The most important results are new constraints on dark matter, dark energy and gravity using information from cosmological surveys, gravitational waves, or a combination of both. I also developed publicly available computational tools and helped build the science case of future experiments.
Dark matter models based on stellar-mass black holes were by the lack of gravitational magnification in type Ia supernovae catalogues. Specifically, black holes heavier than 1% of mass of the sun may represent 30% or less of the dark matter, with weaker limits for lighter objects. This limits include the masses of all black holes detected by LIGO/Virgo and constrains models of high-energy physics that predict the formation of these objects in the early Universe.
In order to study dark energy and gravity with cosmological data, I have continued developing several features of the hi_class code, which are now publicly available. These include 1) the possibility of specifying models using a full description in terms of the scalar field and gravity 2) correct initial conditions for perturbations for non-standard gravity, and 3) a flexible approximation scheme that speeds up the calculations. To date hi_class has been used in 28 publications by different groups, with about 20 different first authors.
Dark energy and gravity theories can be severely constrained by a variety of complementary observations. Unexpectedly, we found Galileon theories compatible with a combination of cosmic microwave background, large-scale structure and their cross-correlation. Viable Galileon predict 1) a high value of the Hubble constant compatible with the directly measured value, improving over standard cosmology and 2) non-standard propagation of gravitational waves, differing significantly from the speed of light. This is proof of the synergies between cosmological surveys and gravitational wave astronomy.
The detection of a neutron star binary merger GW170817 and its electromagnetic counterparts placed very strong bounds on theories that modified the gravitational wave speed, including Galileons. I classified the surviving theories into 1) simple theories with no deviation and 2) complex theories where the speed could be compensated. This compensation has to be robust against change in the cosmological background (e.g. matter structures) for the theory to be viable. In a literature review I addressed other effects affecting the amplitude of the signal, additional polarizations, and the possibility that the gravitational wave signal can oscillate into “hidden” polarizations (analogous to neutrino flavour oscillations). Another project within LISA Cosmology Working Group addressed how well the LISA mission can detect these effects, improving on current limits.
Finally, I have been involved in the planning of future experiments. Besides the work in LISA, I contributed to the Science Case for 3rd generation gravitational wave ground detectors, forecasting their sensitivity to black holes as a dark matter candidate. I also participated in two proposals for space detectors within the European Space Agency’s Voyage 2050 program: 1) a space mission to achieve high angular resolution and source identification and 2) a large-scale space antenna able to measure gravitational waves at very low frequencies and unveil earlier stages of massive black hole inspirals. These experiments will contribute to further advance our understanding of gravity, dark energy and dark matter and answer other deep questions about our Universe in the coming decades.
