**mercredi 5 juin à 15h**, salle 110

Blaise Goutéraux (Nordita) : **From AdS to Ricci-flat : holography and the Gregory-Laflamme instability**

In this talk, we show that a class of asymptotically AdS spacetimes with a planar subspace can be mapped to a class of Ricci flat spacetimes with a transverse round sphere via a generalized dimensional reduction, which involves analytically continuing the number of compact dimensions. In particular, it maps Minkowski spacetime to AdS on a torus and AdS black branes to (asymptotically flat) Schwarzschild black branes. At the linearized level, the holographic stress energy tensor of AdS to the stress energy tensor due to a brane localized in the interior of spacetime with asymptotically flat boundary conditions (and a flat compact subspace). Applying it to the known (nonlinear) solutions describing the hydrodynamic regime in AdS/CFT, we derive the hydrodynamic stress-tensor of asymptotically flat black branes to second order, which is constrained by the parent conformal symmetry. We compute the dispersion relation of the Gregory-Laflamme unstable modes through cubic order in the wavenumber, finding remarkable agreement with numerical data.

**mercredi 10 avril à 16h**, salle 110

Gonzalo J. Olmo (University of Valencia & IFIC Valencia) : **Semiclassical Palatini geons**

I discuss charged black hole solutions in a quadratic extension of the Palatini version of GR. We will see that all such objects can be seen as geons consisting on a wormhole supported by the electromagnetic field. In the lowest part of the charge and mass spectrum, the event horizon may disappear yielding a family of non-singular and topologically stable objects. Such configurations can be naturally seen as black hole remnants. Nonlinear corrections in the electromagnetic sector can lower the mass spectrum of these objects down to the TeV scale, thus bringing Planck scale physics within the reach of particle accelerators in a purely four-dimensional scenario. This construction offers a new window to quantum gravity phenomenology.

**mercredi 27 février à 16h**, salle 110

Takahisa Igata ( Kinki U., Japan and APC, Paris) : **Stable bound orbits of massless particles around a black ring**

**mercredi 20 février à 16h**, salle 110

Ignacy Sawicki (ITP, U. of Heidelberg) : **Modifications of gravity : what can linear cosmology we measure ?**

The difference between various models of dark energy or modifications of
gravity becomes apparent at the level of growth of large-scale structure
in the universe. In addition to measuring the background expansion, we are
now beginning to probe this aspect. The problem that is not particularly
understood currently is the impact of the assumption of a particular model
of dark energy on the interpretation of observations.

I will show that we in principle cannot disentangle the contribution of
dark matter from that of dark energy to neither the evolution of the
cosmological background nor that of large-scale structure unless a model
is assumed in the first place. This degeneracy limits severely the
measurements that we can obtain. I will discuss the total set of
model-independent observables that we can obtain from the sky and will
discuss how we could in principle rule out various classes of models.

**mercredi 6 février à 11h**, salle 110

Simon Ross (Durham U.) : **Holography for Lifshitz spacetimes**

I’ll review the development of a holographic description of field theories with anisotropic scaling symmetries through asymptotically Lifshitz spacetimes. I’ll discuss recent results on alternative boundary conditions for scalar fields in the bulk, which lead to a surprisingly high lower bound on the conformal dimensions of scalar operators in the field theories.

**mercredi 30 janvier à 11h**, salle 110

Minas Tsoukalas (Natl. Tech. U., Athens) : **Conformally coupled scalar fields in diverse dimensions : nontrivial vacua, instantons, and a generalization of the Yamabe problem**

In this talk I will present a generalization of the standard action for a conformally coupled scalar field φ propagating on a fixed Riemannian manifold endowed with a metric in d>2 dimensions, being such that the field equations are of second order. Conformal invariance strongly restricts the possible nonminimal couplings with higher powers of the curvature, so that the action can be mapped into the Lovelock one for a conformally rescaled metric. Therefore, the field equation for the conformally coupled self interacting scalar field can be geometrically formulated as follows : a linear combination of the dimensional continuation of the lower-dimensional Euler densities associated to the rescaled metric is a constant. As a consequence, this result allows to propose a natural generalization of the Yamabe problem. It is also shown that scalar field configurations being such that the rescaled curvature is constant, correspond to nontrivial vacua of the theory since, apart from solving the field equation, they also possess a vanishing stress-energy tensor. In the Euclidean case, it can be explicitly seen that this class of solutions describe instantons, since they are regular everywhere and possess finite action.

