Conference timetable is available at the following link: **Conference programme**

**Keynote speakers contributions**

Speaker | Title | |
---|---|---|

Diego Blas | Filling gaps in GW searches | |

Stefano Liberati | Black hole thermodynamics beyond Lorentz invariance: resilience against all odds | |

Vitor Cardoso | Testing the BH paradigm: from echoes to light rings | |

Christoph Ringeval | Stochastic gravitational waves from cosmic strings | |

Baojiu Li | Structure formation and cosmological tests of modified gravity | |

Mar Bastero-Gil | ||

Licia Verde | Hubble troubles | |

Vincent Desjaques | Dynamical Friction | |

Mariafelicia de Laurentis | First Sagittarius A* Event Horizon Telescope Results. VI. Testing the Black Hole Metric |

## Abstracts

**Filling gaps in GW searches**

*Diego Blas*

Gravitational waves from different frequencies bring information about different phenomena of the Universe. CMB studies, LVK, PTA and LISA-like missions are able to provide relevant information in several bands, but leave some gaps where new signals may be hiding. In this talk, I’ll show how the microHz band can be scanned using binary systems such as the Earth-Moon system or binary pulsars. I will also describe new ideas to reach sensitivity at high frequencies using laboratory detectors, such as electromagnetic cavities.

**Testing the BH paradigm: from echoes to light rings**

*Vitor Cardoso*

One of the most remarkable possibilities of General Relativity concerns gravitational collapse to black holes, leaving behind a geometry with light rings, ergoregions and horizons. These peculiarities are responsible for uniqueness properties and energy extraction mechanisms that turn black holes into ideal laboratories of strong gravity, of particle physics (yes!) and of possible quantum-gravity effects. I will discuss some of the latest progress in testing the black-hole nature of compact objects.I will discuss some of the latest progress in testing the black-hole nature of compact objects.

**Black hole thermodynamics beyond Lorentz invariance: resilience against all odds**

*Stefano Liberati*

A possible signature of the quantum/discrete nature of spacetime at small scales is a breakdown of its local symmetries and in particular of local Lorentz invariance. While a wealth of knowledge has been acquired about departures from such a fundamental symmetry in the matter sector of the Standard Model, not so much is known about the gravitational sector (where it has been suggested that UV Lorentz breaking could be a key ingredient for renormalizability) and current observations leave an interesting parameter space amenable to exploration. In this talk, I will review our knowledge concerning the phenomenological constraints we acquired so far, and then focus on black hole solutions in Lorentz breaking gravity and on the theoretical issue concerning their thermodynamic behaviour. In this sense, I will present a series of recent results pointing to a surprising resilience of Hawking radiation in these settings: a fact that might help expositing what fundamentally lies at the root of black hole radiation and the associated thermodynamic laws.

**Stochastic gravitational waves from cosmic strings**

*Christoph Ringeval*

The advent of gravitational wave astronomy has triggered a renewed interest for the search of relics from the Early Universe.

Cosmic strings are line-like topological defects possibly created during phase transitions at high energies. They could also be more exotic objects created after inflation in theories with extra-dimensions.

They have been searched for, in vain, in the last forty years. In spite of their huge energy density, they have a vanishing Newtonian potential and can only be detected through pure General Relativity effects, which are genuinely small. In this talk, I’ll discuss the recent progresses made in the search of cosmic strings, loops and infinite strings, through their gravitational wave emissions all along the universe history.

**Structure formation and cosmological tests of modified gravity**

*Baojiu Li*

Modified gravity has been proposed as an alternative to a cosmological constant in explaining the observed accelerated expansion of the cosmos. With the upcoming cosmological surveys such as DESI, Euclid and LSST, the precision and statistical power of observational data will be dramatically improved, allowing us to systematically test such models, and hence constrain possible derivations from General Relativity on cosmological scales. In this talk, I will review some recent progresses in the understanding of large-scale structure formation in leading modified gravity models, as well as current and prospective cosmological constraints on them.

*Licia Verde*

The current expansion rate of the Universe is captured by the so-called Hubble constant, which is a key parameter in the, extremely successful, standard model of cosmology. As it is evident from the Friedmann equations, the Hubble constant relates measurements of the expansion history of the Universe to its components.

As such, The Hubble constant can be measured in several different ways: looking at the light of the “early Universe”, looking at bright objects in the “late Universe” (an approach close to Hubble original approach) and other in-between options, each measuring the expansion of the Universe in its peculiar way. Each of these measurements is very precise: error-bars are at the percent level. However, their values do not seem to agree. These are exquisitely sophisticated, and challenging, measurements. Yet one may ask: can this be a signature that the cosmological model starts showing some cracks and that we might need to invoke new physics? I will introduce this modern-day cosmic puzzle, discuss its implications, what this tension has taught us so far and possible future prospects.

**First Sagittarius A* Event Horizon Telescope Results. VI. Testing the Black Hole Metric**

*Mariafelicia de Laurentis*

Astrophysical black holes are expected to be described by the Kerr metric. This is the only stationary, vacuum, axisymmetric metric, without electromagnetic charge, that satisfies Einstein’s equations and does not have pathologies outside of the event horizon. We present new constraints on potential deviations from the Kerr prediction based on 2017 EHT observations of Sagittarius A* (Sgr A*). We calibrate the relationship between the geometrically defined black hole shadow and the observed size of the ring-like images using a library that includes both Kerr and non-Kerr simulations. We use the exquisite prior constraints on the mass-to-distance ratio for Sgr A* to show that the observed image size is within ∼10% of the Kerr predictions. We use these bounds to constrain metrics that are parametrically different from Kerr, as well as the charges of several known spacetimes. To consider alternatives to the presence of an event horizon, we explore the possibility that Sgr A* is a compact object with a surface that either absorbs and thermally reemits incident radiation or partially reflects it. Using the observed image size and the broadband spectrum of Sgr A*, we conclude that a thermal surface can be ruled out and a fully reflective one is unlikely. We compare our results to the broader landscape of gravitational tests. Together with the bounds found for stellar-mass black holes and the M87 black hole, our observations provide further support that the external spacetimes of all black holes are described by the Kerr metric, independent of their mass.

**Dynamical friction**

*Vincent Desjaques*

The gravitational field of a massive object moving in a discrete or continuous medium induces a density wake in that medium. Dynamical friction is the gravitational force exerted on the perturber as a result of the induced density fluctuation. The pioneering study of Chandrasekhar (1943) considered a perturber linearly moving in a collisionless medium. In this talk, I will present an analytic approach to the dynamical friction acting on circularly moving point masses in a gaseous medium. I will show how it can be extended to axion dark matter backgrounds and to eccentric orbits. Some astrophysical implications will also be discussed.