Speakers

I review the theoretical framework of quantum gravity with purely virtual particles (fakeons) and its predictions for the primordial fluctuations of the cosmic microwave background, focusing in particular on the tensor-to-scalar ratio, the spectral tilts, and higher-order corrections. I also comment on other phenomenological predictions, such as the “peak uncertainty” and compare the fakeon approach with alternative frameworks proposed in the literature

I will present a supergravity action whose bosonic part contains the \( f(R) \) gravity. In a perturbative expansion in powers of \( R \), one obtains a small deformation of the Starobinsky cosmological model that solves the problem of initial conditions within the validity of the effective field theory, below the scale of tower of states predicted by the swampland distance conjecture. Moreover, it allows better agreement with the recent cosmological data. A particular example of an underlying microscopic theory with such properties is provided by a four-dimensional heterotic string model containing the Standard Model of particle physics.s.

Multi-agent Artificial Intelligence (AI) systems are touted as powerful co-scientists (e.g. [1]) but can AI do end-to-end science including writing up the research article? Here we present the results of experiments of using AI to build and write up new tests of the Copernican Principle without any human assistance beyond the initial prompt. We conclude with implications and near-term forecasts.

This talk is based on [1]. Considering lepton-flavored Majorana dark matter, it has been shown [2] that the cogenesis of dark matter and the baryon asymmetry can be achieved via conversion-driven freeze-out. We demonstrate that the framework can be extended to quark-philic scenarios, where the matter-antimatter asymmetry is generated %resonantly via baryon-number-conserving CP-violating conversions of a mediator field into quarks and dark matter. The dark matter departs from equilibrium before electroweak sphalerons decoupling, while the mediator %in-equilibrium interactions do not communicate \( B-L \) asymmetry to the sphaleron-active sector, and the asymmetry is transferred only later via decays. Dark matter masses span from few hundred GeV up to the TeV scale.

Waves propagating through a gravitational potential exhibit wave-optics effects when their wavelength is not significantly smaller than the lensing scales [1]. We study the propagation of a scalar wave, governed by the Klein-Gordon equation in curved spacetime, to focus on effects on amplitude and phase, while leaving aside the issue of wave polarization which affects electromagnetic and gravitational waves [2]. Using the Newman-Penrose formalism, we obtain the first corrections beyond the geometric optics in the expansion in the inverse frequency [3]. With the weak field approximation we get an analytic expression for this correction, which we check by solving numerically the equations for study cases.

In the current era of precision cosmology, it is well-known that the present standard \( \Lambda \)CDM model, despite its successes, is an insufficient average of our actual Universe. Given the recorded tensions between theory and observations, it appears that a more precise representation of the local universe is mandatory. Since General Relativity (GR) is the best theory currently available to describe gravitation (verified up to some \( 10^{-15} \) precision in the weak and strong regimes), the new model should agree with its specifications. We have already shown that the Szekeres GR solution possesses all the nice properties allowing it to represent an inhomogeneous small scale universe becoming homogeneous (FLRW) at some large transition scale. Now, a number of observations, which will be described here, lead us to conclude that the distribution of matter and its expansion are anisotropic roughly in the direction of the CMB dipole. While most of the community has been reluctant up to now to use the general Szekeres model, due to its large number of parameters, its axially symmetric version, much more simple to deal with, might be more appealing.

Therefore, taking advantage of both its compatibility with observations and its relative simplicity, we show how it can be used as a new standard cosmological model.

After the Higgs discovery, the question of whether particles beyond those of the Standard Model exist is more pressing than ever. In this context, the scalar sector is particularly promising, since it lies at the core of the internal problems of the Standard Model, while extensions of it allow us to resolve them and can provide explanations for Dark matter, non-zero neutrino masses, inflation etc. In these proceedings, we review the indications for new Higgs bosons at the electroweak scale with masses of \( \approx \)95 GeV and \( \approx \)152 GeV. These excesses are most significant in the di-photon channel but are supported by weaker-than-expected limits in other decay modes. While for the 95\,GeV candidate the production mechanism is mostly unknown, the (hypothetical) 152 GeV Higgs is dominantly produced in association with leptons, \( (b) \) jets and missing energy, pointing towards the Drell-Yan production of an \( SU(2)_L \) triplet with \( Y=0 \). Interestingly, this model predicts \( t\to H^\pm b \) with \( H^\pm\to WZ \), which resembles the signature of \( t\bar{t}Z \) production in the Standard Model and is in fact preferred by current data. Finally, we investigate the possibility that the significant tensions between the Standard Model predictions and the measurements in differential top-quark distributions are due to contamination from new physics involving both the 152 GeV and the 95 GeV scalar.

