The abundance of primordial black holes

• Cristiano Germani (ICC U. Barcelona)

Room: Auditorium C.1 In this talk I will give an overview of the non-linear statistics of primordial black holes pointing out the importance of using the compaction function as the statistical variable.

Hubble-induced phase transitions and Higgs Vacuum Stability

• Javier Rubio (Universidad Complutense de Madrid)

Room: Auditorium C.1 A Hubble-induced phase transition is a natural spontaneous symmetry breaking mechanism, allowing for explosive particle production in non-oscillatory models of inflation involving non-minimally coupled spectator fields. In this talk, I will discuss the impact of this effect on the evolution and stability of the Standard Model Higgs after inflation and the reheating of the Universe, characterizing its dynamics via 3+1-dimensional classical lattice simulations. Phenomenological aspects like the generation of short-lived topological defects and gravitational waves will also be discussed.

Gravitational particle production and freeze-in at stronger coupling

• Oleg Lebedev

Room: Auditorium C.1 I discuss gravitational production of dark matter and how it hinders predictivity of non-thermal dark matter models.

Neutron decay anomaly, dark matter and neutron stars

• Mar Bastero Gil (University of Granada)

Room: Auditorium C.1 The discrepancies in measurements of the lifetime of the neutrons could be resolved considering an extra neutron decay channel into dark matter, with a branching ratio of the order of O(1%). Although the decay channel into a dark fermion $\chi$ plus visible matter has been already experimentally excluded, a dark decay into a dark matter fermion plus either a scalar or dark photon remains still a possibility. In particular, a model with a fermion mass $m_\chi\approx 1$ GeV and a scalar $m_\phi \approx 1-2$ MeV could provide not only the required branching ratio to explain the anomaly but also a good dark matter candidate with the right thermal abundance today, and being consistent with neutron stars physics.

Two-fluid formalism and phenomenology of the dark matter admixed neutron stars

• Oleksii Ivanytskyi (University of Wroclaw)

Room: Auditorium C.1 A two-fluid formalism for curved space-time and phenomenology of the dark matter admixed neutron stars Accumulation of a sizable amount of dark matter in the interiors of neutron stars can significantly modify the observational properties and merger dynamics of these relativistic objects. We propose a two-fluid formalism for the description of a mixture of baryon matter and dark matter existing in curved space-time. The formalism is applied to solving the problem of relativistic hydrostatics and establishes a rigorous scaling between the microscopic parameters of the components, i.e. chemical potentials of baryon matter and dark matter. Furthermore, the formalism is extended to studying gravitational perturbations of the hydrostatic system in order to define the corresponding Love numbers giving direct access to tidal deformability of the dark matter admixed neutron stars. The scenarios of non-interacting fermionic and self-interacting bosonic dark matter are considered in order to constrain the parameters of the dark matter particles. Phenomenological consequences related to the observational mass-radius curves and thermal evolution on neutron stars are also discussed.

Gravitational Waves at Pulsar Timing Arrays and the Early Universe

• Fabrizio Rompineve (IFAE Barcelona)

Room: Auditorium C.1 The recently reported evidence for Gravitational Waves (GWs) in PTA datasets leaves the question of whether the signal is astrophysical or coming from the Early Universe, in the latter case originating from physics beyond the Standard Model. In this talk I provide an overview of PTA results and discuss their relevance for the fundamental physics. Features of cosmological GW backgrounds that can help in addressing the origin of the signal will be discussed, as well as well-motivated scenarios that can be probed at PTAs.

On cosmological clustering of Gravitational Wave events

• José Fonseca (Institute of Astrophysics and Space Sciences)

Room: Auditorium C.1 Traditional large-scale structure surveys estimate the power spectra from galaxy surveys where distances are estimated based on the observed redshift. But not all tracers of dark matter, such as supernovae Type Ia and gravitational wave merger events, have a measured redshift; instead, they provide an observed luminosity distance. Therefore the natural estimator would be the observed number counts in luminosity distance space. In this talk, we would review the full calculation of clustering in luminosity distance space including all light-cone effects, and establish the differences with redshift space. We will also present models for the biases that affect the observed angular power spectrum. Additionally we will discuss the implication of cosmological constraints can have in the properties of BH population responsible for GW events.