**mercredi 16 janvier à 11h**, salle 110

Leandros Perivolaropoulos (Ioannina U., Greece) : **Is there a fundamental cosmic dipole ?**

Some recent and diverse cosmological observations appear to challenge the cosmological principle. These observations include, the claimed spatial variation of the fine structure constant along a particular direction in the sky, the large scale CMB anomalies, the peculiar velocity dark flow and hints for anisotropic cosmic acceleration (dark energy dipole). After a brief review of these observations I will discuss a simple theoretical model (extended topological quintessence) that has the potential to be consistent with the above cosmic asymmetries. The model is based on the recent formation of a global monopole with Hubble scale core whose scalar field is non minimally coupled to electromagnetism. An off center cosmic observer would naturally observe the above observed cosmic asymmetries.

**mercredi 21 novembre à 11h**, salle 110

Luigi Pilo (University of l’Aquila) : **Non-Perturbative Massive Gravitons**

A new family of massive gravity theories is constructed nonpertubatively using Hamiltonian analysis. By using the canonical formalism we show that only five degrees of freedom propagate ; the result is step forward compared with existing ghost free theory which suffer from the the vDVZ discontinuity and strongly coupling. In the new class found, there is no vDVZ and and a preliminary analysis suggests that the cut-off is (M_pl m)^1/2, the largest possible in the absence of fundamental Higgs mechanism for gravity. The phenomenology is promising and, contrary to the existing theories, no strongly coupled physics has to be used to to reproduce solar system tests.

**mercredi 14 novembre à 11h**, salle 110

Viatcheslav Mukhanov (LMU, Munich) : **Why inflation predicts red tilted spectrum ?**

**mercredi 17 octobre à 11h**, salle 110

Vincent Vennin (IAP, Paris) : **Cosmological Inflation and the Quantum Measurement Problem**

According to cosmological inflation, the inhomogeneities in our universe are of quantum mechanical origin. This scenario is phenomenologically very appealing as it solves the puzzles of the standard hot big bang model and naturally explains why the spectrum of cosmological perturbations is almost scale invariant. It is also an ideal playground to constrain new physics and to discuss deep questions among which is the quantum measurement problem in a cosmological context. Indeed, although the large squeezing of the quantum state of the perturbations and the phenomenon of decoherence explain many aspects of the quantum to classical transition, it remains to understand how a specific outcome can be produced in the early universe, in the absence of any observer. Among the solutions currently discussed by the community is the Continuous Spontaneous Localization (CSL) approach to quantum mechanics, which has the advantage to be falsifiable. In this talk, I will apply this theory to inflation, in order to challenge it with further constrains, and more generally to discuss some aspects of the quantum measurement problem in Cosmology.

**mercredi 10 octobre à 11h**, salle 110

Mark Jackson (APC, Paris) : **Effective Field Theory in Inflation**

Although the observed CMB is at very low energy, it encodes ultra high-energy physics in spatial variations of the photon temperature and polarization fluctuations. This effect is believed to be dominated by the initial quantum state of the Universe. I will describe the first theoretical tools by which to construct such a state from fundamental physics. One can then use this technique to reliably calculate corrections to the power spectrum, non-Gaussianity, etc from high-energy physics. We may soon be able to compare these predictions against experiment, allowing one to rule out classes of inflationary models. Now is the critical time to undertake such investigations, with a number of ongoing and planned experiments such as Planck and CMBPol/Inflation Probe poised to collect a wealth of precision data.

**mardi 2 octobre à 11h**, salle 110

Vitor Cardoso (Lisbon, IST) : **Black hole explosions**

The dynamics of black holes play a fundamental role in astrophysics, high energy physics,
fundamental physics and particle physics. I will describe the (frenetic) activity over the last few years
with a special emphasis on black hole bombs and constraints on light bosonic particles.

**mardi 25 septembre à 11h**, salle 110

Roberto Emparan (Barcelona U.) : **Viscous fluid, elastic solid : The effective dynamics of black branes**

**mercredi 19 septembre à 11h**, salle 110

Sander Mooij (Nikhef, Amsterdam) : **Effective action for the Abelian Higgs model in FLRW**

We compute the divergent contributions to the one-loop action of the U(1) Abelian Higgs model. The calculation allows for a
Friedmann-Lemaitre-Robertson-Walker space-time and
a time-dependent expectation value for the scalar field. Treating the
time-dependent masses as two-point interactions, we use the in-in
formalism to compute the first, second and third order graphs that
contribute quadratic and logarithmic divergences to the effective
scalar action. Working in R_xi gauge we show that the result is
gauge invariant upon using the equations of motion.