Due to the non-commutation of spatial averaging and temporal evolution, inhomogeneities and anisotropies (cosmic structures) influence the evolution of the averaged Universe via the cosmological backreaction mechanism. We study the dynamics of backreaction effects and further introduce a relative entropy to characterize the structure formation in the perturbed Universe. We show this entropy increases during the cosmological evolution.
Our universe is homogeneous an isotropic on very large scales. However, when we go down to smaller scales inhomogeneities become more and more important. Given that Einstein equations are non-linear, it is clear than averaging and time-evolution are operations that do not commute. Thus, it is a crucial question to be addressed the importance of the non-commutativity of these two operations when we measure observables of the background cosmology. In General Relativity and a universe dominated by dust, there seems to be a general consensus that these corrections are small. However, when we consider modified cosmologies (either multifluid scenarios or alternative theories of gravity), the issue remains unclear. Moreover, this will need to be addressed to be able to use the next generation of high precision cosmological data to constrain such alternative scenarios.
External collaborators: Peter Dunsby (University of Cape Town), Alvaro de la Cruz Dombriz (Universidad Complutense de Madrid), Diego Saez (University of the Basque Country.
The first measurements of acoustic peaks in the CMB anisotropies strongly suggest that the birth of cosmological fluctuations may have taken place during an early inflationary era of the universe.
In this domain, our activities deal with the construction of explicit models of inflation as well as the extraction of their observable consequences. Our fields of expertise comprise some actively debated subjects as the existence of features (e.g. trans-Planckian effects), inflation with non-minimally coupled scalar fields, DBI- and brane inflation as in the context of String Theory.
For all these theories, we are maintaining various numerical tools such as the ASPIC and FieldInf librairies allowing to compute reheating-consistent predictions for comparison with cosmological data.
External collaborators: Jérôme Martin (IAP, Paris, France), Vincent Vennin (Portsmouth, U.K.), Sébastien Clesse (RWTH, Aachen, Germany).
Based on our knowledge of particle physics at very high energy, cosmic strings are a natural consequence of the symmetry breaking mechanism and are expected to be formed during the cooling of the universe. However, they have not been observed yet and our research is concentrated into the various effects they may have in cosmology. The technical difficulties to deal with such systems are overcome using super-computer numerical simulations. We are focusing our present work to the effects induced in the CMB and in other astrophysical observables.
External collaborators: Jun'ichi Yokoyama (University of Tokyo, Japan), Daisuke Yamauchi (RESCUE, Tokyo, Japan), Mairi Sakellariadou (King's College London, U.K.), Patrick Peter, François Bouchet (Institut d'Astrophysique de Paris, France).
Our expertise on inflation and cosmic strings is involved in the CMB data analysis of the PLANCK satellite.
Our current efforts concern the study of future CMB polarization experiments, ground based, and in space, as the CORE satellite.
We are part of the EUCLID collaboration and interested in the impact of high precision measurements of the matter power spectra of the large scale structures for cosmic inflation.
We are also involved in the LISA project, the giant space interferometer dedicated to gravitational wave astronomy, which should open a new window on cosmic string physics and other early universe phenomena.
Another direction concerns the 21cm cosmological radiation. This radiation is emitted by neutral hydrogen atoms and should shed light into the so-called "dark ages": from the recombination to the reionisation of the universe by the first stars. This new observable is expected to be sensitive to the nature of dark matter as well as to some properties of the inflationary era.
External collaborators: Sébastien Clesse (RWTH, Aachen), V. Vennin (Portsmooth, U.K.), CORE Coll., Euclid Coll., eLISA Coll.
Although the undergoing cosmic acceleration may be explained by a non-vanishing cosmological constant in Einstein gravity, various dynamical effects could very well explain current observations, all dubbed as dark energy.
Quintessence, as a light scalar field minimally coupled to gravity, is a dark energy candidate to explain the recent acceleration of the Universe expansion. The Ratra-Peebles potential and its corrected form in supergravity are under study. Using a modified version of CAMB, including perturbations of the scalar field, we use the latest SNIa and CMB observations to select acceptable points in the parameter space. Starting with the associated matter power spectrum, in collaboration with the LUTh (Paris-Meudon Obs., France) we run N-body simulations of growth of large scale structures where the background evolution is modified by quintessence. We are involved in the Dark Energy Universe Simulation Series (DEUSS) collaboration.
Another dark energy candiate involves cosmic inflation, currently the best explanation of the origin of large scale structures and CMB anisotropies. Similarly, if dark energy is a light scalar field, the current acceleration can be the consequence of quantum fluctuations during cosmic inflation, provided this one occurs at TeV scale.
External collaborators: Jean-Michel Alimi, Yann Rasera, Pier Stefano Corasaniti (Observatoire de Paris-Meudon, France).
