**Upcoming seminars**

**Thursday October 6, 2am**, room 110

Alessandro Fabbri (Enrico Fermi Ctr., Rome & Orsay, LPT) : **Quantum backreaction in rotating BTZ black holes**

**Thursday October 13, 2pm**, room 110

Victor Berezin (INR Moscow) : **Particle creation phenomenology, Dirac sea and the induced Weyl and Einstein-dilaton gravity**

**Past seminars**

**Thursday September 29, 11am**, room 114

Yann Mambrini (LPT Orsay) : **Dark matter : an historical perspective**

I will review in this seminar this historical evolution of the concept
of"Dark Matter", from the first observations of "anomalies" in the 30’s
until the development of the hypothesis of the existence of a dark halo.
The presentation will be entirely based on the original historical
authors, reconstructing (by their ideas but also their mistakes) the
appearance of a dark side of the Universe. The talk will include
discussions of original articles by Poincaré, Zwicky, Oort, Jansky, Gamow,
Peebles, Zeldovich, Bond and many others, opening on the prospect for the
near future.

**Thursday 12 May, 4pm (Note unusual time)**, room 110

Michal Artymowski (Institute of physics, Jagiellonian University) : **Inflation from the almost most genral f(R) theory**

The Starobinsky inflation, which is the simplest form of the f(R) theory has been proven to be perfectly consistent with the data. Nevertheless its ability to generate inflation can be spoiled due to the existence of the higher order terms. In my talk I will present the theory, which consist of all possible higher powers of the Ricci scalar, assumes inflation, and its power spectrum fits the Planck data. I will also investigate the vacuum stability of the theory.

**Thursday 14 April, 2pm**, room 110

Sabir Ramazanov (Gran Sasso Science Institute, Italy) : **Living with ghosts in Horava-Lifshitz gravity**

I will discuss the projectable version of the Horava-Lifshitz model, - the power
counting renormalizable theory of gravity. I will focus on the
particular branch of the model, which exhibits ghost instabilities
in the low energy limit. It turns out that the associated
vaccum decay rates into the Standard Model particles are tiny in a wide range of phenomenologically acceptable parameters, - mainly (but not only) due to the Lorentz violating structure of the model and to the presence of the
finite strong coupling scale. Understood as a cutoff on the spatial momenta of the ghosts, the latter can be raised up to \Lambda 10 TeV. This is by many orders of magnitude higher
than the strong coupling scale in the branch of the model plagued by the tachyons in the infrared limit. At lower momenta, the projectable Horava—Lifshitz
gravity recovers General Relativity supplemented by a fluid characterised by a small positive sound speed squared c^2_s < 10^*-20*, that
could be a promising candidate for the Dark Matter. In the final part of the talk, I will comment
on the possible phenomenological signatures of this Dark Matter on the evolution of large scale structures.

**Wednesday 6 April, 2pm**, room 110

Olivier Minazzoli (Centre Scientifique de Monaco) : **Rethinking the link between matter and geometry**

While geometry and matter actions are usually considered separately before being added up in the total action, it has recently been shown that a theory where matter and geometry cannot be considered separately, classically reduces to general relativity in the dust field limit. In this talk, we will review some of the nice features of this newly proposed surprising theory.

**Thursday 31 March, 2pm**, room 110

Edward Porter (APC Paris) : **Testing General Relativity with the binary black hole GW150914**

On the 14th of September, 2015, the two advanced LIGO detectors made the first direct detection of gravitational waves. The detected signal came from the coalescence of two stellar-mass black holes at a distance of z 0.1 . As well as being the first direct detection of gravitational waves, this observation has also provided concrete proof for the existence of massive stellar-mass black holes. Furthermore, this binary black hole system has finally allowed us to probe the true strong gravitational field. In this presentation, we will look at how this system has been used to test both the predictions of GR, and the robustness of the theory, and, to put constraints on certain alternative theories of gravity.

**Monday 15 February, 2pm**, room 110

Adolfo Cisterna (Universidad Austral de Chile) : **Compact objects in Horndeski theory : The case of nonminimal derivative coupling**

**Wednesday 10 February, 2pm**, room 110

Antonin Coutant (University of Nottingham) : **Almost black holes in flowing water**

When the velocity of flowing water surpasses the speed of surface waves, these propagate in exactly the same way as radiation near a black hole horizon. In particular, it is in theory possible to reproduce in such systems the (classical) analog of the Hawking effect. Unfortunately, it is in practice quite hard to obtain controllable flows that reach the wave velocity. It turns out that if the flow accelerates, even below the threshold velocity, there is still an imprint of the Hawking effect, which has been experimentally observed in Vancouver and in Poitiers. I will describe the nature of this imprint, as well as its spectrum for low frequency waves. In particular, I will show that the production of Hawking-like modes is governed by a new type of horizon, which is reached for complex values of the position. This complex horizon governs both the region of this mode production and its spectrum.

