- Name
- Christophe Ringeval

http://cp3.irmp.ucl.ac.be/Members/ringeval

- Position
- Professor

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- Address
- Centre for Cosmology, Particle Physics and Phenomenology - CP3

Université catholique de Louvain

2, Chemin du Cyclotron - Box L7.01.05

B-1348 Louvain-la-Neuve

Belgium

- Phone
- +32 10 4
**7 2075**

- Office
- E.352

- My personal homepage

- UCL member card

- Postdocs
- Sébastien Clesse, member since September 2007I study inflation, especially models of hybrid inflation and their dynamics. I'm also involved in research in the fields of cosmological perturbations, dark matter, and 21cm cosmology.

- Visitors
- André Füzfa (FUNDP), member since October 2006Professor at FUNDP, working in theoretical cosmology: interpretations of dark energy, observational constraints from structure formation, Hubble diagram, CMB, ... and on complex systems in general relativity: dynamical systems, multi-physics N-body simulations, numerical relativity.

In this respect, we have been involved in the Planck data analysis of the Cosmic Microwave Background (CMB) while our current interests are in CMB polarization measurements, large scale structures (Euclid satellite), gravitational waves (eLISA satellites) and forecasts for future 21cm surveys.

My research directions are diversified and motivated by both theoretical results and observational discoveries.

- I am involved in the following research directions:

#### 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.

#### 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.

#### 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.

#### 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.