We will have several talks by PhD students at the theory group:

1:30 p.m.: Konstantin Leyde,

1:45 p.m.: Edwan Preau,

2:00 p.m .: Valentin Nourry,

2:15 p.m.: Jani Kastikainen,

2:30 p.m.: Thomas Colas,

The abstracts of the talks are:

- Valentin Nourry,

- Jani Kastikainen,

- Thomas Colas,

1:30 p.m.: Konstantin Leyde,

*"A window for cosmic strings"*1:45 p.m.: Edwan Preau,

*"Holographic Baryons"*2:00 p.m .: Valentin Nourry,

*"Holographic stability of a maximally symmetric spacetime"*2:15 p.m.: Jani Kastikainen,

*"Holographic Boundary Conformal Field Theories"*2:30 p.m.: Thomas Colas,

*"Cosmological Master Equations"*The abstracts of the talks are:

- Valentin Nourry,

*"Holographic stability of a maximally symmetric spacetime:"*Maximally symmetric spacetimes are well known to be stable under small metric perturbations if they propagate into the vacuum. This statement is not always verified in the presence of matter. Starobinsky inflation is based on the idea that a quantum Conformal Field Theory (CFT) backreacts in an initial de Sitter spacetime, driving it to a FLRW spacetime after a period of inflation. In this talk, we use AdS/CFT duality to construct correlation functions of the stress-tensor in a strongly coupled CFT with a dynamical metric. These correlations contain all the necessary information to study the stability of spacetimes such as (Anti) de Sitter. Our work is a first step to generalize the Starobinsky model to a non-perturbative strongly coupled CFT. We find the existence of tensor instabilities which exist in addition to the usual scalar inflaton. Depending on the background curvature and a few other parameters of the holographic setup, we show that these tensor perturbations can destabilize (A)dS faster than the usual scalar inflaton. The backreacting CFT gives a possible inflationary model, in which the duration of inflation is fixed by the characteristic time of the most unstable mode.- Jani Kastikainen,

*"Holographic Boundary Conformal Field Theories:"*Recently, conformal field theories with boundaries (BCFTs) and their holographic duals have received renewed attention due to appearing in models of black hole evaporation. These bottom-up models involve an end-of-the-world (EOW) brane that is holographically dual to the boundary of the BCFT. In this talk, I explore the holographic dictionary of a scalar field in the presence of an EOW brane and calculate the BCFT 1-point function of the dual scalar operator.- Thomas Colas,

*"Cosmological Master Equations:"*Nearly scale-invariant, Gaussian and adiabatic scalar perturbations from quantum mechanical origin have been extensively tested in cosmology using CMB and LSS data. Effective field theories aim at providing a systematic way to consider extensions to this adiabatic evolution, incorporating the knowledge of unknown physics in a parametrically controlled manner. In order to grasp the implications of some hidden sector at the quantum level, the formalism needs to incorporate non-unitary effects such as dissipation and decoherence. To achieve this goal, master equations can be a valuable tool. Ubiquitous in quantum optics where they describe the effects of an almost unspecified environment on the evolution of measurable degrees of freedom, they rely on assumptions that do not straightforwardly extend to cosmology where the background is curved and dynamical, the Hamiltonian time-dependent and the environment out-of-equilibrium. In this talk, I will present their implementation in cosmology and benchmark their efficiency on solvable models.## Dates:

Tuesday, 7 June, 2022 - 13:15 to 15:00

## Localisation / Location:

APC

## Salle / Local:

seminar room 483A, contact roperpol@apc.in2p3.fr for Zoom meeting details

- Séminaire

## Equipe(s) organisatrice(s) / Organizing team(s):

- Théorie