CNRS Astroparticle and Cosmology Lab

Paris, France

CNRS Astroparticle and Cosmology Lab

Paris, France
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Li W.,University of Paris Pantheon Sorbonne | Li W.,CNRS Astroparticle and Cosmology Lab
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2016

A set of novel derivative terms for spin-2 fields are proposed. They are the wedge products of curvature two-forms and vielbeins. In this work, we investigate the properties of novel two-derivative terms in the context of bigravity. Based on a minisuperspace analysis, we identify a large class of bigravity models where the Boulware-Deser ghost could be absent. We give a new perspective that Weyl gravity and new massive gravity belong to this class of bigravity models involving novel derivative terms. This is related to the fact that this class of models contains spin-2 ghosts. In addition, we discuss the UV cutoff scales, dynamical symmetric conditions, and novel higher-derivative terms. © 2016 American Physical Society.

Parentani R.,University Paris - Sud | Serreau J.,CNRS Astroparticle and Cosmology Lab
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We propose a new approach to compute correlators of quantum fields in de Sitter space. It is based on nonequilibrium field theory techniques, and exploits de Sitter symmetries so as to partially reduce the number of independent variables of n-point functions in a manner that preserves the usefulness of a momentum representation, e.g., for writing spatial convolution integrals as simple products. In this representation, the two-point function of a scalar field only depends on two physical momenta, and the corresponding Schwinger-Dyson evolution equations take the form of momentum flow equations. Moreover, standard diagrammatic rules can be entirely formulated in this representation. The method is suitable for analytical approximations as well as numerical implementations. In forthcoming publications, we apply it to resum infrared logarithmic terms appearing in the perturbative calculation of vertex and correlation functions. © 2013 American Physical Society.

Deffayet C.,CNRS Astroparticle and Cosmology Lab | Jacobson T.,University of Maryland University College
Classical and Quantum Gravity | Year: 2012

We discuss the structure of horizons in spacetimes with two metrics, with applications to the Vainshtein mechanism and other examples. We show, without using the field equations, that if the two metrics are static, spherically symmetric, nonsingular and diagonal in a common coordinate system, then a Killing horizon for one must also be a Killing horizon for the other. We then generalize this result to the axisymmetric case. We also show that the surface gravities must agree if the bifurcation surface in one spacetime lies smoothly in the interior of the spacetime of the other metric. These results imply for example that the Vainshtein mechanism of nonlinear massive gravity theories cannot work to recover black holes if the dynamical metric and the non-dynamical flat metric are both diagonal. They also explain the global structure of some known solutions of bigravity theories with one diagonal and one non-diagonal metric, in which the bifurcation surface of the Killing field lies in the interior of one spacetime and on the conformal boundary of the other. © 2012 IOP Publishing Ltd.

Vaananen D.,CNRS Astroparticle and Cosmology Lab | Volpe C.,CNRS Astroparticle and Cosmology Lab
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We linearize the neutrino mean-field evolution equations describing the neutrino propagation in a background of matter and of neutrinos, using techniques from many-body microscopic approaches. The procedure leads to an eigenvalue equation that allows us to identify instabilities in the evolution, associated with a change of the curvature of the neutrino energy-density surface. Our result includes all contributions from the neutrino Hamiltonian and is generalizable to linearize the equations of motion at an arbitrary point of the evolution. We then consider the extended equations that comprise the normal mean field as well as the abnormal mean field that is associated with neutrino-antineutrino pairing correlations. We first rederive the extended neutrino Hamiltonian and show that such a Hamiltonian can be diagonalized by introducing a generalized Bogoliubov-Valatin transformation with quasiparticle operators that mix neutrinos and antineutrinos. We give the eigenvalue equations that determine the energies of the quasiparticle eigenstates. Finally we derive the eigenvalue equation of the extended equations of motion, valid in the small amplitude approximation. Our results apply to an arbitrary number of neutrino families. © 2013 American Physical Society.

Francia D.,CNRS Astroparticle and Cosmology Lab
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2010

Unconstrained local Lagrangians for higher-spin gauge theories are bound to involve auxiliary fields, whose integration in the partition function generates geometric, effective actions expressed in terms of curvatures. When applied to the triplets, emerging from the tensionless limit of open string field theory, the same procedure yields interesting alternative forms of geometric Lagrangians, expressible for both bosons and fermions as squares of field-strengths. This shows that higher-spin curvatures might play a role in the dynamics, regardless of whether the Fronsdal-Labastida constraints are assumed or forgone. © 2010 Elsevier B.V. All rights reserved.

