Théorie

Hamiltonian vs stability in alternative theories of gravity 

When a Hamiltonian density is bounded by below, we know that
the lowest-energy state must be stable. One is often tempted
to reverse the theorem and therefore believe that an unbounded
Hamiltonian density always implies an instability. The main
purpose of this talk is to pedagogically explain why this is
erroneous. Stability is indeed a coordinate-independent property,
whereas the Hamiltonian density does depend on the choice of
coordinates. In alternative theories of gravity, like k-essence
or Horndeski theories, the correct stability criterion is

Carrollian fluids and flat holography

Various extensions of the AdS/CFT correspondence beyond the realm of asymptotically anti-de Sitter spacetimes have been proposed. Among those, the asymptotically flat spacetimes have played a distinguished and recently revived role. With the exception of three spacetime dimensions, and despite the wide interest and the precise knowledge of the asymptotic symmetries, no concrete proposal was available.

Stochastic inflation, primordial black holes, and beyond the slow-roll approximation

I will explain how primordial black holes can form from
perturbations seeded during inflation and how their abundance can be
calculated in the framework of stochastic inflation. This formalism
incorporates quantum backreaction of the small-wavelength fluctuations
on the large distances dynamics of the Universe. If quantum
corrections are small, the probability distribution of density
fluctuations is well approximated by a Gaussian. If they are large,
the PDF has a different profile with a longer tail and leads to

Holographic collisions across a phase transition

We use holography to mimic heavy ion collisions and obtain new qualitative insights possibly relevant for QCD. Our studies are motivated by the extensive experimental efforts devoted to the search of the conjectured critical point in the QCD phase diagram. Holographically, we perform collisions in strongly-coupled gauge theories with thermal phase transitions. We find that near a second order phase transition almost all the energy of the projectiles is deposited into a long-lived, quasi-static blob of energy.

Two modified gravity theories with few degrees of freedom

In this talk, I will present two specific modifications of gravity in 
which the number of degrees of freedom is minimized. I will focus on 
their construction, showing how different methods can be used to 
accomplish the same goal: keeping a low number of degrees of freedom. 
The first theory that I will describe is the minimal theory of 
quasidilaton massive gravity, a Lorentz-breaking theory of massive 
gravity + scalar field. As a second example, I will describe a class of 
modified gravity theories that propagate only the usual two 

Perturbative study of infrared QCD

Quantum Chromodynamics, the microscopic theory of strong interaction, is asymptotically free. As a consequence, perturbation theory is a useful tool to study high energy phenomena. In the opposite regime, the infrared, QCD is believed to be non-perturbative. However, last decade lattice simulations have showed that the coupling constant is finite in the infrared and not so big. That makes us think that some features of infrared QCD could be understood using perturbation theory. Moreover, the completely gauge-fixed Lagrangian is not known in the infrared.

Testing relativistic vacuum decay with cold atoms

Vacuum decay is a prominent example of strongly nonlinear effects in quantum field theory with potentially important implications for cosmology, relating to phase transitions in the early universe or the supposed metastability of the current Higgs vacuum. Although a general theoretical description was laid out in the 80s by Sidney Coleman and his collaborators, fundamental questions pertaining to the back-reaction of true vacuum bubbles on space-time curvature and their correlations remain so far unanswered, calling for different approaches to the problem.

Gravitational Generation of Dark Matter

In this talk I will discuss several working models for producing dark matter (DM), which require no couplings between the visible sector and DM besides (classical) gravity. In particular, I will show that contrary to common wisdom the current observational bounds do not substantially limit the parameter space nor cosmological history, but rather that gravitational particle production can quite naturally lead to a DM abundance in agreement with observations. Based on arXiv:1808.08236 and arXiv:(to be uploaded…).

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