Stage IV Large Scale Structure Surveys are ushering in a new era of precision cosmology!
In this talk, I will explore the effort to test gravity on cosmological scales, highlighting the theoretical advancements aimed at constructing an optimal framework. I will also touch on the synergy with gravitational wave surveys. Additionally, I will provide a detailed review of recent findings based on...
We analyze theories that do not have a de Sitter vacuum and cannot lead to slow-roll quintessence, but which nevertheless support a transient era of accelerated cosmological expansion due to interactions between a scalar ϕ and either a hidden sector thermal bath, which evolves as dark radiation, or an extremely light component of dark matter. We show that simple models can explain the...
Cosmological constraints on modified gravity typically need to assume parametrisations that heavily restrict the functional forms of the independent degrees of freedom. They are also subject to assumptions about the background expansion history, the speed of gravitational waves, and theoretical priors such as shift symmetry and stability in a gravitational wave background. I show the impact of...
Axion-dilaton models provide a well-motivated, minimal class of models
for which kinetic interactions between multiple scalar fields and their
predictions can be explored, in particular in late time cosmology. I will present the cosmological implications of these interactions when prescribing an axion and a dilaton field to describe dark matter and dark energy, respectively, including the...
The matter power spectrum, $P(k)$, is one of the fundamental quantities in the study of large-scale structure in cosmology. In this talk, I will study its small-scale asymptotic limit, and give a theoretical argument to the effect that, for cold dark matter in $d$ spatial dimensions, $P(k)$ has a universal $k^{-d}$ asymptotic scaling with the wave-number $k$, for $k \gg k_{\rm nl}$, where...
By directly probing the initial conditions of our universe, cosmological surveys offer us a unique observational handle on quantum field theory in curved spacetime with dynamical gravity and might even allow us to glean information about a full theory of quantum gravity. Here I will report on recent progress in the study of the natural observables in the problem, namely cosmological...
Modifications of General Relativity have been widely studied to alleviate cosmological tensions. Most of these models lead to an effective strengthening of gravity and enhanced growth. However, the S8 tension — arising from weak-lensing observations that suggest less structure formation than predicted by ΛCDM — points to a different scenario. In this talk, we will investigate stable subclasses...
Coupled dark sector models have gained significant attention, motivated by recent advances in cosmology and the pressing need to address unresolved puzzles. In this talk, I will review recent work on coupled scalar dark sector models involving scalar couplings arising either from their kinetic terms, or from their scalar potential. Using a dynamical systems approach, I will show that certain...
The cosmic microwave backround (CMB) and baryon acoustic oscillations (BAO) provide precise benchmarks for measuring the expansion history of the universe. In particular, the CMB angular scale measurement $\theta_*$, which determines the ratio of the sound horizon to the angular diameter distance to the last scattering surface, offers a robust constraint on cosmological models independent of...
A major challenge in the analysis of Cosmic Microwave Background (CMB) data is posed by the presence of Galactic foregrounds, especially thermal dust emission. Both the search for primordial B-modes and measurements of structure growth rely on foreground modelling, for which most works implicitly assume that all sky components follow Gaussian statistics. However, we know that this is a poor...
The discrepancy between local measurements of the Hubble constant and inferences from CMB and galaxy clustering data, known as the 'Hubble tension', has motivated numerous models introducing additional components active before recombination. While many such models have been proposed, none are currently strongly favoured by data. This highlights the critical role of upcoming CMB experiments,...
Understanding the accelerated expansion of the Universe remains as one of the key challenges in cosmology. The main candidates to explain this observation, which do not rely on a cosmological constant, are dark energy and modifications of General Relativity, but they require robust tests on cosmological scales. The Dark Energy Spectroscopic Instrument offers unprecedented precision in...
Accurate predictions of weak lensing observables are essential for understanding the large-scale structure of the Universe and probing the nature of gravity. In this talk, I will present a lightcone implementation to generate maps of the weak lensing convergence field using the COmoving Lagrangian Acceleration (COLA) method. The lightcone is constructed in spherical shells from the source to...
Abstract: I discuss in a (hopefully) pedagogical manner the possibility of extra dimensions of space and the constraints on their sizes and detection. In particular we will distinguish between rigid and non-rigid extra dimensions, depending on how easily the extra dimensions can fluctuate. Without going to full-fledged string- or M-theory we can already deduce such properties using 1) entropy...
