Periodic Reporting for period 1 - FunGraW (Fundamental physics in the era of gravitational-wave astronomy)
Periodo di rendicontazione: 2019-01-01 al 2020-12-31
The direct detection of gravitational waves (GWs) by the LIGO and Virgo interferometers is one of the greatest achievements of modern science. A significant effort is now in place to further increase the sensitivity of current detectors and prepare for the construction of future ground and space-based GW detectors. These developments promise to open an era of precision GW physics and have the potential to revolutionise our understanding of astrophysics, cosmology and fundamental physics.
The observation of GWs gives us the unique opportunity to observe and study with great precision an otherwise invisible side of the Universe, with a tremendous potential for new and unexpected discoveries. Indeed, such observations will allow us to probe the highly-dynamical and nonlinear regime of Einstein’s theory of general relativity (GR) to unprecedented levels and could also have profound implications for particle physics, potentially helping to uncover the nature of dark matter. This, however, requires a significant theoretical effort in order to interpret the observations in view of our best theories. The FunGraW project aimed precisely at joining this effort by developing theoretical tools that can help tackling questions such as:
i) Can we use GW observations to probe the existence of new particles, such as ultralight bosons, that could possibly explain the nature of dark matter?
ii) Can GWs provide conclusive evidence for the existence of BHs as described in GR and rule out alternative models?
The observation of GWs gives us the unique opportunity to observe and study with great precision an otherwise invisible side of the Universe, with a tremendous potential for new and unexpected discoveries. Indeed, such observations will allow us to probe the highly-dynamical and nonlinear regime of Einstein’s theory of general relativity (GR) to unprecedented levels and could also have profound implications for particle physics, potentially helping to uncover the nature of dark matter. This, however, requires a significant theoretical effort in order to interpret the observations in view of our best theories. The FunGraW project aimed precisely at joining this effort by developing theoretical tools that can help tackling questions such as:
i) Can we use GW observations to probe the existence of new particles, such as ultralight bosons, that could possibly explain the nature of dark matter?
ii) Can GWs provide conclusive evidence for the existence of BHs as described in GR and rule out alternative models?
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).
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. 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.
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).
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. 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.
The work done during the FunGraW project demonstrates that GWs are extraordinary tools to probe fundamental physics. In particular we were able to make important progress beyond the state of the art by:
i) Closing important gaps in our knowledge on the physics of ultralight bosons around black holes, either in isolation or in binaries;
ii) Firmly establishing that current and future ground-based GW detectors have a very good chance of discovering or constraining ultralight bosons by searching for GW signatures from boson clouds around spinning BHs;
iii) Obtaining among the first bounds on ultralight bosons using GW data from the LIGO and Virgo observatories;
iv) Showing that, by measuring the impact of tidal heating in EMRIs detected by LISA, we will be able to obtain among the strongest constraints on alternatives to BHs or, in a more speculative but exciting scenario, obtain evidence for new physics appearing at the horizon scale;
v) Showing that there is so far no evidence for higher curvature corrections to GR at scales comparable with the size of the BH binaries detected by LIGO and Virgo.
These results have an impact for both the GW community and the fundamental physics community. Ultimately they will also contribute to the worldwide effort in developing the full scientific potential of GW astronomy and to build a strong science case for the next generation of ground-based GW detectors, such as the Einstein Telescope whose conceptual design was supported by the European Commission, and for ESA’s forthcoming LISA mission, planned to be launched in 2034.
i) Closing important gaps in our knowledge on the physics of ultralight bosons around black holes, either in isolation or in binaries;
ii) Firmly establishing that current and future ground-based GW detectors have a very good chance of discovering or constraining ultralight bosons by searching for GW signatures from boson clouds around spinning BHs;
iii) Obtaining among the first bounds on ultralight bosons using GW data from the LIGO and Virgo observatories;
iv) Showing that, by measuring the impact of tidal heating in EMRIs detected by LISA, we will be able to obtain among the strongest constraints on alternatives to BHs or, in a more speculative but exciting scenario, obtain evidence for new physics appearing at the horizon scale;
v) Showing that there is so far no evidence for higher curvature corrections to GR at scales comparable with the size of the BH binaries detected by LIGO and Virgo.
These results have an impact for both the GW community and the fundamental physics community. Ultimately they will also contribute to the worldwide effort in developing the full scientific potential of GW astronomy and to build a strong science case for the next generation of ground-based GW detectors, such as the Einstein Telescope whose conceptual design was supported by the European Commission, and for ESA’s forthcoming LISA mission, planned to be launched in 2034.