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Large scale structure of the Universe and fundamental physics

Periodic Reporting for period 2 - NLO-CO (Large scale structure of the Universe and fundamental physics)

Reporting period: 2019-11-01 to 2020-10-31

Detailed astronomical observations have established a standard model for cosmology, accounting for the Universe’s contents and the laws that govern its dynamics. If Einstein’s theory is assumed to be a valid description of gravity in the cosmos, two new components, dark matter and dark energy, need to dominate the evolution of the Universe while evading detection on terrestrial experiments. Together, these dark components make up 95% of the energy in the Universe today.

The objective of this action is to advance our knowledge of the Universe through the study of theories for gravity, dark energy and dark matter. The predictions of these theories are compared with data to identify viable scenarios which may differ from the standard paradigm. To address the great variety of theories involved, it is necessary to craft flexible computational tools adaptable to many different models. This theoretical, computational and data-interpretation work is crucial to understanding our Universe and the laws of nature.

Identifying the nature of dark matter and dark energy is a driver for science and innovation, involving many different experiments across the globe and in space. Progress in fundamental physics drives technical developments in the short term, necessary to implement novel experiments, while it will lead to fundamentally new applications in the long term. Finally, the topics of cosmology and fundamental physics connect with some of the deepest questions known to humanity. The search for answers trains many scientifically skilled workers, many of whom will apply their training beyond purely academic goals, helping society solve its most immediate and pressing challenges.
The project has focused on the analysis of gravitational theories as dark energy models, as well as one class of dark matter candidates. I have compared these models predictions against cosmological observations and new gravitational wave data.

My work on gravity addressed Horndeski’s theory, a very general framework extending Einstein gravity by including a scalar field. This theory encompasses a sizeable fraction of models proposed in the literature, allowing both to study its general properties and analyze specific realizations in detail. A very interesting model of dark energy, known as Galileon, can explain most cosmological observations. I implemented Galileon into hi_class and studied its implications in detail, including the compatibility with data from a variety of cosmological observations. I also analyzed other dark energy and gravity theories using hi_class, including a model connecting the physics of early and late-time acceleration and different parameterizations of the fundamental properties of gravity.

The recent detection of gravitational waves represents a great new opportunity for the study of gravity and dark energy. I investigated the implications of gravitational wave observations for theories of gravity and dark energy, some of which predict substantial deviations in how gravitational waves travel through the Universe. I considered several effects, how they could be tested and used to constrain the properties of gravity and dark energy. Among them, I focused on the speed of gravitational waves, a property that was tested at great accuracy, placing phenomenal bounds on large classes of dark energy and gravity theories.

Gravitational wave astronomy also had important implications for dark matter. It was argued that black holes detected through gravitational waves were sufficiently abundant and massive to provide all the dark matter in the Universe. I addressed this scenario by studying the gravitational magnification effect that these black holes would exert on Type Ia supernovae, a very regular stellar explosion used to measure the cosmic expansion.
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.
Speed of gravitational waves in Galileon gravity.
Galileon gravity as a solution to the Hubble problem.
Constraints on the abundance of black holes as dark matter from lack of lensed supernovae
Gravitational lensing of a supernova (low left) and galaxy by a black hole