This project is composed of three main themes to achieve the objectives.
In theme 1 “Theoretical models”, we studied theoretical consistencies of various theoretical models such as vector Galileon, non-local gravity and quasi-dilaton and generalised massive gravity models. Among them, generalised massive gravity models emerge as promising candidate to explain the accelerated expansion of the universe without the cosmological constant. We have made significant contributions to the development of new classes of scalar tensor theory of gravity. These theories have found a special place after almost simultaneous detections of gravitational waves and short gamma-ray bursts from a neutron star merger on 17 August 2017, which put a stringent constraint on the difference between the speed of gravitational waves and that of light. A subclass of these new theories satisfies the condition that these two speeds are equal thus it provides viable modified gravity models that can be tested by cosmological and astrophysical observations. We also studied early modified gravity models as a possible solution to the Hubble constant tension, the discrepancy of the measurement of the Hubble constant from local and cosmological measurements.
In theme 2 “Non-linear clustering”, we developed a perturbation theory approach to predict quasi non-linear power spectra in the redshift space. We also developed a fast approximate N-body method based on a combination of second order Lagrangian perturbation theory and particle mesh simulations. The combination of the semi-analytic approach based on the perturbation theory, fast approximate simulations and full N-body simulations opens a way to test modified gravity models in a consistent way including non-linear scales on which the bulk of information is available. We generated galaxy mock catalogues using the fast methods and showed that they can match the mocks based on full N-body simulations. As a co-lead of the work package on non-standard cosmological simulations, the PI has made an important contribution to the Euclid mission activities.
In theme 3 “Cosmological tests of gravity”, we studied constraints on general classes of scalar tensor theory identified in theme 1. The screening mechanism does not work inside matter in some of these models and we looked for novel probes of these theories by deriving modified properties of stars, galaxies and clusters of galaxies. We also studied neutron stars to provide strong field tests of these theories.
We studied the performance of theoretical templates to constrain a modified gravity parameter using redshift space power spectra. We showed that the theoretical templates developed in theme 2 can reproduce the input modified gravity model parameter in an unbiased way.
For the tests of gravity on linear scales, we performed reconstruction of three time-dependent functions describing the expansion history of the Universe and gravitational effects on light and matter in the large-scale structure. Using the combination of latest cosmological observations, we have shown that current data can already constrain 15 combined modes of these three functions with respect to the theoretical prior. We showed that reconstruction of these three functions enables us to identify the phenomenological features that alternative theories would need to have in order to ease some of the tensions between datasets within the standard model, and deduce important constraints on broad classes of modified gravity models.
