# Théorie

# Higher-Spin Asymptotic Symmetries, Charges and Soft Theorems

# Transport effects due to quantum anomalies

# 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

# TBA

# Multi-Messenger gamma-ray and neutrino signals from Galactic and extragalactic sources

# 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