Théorie

Gravitational waves from first-order phase transitions: Towards a better prediction

Over the next few decades, we will have an exciting opportunity to test gravitational waves (GWs) from the early Universe with space interferometries. In this talk, we focus on GWs from first-order phase transitions and present recent efforts to improve the prediction on the GW spectrum. We first present an efficient numerical scheme to calculate GWs from sound waves (under the assumption that the system is in the linear regime) based on 2010.00971 (with T.Konstandin and H.Rubira).

Exploring early-universe physics with stochastic gravitational-wave data

The stochastic gravitational-wave background is a superposition of many astrophysical and cosmological sources, such as unresolved compact binaries, cosmic strings, and phase transitions in the early Universe. We highlight the importance of source separation in the case of a detection. By separating the individual sources, we can reveal remnants of early-universe processes. We use the data from the third LVK observing run to explore the parameter space of first-order phase transition models. We then investigate signs of parity violation in gravitational-wave data.

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.

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