Further bright sirens -- gravitational wave events with electromagnetic counterparts -- are keenly awaited, but proving elusive. The exceptional event GW170817 had a profound impact on the landscape of viable cosmological extensions of General Relativity (GR); can we expect this kind of shift to be repeated in the next decade? In this work we will assess the potential constraints from bright...
Recent Pulsar Timing Array (PTA) observations provide strong evidence for a stochastic gravitational wave background (SGWB), potentially originating from astrophysical sources or early universe phenomena. If the SGWB is cosmological, our relative motion with respect to the SGWB rest frame induces a kinematic anisotropy, which could dominate over intrinsic anisotropies, similar to the cosmic...
LISA and ET will operate in different frequency ranges but with comparable integrated sensitivities to a stochastic GW background (SGWB). We explore their synergies in detecting cosmological SGWBs with large amplitudes and broad frequency spectra, arising from sources such as cosmological phase transitions, cosmic strings, and primordial inflation. By combining measurements from both...
Cosmological first-order phase transitions may have generated an observable gravitational wave background, offering a unique probe of beyond-Standard-Model physics. A crucial step in predicting this background is the reliable computation of bubble nucleation rates. In this talk, I will give an overview of recent advancements in perturbative high-temperature nucleation rate calculations. These...
The Dark Energy Spectroscopic Instrument (DESI) is the first of a new generation of Dark Energy experiments, and probes evolution in the universe using galaxy clustering. Within the galaxy clustering signal, the projected location of the Baryon Acoustic Oscillations (BAO) acts as a standard ruler to map cosmic evolution. I will present the latest BAO results from the DESI Data Release 2 (DR2)...
We write a closed form expression for the metric perturbation around de Sitter that describes gravitational radiation from a compact and slowly varying source, in terms of a consistent multipolar expansion at quadrupolar order. We show that the corresponding displacement memory effect with both the even and odd parities is at a higher order in the radial expansion compared to their flat...
Primordial perturbations observed on the CMB are thought to come from cosmological correlators during inflation. While massive (spinning) fields lead to vanishing correlators, their interaction with massless ones can alter the massless field correlators. These changes can be used to infer existence of massive spinning fields during inflation. We will compare two main models of massive spinning...
A key step in the comparison between inflationary predictions and cosmological observations resides in the computation of primordial correlators.
Numerical methods have been developed, which allow to overcome some of the difficulties arising in analytical calculations when the models considered are complex.
The \texttt{PyTransport} package, which implements the transport formalism, allows...
Although ΛCDM has been a successful cosmological model, there is a 5σ tension between $H_0$ values inferred from the cosmic microwave background (CMB) data by Planck and those measured directly by SH0ES. Apart from the Hubble tension, debates also extend to tensions on other cosmological parameters. Many alternative models are proposed to reconcile these tensions in cosmology. I will focus on...
Gravitational-wave bursts are a class of transient gravitational-wave signals which have unknown or very difficult to model signal morphologies. Likely progenitors of gravitational-wave bursts include core-collapse supernovae, cosmic string cusps, pulsar glitches, and black hole encounters. By definition, gravitaitonal-wave bursts cover a broad parameter space which poses a significant...
Historically, studies of the merger stage of a black hole binary have centred on fully nonlinear numerical relativity simulations. However, nonlinear black hole perturbation theory provides powerful insights into the ringdown regime immediately following merger, and perturbative self-force theory has proved highly accurate in describing asymmetric binary inspirals even for mass ratios not too...
Gravitational self-force theory (GSF) has proved to be a viable method of solving the general relativistic 2-body problem for asymmetric binaries, with state-of-the-art GSF inspiral waveforms now exhibiting minimal phase error across all mass ratios smaller than ~1/10. Recent work has extended these GSF inspiral waveforms to include beyond-GR effects in a broad class of scalar-tensor theories....
Supermassive black hole binaries (SMBHBs) are exceptional multi-messenger sources, since they emit bright electromagnetic (EM) radiation and low-frequency gravitational waves (GWs). On the EM side, SMBHB can be detected as quasars with periodic variability in time-domain surveys. Several promising candidates have already been identified and many more discoveries are expected with the Rubin...
