Black hole perturbations in modified gravity

General relativity can be tested at many scales using various physical systems. A particularly interesting probe is the study of the ringdown phase of a binary black hole merger, during which the newly-formed black hole emits gravitational waves at given frequencies called its quasinormal modes. Such modes depend heavily on the theory of gravity underlying the solution and can thus be used to test GR and put constraints on modified gravity theories.

New ideas on quenching and detecting BH rotational superradiances

In this talk I’ll discuss two recent results on BH superradiance: first, I will describe how BH photon superradiance is typically quenched by interactions of the photon cloud with the ambient electrons. Second, I will explain how an axionic cloud may impact the CMB if it decays into low energy photons which quickly heat and ionise the surrounding medium to Mpc scales.

Cosmic String Interpretation of NANOGrav Pulsar Timing Data and its impact on Cosmic Archaeology with Gravitational Waves

The NANOGrav Collaboration has recently reported strong evidence for a stochastic common-spectrum process, which we interpret as a SGWB in the framework of cosmic strings. The possible NANOGrav signal would correspond to a string tension Gμ∈(4×10^{−11},10^{−10}) at the 68% confidence level, with a different frequency dependence from supermassive black hole mergers.

A numerical approach to stochastic inflation and primordial black holes

Primordial black holes are a dark matter candidate, which may originate from strong perturbations created during inflation. These perturbations can be studied using the formalism of stochastic inflation. I present a numerical approach to this problem, where the stochastic dynamics is solved by generating a large number of random realizations. This makes it possible to go beyond analytical approximations and take into account additional effects such as backreaction between the perturbations and the background.

From pulsars to supermassive black holes and gravitational waves in between

Pulsar Timing Arrays (PTAs) aim to detect nHz gravitational waves (GWs) from supermassive black hole binaries (SMBHBs). This is done by looking for correlated variations of the Time of Arrivals (TOA) across an array of ultra-stable millisecond pulsars. Comparing the predicted TOAs from our timing model against the measured TOAs gives us the residuals. These contain the imprint of GWs, but also other effects and sources of noise processes.

Detecting quantum entanglement in the sky

Although inflation is broadly accepted to be the standard paradigm for early universe cosmology, many of its quantum properties remain unknown. For instance, the crowning glory of inflation lies in explaining late-time macroscopic inhomogeneities as arising from tiny quantum fluctuations; however, most of the established literature ignores the crucial role that entanglement between the modes of the fluctuating field plays in its observable predictions.

Stochastic Gravitational Wave Backgrounds (SGWB) characterization with LISA

In this seminar I will discuss Stochastic Gravitational Wave Backgrounds (SGWB) characterization with LISA. After a general introduction on SGWB detection, I will explain the peculiar features of LISA with a focus on the response function and on the noise spectrum. The core of my talk will be the presentation of two different methods for model independent SGWB frequency reconstruction:
- The so-called ``binning method'' (1906.09244, 2009.11845) based on the idea of approximating the signal with a piecewise-defined function where each sub-function is a power law.

Primordial black holes from metric preheating: mass fraction in the excursion-set approach

We calculate the mass distribution of Primordial Black Holes (PBHs) produced during metric preheating. After inflation, the oscillations of the inflaton at the bottom of its potential source a parametric resonant instability for small-scale scalar perturbations, that may collapse into black holes. After reviewing in a pedagogical way different techniques that have been developed in the literature to compute mass distributions of PBHs, we focus on the excursion-set approach.


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