Talks

I will introduce mochi_class, a refined version of the popular Einstein-Boltzmann solver hi_class optimised for calculations within Horndeski's gravity framework. Thanks to (i) a re-parametrisation of Horndeski functions, (ii) a numerically stable quasi-static approximation, and (iii) support for time-dependent inputs, mochi_class enhances hi_class capabilities and nicely complements other public Einstein-Boltzmann solvers. Additionally, I will present a non-parametric approach that, when integrated with Principal Component Analysis, can effectively reconstruct Horndeski functions from well-studied modified gravity models, extending the exploration of scalar-tensor theories beyond conventional parametrisations. I will conclude by highlighting practical applications where mochi_class can prove instrumental in analysing current and forthcoming large-scale structure data.

We present a comprehensive joint analysis of two distinct methodologies for measuring the masses of galaxy clusters: hydrostatic measurements and caustic techniques. We show that by including cluster-specific assumptions obtained from hydrostatic measurements in the caustic method, the potential mass bias between these approaches can be significantly reduced. While this may appear to diminish the caustic method as a technique independent of the dynamical state of a cluster, it provides a means to refine mass constraints and offers an avenue for scrutinizing modifications to gravity. Applying this approach to two well-observed massive galaxy clusters A2029 and A2142, we find no discernible mass bias, affirming the method's validity. We draw a similar conclusion when applying this approach to modified gravity models. Specifically, our implementation allows us to investigate Chameleon and Vainshtein screening mechanisms, enhancing our understanding of these modified gravity scenarios. Furthermore, we explore the prospect of achieving more precise constraints with fewer systematic errors by exclusively employing the caustic method to constrain screening mechanisms on a larger scale, encompassing several hundred stacked galaxy clusters.

Since the discovery of the accelerated expansion of the universe, significant progress has been made to develop modified gravity theories as alternatives to dark energy and these have been developed into tests of General Relativity itself via cosmological observations. These models share common properties such as screening mechanisms they use to evade the stringent Solar System tests. In this talk, I will review recent status of observational tests of screened modified gravity models and discuss the prospect of cosmological tests of gravity from ongoing surveys such as Euclid.

The coming years will see an amazing increase in data on the large-scale structure of the Universe, ushering in a new phase for "precision cosmology". One of the major questions in fundamental physics concerns the nature of the dark energy, and the new data may help to shed light on this issue. But in order to unlock the full power of the future data to test alternative models like Horndeski Gravity, we need theoretical predictions that are as accurate as the new observations on all scales, including non-linear scales. In my presentation I will introduce our relativistic N-body code for cosmological simulations, gevolution, and how we are using it to look at non-linear effects in the Universe. In particular I will discuss our k-essence simulations, how to use them for cosmology, and what can happen when dark energy clustering becomes non-linear in models with low speed of sound.

This talk will introduce scalar-tensor theories of gravity that contain a single scalar degree of freedom in addition to the usual tensor modes. These theories constitute the very broad family of Degenerate Higher-Order Scalar-Tensor (DHOST) theories, which include and extend Horndeski theories. Cosmological aspects of these theories will then be discussed. Finally, I will also present some results concerning black hole perturbations in the context of these models of modified gravity.

I will present the class of effective field theories of dark energy, which aim to reproduce a dark energy-like phenomenology by modifying general relativity with the addition of a scalar graviton. I will review how non-linearities can "screen" local scales from scalar effects, therefore allowing these theories to pass existing solar-system experimental tests. I will then present fully relativistic simulations of gravitational wave generation in these theories in 1+1 dimensions (stellar oscillations and collapse) and 3+1 dimensions (binary neutron stars). I will show that screening tends to suppress the (subdominant) dipole scalar emission in binary neutron star systems, but it fails to quench monopole scalar emission in gravitational collapse, and quadrupole scalar emission in binaries. This opens the way to the exciting possibility of testing dark energy with gravitational wave data.

Recently, there has been much interest in black hole echoes, based on the idea that there may be some mechanism (e.g., from quantum gravity) that waves/fields falling into a black hole could partially reflect off of an interface before reaching the horizon. There does not seem to be a good understanding of how to properly model a reflecting surface in numerical relativity, as the vast majority of the literature avoids the implementation of artificial boundaries, or applies transmitting boundary conditions. Here, we present a framework for reflecting a scalar field in a fully dynamical spherically symmetric spacetime, and implement it numerically. We study the evolution of a wave packet in this situation and its numerical convergence, including when the location of a reflecting boundary is very close to the horizon of a black hole. This opens the door to model exotic near-horizon physics within full numerical relativity.

The non-linear dynamics of gravitational wave propagation in spacetime can contain drastic new phenomenology that is absent from the linearised theory. In this talk, I will probe the non-linear radiative regime of Horndeski gravity by making use of disformal field redefinition. I will discuss how disformal transformations alter the properties of congruences of geodesics and in particular how they can generate disformal gravitational waves at the fully non-linear level. I will illustrate this effect by presenting a new exact radiative solution in Horndeski gravity describing a scalar pulse. Analysing the non-linear dynamics of this new radiative solution will show that it contains tensorial gravitational waves generated by a purely time-dependent scalar monopole. This intriguing result is made possible by the higher-order nature of Horndeski gravity.

Gravitational waves from black hole binary mergers can tell us a lot about the physics of the system. At the late part of the graviational wave signal, GR predicts the presence of characteristic frequencies (called quasinormal modes) in the signal. Measuring multiple quasinormal modes is a strong consistency test for GR. Here we probe the regime where a signal can be described entirely by quasinormal modes. We consider a higher order effect, where the remnant black hole is absorbing some radiation and so has a changing mass and spin. We test the contribution of this effect to the signal in a physically relevant scenario. We find evidence that this effect causes other mode excitations as well as a changing frequency contribution.

Observational constraints on time-varying dark energy are commonly presented in terms of the two CPL parameters $w_0$ and $w_a$. Recent observations favor a sector of this parameter space in which $w_0 > -1$ and $w_0 + w_a < -1$, suggesting that the equation of state underwent a transition from violating the null energy condition (NEC) at early times to obeying it at late times. In this talk, I will demonstrate that this initial impression is misleading, by showing that simple quintessence models satisfying the NEC at all times predict an observational preference for the same sector. The upshot is that the CPL parameterization is simultaneously useful for detecting deviations from cosmological-constant dynamics ($w = -1$) but unreliable for predicting the true behavior of $w(z)$.

Exploring the structure of compact objects in modified theories of gravity is mandatory to parametrize the possible deviations w.r.t general relativity and confront these theories to the current and future observations. While important efforts have been devoted to understand the phenomenology of stars and black holes, it is still a challenging task to provide new exact analytical solutions describing rotating black hole in such theories. In this talk, I propose to recent efforts to construct such solutions. Concretely, I will review how one can mix the disformal field redefinitions affect the Petrov type of a given gravitational field and how this can be used to constrain the derivation of rotating black hole. Then, I will review the main properties of a new solution of a subset of Horndeski theories called the disformal Kerr black hole and comment on the most promising directions to derive exact rotating black hole solutions in scalar-tensor theories. This talk will be based on the two articles: https://inspirehep.net/literature/1800972, https://inspirehep.net/literature/1877661