Research
My research lies at the intersection of Fundamental Physics, Cosmology and Astrophysics. My main goal is to learn about the Universe with gravitational waves (GWs). In particular, I work to unveil the nature of Dark Energy, test general relativity, search for GW lensing, accelerate discoveries with machine learning, and understand the origin of the observed black holes.
Below, I summarize my main research topics, with a list of selected publications for each.
Gravitational waves are natural standard sirens, since their amplitude is inversely proportional to the luminosity distance, directly encoding the cosmic expansion history. However, to extract the cosmological parameters one needs additional redshift information. An electromagnetic counterpart can provide this information, but binary neutron stars have proven to be rare and their multi-messenger identification becomes more challenging at high redshift. Analyses of the binary black hole mass distribution provide a promising alternative, since any feature in their mass distribution can be used to calibrate the masses and break the redshift degeneracy. In particular, the theory of stellar evolution predicts a pair-instability supernova mass gap whose edges can be used to probe the cosmological model using black holes below and above the gap.
Selected publications:
Gravitational wave astronomy has opened an exciting new laboratory for the theory of gravity. It uniquely probes the strong-gravity regime, the cosmological propagation of GWs, and their couplings to fields beyond the Standard Model. Tests of GW propagation are powerful tools to constrain dark energy models. The bounds on the speed of GWs are an excellent example of this, dramatically reducing the landscape of viable theories. However, cosmological propagation is just the beginning. By studying the behavior of GWs traveling near massive objects, one can probe even further the nature of the gravitational degrees of freedom. Beyond general relativity, GW lensing can scramble the signal and produce echoes and birefringence. All of these effects can be probed without the need for electromagnetic counterparts.
Selected publications:
Just as light from a distant supernova or galaxy is affected by the intervening matter between the source and the observer, gravitational waves from the coalescence of compact binaries can be lensed. If strong enough, GW lensing can produce multiple images. We have searched for such lensing events in the LIGO/Virgo data but have not found any conclusive evidence. However, as detector sensitivity improves, increasing both the number of events and their distance range, we expect to have the first GW strong lensing identification in the coming observing runs. Moreover, strongly lensed events can be identified by a single image due to the waveform distortions induced by the lens. GW lensing studies provide a unique probe of the dark matter in the Universe and the origin of the observed black holes.
Selected publications:
- A deep dive into GW231123, with Juno Chan, Rico Lo, Joey Bowman, Lorena Magaña Zertuche and Luka Vujeva (PRDL)
- GW lensing by dark matter subhalos, with Luka Vujeva, Daniel Gilman, Srashti Goyal and Miguel Zumalacárregui (PRD)
- Identifying strongly lensed GWs through their phase consistency, with Wayne Hu and Rico Lo (PRD)
- LIGO-Virgo O3b lensing search, with the LIGO-Virgo Collaboration (ApJ, science summary)
- LIGO-Virgo O3a lensing search, with the LIGO-Virgo Collaboration (ApJ, science summary)
- Phase effects for strong lensing, with Daniel Holz, Wayne Hu, Macarena Lagos and Bob Wald (PRD)
Searching for new physics inevitably involves dealing with data. In the case of gravitational waves, catalogs are growing rapidly, pushing traditional analysis methods to their limits. My research uses novel deep-learning algorithms to accelerate such discoveries.
Selected publications:
More messengers are always better, and when the source is a merging binary neutron star, the result can be beautiful. However, the new era of multi-messenger astronomy is not only about simultaneously observing the same source with different signals, but also about coordinating efforts, sharing resources and learning from other communities.
Selected publications:
- Multi-messenger gravitational lensing, with Graham Smith et al. (Phil. Trans. R. Soc. A)
- Dark energy in light of multi-messenger gravitational-wave astronomy, invited review with Miguel Zumalacárregui (Front. Astron. Space Sci.)
The inflationary paradigm provides a beautiful explanation of the large-scale structure of the Universe, which has been exquisitely tested by the cosmic microwave background. Nevertheless, little is known about the primordial perturbations at small scales and their contribution to the dark matter. If those fluctuations are enhanced by the inflationary dynamics, then they could form primordial black holes that would naturally form part of the dark matter budget. In fact, such enhancement of the fluctuations occurs whenever the inflaton undergoes a period dominated by quantum backreaction, leading to an exponential tail in the primordial spectrum. In other words, constraining the abundance of primordial black holes and the small-scale structure of the Universe in general probes a novel region of the inflationary period.
Selected publications:
- Could El Gordo be hinting at quantum diffusion?, with Juan García-Bellido and Vincent Vennin (PRL)
- The exponential tails of inflationary fluctuations, with Juan García-Bellido and Vincent Vennin (JCAP)
- Primordial black holes in critical Higgs inflation, with Juan García-Bellido and Ester Ruiz Morales (PLB)