Teruaki Suyama (The University of Tokyo, Japan), Tomo Takahashi (Saga University, Japan), Masahide Yamaguchi (Tokyo Institute of Technology, Japan), Shuichiro Yokoyama (Nagoya University, Japan).
It is possible to construct classical models of extra-dimensions based on Field Theory and General Relativity. The goal is to gain deeper understanding into these systems based on tractable and well known theories. In particular, the so-called Randall-Sundrum (RS) models in various dimensions can be modelised as hyper-dimensional topological defects. Our present studies concern the realisation of the Dvali-Gabadadze-Porrati mechanism inside hyper-dimensional monopoles, in which four-dimensional gravity can be obtained by trapping gravitons.
External collaborators: Antonio De Felice (The University of Tokyo, Japan).
The statistical properties of large scale structures contain a large amount of information on cosmological observables. The abundance of halos of given mass is sensitive to various cosmological observables such as the equation of state of dark energy, to the amount of primordial non-Gaussianity as well as to the mass and cross section of the dark matter particles. Various of our activities and research are equally focused to the future EUCLID satellite mission.
External collaborators: M. Musso.
The recent acceleration of the universe is explained in the standard model by the presence of a non-vanishing cosmological constant. However, one may also question the validity of General Relativity on length scales that have never been so accurately tested so far. However, it is not trivial to modify gravity and to build at the same time a model which can survive all the experimental tests. In collaboration with S. Capozziello, we looked for cosmological exact solutions for modifications of gravity in the form f(R) where f possesses a constant of motion during the evolution of the universe. In the future we plan to look for the subset of solutions which can describe experimental data from radiation domination up to today's accelerated expansion.
I have been working also in the so called f(G) gravity. This is a work made in collaboration with Shinji Tsujikawa, which aims to give some necessary conditions for a cosmologically viable f(G) gravity, where G is the Gauss-Bonnet scalar, that is a particular quadratic combination of the Riemann tensor. This scalar, G, has the property that, if not coupled, it can be written as a total derivative. Along with these conditions, we provide also some toy-models which fulfill them.
Born-Infeld inspired theories. Although General Relativity has proven to be very successful in the scales where it has been tested, when going to high curvature regimes it is commons the appearance of singularities like the Big Bang and/or black holes singularities. This motivates the modification of gravity in such a regime to try to regularize those singularities. We study a natural extension of these models and study their predictions in cosmology and astrophysics
External collaborators: Jose Beltran Jimenez (CPT, Université de Marseille), Lavinia Heisenberg (University of Stockholm), Gonzalo Olmo (University of Valencia).
The forthcoming cosmological experiments should provide new insights on the amount of non-Gaussianity eventually present in the Cosmic Microwave Background fluctuations and large scale structures surveys. We study various early universe models that could potentially let some imprints in these observables and especially cosmic strings.
External collaborators: Teruaki Suyama (The University of Tokyo, Japan), Mark Hindmarsh (Sussex University, U.K.), Stéphane Colombi, François Bouchet (Institut d'Astrophysique de Paris, France).
Observations show that magnetic fields are present everywhere in the universe. Planets, galaxies, clusters carry magnetic fields of varying strength and coherence size. There are evidences of their presence also in the intergalactic medium and this strongly suggests that their origin might be primordial. A promising candidate for the generation of primordial magnetic fields is inflation. Our work concerns the construction of efficient inflationary mechanisms which could produce the large-scale magnetic fields observed today and it deals in particular with the possible effects that such mechanisms have on the physics of the universe after inflation.
External collaborators: Chiara Caprini (CEA Saclay, France), Teruaki Suyama (The University of Tokyo, Japan).
When computing cosmological predictions it is often assumed that reionisation is homogenous and completely described by only one parameter, it's optical depth. However, reionisation is driven by the local collapse of matter and therefore highly inhomogeneous.
The above method is therefore only an approximation and large corrections can be expected for quantities which depend on the exact dynamics of reionisation.
We study more realistic models on reionisation and their impact on the cosmic microwave background, especially in polarization.
We work on the development and update of the numerical code SONG which solves the dynamics of the primordial Universe after Inflation. The computational methods used are comparable to the ones employed in the public codes CLASS and CAMB, but we solve the equations of motion beyond the linear order approximation, providing greater precision.
This is crucial for several dynamical effects which are absent in the leading order equations such as the generation of B-mode polarization and non-Gaussianity.
Furthermore, the code plays a central role in the recently developed Newtonian motion gauge framework. In this framework, a Newtonian N-body simulation can be promoted to a full relativistic simulation by interpreting it on the space-time of a specific Newtonian motion gauge. SONG can be used to compute the structure of these space-times up to second order in perturbation theory, thereby including for example the impact of relativity on the dark matter bispectrum.
External collaborators: Guido W. Pettinari, Thomas Tram, Cyril Pitrou (IAP, France).