Gravitational microlensing with extended dark matter structures [remote]

• Djuna Croon (IPPP Durham)

Room: Auditorium C.1 Many models of dark matter feature small scale substructure in the form of sub haloes, boson stars, or miniclusters. In this talk, I will describe how gravitational microlensing can be used to probe such models. In particular, I will explain how existing techniques used to constrain point-like objects (such as primordial black holes) can be adapted to find constraints on several different extended objects, using data from EROS-II, OGLE, and Subaru-HSC experiments. For particular mass distributions, such as those expected from boson stars, optical effects such as caustics imply a unique shape of the light curve, which may be looked for using machine learning methods. I will explain under what circumstances positive detections of such objects can be made and describe an ongoing analysis for their detection using the Vera Rubin Observatory’s Legacy Survey of Space and Time.

Cosmology with the Einstein Telescope

• IVAN DE MARTINO (Universidad de Salamanca)

Room: Auditorium C.1 Gravitational waves opened a new avenue to explore the Universe and to understand its evolution. Here, I will show the potential of third-generation gravitational wave detectors, such as the Einstein telescope in studying the underlying cosmological model and measuring the cosmological parameters with unprecedented accuracy on the Hubble constant which will aim to solve the well-known Hubble tension.

Heat kernel and Revelations

• Vincenzo Vitagliano

Room: Auditorium C.1 Heat-kernel techniques provide a standard toolkit for calculating functional determinants of Laplace-type operators, facilitating the evaluation of contributions at one loop to the vacuum effective action. While typically perturbative, the coincidence limit of the heat-kernel expansion in proper-time enjoys in some cases a resummed version, paving the way for the analysis of various nonperturbative aspects of quantum field theories. I will discuss some recent results in this direction, in curved and flat spaces, and present several physically relevant applications.

Space-time symmetry breaking and the mystery of dark matter

• João Magueijo (Imperial College London)

Room: Auditorium C.1 If 4D diffeomorphism invariance were to break down to 3D diffeomorphisms on the leaves of a preferred foliation something dramatic would happen to the Universe, even after full 4D diffeo symmetry is restored. The usual algebra of constraints of General Relativity would feel the symmetry breaking effects for ever after the end of the symmetry breaking phase in the form of a leftover violation of the Hamiltonian constraint. We show that this violation can be seen as a form of dark matter, with the Dirac algebra and the geodesic nature of the foliation determining whether this is something almost equivalent to cold dark matter, or an entirely new beast altogether. Scenarios where this early tragedy in the life of the Universe comes to pass include the mainstream Horava-Lishitz gravity model but also theories with evolution in the laws of physics (in terms of global time variables generalizing 4-volume time in unimodular theories), or any other theory with global or Machian interactions. We provide examples of astrophysical implications.

TDiff scalar fields: symmetry restoration and model selection

• Prado Martín-Moruno

Room: Auditorium C.1 As a case of potential interest for the dark sector, in this talk a scalar field action that is invariant only under transverse diffeomorphism will be considered. In the first place, some gravitational properties of this minimally coupled field will be discussed in the kinetic and potential domination regimes. Then, the idea of restoring the full diffeomorphism invariance by the introduction of additional fields will be explored. The reformulation with the full symmetry allows one to analyze the gravitational properties of the theory beyond those regimes.

Searching for fifth forces and modifications of gravity

• Clare Burrage (University of Nottingham)

Room: Auditorium C.1 Light scalar fields are often considered as part of explanations for dark energy and dark matter. They also appear in theories in which gravity is modified on cosmological scales. If the scalar field theory has a screening mechanism, the fifth forces that the scalar mediates can be significant in the cosmological vacuum but suppressed on Earth and in the solar system. I will discuss how this gives rise to novel signatures that could enable us to detect fifth forces, but also the challenges involved in making accurate theoretical predictions for these non-linear theories.

Constraining 3-form dark energy models

• Mariam Bouhmadi-López (Ikerbasque-University of the Basque Country)

Room: Auditorium C.1 In the present talk, we reanalise 3-form dark energy (DE) models. Those models are well known to have a phantom-like behaviour. In particular, they may lead to an abrupt late-time cosmological event which is known as the little sibling of the Big Rip (LSBR) much smoother than a Big Rip singularity. We will present cosmological constraints on the model using Planck, DESI, Pantheon+, SH0ES and DESY1. The combined dataset suggests a scenario where 3 form climbed the potential at 2<z<4, successfully bridging late time and early time datasets. As the result the infamous H_0 tension is relieved from 5 sigma in the LambdaCDM model to 3 sigma in the 3 form model, without sacrificing the sigma-8 tension.

