Two important ESA missions are shaping the future of cosmology and astrophysics. One is Euclid, a space telescope designed to measure the galaxy shapes and positions up to high redshift. The correct interpretation of its data will test the seeds of primordial cosmological perturbations and will establish the impact of dark components--such as massive neutrinos and dark matter--and the presence of modifications of gravity, on the dynamics of the large-scale structure. The other mission is LISA, a space-based interferometer designed to detect gravitational waves produced by coalescing super massive black holes binaries or strong first-order phase transitions in the early universe. In this context, a major part of our activity in cosmology and gravity focused on theoretical aspects relevant for these missions: the study of the dynamics of the large-scale structure (LSS) of the universe, the modelling and parametrization of modified gravity theories and the predictions of gravitational waves spectra emitted at early times or from compact binary inspirals.
A large part of the information in the LSS resides on short scales, where density perturbations become large and enter the nonlinear regime. This is studied with N-body simulations, which have been used to reconstruct the response function measuring the impact of the small-scales on the large-scale physics. But to treat the nonlinear regime, we have developed several complementary techniques that rely on well-understood mathematical constructions, such as the large deviation principle. Other approaches, developed for standard gravity and applied to make predictions in modified gravity scenarios, use standard perturbation theory on large scales and regularize the UV regime either with a phenomenological halo model or with semi-analytic resummation techniques.
The major goal of Euclid is to shed light on the origin of the cosmic acceleration and test general relativity on cosmological scales. At IPhT we have developed new modified gravity theories and studied their phenomenological consequences. One way to modify gravity is to weaken it on large scales, by giving the graviton a tiny mass. We have studied the cosmology of the most viable realization of this theory, called bigravity, which involves two dynamical metrics coupled via a potential term designed to avoid instabilities. Another way to modify gravity is to add a scalar interaction. The fifth force exchanged by this scalar must be screened on Solar System scales by nonlinear self-interactions. We have studied the cosmological effects of modified gravity models with screening. Moreover, due to the large number of models of modified gravity in the literature, we have developed a unifying framework to compare them with data in terms of a minimal number of parameters, now adopted by most collaborations to parametrize deviation from general relativity on large scales.
The recent detection of gravitational waves by the LIGO/Virgo collaboration has opened a new window on the Universe. Although undetectable by current or planned interferometers, the search for the relic gravitational wave background produced by inflation has become the major activity of many cosmic microwave background polarization experiments. The simultaneous observation of gravitational waves and gamma ray bursts from the merger of two neutron stars, implying that gravity travels at the same speed as light, had dramatic consequences on modified gravity theories. We have shown that this observation strongly constrains the matter coupling in bigravity theories, while it rules out a large portion of the parameter space in scalar-tensor theories. For both these theories, we have derived the most general classes that leave the speed of gravity unaffected and we have explored their consequences for structure formation.
|Nicolas Van de Rijt
Our weekly seminar takes place every Tuesday at 16:00.
Postdoctoral positions are available each year in the Fall. Check this page or contact any staff member of the group.
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The full postal adress of IPhT is: Institut de Physique Théorique, CEA/Saclay, Bat 774 Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France.
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