In this talk, I will present recent developments aimed at simulating black hole binaries beyond general relativity in Spectre, an open-source numerical relativity code by the SXS Collaboration. For concreteness, I will focus on scalar Gauss-Bonnet (sGB) gravity. I describe results derived from a parameter space exploration of a model of sGB gravity using initial data sequences of equal-mass...
Black hole quasi-normal modes (QNMs) provide a powerful probe of deviations from General Relativity. We analyze the relative contributions of shifts in the QNM spectrum of Kerr black holes and the presence of additional modes sourced by extra fields in a theory-agnostic framework. By exploring different regimes, we identify when each effect dominates and propose suitable ansätze for the...
In this talk, I will precent recent work on gravitational waves generated by compact binaires in a class of scalar tensor theories within the post-Newtonian (PN) approximation. I will briefly review the waveform generation formalism in these theories and waveforms for circular orbits. I will then discuss recent work on extending these waveforms to elliptic and hyperbolic orbits. I will first...
Gravitational wave memory is a non-oscillatory feature of gravitational wave signals which both probes nonlinearities appearing in Einstein's equations and is connected to the asymptotic structure of isolated gravitating systems. In recent years, in addition to the usual "displacement" memory, which appears as a DC offset in the gravitational wave signal, other "higher" memory effects have...
In this talk, I will discuss our work in modelling neutron stars using the recently developed formulation for relativistic dissipative hydrodynamics, known as the BDNK theory. By performing numerical simulations of neutron stars in spherically symmetric spacetimes, we will study how incorporating dissipation using the BDNK theory affects the physical predictions of the system.
Gravitational-wave (GW) astronomy is an established field that is rapidly expanding with increasing detections from merging compact binary systems. The next generation of GW detectors promises a tenfold increase in sensitivity, leading to a thousandfold increase in the observable volume of the Universe and a corresponding rise in detection rates. This growing dataset provides a unique...
The Penrose process consists of transferring energy from a black hole to infinity. This process can be studied in a combined description with the Bañados-Silk-West (BSW) mechanism, which uses collisions of ingoing particles at the event horizon of a black hole to locally produce large amounts of energy. In this talk, the blending of the Penrose process with BSW mechanism is described for a $d$...
Scalar-tensor theories are a popular extension of gravity where an extra scalar degree of freedom non-minimally couples to the gravitational sector. Despite existing experimental tests for such modifications from general relativity, there is still no conclusive evidence for or against these theories. A possible reason for this is the presence of screening mechanisms, which can hide the scalar...
In this talk, I will present the Kaluza-Klein spectrum of background perturbations to Freund-Rubin compactifications, which are the near-horizon geometry of a class of extremal black branes. The purpose of this is twofold.
The effective masses of fields in the near-horizon geometry determine the scaling exponents of (tidal) deformations to extremal black branes, so the spectrum...
Quantum field theories on curved space have a diverging effective action that depends purely on the polynomials of the curvature tensor. One needs to add higher derivative terms in the bare gravitational action to renormalize these divergences. In this setting it becomes reasonable to calculate how the gravitational couplings run with the cut-off scale. It turns out that for a Standard Model...
Understanding gauge and frame dependence is crucial for comparing black hole perturbation theory results at future null infinity. At second order, new challenges arise: gauge-invariant quantities in linear theory lose their invariance, and the nonlinear source terms can lead to infrared divergences. We address these issues by constructing an invariant second-order field equation in the...
It has long been understood that certain theories of ghost free massive gravity and their multi-graviton extensions can be thought of as arising from a higher dimensional theory of gravity, upon discretising the extra dimension. However, this correspondence between standard multi-gravity and extra dimensional gravity holds only when one discretises the extra dimension after gauge fixing the...
The gravitational deflection of light is a key phenomenon for testing gravitational theories. Recently, a novel method was introduced to compute the angular deflection in non-asymptotically flat spacetimes, based on the construction of null geodesic polygons. Building on this approach, we apply this technique to analyze the angular difference in null geodesic triangles, providing a systematic...
