark energy" necessary to make observations consistent with general relativity and the cosmological principle have not helped narrow this space much in the past 16 years.…”

Generic searches for alternative gravitational wave polarizations with networks of interferometric detectors

“…A commonly used application for LIGO-Virgo gravitational wave searches is X-Pipeline [31,34], which assumes that only tensor polarizations can be present, and then compares the null energy (essentially the square of the null stream) with other combinations of detector outputs to search for GW signals that are in accordance with GR. As pointed out in [19,[35][36][37][38][39], null streams can also be used to study a signal's nontensorial polarization content that may result from a GR violation; notably, in [37] an upper bound was put on vector modes in GW170817.…”

“…Suppose that in a given signal, alternative polarizations are in fact present, mixed with tensor polarizations. Then to determine the precise nature and relative contributions of the additional modes, in general one would need a network of at least five detectors in addition to the sky position [19,35,39,40]. 1 Although in the near future KAGRA [42] will join the discovery efforts, and LIGO-India [43] is about to be built, for now only the two LIGO interferometers and Virgo are making regular detections.…”