Séminaire

Within the framework of the inflationary paradigm, it is well-known that correlation functions (or in general bi-linear observables) of quantum fields on a curved background suffer from divergences. In general, the presence of ultraviolet (UV) divergences due to fluctuations on arbitrary short scales is a common aspect of quantum field theory.

I will discuss classes of hidden symmetries of gravity and their consequences for black holes and compact objects. 

I will discuss our recent work Phys.Rev.Lett. 128 (2022) 4, 041301 in which we present a simple class of mechanical models where a canonical degree freedom interacts with another one with a negative kinetic term, i.e., with a ghost. We prove analytically that the classical motion of the system is completely stable for all initial conditions, notwithstanding that the conserved Hamiltonian is unbounded from below and above. Numerical computations fully supported this.

Light dark matter candidates, such as axions and hidden photons, call for new ideas in direct detection. I discuss the recently proposed strategy of searching for e.g. axions using tunable cryogenic plasmas. The plasma haloscope enables resonant conversion by matching the axion mass to a plasma frequency, therefore converting axions to plasmons. Metamaterials are promising candidates, as the plasma frequency can be tuned. Besides axions, other dark matter candidates, such as hidden photons and scalars, can be successfully targeted with a plasma haloscope.

Black holes with masses ~1 Msun cannot be produced via stellar evolution. A popular scenario of their formation involves transmutation of neutron stars - by accumulation of dark matter triggering gravitational collapse in the star centers. We show that this scenario can be realized in the models of bosonic dark matter despite the apparently contradicting requirements on the interactions of dark matter particles: on the one hand, they should couple to neutrons strongly enough to be captured inside the neutron stars, on the other, their loop-induced self-interactions impede collapse.

Faint extended elliptically shaped ultra-diffuse galaxies and slightly brighter and more compact dwarf elliptical and lenticular stellar systems are common in galaxy clusters. Their poorly constrained evolutionary paths can be studied by identifying young ultra-diffuse galaxy and dwarf elliptical analogues populated with bright, massive stars. Using data mining we identified 11 such low-mass (2 × 10^8 M⊙ < M* < 2 × 10^9 M⊙) galaxies with large half-light radii (2.0 kpc < re < 5 kpc) and recently quenched star formation in the Coma and Abell 2147 galaxy clusters.

Langevin equation, Markovian process, multiplicative noise, Fokker-Planck equation, Schwinger-Keldysh formalism and much more... During this seminar, almost organised as a lecture, you will embark on a journey into the world of stochastic processes, with focus on their applications in the early Universe in the context of so-called stochastic inflation.

Upcoming Large-Scale Structure surveys will likely become the next leading sources of cosmological information, making it crucial to have a precise understanding of the influence of baryons on cosmological observables. The Effective Field Theory of Large-Scale Structure (EFTofLSS) provides a consistent way to predict the clustering of dark matter and baryons on large scales, where their leading corrections in perturbation theory are given by a simple and calculable functional form even after the onset of baryonic processes.