The self dual action: Ashtekar variables without gauge fixing

• Jesus Fernando Barbero Gonzalez (Instituto de Estructura de la Materia-CSIC)

Room: Auditorium C.1 I will discuss how to use the GNH method to study the Hamiltonian formulation of the Euclidean self-dual action. This action can be used to arrive at the complex Ashtekar formulation for Lorentzian General Relativity or a real connection formulation for Euclidean General Relativity. I will show how one can get the Ashtekar formulation for Euclidean gravity without using any gauge fixing and compare this derivation with the one corresponding to the Holst action.

Navigating the Perils of Affinesia

• José Beltrán-Jimenez (U. Salamanca)

Room: Auditorium C.1 In this talk I will review some of the pathologies that typically arise in modified gravity theories. I will adopt a geometrical perspective to modifications of GR taking the Geometrical Trinity of Gravity as starting point and will mainly focus on teleparallel theories. In particular, I will discuss the unavoidable presence of ghosts in the so-called f(Q) theories. To end with a positive message, a class of ghost-free theories in the symmetric teleparallel framework that is based on a TDiff symmetry will be introduced.

Numerical study of the Hawking and dynamical Casimir effects

• Javier Olmedo (Universidad de Granada)

Room: Auditorium C.1 In this presentation we will explore, with the help of analytical and numerical tools, the dynamics of a quantum free field enclosed in a cavity with moving boundaries. The nontrivial dynamics of the mirrors yields to mode-mixing and particle production. We will then discuss how to simulate the Hawking effect and some intriguing phenomena associated with the dynamical Casimir effect.

Black holes in effective field theories – dynamics and new observational signatures

• Daniela Doneva (University of Tuebingen)

Room: Auditorium C.1 Black holes in effective field theories, such as scalar-Gauss-Bonnet theory, offer interesting alternatives to the Kerr solution that can be tested observationally. In the present talk, we will discuss some of the most interesting examples of such beyond-Kerr black holes. Special attention will be paid to the progress in studying their dynamics in cases when they are isolated and when put in a binary. Such studies shed light on the nonlinear stability and potential loss of hyperbolicity of beyond-Kerr black holes from a theoretical perspective. On the other hand, they are the ultimate tools to predict gravitational wave emission of black hole coalescence that can deviate significantly from GR, especially in the nonlinear phase of the merger.

Weak cosmic censorship with quantum-corrected black holes

• Jorge Rocha

Room: Auditorium C.1 The weak cosmic censorship conjecture asserts that naked singularities cannot be generically produced when regular initial data is evolved using the classical equations of General Relativity, and assuming the matter content is physically reasonable. Whatever curvature singularities arise — if they arise — are con- cealed within black hole event horizons. A potential violation of this hypothesis was investigated fifty years ago by Wald. In one of the versions considered, it consisted in trying to overspin an extremal rotating black hole by throwing at it a test particle with large angular momentum. Alas, what was found was that either the particle slings around the black hole or, if it falls through the horizon, it does not cause overspining. In any case, the horizon is preserved and no singularity is exposed. In this talk I will discuss how this test of the weak cosmic censorship performs on quantum-corrected black holes recently discovered. Specifically, I will analyze the effects of dropping a test particle into an extremal quantum rotating BTZ black hole, whose three-dimensional metric captures the exact backreaction from strongly coupled quantum conformal fields.

Greybody factors of string-corrected black holes

• Filipe Moura (ISCTE - Instituto Universitário de Lisboa)

Room: Auditorium C.1 We compute analytically greybody factors for asymptotically flat spherically symmetric black holes with stringy higher derivative corrections in d dimensions in the high frequency limit. Our calculations include both the eikonal limit — where the real part of the frequency of the scattered wave is much larger than the imaginary part — and the highly damped case — where the imaginary part of the frequency is much larger than the real part —, addressing the emission of gravitons and test scalar fields, and yielding full transmission and reflection scattering coefficients.

