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

Lorenzo Sorbo (Massachusetts U., Amherst) : **Nonstandard tensor modes from inflation**

In several models, the inflaton is coupled to other fields in such a way that nonperturbative phenomena can lead to production of particles during inflation. These particles can leave a trace of their existence on the spectrum of primordial tensors. I will describe how such tensor modes can inherit nonstandard properties, such as a net chirality, a blue tilt, or a feature in their power spectrum. I will also discuss the possibility that such tensor modes might be directly detected by ground-based gravitational interferometers such as advanced LIGO. A significant part of the talk will focus on the way such effects can be induced by a axion-like coupling of the inflaton to gauge fields, but I will also discuss other kinds of coupling of the inflaton to matter.

**mardi 7 juin à 11h**, salle 110

Monica Guica (LPTHE) : **Microscopic Realization of the Kerr/CFT correspondence**

The Kerr/CFT correspondence is a proposed microscopic description of
extremal black holes in terms of a conformal field theory (CFT). Despite
its universal success in explaining the entropy of extremal black holes,
the exact nature of the CFT remains rather mysterious. I will describe a
particular embedding of Kerr/CFT into string theory, according to which
the ’CFT’ is a deformation of a known CFT_2 (e.g. the D1-D5 CFT) by a
specific operator, which we write down explicitly. In addition, there is
a finite temperature for the right-movers turned on. I will argue that the
operator deformation is in fact exact and will draw parallels between the
background we use to study Kerr/CFT and those dual to 2d dipole theories.

**mercredi 1er juin à 11h**, salle 110

Tassos Petkou (University of Crete) : **Holographic Fluids in Randers-Zermelo geometries**

I discuss 2+1 dimensional holographic fluids that live on background non-trivial stationary metrics. Examples are the fluids in the boundary of Kerr-Taub-NUT AdS4 metrics. Using the Randers form of the boundary metric we describe fluids with non-trivial vorticity. Using the Zermelo form of the boundary metric we can describe moving media. I discuss briefly the relevance of the results to the holographic description of analog gravity systems as well as of rotating atomic gases.

**mardi 17 mai à 11h**, salle 110

Alessandro Fabbri (Valencia U.) : ** Testing Hawking particle creation by black holes through correlation measurements**

Hawking’s prediction of thermal radiation by black holes has been shown by
Unruh to be expected also in condensed matter systems. We show here that in a
black hole-like con-guration realised in a BEC this particle creation does indeed
take place and can be unambiguously identi-fied via a characteristic pattern
in the density-density correlations. This opens the concrete possibility of the
experimental veri-cation of this e-ffect.

**mercredi 11 mai à 11h**, salle 110

Bert Vercnocke (CEA Saclay) : **Goedel space and chronology protection in string theory**

Put forth in the nineteen fourties by Kurt Goedel, the Goedel universe has intrigued ever since. Sourced by seemingly innocuous matter, it describes a solution to Einstein’s equations which contains closed timelike curves (CTCs). Goedel space has also appeared in various guises and dimensionalities in string theory. In this talk, I will review recent work on three-dimensional spaces consisting of a Goedel inner part and a locally anti-de Sitter outer part, separated by a domain wall. Such a construction should hopefully eliminate the CTCs in the Goedel part of the geometry, while shedding light on the properties of the locally AdS geometry. I present applications in black hole microstate counting, in relating the presence (absence) of CTCs (’chronology protection’) to (not) having unitarity in the CFT dual to the locally AdS space, and finally supersymmetric constructions of such Goedel/AdS spaces in string theory. The latter have interesting consequences, such as a derivation of the
stringy exclusion principle from demanding absence of CTCs in the bulk spacetime perspective.

**jeudi 28 avril à 11h**, salle 110

Miguel F. Paulos : **Holographic c-functions and black holes as RG flows**

We consider domain-wall and AdS-black hole backgrounds in Lovelock theories of gravity. We construct a ``c-function’’ for these, which we argue counts the number of effective fields at a given scale - on AdS spaces it is related to the Euler anomaly, and at a black hole horizon it is proportional to the entropy. The monotonicity properties of are controlled by the ``gravitational field’’ which has opposite signs in the domain-wall and black hole backgrounds, due to the presence of negative (positive) energy in the former (latter). We show how can be written as the ratio of the Wald entropy to a quantity resembling an effective phase space volume.

