The cosmology group has various themes of research, aiming at relating observations and experiments on the primordial Universe with the most recetn ideas on theoretical physics. They have active collaborations with
LAL and IAS experimental groups.

### Neutrinos and leptogenesis

With three families of neutrinos, it is very difficult to create the asymmetry needed for leptogenesis at the TeV scale. A remedy proposed by A. Abada is a four-singlet model (right-handed neutrinos) with 4 families. It is possible to generate a satisfying asymmetry for leptogenesis, compatible with the current observations on the ratio of the densities of baryons and photons and on the left-handed neutrinos.

### Primordial Universe

The Universe is known to be expanding, and it was hotter and denser in the past. We can say that in the past, 1) the Universe reached temperatures above 10^10 deg Kelvin, 2) it was in thermica equilibrium at 10^-5, and 3) it was homogenous and isotropic at 10^-5 près, due to the scale invariance of the inhomogeneity spectrum.
We still have to design cosmological scenarios of the Univers for the period beofre the cosmological standard model. Indeed, this model does not account for the origin of the three previous points, which must be considered as initial conditions.

The scenario of cosmological inflation (accelerated expansion) is the only one available to understand these properties, and more specifically the observed properties of the inhomogeneity spectrum. Most of the work of the cosmology group deal with inflation.

### Black holes

We study exact solutions of black holes in dimensions above four, as well as the generalisation of classical theorems for black holes to higher dimensions and their application to extra-dimensions. We work in particular on static black holes with axial symmetry, wich generalise accelerating or rotating black holes in 4 dimensions. These solutions may exhibit exotic topologies, they may describe the gravitation of a black hole localised on a brane and they may have a holographic explanation through the AdS/CFT correspondence.

### Microwave background

We are interested in the predictions of models for the primordial Universe concerning the anisotropies of temperature and polarisation for the microwave background, since they constitute particularly strong constrains for cosmological models. It has (and will) become more and more difficult to propose new models without contradicting the available data (the simple inflation models seem to cope with all the data currently available).

LPT is part of the HFI collaboration on the PLANCK satellite to be launched in 2008. We work closely with the PLANCK teams at IAS, LAL (Orsay) and IAP (Paris).

### Extra-dimensions and modification of gravity

The cosmology group is interested in gravity in extradimensions, and in particular in the modifications that can be brought to Einstein theory both in infrared and ultraviolet regimes (very large and very small distances). This research in the domain of high-energies where gravity becomes comparable to the other fundamental forces is motivated on the one hand by theories proposing the unification of gravity with the other interactions (such as string theories living in more than 4 dimensions), and on the other hand, by recent observations showing that our Universe is accelerating.

Therefore, either the main part of our Universe is not made of observable matter, or gravity itself is modified at these scales. The group studies theories which generalises Einstein theory at 4 dimensions
(exact solutions, properties, stability).
We work actively on brane universes where our 4-dimensional universe is a subspace of the whole space-time. We want to establish cosmological perturbations of branes and exact solutions corresponding to black holes localised on a brane, with the aim of confronting these results with cosmological observations.

### Astroparticles

Y. Mambrini focuses models inspired by string theory, which are situated in a more general framework of supersymmetric theories. Astroparticle and cosmological observations provide information on these theories, complementarily to accelerator experiments, in order to constrain the parameters of the underlying theory. Y. Mambrini is interested in dark matter in cosmology : it could be a weakly interactive massive particle, such as the lightest supersymmetric particle. Such a particle, linked to supersymmetric models, could be detected in high-energy colliders.