Theoretical physics: gravitation and cosmology
Astroparticles
The detection, a few decades ago, of cosmic rays with energies that can exceed 1020 eV confronted us with some of the most interesting and challenging questions in astrophysics: Where do they come from? How can they be accelerated to such high energies? What do they tell us about these extreme cosmic accelerators? In spite of all these years of experimental and theoretical endeavor, these questions remain unanswered. The measurement of a flux suppression at the highest energies, reminiscent of the "GZK cut-off" produced by the interaction of particles with the cosmic microwave background photons for propagations over intergalactic scales, has appeased the debate concerning the extragalactic provenance of ultrahigh energy cosmic rays (UHECRs). This feature not only suggests that UHECRs would originate outside of our Galaxy, but also that the sources of the highest energy particles should be located within ~100 Mpc distance, in our local Universe. However, the sources remain a mystery and results from the Auger Observatory on the arrival directions and chemical composition of UHECRs make the picture even more puzzling. Indeed, no powerful counterpart sources are observed in the arrival directions of the highest energy particles (where the deflections due to magnetic fields should become negligible), and the measurements of the chemical composition seem to indicate an unexpected trend towards heavy nuclei at the highest energies.
Candidate sources of UHECRs range from the birth of pulsars to explosions related to long-duration gamma-ray bursts or to events in active galactic nuclei. Working at the interface of cosmology, high energy astrophysics, plasma physics and particle physics, the research group is trying to unveil some of this mystery.
Permanent researchers: Kumiko Kotera, Martin Lemoine
Primordial Cosmology
In the very early phases of the universe, i.e. when it was much smaller and therefore much more energetic than it currently is, the physics involved was probably quite different from what is known at lower energies. Large scale structures were formed from the initial conditions set by the physics relevant at this scale, which thus can be probed. Grand Unified Theories or Superstring/M theory, for instance, are expected to play a crucial role at such scales, primordial cosmology being, as yet, the only known testing ground for those. Some relevant scales could even have passed through a stage during which their energy would have exceeded the Planck scale, hence leading, again, to a unique probe for this regime; this is the so-called trans-Planckian problem of cosmology.
This research allows, among other things, to calculate the shape and properties of the primordial perturbations, in view of comparing them to current or future observational data (see for instance those that have been produced by the Planck mission [link]): the result depends on the theory involved. Those involved in particular inflationary models that can be studied with extremely accurate methods first developed in the field of atomic physics. Although current data clearly favor this category of models, one needs to understand better the alternative scenarios, would it be only to infirm their validity, hence somehow confirming the inflationary paradigm. Alternative models comprise those having topological defects such as cosmic strings (note that these objects could be formed at the end of the inflation phase or right after it, with, again, cosmological consequences that have not all been obtained), or a contraction phase connecting the currently expanding one by a bounce, which thus comes as a replacement of the Big-Bang. Other models are even more extreme in the vision of the Universe they propose: it could consist of a "brane", i.e. a 3-dimensional object on which we would be living, itself moving in a higher dimensional spacetime.
Permanent researchers: Martin Lemoine, Jérôme Martin, Patrick Peter, Cyril Pitrou, Sebastien Renaux-Petel, Jean-Philippe Uzan
Relativistic gravitation
The theory that describes the gravitational interaction is general relativity, in which matter distorts spacetime: there is no force in this framework. Studies done in the group concern gravitational waves emitted by neutron star or black hole binary systems, the question of coalescence of black holes, test of alternative theories of gravity, e.g. including extra scalar fields [link], and internal structure of neutron stars. Part of the group activity fits in the framework of the VIRGO project aiming at detecting gravitational waves.
Permanent researchers: Luc Blanchet, Gilles Esposito-Farèse, Guillaume Faye
Alternative theories of gravitation
The theory of General Relativity is theoretically well-motivated and experimentally well-tested. It is however possible that the theory be modified at scales larger or smaller than those for which its validity has been evidenced. This type of modifications (e.g., MOND-type theories) could explain in particular the currently observed acceleration of the expansion of the Universe, and/or provide an alternative to Dark Matter theories.
However, potential modifications of General Relativity are highly constrained, both at classical and quantum levels (stability, unitarity...). The research undertaken at IAP aims at better characterizing the viable theories from this point of view.
Permanent researchers: Luc Blanchet, Cédric Deffayet, Gilles Esposito-Farèse