I discuss a negative kinetic energy oscillator (ghost) which interact with a positive energy one. The motion of this system is shown to be classically fully stable, due to integrability. Extension to non integrable analog cases, field theories and quantization will also be discussed.

We live in an exceptional era of precision cosmology, marked by rapidly advancing observational probes that explore the Universe across many length scales. While these experiments offer clues about the geometry, dynamics, and large-scale structure of the cosmos, they still leave the origin and much of the evolution and structure of the Universe unknown. One of the key steps to answer these questions is to uncover the building blocks of theoretical models in both linear and non-linear regimes.

In this talk, based on [1], I will present methods for uncovering physical degrees of freedom, outlining the standard approaches and presenting an alternative, simple and straightforward way. Following [2], I will then show how one can naturally connect this approach to machine learning. Finally, I will introduce the framework of constrained gravity that was recently introduced in [3], in which gravity is modified through non-propagating external fields, and discuss how such approaches can help address long-standing challenges in fundamental physics and open new directions across disciplines.

Cosmology remains strongly tied to model-dependent analyses, with the \( \Lambda \)CDM paradigm playing an important role at various stages from data reduction to interpretation. While this framework has been highly successful, it also limits our ability to interpret deviations from the \( \Lambda \)CDM model present in current data. In this talk, I will present the theoretical framework for interpreting data with minimal model assumptions and some of the results obtained so far using this framework. The talk is based on e.g. [1,2,3].

In this talk I propose a novel reinterpretation of spacetime as an effective optical medium capable of exhibiting negative refraction. I show that this phenomenon is not a coordinate artifact, but rather a covariant property of energy-momentum flow, arising when phase propagation and energy transport become oppositely directed. I argue that such a regime is forbidden in minimally coupled general relativity and instead requires non-minimal disformal couplings, which induce an intrinsically anisotropic optical geometry. Within this framework, spacetime can support localized, finite-energy configurations that emerge directly from its optical structure. These objects, which I identify as gravitational metamaterials, behave as particle-like excitations whose effective dynamics, in the weak-field limit, naturally reproduces cold dark matter phenomenology. This suggests a new perspective in which dark matter is not a fundamental component but an emergent geometric manifestation with exotic optical properties. To test this paradigm, I discuss how negative refraction modifies gravitational lensing, potentially leading to observable signatures that distinguish these configurations from standard compact objects and their mimickers.

Inflation is conventionally described in terms of fundamental scalar fields whose microscopic origin remains unknown. We propose [1,2] a fundamentally different mechanism in which inflation emerges from fermion condensates generated by torsion-induced four-fermion interactions in Einstein–Cartan–Holst gravity. Starting from a purely fermionic theory, we derive a two-condensate effective description whose inflationary dynamics resembles hybrid inflation but exhibits a qualitatively new exit mechanism.

The central result is the emergence of a density-driven waterfall transition. During inflation, axial charge production continuously increases the fermion density and the associated chiral chemical potential. Solving the finite-density gap equation, we show that above a critical axial density the condensate no longer admits a symmetry-broken solution. Inflation therefore terminates through the evaporation of the condensate itself rather than through the inflaton crossing a critical field value. This mechanism naturally triggers rapid energy transfer to fermionic degrees of freedom and provides a microscopic realization of instant preheating.

The post-transition condensate is unstable to fragmentation into charged non-topological solitons. These objects can subsequently collapse into primordial black holes, generating a calculable relic population whose properties are determined by the same condensate dynamics that drives inflation [3]. Because the underlying theory is intrinsically chiral and connected to dynamical Chern–Simons gravity, the scenario simultaneously predicts parity-violating signatures [4].

The framework unifies inflation, reheating, soliton formation, primordial-black-hole production, and parity-violating phenomenology within a single gravitationally induced fermion-condensation mechanism. It provides a composite alternative to scalar-field inflation and identifies correlated observational signatures that can test the role of spacetime torsion in the early Universe.

Cosmic defects are interesting objects that could arise in cosmological phase transitions, having important implications for cosmology and gravitational waves. In this talk, I will first review the case of cosmic defects that are metastable [1], in particular for theories with symmetry-breaking patterns analogous to the electroweak sector of the Standard Model. Next, I will focus on metastable cosmic strings arising in a concrete dark sector model, which have recently been considered as candidates for explaining the gravitational-wave background observed by pulsar timing arrays (PTA). I will discuss how they can decay quantum mechanically, and I will then present an analysis of their classical stability, where I will show that classical instabilities can affect the parameter space relevant for explaining the PTA signal [2]. I will conclude by outlining future directions in the study of metastable defects.

