Line of research overview

Gravitational Wave Astronomy: A Paradigm-Shifting Window into the Cosmos

Principal Investigator: Enrico Barausse

Abstract:

Gravitational wave (GW) astronomy is revolutionizing our understanding of the universe’s most cataclysmic events. The observation of GWs from binary mergers by LIGO and Virgo has already provided unparalleled insights into the properties and astrophysical formation channels of neutron stars and stellar-mass black holes, probing general relativity in the strong-field regime. The anticipated advances with third-generation detectors such as the Einstein Telescope and Cosmic Explorer, and space missions like LISA, are expected to extend these observations to higher precision and lower frequency bands. At the same time, pulsar timing array experiments have already detected evidence for a possible stochastic background of GWs in the nHz band, possibly from the mergers of supermassive black holes with masses of billions of solar masses.

Status of project and perspectives:

The group has made substantial contributions to various aspects of GW astronomy. Some very recent highlights include the group’s contribution to the interpretation of the European Pulsar Timing Array (EPTA) data, which provide evidence for a stochastic background of GWs at nHz frequencies. In more detail, we led the effort to place bounds on the presence of possible scalar/pseudoscalar ultralight dark matter particles (fuzzy dark matter) in the EPTA data, and we leveraged previous work by Barausse and collaborators on semi-analytic galaxy formation models to show that if the pulsar timing array signal is due to a population of supermassive black hole binaries, then the final parsec problem must be efficiently solved. In a series of papers from 2020 to 2023, our group also studied the generation of GWs in effective field theories (EFTs) of Dark Energy. The latter are an effort to classify in a coherent framework the large zoo of gravitational theories that try to reproduce cosmological observables without any actual Dark Energy/cosmological constant, but at the price of extending general relativity (GR) by introducing an additional scalar graviton. These theories are intrinsically non-linear and non-perturbative (and thus hard to handle), and they have received much attention because they could provide different observational signatures from the LCDM model in cosmological experiments. While some of these theories are constrained by the LIGO/Virgo results on the propagation speed of GWs, a large class — including in particular theories with derivative self interactions — remains viable under the commonly made assumption that the generation of GWs behaves like in GR. This assumption relies on the (alleged) presence of nonlinear screening mechanisms in these theories, which make them indistinguishable from GR in quasi-static and spherical configurations. Our work showed for the first time that these screening mechanisms are inefficient at masking deviations from GR away from quasi-static situations. In more detail, we performed the first 3+1 numerical relativity simulations in these theories (for gravitational collapse and neutron star binaries). We find that future GW detectors will be able to detect subtle but measurable low-frequency deviations away from the GR predictions, thus confirming or ruling out a modified gravity origin for Dark Energy.

The group’s future research lines will continue and complement the aforementioned themes(tests of GR/screening, pulsar-timing array experiments), but will also include various aspects of the LISA science case. In more detail, we will work on the development of astrophysical models for the population of massive black hole binaries observable with LISA. The detection of GWs from these systems will provide critical insights into the evolution of galaxies, as the formation and merger of massive black holes are closely linked to the hierarchical growth of structures in the universe. The gravitational waveforms captured by LISA will allow for precise measurements of the system’s parameters, including masses, spins, and the distance to the source. These observations are not only pivotal for understanding the astrophysics of black hole formation and merger histories, but also for testing GR in the strong-field regime.