Serreau J.,CNRS Astroparticle and Cosmology Lab | Volpe C.,CNRS Astroparticle and Cosmology Lab
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2014

We derive the most general evolution equations describing in-medium (anti)neutrino propagation in the mean-field approximation. In particular, we consider various types of neutrino-antineutrino mixing, for both Dirac and Majorana fields, resulting either from nontrivial pair correlations or from helicity coherence due to the nonvanishing neutrino masses. We show that, unless the medium is spatially homogeneous and isotropic, these correlations are sourced by the usual neutrino and antineutrino densities. This may be of importance in astrophysical environments such as core-collapse supernovae. © 2014 American Physical Society.

Ferreira J.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory | Casse F.,CNRS Astroparticle and Cosmology Lab
Monthly Notices of the Royal Astronomical Society | Year: 2013

We investigate under which conditions cold, fan-shaped winds can be steadily launched from thin (Keplerian) accretion discs. Such winds are magnetocentrifugal winds launched from a thin annulus in the disc along open magnetic field lines that fan out above the disc. In principle, such winds could be found in two situations: (1) at the interface between an inner jet emitting disc, which is itself powering magnetocentrifugally driven winds, and an outer standard accretion disc; (2) at the interface between an inner closed stellar magnetosphere and the outer standard accretion disc. We refer to terminal or T-winds as the former kind and to magnetospheric or M-winds as the latter. The full set of resistive and viscous steady-state magnetohydrodynamic (MHD) equations are analysed for the disc (the annulus), which allow us to derive general expressions valid for both configurations. We find that, under the framework of our analysis, the only source of energy able to power any kind of fan-shaped winds is the viscous transport of rotational energy coming below the inner radii. Using standard local a prescriptions for the anomalous (turbulent) transport of angular momentum and magnetic fields in the disc, we derive the strength of the transport coefficients that are needed to steadily sustain the global configuration. It turns out that in order for these winds to be dynamically relevant and explain observed jets, the disc coefficients must be far much larger than values expected from current knowledge of turbulence occurring inside protostellar discs. Either the current view on MHD turbulence must be deeply reconsidered or steady-state fan-shaped winds are never realized in nature. The latter hypothesis seems to be consistent with current numerical simulations. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Allard D.,CNRS Astroparticle and Cosmology Lab
Astroparticle Physics | Year: 2012

In this paper we review the extragalactic propagation of ultrahigh energy cosmic-ray (UHECR). We present the different energy loss processes of protons and nuclei, and their expected influence on energy evolution of the UHECR spectrum and composition. We discuss the possible implications of the recent composition analyses provided by the Pierre Auger Observatory. The production of secondary cosmogenic neutrinos and photons and the constraints their observation would imply for the UHECRs origin are also addressed. Finally, we conclude by briefly discussing the relevance of a multi messenger approach for solving the mystery of UHECRs. © 2012 Elsevier B.V. All rights reserved.

Volpe C.,CNRS Astroparticle and Cosmology Lab
Annalen der Physik | Year: 2013

Neutrinos of astrophysical origin are messengers produced in stars, in explosive phenomena like core-collapse supernovae, in the accretion disks around black holes, or in the Earth's atmosphere. Their fluxes and spectra encode information on the environments that produce them. Such fluxes are modified in characteristic ways when neutrinos traverse a medium. Here the current understanding of neutrino flavour conversion in media is summarized. The importance of this domain for astrophysical observations is emphasized. Examples are given of the fundamental properties that research into astrophysical neutrinos has uncovered, or might reveal in the future. © 2013 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Francia D.,CNRS Astroparticle and Cosmology Lab
Journal of Physics: Conference Series | Year: 2010

Equations of motion for free higher-spin gauge fields of any symmetry can be formulated in terms of linearised curvatures. On the other hand, gauge invariance alone does not fix the form of the corresponding actions which, in addition, either contain higher derivatives or involve inverse powers of the d'Alembertian operator, thus introducing possible subtleties in degrees of freedom count. We suggest a path to avoid ambiguities, starting from local, unconstrained Lagrangians previously proposed, and integrating out the auxiliary fields from the functional integral, thus generating a unique non-local theory expressed in terms of curvatures. © 2010 IOP Publishing Ltd.

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