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.

Research themes

Large scale structures

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.  

Modified gravity

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.

Gravitational waves

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. 

Researchers involved

Permanent and emeritus researchers

PhD students

Postdoctoral researchers

Former staff members

Former Postdoctoral researchers

Former graduate students

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Networking, collaborations & fundings

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Seminars

Our weekly seminar takes place every Tuesday at 16:00.

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Events

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Jobs

Postdoctoral positions are available each year in the Fall. Check this page or contact any staff member of the group.

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Contact

Each member of the group can be contacted via email at name.surname@ipht.fr . 

The full postal adress of IPhT is: Institut de Physique Théorique,  CEA/Saclay, Bat 774 Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France.  

Here are directions to the IPhT.