The work done during the FunGraW project can be divided in two main parts, each one addressing one of the questions mentioned above.
The first part aimed at studying in detail the physics of ultralight bosons around spinning BHs with particular focus on GW signatures from these systems. Among the most important outcomes for this part are:
i) Using analytical techniques, we obtained new results concerning the superradiant instability of spinning BHs in the presence of ultralight tensor bosons. We showed that, just like scalar bosons and vector bosons, spinning BHs in the presence of ultralight tensor particles can also be unstable and emit continuous GWs due to the formation of an oscillating bosonic condensate grown out of the instability. The detection of these potential GW sources is specially promising for future detectors, such as the forthcoming ESA's space-based GW detector LISA and next generation of ground-based detectors.
ii) We demonstrated that, by targeting specific sources, current and future ground-based GW detectors will be able to efficiently search for continuous GW signals emitted by scalar boson clouds from BHs formed by mergers observed with LIGO and Virgo or other known BHs, such as those in X-ray binaries. Those results allowed us to obtain bounds on ultralight scalar bosons by performing the first direct search for continuous GW signals targeted at the Cygnus X-1 binary system, using data from Advanced LIGO's second observing run.
iii) We obtained the first bounds on ultralight vector bosons from a GW search, by searching for the stochastic GW background emitted by vector boson clouds in data from Advanced LIGO's first and second observing run.
iv) We studied how the superradiant instability would be affected if ultralight scalar bosons couple to photons through an axionic coupling, showing that for sufficiently strong interactions a laser-like emission from boson clouds can occur. These results might have an impact on GW searches for these particles and could potentially lead to new ways to look for the existence of axionic particles.
v) We developed a Bayesian model selection methodology that can be used to unequivocally infer the presence of a boson cloud around supermassive BHs, by using the detection of the GW signal emitted by extreme mass-ratio inspirals (EMRIs), i.e. systems where a stellar-mass BH orbits a massive BH with millions of solar masses. Such systems are among the most promising sources for the LISA mission.
vi) We developed a Newtonian framework to study eccentric BH binaries where one BH is surrounded by a boson cloud. These results extend previous work that had focused on circular orbits and show that, for eccentric orbits, resonant tidal perturbations on the cloud can occur for a wider range of orbital frequencies than for circular orbits.
vii) We wrote the second edition of the monograph “Superradiance” (Springer), which includes all the new results obtained during the FunGraW project, updates the state-of-the-art of the field and provides a summary on the bounds on ultralight fields (see
https://web.uniroma1.it/gmunu/resources(odnośnik otworzy się w nowym oknie)).
In the second part we studied some specific signatures that can be used to probe the nature of very compact objects and constrain beyond-GR theories with GW detectors. Among the main results are:
i) We studied the impact of tidal heating in the GWs emitted by EMRIs, showing that, by measuring the effect of tidal heating in gravitational waveforms, the future LISA mission will be able to obtain stringent constraints on the reflectivity of the central massive compact object.
ii) We computed post-Newtonian corrections to the GWs emitted by BH binaries in an effective field-theory (EFT) based extension of General Relativity constructed only with higher curvature corrections and used these results to obtain the first constraints on this EFT, using data from the LIGO and Virgo detectors.
All these results were published in major peer-reviewed international journals and are openly accessible in the arXiv repository:
http://www.arxiv.org(odnośnik otworzy się w nowym oknie). Some of the results were presented at major international conferences of the field, including GR22/Amaldi13 and the 13th International LISA Symposium, and various smaller workshops and invited talks.
