Gravitational Universe: Challenges and Opportunities

Project Information

GRU

Grant agreement ID: 101007855

DOI

10.3030/101007855

Project closed

EC signature date 1 December 2020

Start date 1 October 2021

End date 30 September 2025

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 280 600,00

EU contribution

€ 280 600,00

Coordinated by

UNIVERSITA DI PISA
Italy

Periodic Reporting for period 2 - GRU (Gravitational Universe: Challenges and Opportunities)

Reporting period: 2023-10-01 to 2025-09-30

Summary of the context and overall objectives of the project

The landmark detection of gravitational waves (GWs) emitted by merging black-hole (BH) and neutron-star binaries has opened a new era in physics, giving us access to the strong-field regime of the gravitational interaction. In this new observational window, we could learn that general relativity is not adequate to describe the strong-field regime of gravity; new fundamental fields beyond the standard model, or new kinds of compact objects, could challenge current understanding of dark matter and dark energy.
In order to reach these results, an effort is needed by the theoretical and the data-analysis communities. On one hand, we need to develop new data-analysis algorithms, to extract meaningful results from the impressive amount of data which will be delivered. On the other hand, we need to develop theoretical models of GW sources which do not assume the standard theories and paradigms which we want to test, in order to perform non-biased tests and to use observational results to address fundamental problems. To this aim, scientists with different expertise and addressing different challenges should exchange their views and learn from each other’s problems. The proposed studies can be loosely classified into four groups with considerable overlap: i) Perturbation techniques in general relativity and beyond ii) Non-linear simulations of binary compact objects beyond general relativity iii) GWs from the dark universe iv) Data analysis for GW experiments.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

WP1:
In this WP we developed perturbative techniques, to describe the coalescence of compact binary systems beyond GR. In the last two years of the project, we have been studying multipolar PN formalism and spacetime perturbation theory, mostly to study compact binaries in scalar-tensor theories of gravity.

WP2:
In this WP we developed NR techniques, and perform NR simulations, to study compact binary coalescences beyond GR, or beyond the BH hypothesis. Moreover, we have found - using NR techniques - that even in GR there are features of the binary BH coalescence which were not yet understood.

WP3:
In this WP we studied how DM structures or compact dark objects would affect the GW signal from compact binaries. To this aim, we studied the dynamics of BHs and their GW emission in presence of DM, described either as a fluid or (in the so-called “fuzzy DM model”) as an ultra-light scalar field. Moreover, we studied how the dynamics of BHs and their GW emission is affected by environmental effects like accretion and dynamical friction.

WP4:
In this WP we developed data analysis techniques applicable to present and upcoming GW detectors.

See Technical Report for details.

Dissemination has been done through the afrorementioned articles, and through participation to international conferences (some of which organized by ourselves), see Technical Report for details. Outreach activities include movies, participation to podcasts, and participation to the European Reaearchers' Night, see Tecnical Report for details.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Our results go substantially beyond the state of the art. For instance:

• We have studied tidal effects to next-to-next-to leading PN order in scalar-tensor theories of gravity, studying the conservative dynamics and computing the conserved quantities.
• We have computed the tidal contributions to the full gravitational waveform in general relativity, to 2.5PN order.
• We have studied the dynamics of EMRIs in presence of scalar fields (either GR modifications or fundamental fields), extending previous results to non-equatorial orbits and to time-dependent scalar fields.
• We have studied QNMs for a wide range of BH mimickers, from regular black holes to horizonless objects, finding that their detection by next generation interferometers might be sensible to macroscopic values of the regularization parameter.
• We have clarified the phenomenon of spectral instability.
• We have studied quadratic QNMs, computing their amplitudes for Schwarzschild BHs, finding the ratio between their amplitudes and those of linear QNMs, and assessing their detectabiltiy by present and future GW detectors.
• We have computed, using different approaches (one of which developed by ourselves) the QNMs of GR with higher-derivative corrections, i.e. including in the action (in an EFT framework) quadratic and cubic terms in the curvature tensor.
• We have studied the excitation factors of QNMs for a spinning BH in GR, reconstructing the ringdown waveform in terms of the QNMs.
• We have studied the avoided crossing of modes in Kerr and Kerr-de Sitter BHs.
• We have studied the ringdown of dynamical spacetimes, focussing on the dynamical consequences of a change of mass due to accretion, and on dynamical lensing.
• We have addressed the problem of well-posedness of modified gravity theories with higher derivatives, applying an approach we had developed in the first part of the project, to a class of scalar-tensor theories, the K-Essence theories.
• Using NR simulation we studied the tail signal at the end of the ringdown, finding that it is stronger than previously expected, and within reach of GW detector measurements.
• We developed a novel method to construct initial data NR simulations, based on hyperboloidal slices and leads to simulations with increased numerical stability.
• We studied the nonlinear merger of two boson stars and characterized the corresponding gravitational-wave signal.
• We performed NR simulations to study the nonlinear ringdown of hairy black holes displaying features typically associated with horizonless objects, such as echoes.
• We have investigated the fuzzy DM model, studying the mixing between ultra-light bosons (e.g.axions or dark photons) and the photon.
• We have studied the superradiant instability of magnetically-charged, rotating BHs finding that a magnetic charge significantly reduces the instability timescale
• We studied the possibility of using the motion of S2 to constrain vector clouds around SgrA*, finding novel bounds on the vector mass from GRAVITY observations.
• We studied how PTA observations (NANOGrav and EPTA) can be intepreted as due to GWs induced by scalar fields or to primordial BHs, and how they can constrain modified gravity theories and ultra-light dark matter models.
• We studied how high-frequency GW detectors (based on cavities and circuits) may be used to detect ultra-light dark matter.
• We developed a data-analysis framework to forecast the sensitivity of future PTA configurations and to assess the parameter estimation of the stochastic GR background.
• We developed models of the binary BH population to find the expected statistical distribution of LISA sources.
• We studied the statistical properties of the GW signal from primordial BHs.
• We developed data-analysis models to study the ringdown. One of these models addresses the computation of the Bayes factor for the presence of the first overtone in the ringdown signal.
• We made an exhaustive analysis of possible causes of false GR violations in GW observations.
• We studied how, in the analysis of the GW signal from LISA binaries, the effect of stochastic gas torques affects the estimation of parameters.

SNR required to distinguish halo in ringdown signal [PRL 134 081402 2025]

Principal false causes of GR violation in GW data [Scipost Ph.Comm.Rep. 5 2025]

GW amplitude of ringdown [PRL135, 171401 2025]