Série :
Event Type: Seminar
Domaine(s) scientifique(s) : Cosmology
Date de fin : 2022-07-25
Speaker(s): Sergey Sibiryakov Valentin Boyanov Savov
Sergey Sibiryakov, McMaster University and Perimeter Institute
Condensation and Evaporation of Boson Stars
Axion-like particles, including the QCD axion, are well-motivated dark matter candidates. Numerical simulations have revealed coherent soliton configurations, also known as boson stars, in the centers of axion halos. We study evolution of axion solitons immersed into a gas of axion waves with Maxwellian velocity distribution. Combining analytical approach with controlled numerical simulations we find that heavy solitons grow by condensation of axions from the gas, while light solitons evaporate. We deduce the parametric dependence of the soliton growth/evaporation rate and show that it is proportional to the rate of the kinetic relaxation in the gas. The proportionality coefficient is controlled by the product of the soliton radius and the typical gas momentum or, equivalently, the ratio of the gas and soliton virial temperatures. We discuss the asymptotics of the rate when this parameter is large or small.
Valentin Savov, Universidad Complutense de Madrid
The pseudospectrum as a tool for studying perturbations of black holes and horizonless compact objects
The dynamical evolution of systems containing black holes and compact stellar configurations has experienced a renewed interest after the historic detection of gravitational waves 7 years ago. An important part of this study boils down to the analysis of linear wave equations which describe the evolution of first-order perturbations in these spacetimes. A spectral analysis of these equations reveals the quasinormal modes (QNMs) which encode the universal ringdown behaviour of each of these objects. However, it has been noted that the non-self-adjoint nature of the operator describing this linear evolution implies that a spectral analysis may be insufficient for capturing the full complexity of the problem. In this talk I will briefly introduce the notion of ``pseudospectrum", and show how it can be used to obtain more information about the evolution of perturbations in dissipative systems. Particularly, I will show how most of the QNM spectrum of black holes and other compact objects is unstable under potential environmental perturbations to the problem, as well as how the early-time behaviour of the system can differ greatly form what the picture painted by QNMs alone would suggest.
Zoom Link: https://pitp.zoom.us/j/93637180996