My research interests  :

Bibliographic numbering refers to the publication list in my curriculum vitae.


Hybrid inflation : Multi-field dynamics and cosmological constraints


Among the zoo of inflationary models, the hybrid class is particularly interesting because it can be embedded in various high energy frameworks like supersymmetry/supergravity, grand unified theories, extra-dimensions or string theory.  In hybrid models, inflation is realised along a quasi-flat valley of the potential.  In the usual description, inflation ends abruptly when an auxiliary field develops a tachyonic waterfall instability.  During the waterfall, the long wavelength transverse perturbations grow exponentially so that the linear theory quickly breaks.  This induces a rapid energy transfer of the potential energy of the fields into the energy of inhomogeneous classical waves.  This process is called the tachyonic preheating.  My research work in this field concerns three issues, which are briefly described below.

Problem of initial field values:  In order to generate a sufficiently long phase of inflation, the two-field trajectories were usually required to be initially fine-tuned in a very narrow band along the inflationary valley or in some isolated points outside it.   From a more precise investigation of the dynamics, I have shown with C. Ringeval and J. Rocher [11-14] that the original hybrid model does not suffer from any fine-tuning problem. We have shown that the set of successful initial field values possesses a fractal boundary exploring the whole field space. By performing a MCMC analysis of the parameter space, we have shown that inflation is actually generic and more probable by starting outside of the inflationary valley. Natural bounds on potential parameters have been deduced. The genericity of our results has been confirmed for 5 other hybrid models from various frameworks, namely the SUSY/SUGRA F-term, smooth and shifted hybrid models, as well as radion assisted gauge inflation. 


Violation of slow-roll conditions along the valley:  We have shown that slow-roll violations can modify strongly the slow-roll dynamics along the valley [11].  In that case, super-Planckian initial field values are needed but the resulting scalar power spectrum is generically red, possibly in agreement with observations.  With J. Reckier we have established new Planck/BICEP2 constraints on the hybrid model in the large field regime [2], taking account those slow-roll violations.  

The waterfall phase:  I have identified a new inflationary regime for which more than 60 e-folds are generated classically during the waterfall phase [10]. This new scenario takes place in a large part of the parameter space of the original model. When it occurs, topological defects are conveniently stretched outside the observable Universe and the power spectrum of scalar perturbations is modified. With B. Garbrecht and Y. Zhu, I have extended this analysis to the F-term and D-term models and calculated the power spectrum of curvature perturbations and the level of non-Gaussianity in this regime [5,8]. With J. Garcia-Bellido, I study a similar regime in which large curvature perturbations lead to the formation of primordial black holes that can be identified to the dark matter.  


Cosmology with the 21-cm cosmic background


In the field of 21-cm cosmology, we have forecast with C. Ringeval, M. Tytgat, H. Tashiro and L. Lopez [7] possible future bounds on the cosmological and reionisation parameters, as well as on the primordial power spectrum from inflation. For this purpose, we have used and compared Fisher matrix and Bayesian MCMC methods. I have also studied with A.C. Davis and P. Brax the signatures on the 21-cm power spectrum of several modified gravity models, namely the f(R), chameleons, symmetron and dilaton models [6]. 

Linear perturbations in Modified Gravity


In collaboration with A.C Davis, P. Brax, C. van de Bruck and G. Sculthorpe, we have studied the linear perturbation dynamics of some specific models of early modified gravity, and their signatures on the CMB angular power spectrum and on the amplitude of the baryon acoustic oscillations [4]. I have also studied the linear perturbation dynamics of several models and their signatures on the 21cm spectrum at reionization (see above). Finally I have studied with P. Brax, B. Li, P. Valageas and A. Barreira the cosmological signatures of the K-mouflage model in presence of massive neutrinos [1].     


Observational signatures of non-singular bouncing cosmologies:


Models of inflation alone do not solve the initial singularity problem and a quantum theory of gravitation is needed to describe the Universe at the Planck energy scale. On the other hand, models of closed Universe performing a classical bounce avoid the initial singularity but do not explain the flatness of the Universe. In this context, some scenarios in which a classical bounce is followed by an inflationary era have been proposed. But the signatures of such models in the initial power spectrum were relatively blurred. With my collaborators M. Lilley and L. Lorenz [9], we have studied of a model of classical bounce followed by inflation and calculated the resulting signatures on the scalar power spectrum, taking the form of superimposed oscillations.


Testing inflation and curvaton scenarios with CMB distortions:


Prior to recombination, Silk damping causes the dissipation of the energy of acoustic waves into the CMB monopole, resulting in CMB spectral distortions. These can be used to probe the primordial scalar power spectrum on smaller scales than it is possible with CMB anisotropies. With B. Garbrecht and Y. Zhu, I have studied the detectability of CMB distortions generated by a broad variety of inflation and curvaton models [3].  

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