Gravitational wave astronomy will play a transformative role in astrophysics; can it do the same for particle physics? An ultralight bosonic field will extract mass and angular momentum from a rapidly spinning black hole, forming a gravitationally bound condensate reminiscent of the hydrogen atom. This "gravitational atom" will have nontrivial dynamics if it is part of a binary inspiral. I...

Since the discovery of pulsar 50 years ago, they have proven to be very useful astronomical objects. Spinning neutron stars with a very stable rotation period as low as a few milliseconds, emitting radio pulses, similar to a lighthouse. These periodic radio signals can be detected and timed on Earth with an accurate timing model describing the entire process from emission at the pulsar through...

In this talk I will discuss the decay of gravitational waves (GW) into dark energy fluctuations π in the context of the EFT of Dark Energy.

In such theories, the time-dependence of the Dark Energy (DE) field spontaneously breaks Lorentz invariance. Therefore as for light in a material, GW travelling in the cosmic medium are affected by dispersion phenomena and can decay into DE fluctuations....

Stochastic gravitational wave backgrounds induce correlated patterns in the redshift and astrometric shifts of objects on the sky. The astrometric equivalent of the Hellings-Downs curve depends on the polarization content as the group velocity of the GWs making up the stochastic background. I will explain how these results relate to the measurements taken by the Gaia mission and how they can...

At some length scale, Einstein's theory of general relativity (GR) must break down and be reconciled with quantum mechanics in a quantum theory of gravity. Binary black hole mergers probe the strong field, non-linear, highly dynamical regime of gravity, and thus gravitational waves from these systems could contain beyond-GR signatures. While LIGO presently performs model-independent and...

The groundbreaking detection of gravitational waves by LIGO has opened up a brand new window into observational cosmology, catalysing research into gravitational wave signatures from a wide range of astrophysical and cosmological sources. In this work, we calculate accurate radiative signatures from topological `cosmic’ strings, implementing adaptive mesh refinement (AMR) simulations of global...

We investigate the correlation between the distribution of galaxies and the predicted gravitational-wave background of astrophysical origin. We show that the large angular scale anisotropies of this background are dominated by nearby non-linear structure, which depends on the notoriously hard to model galaxy power spectrum at small scales. In contrast, we report that the cross-correlation of...

We propose a multi-messenger probe of QCD axion dark matter (DM) based on observations of black hole-neutron star binary inspirals. It is suggested that a dense DM spike may grow around intermediate mass black holes. The presence of such a spike produces two unique effects: a distinct phase shift in the gravitational wave strain during the inspiral period and an enhancement of the radio...

Dark matter may induce apparent temporal variations in the physical ``constants'', including the electromagnetic fine-structure constant and fermion masses. In particular, a coherently oscillating classical dark-matter field may induce apparent oscillations of physical constants in time, while the passage of macroscopic dark-matter objects (such as topological defects) may induce apparent...

Atom interferometry promises to extend the detection bandwidth of GW detectors in the mid-frequency band (10 mHz - 10 Hz), where Earth based optical detectors are limited by low frequency gravity noise. Adopting as probes arrays of atomic ensembles in free fall, and tracking their motion on geodesics with atom interferometry allows the suppression of Newtonian Noise, enables low frequency...

In this talk I will address a theoretical underpinning of potential quantum modifications to classical GR black holes from the perspective of string theory. I will discuss theoretical motivations, new insights over the last two decades, and give a view on observational consequences.

I will talk about GW signatures from the collisions of exotic compact objects (ECOs). I will argue that getting good high quality signatures require good control over the initial conditions which is currently one of the key challenges. I will also discuss whether ECOs can act as black hole mimics.

One of the most exciting targets for current and future gravitational-wave (GW) observatories is the stochastic GW background (SGWB)---a persistent all-sky signal, sourced by the incoherent emission of GWs from many independent sources throughout the history of the Universe. In particular, the SGWB is a sensitive probe of cosmic strings: line-like topological defects formed through spontaneous...

I will outline our current understanding on how massive black holes, routinely found in galaxy centers, form in galaxies in the first billion years of the Universe and how they form binaries, eventually coalescing emitting gravitational waves in the frequency range observable by LISA and PTAs.