Séminaire

Scalar perturbations from inflationary magnetogenesis

Primordial non-Gaussianities, though yet unobserved, remain an important observable since they can help differentiate various models of inflation. This necessitates a deep understanding of the various processes that could contribute to these non-Gaussianities, with inflationary magnetogenesis being one of them. Often, the spectrum and the bispectrum of the perturbations produced during inflation are studied under the assumption that the metric perturbations can be neglected and that all the relevant physics resides in the coupling of the inflaton and the gauge fields.

An obstruction to positivity bounds with spontaneously broken Lorentz invariance

Positivity bounds are a set of consistency conditions that EFTs shall obey in order to admit a sensible UV completion. Generically, they arise as inequalities for the Wilson coefficients of the low-energy theory and, in the Lorentz invariant framework, they are extracted exploiting the analyticity properties of the S-Matrix. 

The (micro-)physics of neutron stars from multi-messenger observations

The first detection of gravitational waves from a merging neutron star
binary system and the accompanying observations of electromagnetic
counterparts in 2017 demonstrated the enormous potential of
multi-messenger astronomy for understanding the properties of
ultra-dense (and hot) matter. Neutron stars --relict of the
gravitational collapse and subsequent supernova explosion of a massive
star at the end of his life-- comprise the highest densities of matter
that can stably exist in the Universe. During this talk, I will

Fantastic higher-curvature gravities and where to find them

Higher-curvature gravities are theories of gravity including terms of higher order in the space-time curvature. They arise naturally when considering an effective approach to gravity. In this talk, I will focus on higher-curvature gravities satisfying two properties: the differential order of their equations of motion gets reduced when restricted to certain specific backgrounds and form a basis for the space of effective theories of gravity.

Some Aspects of Complexity and Krylov Complexity

A review of the concept of Complexity as have emerged in studying long time properties of systems including blackholes will be presented. Some emphasis will be made on describing features of a particular form of Complexity, the Krylov Complexity. These include its features when interpolating among various types of systems: free, strongly integrable and chaotic ones. Finally I will present a case where we have shown that the value of the Krylov Complexity, a quantum information concept, is equal to the value of a geodesic in a certain background, a geometric concept.

New model of the coherent magnetic halo of the Milky Way

Recent catalog of Faraday rotation measures (RM) of extragalactic sources together with the synchrotron polarization data from WMAP and Planck provide us with the wealth of information on the Galactic magnetic field (GMF). In this talk, we will present a new model of the regular GMF outside of the thin disk. The model is based on several phenomenological components of the GMF -- the spiral arms, the toroidal halo, the X-shaped field and the compressed field of the Local Bubble wall.

Confronting the Cosmological Principle: How secure are the foundations of the cosmos?

I will present our recent work using quasars and radio galaxies to confront the cosmological principle, the idea that sits at the heart of modern cosmological theories. Whilst the cosmological principle states that the universe is isotropic and homogeneous, our observations of the Cosmic Microwave Background (CMB) reveal a pronounced dipole, interpreted as our kinematic departure from the local Hubble flow.

Multi-metric black holes and the Gregory-Laflamme instability

Multi-metric gravity is the umbrella term for a class of modified gravitational theories, motivated by a number of problems at the interface between gravity and particle physics, that extend general relativity (GR) via the inclusion of additional interacting massive spin-2 fields beyond the single massless graviton of GR. Nonlinearly, the extra interactions manifest as a framework where multiple metric tensors interact with one another on the same spacetime manifold (hence the name).

Wave optics lensing of gravitational waves in LISA-band triple systems

One of the major predictions of Einstein’s general relativity is gravitational lensing, the deflection or amplification of light by mass distributions. In my talk, I focus on gravitational wave lensing in wave optics (very long wavelength), as opposed to the standard geometric optics. I show how a supermassive black hole acts as a wave optics lens, for triple systems in in the regime of the LISA mission.

Séminaire