Relativity and Compact Objects (ROC Group)

Numerical Relativity

01 Strong-field tests of gravity

The upcoming of new instruments (GRAVITY, ALMA, ...) with very high angular resolution, in particular for the observation of the central black hole in our Galaxy, is an opportunity to test our knowledge of gravitation. Indeed, this is the first time we are able to directly observe the near vicinity of a black hole, with possibly an image of its accretion disc, as well as the trajectories of the stars grazing the horizon. It is then possible to test two essential points in our understanding of gravitational force: the theory of General Relativity, written by Albert Einstein in 1915, and the very concept of black hole.

The ROC team tries to predict the images and the trajectories that can be observed by those instruments, assuming other theories for gravitation than general relativity, or alternative models to the black hole (boson stars, gravastars, ...). This is done using numerical tools, as Gyoto, that have been developed in our team, but are open to the whole scientific community.

02 Gravitational waves

Gravitational waves are another potentially observable consequence of General Relativity. They already have been indirectly detected with binary pulsar timing and since several years, they are directly sought with large interferometric detectors (Virgo, LIGO, ...). The current improvement of the sensitivity of these instruments shall certainly allow for a first detection of gravitational waves in a near future. The launching of a space-based detector (eLISA) by the European Space Agency (ESA), shall be the opportunity to study later other wavelengths of space-time ripples, and to obtain informations about the history of our Universe.

Activity in ROC team deals with the prediction of expected signals, in order to optimize the processing of the data from the instruments and improve the probability of gravitational wave detection. The approaches used are both numeric (with LORENE and Kadath libraries) and analytic, with highly efficient perturbative techniques. The systems that are studied mainly consist of two compact objects: black hole binaries, neutron star binaries or mixed ones. These are the most promising astrophysical sources for the emission of gravitational waves.

03 Compact objects and supernovae

Within the theory of General Relativity, part of the team’s activity is devoted to compact objects. Thus, nuclear physics are used in order to describe the interior of neutron stars. An open problem is the composition and the equation of state of this nuclear matter, where density can overcome that of atomic nuclei. Nucleons (protons and neutrons) could, for instance, dissolve into quarks. Comparing observations with theoretical predictions from our team’s models, we expect to obtain informations on hadronic physics, which would be complementary to those obtained with particle accelerators. This is a contribution from astrophysics to fundamental physics. Black holes constitute a second type of compact objects. They are studied as sources of gravitational radiation (see above), but also through their effects on their nearest surroundings.