Conveners
Flash talks
- David Stefanyszyn (University of Nottingham)
Flash talks
- Chair: Miguel Bezares
In this talk I will review how modified gravity parametrisations can, in principle, be exploited to not only test general relativity, but also tackle the problematic tensions that riddle cosmology in its current state, considering these issues by the point of view of both background observables and perturbations. More precisely, I will aim to provide insight into questions such as: to which...
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...
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...
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...
In the era of upcoming cosmic surveys, the bright sky will be more revealing than ever, allowing us to disentangle the most intriguing mysteries of the origins, content, and evolution of the universe.
In this talk, I will highlight the fundamentals of extended gravity theories, and I will focus on the quintessence, probabilistic gravity, and functors of action theories (FAT). I will emphasize...
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 (GWs) can undoubtedly serve as a messenger from the early Universe and a novel probe of the underlying gravity theory. In this talk, motivated by one-loop vacuum-polarization effects on curved spacetime, we investigate a gravitational theory with non-minimal curvature-electromagnetic coupling terms of the form $\xi \frac{R}{M_{p}^2}F_{\mu\nu}F^{\mu\nu}$, where $M_p$ is the...
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...