**mercredi 6 avril à 11h**, salle 110

Francesco Nitti (APC, Paris) : **The thermodynamics of gauge/gravity duality (CONTINUED)**

**mercredi 30 mars à 11h**, salle 110

Francesco Nitti (APC, Paris) : **The thermodynamics of gauge/gravity duality**

**mardi 22 mars à 10h30**, salle 110

Thomas Sotiriou (DAMTP, Cambridge) : **Black holes in Einstein-aether and Horava-Lifshitz gravity**

**mercredi 16 mars à 11h**, salle 110

Jean-Francois Dufaux (APC, Paris) : **Gravitational Waves from Preheating After Inflation : Overview and Recent Results**

There can be several instances in the early universe where a scalar field condensate decays in a violent and highly inhomogeneous way. A major example is the process of preheating after inflation, when the particle content of the universe is produced by the decay of the inflaton and then evolve towards the thermal bath of the Hot Big Bang. These phenomena generate stochastic backgrounds of gravitational waves, which carry unique informations about their dynamics and the underlying high-energy physics, and which could be observable by interferometric experiments like LIGO/VIRGO and LISA.
In this talk, I will briefly review the analytical and numerical methods that we have developed to compute the gravity wave spectrum produced by such cosmological sources. I will then discuss the gravitational wave signals emitted by preheating in three main classes of inflationary models. I will also consider a different but related source of gravitational waves : the non-perturbative decay of flat direction condensates in supersymmetric theories. I will then discuss models where not only scalar fields, but also gauge fields play an important role in the dynamics. I will show that they lead to specific signatures in the gravity wave spectrum and I will correlate these new features with the dynamics of cosmic string configurations.

**mercredi 2 mars à 11h**, salle 110

Gwenael Giacinti (APC, Paris) : **Ultra-High Energy Cosmic Rays : Present status of the field**

I will present an up-to-date overview of the field of ultra-high energy cosmic rays. Its current status and the remaining open questions will be reviewed. Recent experimental data will be presented and their implications for theoretical physics and astrophysics will be discussed. I will also show the prospects for the near future.

**Vendredi 3 Décembre à 14h30**, salle 110

Jeff Steinhauer (Technion, Haifa) : **A sonic black hole and an oscillating soliton/vortex ring in a Bose-Einstein condensate**

Two of our recent experiments will be discussed. Firstly, we have
created an analogue of a black hole in a Bose-Einstein condensate. In
this sonic black hole, sound waves, rather than light waves, cannot
escape the event horizon. A step-like potential accelerates the flow of
the condensate to velocities which cross and exceed the speed of sound
by an order of magnitude. The Landau critical velocity is therefore
surpassed. The point where the flow velocity equals the speed of sound
is the sonic event horizon. The effective gravity is determined from
the profiles of the velocity and speed of sound. A simulation finds
negative energy excitations, by means of Bragg spectroscopy. In the
second experiment, We see evidence that the solitons evolve periodically
between vortex rings and solitons. The stable, periodic evolution is in
sharp contrast to the behaviour seen in previous experiments in which
the solitons decay irreversibly into vortex rings through the so-called
snake instability7-13. The evolution can be understood in terms of
conservation of mass and energy in a narrow condensate.

**Mardi 30 Novembre à 11h**, salle 110

Sudipta Sarkar (Maryland) : **Membrane Paradigm in f(R) gravity**

The membrane paradigm is the remarkable view that, to an external observer,
a black hole appears to behave exactly like a dynamical fluid membrane,
obeying pre-relativistic equations. We extend the existing membrane
formalism for General Relativity to black holes in any general f(R)-theory
of gravity and derive the stress tensor and various transport coefficients
of the membrane fluid. The membrane turns out to behave like a non-Newtonian
fluid in case of any f(R) gravity except for the case of General Relativity.
We also derive dynamical equations describing the fluid membrane. This
result establishes the general analogy of the properties of black hole
horizon with fluid mechanics beyond General Relativity to any general f(R)
gravity theory.

**Mercredi 24 Novembre à 11h**, salle 110

Julian Adamek (Würzburg Univ.) : **The shapeshifting Universe - anisotropic cosmologies from gravitational
tunneling and their observational signatures**

Higher dimensional gravity theories such as string theory may give rise to
a vast multitude of different vacuum configurations with varying numbers
of microscopic dimensions. It is possible that gravitational tunneling
induces transitions between these configurations, very similar to
first-order phase transitions in conventional false vacuum decay. If some
microscopic dimensions decompactify in such a process, this may be
perceived as an anisotropic cosmology from within the "bubble universe."
These ideas lead to interesting phenomenological questions which establish
the connection to cosmological observations. In particular, I will focus
on the CMB as a decent probe for the signatures of anisotropic cosmology
and discuss the constraints placed by observations on the parameters which
characterize the anisotropy. Furthermore, anisotropic cosmology may be
related to some of the anomalies already found in present experiments such
as WMAP.