CNRS/CEA Research Directors and Associate Professors

IHES Annual Report 2020 | 21

Cédric DEFFAYET Physics, CNRS research director, Institut d'Astrophysique de Paris, lecturer at École Polytechnique

In 2020, CÉDRIC DEFFAYET mainly explored two different research directions: first the problem of matter coupling in certain extended scalar-tensor theories

where some constraints reduce the number of propagating degrees of freedom with respect to the naive expectation;

secondly, certain topological defects appearing in scalar theories of k-essence type.

Concerning the first direction, it was shown for the first time that matter coupling generally preserves the degrees of freedom count in the absence of matter, whereas

in other cases it could dramatically change this count. This work also paves the way for the building of theories with a derivative coupling to fermions but no propagation of pathological degrees of freedom. In the second direction, new types of domain walls have been discovered. These domain walls are locally stable and not supported by a potential. They could play a role in the primordial universe.

SCIENTIFIC ACTIVITY

PUBLICATIONS

With S. Garcia-Saenz Degeneracy, Matter Coupling, and Disformal Transformations in Scalar tensor Theories Phys.Rev. D102 (2020), 064037, pre-publication arXiv: 2004.11619.

With F. Larroutou Domain Walls without a Potential Pre-publication arXiv: 2009.00404.

DISTINCTIONS

CNRS Bronze Medal (2007) Prix Thibaud, Académie des sciences de Paris (2010) ERC Starting Grant "New paradigms for InfraRed modification of Gravity" (NIRG) (2013)

Editor of: J. Cosmology and Astroparticle Physics

CONFERENCES

Austria Gravitational Physics Group, Universität Wien, Vienna (23 January) Topics in massive and partially massless gravity (seminar)

France Spontaneous Workshop XIV, Hot Topics in Modern Cosmology, Institut d'Études Scientifiques de Cargèse (3-9 May) Spontaneous Workshop on Cosmology 14 (conference postponed to 2021)

Particle Astrophysics and Cosmology Including Fundamental InteraCtions (PACIFIC) 2020, Moorea (31 August-5 September) (conference postponed to 2021)