# Théorie

# Vacuum stability without Supersymmetry: Brane dynamics, bubbles and holography

n this talk I will present some recent results on instabilities of anti-de Sitter flux compactifications in effective field theories arising from non-supersymmetric string models, namely the USp(32) and U(32) orientifold models and the SO(16) x SO(16) heterotic string. We focus on non-perturbative instabilities, framing the vacua in terms of brane stacks, analyzing their back-reacted geometry and reproducing AdS its near-horizon limit.

# D-brane model building and phenomenology

# Miniworkshop on Holographic QCD at high densities, neutron stars and gravitational waves

This one-day workshop will bring together theorists working on the AdS/CFT correspondence, nuclear physics, and General Relativity for an interdisciplinarly exchange aimed at developing a consistent holographic picture of QCD matter at high density, from model building to the description of compact objects and observation via gravitational wave signals. Invited speakers include: Umut Gursoy (Utrecht U); Matti Jarvinen (Utrecht U. and Tel Aviv U.); Niko Jokela (Helsinki U.); Micaela Oertel (LUTH); Eric Gourgoulhon (LUTH).

Programme:

# Singularity avoidance, clocks and primordial power spectrum

There are ways to cure the singularity problem in cosmology by means of quantum effects. In the process, one encounters the question of time, which can for instance be addressed by using an internal degree of freedom. Applying such a method to an anisotropic Bianchi I model where the shear serves as a clock, I show that some generic features can be obtained for the trajectory of the average scale factor, all avoiding the singularity and reaching similar asymptotic states, deemed semi-classical. Using a trajectory approach yields similar

# Non-Fierz-Pauli bimetric theory from quadratic curvature gravity on Einstein manifolds

I will show that, in four-dimensional spacetimes with an arbitrary Einstein metric, with and without a cosmological constant, perturbative dynamical degrees of freedom in generic quadratic-curvature gravity can be decoupled into massless and massive parts. The massive part has the structure iden-ical to, modulo the

# Stability properties in mimetic gravity theories

energy, we analyze the cosmological applications and the stability

conditions of Mimetic Gravity models. In connection to the fact that

these models have been recently shown to be plagued by instabilities, we

show that for the original mimetic matter model the ghost/tachyon instability

for perturbations around the FLRW background can be interpreted as the

standard Jeans instability of dust.

Proposed as a solution for the instability problems, we discuss mimetic