Black holes, gravitational waves and fundamental physics: a roadmap

Abstract

The grand challenges of contemporary fundamental physics—dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem—all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress. This write-up is an initiative taken within the framework of the European Action on ‘Black holes, Gravitational waves and Fundamental Physics’.

Dark energy gravitational wave observations and ice age periodicity

“…In the current work, we have described how gravitational waves with an amplitude h 0 ∼ 10 −2 and with a time period T ∼ 10 5 years which corresponds to a frequency of ∼ 10 −13 Hz can arise out of dark energy dynamics. The value of h 0 ∼ 10 −2 at the low frequencies of ∼ 10 −13 Hz is compatible with other constraints on gravitational waves -please see for example [38] for a thorough discussion on existing constraints. While it is reasonable to expect the background of gravitational waves as a result of the dark energy dynamics, it is fair to say that the observation of such waves (other than through the variation of earth's orbit leading to the ice age periodicity) will be very challenging.…”

Axial quasinormal modes of neutron stars in R2 gravity

“…Theoretically the ringdown phase can be studied in terms of quasi-normal modes (QNMs) [11][12][13][14]. Observationally, however, the precision of the detected signals is not yet sufficient to extract the QNMs, although the next generation of instruments is expected to achieve the necessary sensitivity [15,16].…”

Growth of Supermassive Black Hole Seeds in ETG Star-forming Progenitors: Multiple Merging of Stellar Compact Remnants via Gaseous Dynamical Friction and Gravitational-wave Emission

“…The contribution to the early BH growth from migration of compact remnants by gaseous dynamical friction in high-z ETG progenitors could hardly be probed via standard electromagnetic observations; even if it were present, luminous emission would be too weak and likely strongly dimmed by the very gas and dust-rich environment to be ever detected. However, we will show that the repeated mergers of the compact remnants with the accumulating central BH mass can originate detectable GW signals (e.g., Barausse 2012;Barack et al 2019). Specifically, in this Section we aim to compute the cosmicintegrated GW rate density of these events as a function of redshift, and their detectability with the future ET and LISA detectors.…”