Alternative theories for dark matter and dark energy – THALES

The nature of dark matter and dark energy is one of the most complex enigmas of the standard cosmological model. The aim of our project is to quantify, from the theoretical viewpoint confronted with observational data, the robustness of this model along three complementary directions heading towards new possible solutions to the dark sector problem:

(i) Modified gravity theories. Several extensions of general relativity have been proposed to reproduce the various observations suggesting the existence of dark matter or dark energy, for instance tensor-vector-scalar (TeVeS) theories or the Dvali-Gabadadze-Porrati (DGP) brane model, but none of them is fully satisfactory. We intend to construct extensions which would be both theoretically well-defined (for instance with no ghost nor tachyon) and consistent with local constraints on deviations from general relativity (solar system, binary pulsars). We will notably focus on the classes of nonlocal models and massive gravity theories.

(ii) Alternative models of dark matter. Related to the modified Newtonian dynamics (MOND) phenomenology like in direction (i), but in a complementary direction, we wish to analyze in depth and relate to some fundamental physics a model of dark matter based on the concept of "gravitational polarization" of a dipolar fluid, which allows to reproduce both MOND at the galaxy scale and cold dark matter at the cosmological scale.

(iii) Tests of the cosmological-model hypotheses. Both directions (i) and (ii) will help us construct and test a post-Lambda-CDM formalism, which will allow us not only to determine the preferred theories, but above all to extend the tests of fundamental laws to the cosmological scales. This formalism is associated with various null tests based on existing correlations between the predictions of the models. Any deviation from one of them would point out either a violation of the Copernican principle, or the need for an extension of general relativity, or some properties of the dark sector, without the need to specify any precise model.

This ANR project brings together the main French theoretical physicists working on extensions of general relativity and their observational tests, from the viewpoints of both local physics and cosmology. They have been working on the subject during several years, and some of their results are already internationally acknowledged in this field. The project's ambition is to connect the best scientists in France and abroad, and to federate a working community sharing notably their expertise in relativistic theories of gravity. This goal depends on strong international collaborations. The grant we ask will be mainly devoted to travel (visits and conferences), invitations of foreign scientists, and the organization of an international workshop.