## Cosmology and General Relativity

Our data analysis activities have been focused on the Planck satellite measurements and are now turning to the next generation of CMB polarization experiments, such as the CORE satellite. We are also part of the EUCLID and LISA missions, with measurements of the large scale structure of the Universe and gravitational waves, respectively.

Our works on late-time cosmology concern the reionization era and the development of numerical codes to solve for the cosmological perturbations at second order. For this we have developed the SONG code. In particular, our results are used to implement General Relativity effects within the numerical simulations of the formation of the large scale structures.

With respect to early universe cosmology, we explore the observable consequences of Cosmic Inflation and the existence of topological defects, and maintain the public numerical libraries ASPIC and FieldInf allowing to solve any slow-roll and multifield inflationary models.

Another part of our theoretical activities is concerned with modified gravity theories as well as supersymmetric model building for dark matter candidates.

### Members

#### Professors

##### Chiara Arina

Jean-Marc Gérard

Christophe Ringeval

#### Postdocs

##### Sébastien Clesse

Michele Lucente

#### Visitors

##### André Füzfa

### Projects

#### Cosmic inflation

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).

#### Cosmic strings

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).

#### Cosmological data

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.

#### Dark matter and neutrino masses in SUSY models

We study supersymmetric models in which modifications of the neutrino sector, to include a mass term, are connected to the dark matter sector. We analyse if the dark matter particles can be good dark matter candidates by considering cosmological and astrophysical constraints, as well as if the new neutrino sector can accommodate neutrino data.

External collaborators: Suchitat Kulkarni (OAW Wien).

#### Modified gravity

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).

#### Reionisation

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.

#### Scalar-tensor theories of gravitation

The strong equivalence principle (SEP) does not hold anymore in various extensions of General Relativity. Its violations can be revealed by the non-universality of free-falling for compact objects and we have developed a generic and effective way to test the SEP in the Cosmic Microwave Background (CMB). A violation of the SEP indeed alters the amplitude of the acoustic oscillations in the primeval plasma. Using the WMAP data, we have contrained a possible SEP violation for the baryons.

Our interests are also focused on the scalar-tensor theories of gravitation and their cosmologies. In a more specific way, we are currently studying a model where the scalar sector is conformally invariant. The effective fluid related to the non-minimally coupled scalar field differs from the other cosmological fluids of radiation by its very particular anisotropic pressure and we are studying its impact on the CMB anisotropies by modifying the CAMB code.

#### SONG -- Simulations of the early Universe

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).

Show past projects.

### Publications

#### 2017

*A. Vafaei Sadr, S. Movahed, M. Farhang, C. Ringeval and F. R. Bouchet*

[Abstract] [PDF]

Refereed paper. 3rd October.

*Christophe Ringeval and Teruaki Suyama*

[Abstract] [PDF]

Refereed paper. 13th September.

*Christian Fidler and Christophe Ringeval*

[Abstract] [PDF]

Refereed paper. 6th September.