**Thursday 4 February, 2pm**, room 110

Mokhtar Hassaine (University of Talca, Chile) : **Chern-Simons supergravity in 11 dimensions**

**Wednesday 16 December, 2pm**, room 110

Sébastien Renaux-Petel (IAP Paris) : **Geometrical destabilization of heavy scalar fields during inflation**

We show the existence of a general mechanism by which heavy scalar fields can be destabilized during inflation. It relies on the fact that the effective mass of fluctuations orthogonal to the inflationary direction contains a contribution proportional to the curvature tensor of the field space metric, and that it can render the entropic fluctuations tachyonic. We describe a simple and rather universal setup in which apparently benign higher-order operators trigger this instability. This phenomenon can prematurely end inflation and have important observational consequences, sometimes excluding models that would otherwise perfectly fit the data. More generally, it modifies the interpretation of cosmological constraints in terms of fundamental physics.

**Tuesday 15 December, 2pm**, room 110

Jeff Steinhauer (Technion University, Israel) : **Observation of thermal Hawking radiation and its entanglement in an analogue black hole**

We observe a thermal distribution of Hawking radiation, stimulated by quantum vacuum fluctuations, emanating from an analogue black hole. This confirms Hawking’s prediction regarding black hole thermodynamics. The thermal distribution is accompanied by correlations between the Hawking particles outside the black hole and the partner particles inside. We find that the high energy pairs of Hawking and partner particles are entangled, while the low energy pairs are not. This demonstrates the quantum nature of the Hawking radiation, and shows that there is indeed an issue of information loss. The observation of Hawking radiation reported here verifies Hawking’s calculation, which is viewed as a milestone in the quest for quantum gravity.

**Wednesday 2 December, 2pm**, room 110

Shinji Mukohyama (Kyoto U) : **Massive gravity and cosmology**

The search for a consistent theory of finite-range gravity
is a longstanding problem and well motivated by both theoretical and
observational considerations. On the theoretical side, whether there
exists such a consistent extension of general relativity by a mass
term is a basic question of classical field theory. After Fierz and
Pauli’s pioneering attempt in 1939, this issue has been attracting a
great deal of interest. On the observational side, continuing
experimental probes of gravity have revealed new unexpected phenomena
at large scales. One of the most profound discovery is the cosmic
acceleration, which was found in 1998. The extremely tiny energy-scale
associated with the cosmic acceleration hints that gravity might need
to be modified in the infrared. The massive gravity is one of the most
interesting attempts in this direction. In this talk, after reviewing
the history and recent developments of massive gravity, I will
describe cosmological solutions and their stability.

**Wednesday 18 November, 2pm**, room 110

Enrico Barausse (IAP Paris) : **Gravitational-wave emission in shift-symmetric Horndeski theories**

Gravity theories beyond General Relativity typically predict dipolar gravitational emission by compact-star binaries. This emission is sourced by sensitivity parameters depending on the stellar compactness. We introduce a general formalism to calculate these parameters, and show that in shift-symmetric Horndeski theories stellar sensitivities and dipolar radiation vanish, provided that the binary’s dynamics is perturbative (i.e. the post-Newtonian formalism is applicable) and cosmological-expansion effects can be neglected. This allows reproducing the binary-pulsar observed orbital decay.

**Wednesday 4 November, 2pm**, room 110

Florent Michel (LPT Orsay) : **Lorentz-violating theories, universal horizons, and Hawking radiation**

Although Lorentz invariance has been verified with
an astonishing accuracy in the matter sector, bounds concerning the
gravitational sector remain relatively weak. This leaves room for
Lorentz-violating theories of gravitation, which are expected to have
better renormalizability properties than standard general relativity. In
this talk I will briefly review the main properties of two of the most
successful ones, namely Hoava-Lifshitz gravity and Einstein-æther,
focusing on black-hole solutions. Using a simple model, I will show the
particular structure of those solutions and their relation with Hawking
radiation, pointing to a possible instability.

**Wednesday 14 October, 2pm**, room 110

Francesco Nitti (APC Paris) : **Holographic RG flows and quantum effective actions**

**Wednesday 30 September, 2pm**, room 110

Wenliang LI (APC Paris) : **Unifying ghost-free Lorentz-invariant Lagrangians**

We will present a novel unifying framework for Lorentz-invariant Lagrangian field theories that lead to second order equations of motion. The key ingredient is the antisymmetric Kronecker delta. Then we reformulate the general ghost-free Lagrangians in the language of differential forms. The absence of higher order equations of motion stems from the basic fact that every exact form is closed. All well-known ghost-free Lagrangian theories for spin-0, spin-1, spin-2 fields have natural formulations in this framework. We propose many new ghost-free Lagrangians. The most interesting examples are novel nonlinear kinetic terms for graviton, which will be discussed in detail.