Merge many times

• Davide Gerosa

Room: Auditorium C.1 The mergers we observe with LIGO and Virgo are ultimately driven by the emission of energy and angular momentum via gravitational waves. However, relativity alone is not sufficient to explain how compact objects pair and merge. For instance, black-hole binaries of ~10Msun orbiting at separations >~10Rsun will take more than a Hubble time to merge under gravitational radiation reaction. Additional mechanisms of astrophysical nature are needed to explain the LIGO/Virgo events. Conceiving plausible scenarios for the formation and evolution of binary black holes is a major industry in the field and still presents considerable challenges. But challenges reveal opportunities for discovery! I present an update on the formation-channel problem in gravitational-wave astronomy with a specific focus on hierarchical black-hole mergers. These provide, in my opinion, an orthogonal, but at the same time complementary, direction to the usual “field vs clusters” debate.

Evolution of creases on the event horizon of a black hole merger

• Robie Hennigar (ICCUB, Universitat de Barcelona)

Room: Auditorium C.1 The event horizon of a dynamical black hole is generically nonsmooth. The types of nonsmooth structures that can arise on the event horizon of a dynamical black hole have been recently classified 2303.15512. The most important type of nonsmooth structures were found to be crease points and caustics. In this talk, I will discuss how creases and caustics arise and evolve on the event horizon of a black hole merger. The study is carried out in the (strict) extreme mass ratio limit, for which constructing the event horizon reduces to finding a null hypersurface that asymptotes to a Rindler horizon in the Kerr spacetime. The construction allows for a quantitive study of geometrical properties of the crease set and a comparison with the predictions of an exact local model. Based on work with Maxime Gadioux and Harvey Reall.

Black hole binaries in an expanding Universe

• Óscar Dias (U. Southampton)

Room: Auditorium C.1 According to the Λ-Cold Dark Matter (ΛCDM) model, a positive cosmological constant explains the accelerated expansion of the Universe. We start by constructing a static solution of general relativity with a positive cosmological constant that consists of two (or more) static black holes whose gravitational attraction is balanced by the cosmic expansion of the de Sitter background. Then, we extend our analysis and establish the existence of stationary, spinning black binaries in a de Sitter universe and analyse their properties (there is no quadrupole momenta, no radiation). We consider identical black holes with either aligned or anti-aligned spins which maximize the spin-spin repulsion or attraction, respectively. We discuss the prospect that spin-spin interactions can stabilize the binaries. Our solutions establish continuous non-uniqueness in general relativity without matter (we have several solutions with the same cosmological entropy and angular momentum) for the first time in four dimensions. They evade assumptions of mathematical theorems that would otherwise rule out their existence. They also provide initial data for the spinning binary merger problem (when orbital angular momentum is added).

Kerr-de Sitter and algebraically special metrics with prescribed asymptotics in all dimensions

• CARLOS PEON NIETO (PROFESOR AYUDANTE DOCTOR)

Room: Auditorium C.1 We provide a classification of arbitrary dimensional $(\Lambda>0)$-vacuum metrics corresponding to type II metrics in the Coley et al. classification of Weyl tensors, which also admit a conformal extension with a locally conformally flat boundary. These metrics are found to coincide with a previously known class that generalizes Kerr-de Sitter metrics. To identify both classes, we conduct a detailed analysis of the fall-off behavior of the Weyl tensor and its relation to the asymptotic initial data.

Quasinormal mode spectrum of the AdS black hole with the Robin boundary condition

• Keiju Murata

Room: Auditorium C.1 We study the quasinormal mode (QNM) spectrum of an asymptotically AdS black hole with the Robin boundary condition at infinity. We consider the Schwarzshild-AdS$_4$ with the flat event horizon as the background spacetime and study its scalar field perturbation. Denoting leading coefficients of slow- and fast-decay modes of the scalar field at infinity as $\phi_1$ and $\phi_2$, respectively, we assume a linear relation between them as $\phi_2 = \cot(\theta/2) \phi_1$, where $\theta$ is a constant called the Robin parameter and periodic under $\theta\sim\theta+2\pi$. In a certain range of the Robin parameter, there is an instability driven by the boundary condition. We also find the holonomy in the QNM spectrum under the parametric cycle of the boundary condition: $\theta=0\to2\pi$. After the one-cycle, $n$-th overtone of the QNM moves to $(n-1)$-th overtone. The fundamental tone of the QNM is swept out to the infinity in the complex plane.