I will present a newly established, Nottingham-based experimental platform for simulating rotating curved spacetimes in superfluid helium - a quantum liquid with vanishing viscosity. The effective curved spacetime, induced by the most extensive quantum vortex flows ever created, is probed via micrometre-scale surface waves. These reveal intricate wave-vortex interactions, including the...
Axions are a hypothetical class of particle predicted in a variety of settings and of utility in solving many mysteries of theoretical physics, most notably as dark matter candidates and solving the strong CP problem. I will describe recent dramatic progress in understanding what string theory predicts about the properties of axions, and the door this opens to test quantum gravity. I will thus...
The inaccurate modeling of the gravitational wave templates that are used for analysing gravitational wave signals can lead to a systematic bias in the parameter estimation. This inaccuracy can be related to the lack of terms in the waveform coming from fundamental physics or astrophysical environments or to some truncation in perturbation theory. This issue is going to be more relevant for...
The gravitational waves emitted by a perturbed black hole ringing down are well
described by damped sinusoids, whose frequencies are the so-called quasinormal
modes. Until recently first-order black hole perturbation theory was used to calculate
these frequencies. However, it is now clear that second-order effects are also
necessary to model the gravitational-wave signal during the...
We will discuss the black hole stability problem and discuss results on asymptotics of linear waves. We will present results for extremal and sub-extremal black holes.
The merger of compact objects offer the opportunity to explore and put General Relativity to test. This talk will discuss particular examples and connect them with broader aspects from observations to other research fronts in physics.
Black holes serve as key testing grounds for quantum gravity due to their singular nature and have been extensively studied in various quantum gravity approaches. In this talk, I apply the Henneaux-Teitelboim formulation of unimodular gravity to the symmetry-reduced Schwarzschild-(Anti-)de Sitter model. We perform a canonical quantization, leading to a Wheeler-DeWitt equation that takes the...
We present recent advancements in the 3+1 formalism within two reformulations of general relativity: the teleparallel equivalent of general relativity, and the symmetric teleparallel equivalent of general relativity. Both theories are based on the torsion and nonmetricity of a flat connection, respectively, and their Lagrangians are expressed in terms of the torsion scalar T and the...
In general relativity, inflationary models with a non-zero background curvature require additional parameters or fine-tuning compared to flat inflation. For this reason, there is no consensus on the primordial power spectrum that should be considered at large scales in a curved Universe. I will present a model of curved inflation not requiring additional parameters and in which the usual...
Precision tests of General Relativity (GR) are a cornerstone of modern physics, however they are typically discipline and context specific. Based on the successful Parameterised Post Newtonian (PPN) approach, I will present a holistic framework for constraining theory agnostic modifications to GR that allows astrophysical and Solar system tests to be combined with cosmological tests using a...
Gravitational waves (GWs) have rapidly become important cosmological probes since their first detection in 2015. As the number of detected events continues to rise, upcoming instruments like Einstein Telescope (ET) and Cosmic Explorer (CE) will observe millions of compact binary (CB) mergers. As GWs carry information on their luminosity distance, but remain uninformative about their redshifts,...
Gravitational waves provide a new observational tool to study the
universe. Second-order cosmological perturbation theory allows to
study gravitational waves sourced by terms quadratic in first order
quantities. For example, so-called scalar induced gravitational waves
are sourced by first order scalar fluctuations and have been studied
extensively. In this presentation I discuss the...
Within the framework of hybrid metric-Palatini gravity, we incorporate non-localities introduced via the inverse of the d'Alembert operators acting on the scalar curvature. We analyse the dynamical structure of the theory and, adopting a scalar-tensor perspective, assess the stability conditions to ensure the absence of ghost instabilities. Focusing on a special class of well-defined hybrid...
Stage IV large scale structure surveys are promising probes of gravity on cosmological scales. Due to the vast model-space in the modified gravity literature, model-independent parameterisations represent useful and scalable ways to test extensions of ΛCDM. In this talk, I will present recent work that shows how one can consistently parameterise and constrain modified gravity models with a...
Identifying useful flat-space limits for cosmological correlators—where they can be expressed in terms of observables in Minkowski space—is nontrivial due to their scale-invariant nature. In this talk, I present a massive flat-space limit in which cosmological correlators, induced by the exchange of heavy fields, can be expressed in terms of massive Feynman graphs in flat space. As a...