I will discuss the diagrammatic approach to the conservative dynamics of coalescing binary systems within the effective field theory framework applied to General Relativity. In this setting, the Post-Newtonian expansion of the interaction potential can be computed systematically using techniques familiar from quantum field theory.

Focusing on the static sector, I will present the evaluation of the sixth-order Post-Newtonian correction, which requires the computation of six-loop two-point Feynman integrals. I will review the main ingredients of the calculation and outline how the structure of the contributing diagrams enables a controlled organization of the computation. I will also discuss how a factorization theorem for the static potential can be exploited to extend the analysis to higher Post-Newtonian orders.

Finally, I will comment on general structural properties of Feynman integrals arising in General Relativity contexts, including their vector-space structure and the emergence of linear relations, differential equations, and quadratic relations from Intersection Theory of de Rham cohomology groups, as well as their connections with D-module theory.

Although quantum gravity has been studied extensively, it has no empirical evidence yet. This makes “is gravity quantum?" one of the most important open questions of our time. I will explain a protocol “spin entanglement witness for quantum gravity," to test the quantum nature of gravity in a lab. It exploits quantum information principles and combines a quantum spin with cooling and trapping technologies. It is based on entangling two neutral quantum masses solely through their gravitational interaction, while all other interactions, e.g., electromagnetic (EM) interactions, are mitigated. It proves the quantum nature of gravity, as classical gravity cannot mediate quantum correlations (entanglement). The potentially realisable protocol requires addressing a rich set of challenges: mitigating EM interactions and background noise, creating spatial quantum superpositions of massive objects, and measuring spin correlations to witness entanglement. We must also protect the quantum superpositions from heating, blackbody radiation, acceleration, seismic noise, and gravity-gradient noise, see Refs.[1,2,3].

I will discuss a general approach to quantum field theory in de Sitter and anti-de Sitter spaces based on harmonic analysis and plane-wave representations. I will then focus on recent results concerning quantum fields in anti-de Sitter space.

In particular, I will present a new, manifestly covariant plane-wave expansion of the two-point Wightman functions of scalar fields in AdS. The construction uses holomorphic plane waves defined globally on the universal covering of AdS and leads to integral representations of the two-point functions in terms of relative homology cycles. In Poincaré coordinates, these representations diagonalize into a Källén–Lehmann superposition of lower-dimensional Minkowski two-point functions, with a weight expressed in terms of Bessel functions.

I will also discuss applications of the plane-wave representation to the Feynman propagator and to the relation between Euclidean and Lorentzian AdS quantum field theory, including a new integral representation that permits the Wick rotation of Euclidean Feynman diagrams into Lorentzian integrals while preserving full AdS covariance.

We proposes a new avenue for understanding the cosmological singularity. The standard cosmological model contains a generic initial singularity usually referred to as the big bang. Herein, we present a novel idea to extend the description of our Universe beyond this limit. The proposal relies on rewriting physics in a purely Riemannian, i.e. locally Euclidean, 4-dimensional space and the emergence of Lorentzian patches owing to the interaction of all matter fields to a clock field that is responsible for a signature change. If our universe is contained within one of these patches, the initial singularity is replaced by a smooth boundary on which the signature of the physical metric flips. In this talk, we first define the model and draw the necessary conditions on its arbitrary functions for solutions to exist. Next, we prove the existence of solutions that lead to an emergent universe with a primordial (almost) de Sitter phase. To finish, we discuss the consequences of this construction for the universe on scales much larger than our observable universe: a large “Euclidean sea” in which Lorentzian islands locally emerge and host an expanding universe potentially similar to ours. While speculative, this scenario has specific features that can be tested, and this talk sets the basis for further phenomenological investigations.

Wave properties are modified by the medium through which the waves propagate. For example, photons propagating in a thermal gas exhibit collective excitations: a gap known as the plasmon mass arises, and Landau damping appears. Motivated by this phenomenon, in this talk I discuss the collective properties of gravitational waves in a cosmological medium, such as a thermal radiation gas in the early universe. In Ref.[Ota:2023iyh], we used the in-in formalism and pointed out that, although the one-loop linear response of gravitational waves is suppressed by \( M_{\rm pl}^{-2} \), the thermal energy density \( \rho\sim H^2 M_{\rm pl}^{2} \) cancels the Planck suppression. The linear response effect is then dominant on super-Hubble scales, indicating a secular instability of gravitational waves beyond the thermal soft-scale known in the context of thermal gauge theories. In a follow-up work[Ota:2025rll], I constructed a real-time linear response framework for gravitational perturbations in a dynamical background and proposed a Ward identity diagnosis of diffeomorphism invariance on an off-shell background, which constrains the secular instability reported in the previous reference. We also showed that the gravitational plasmon mass indeed arises in the adiabatic dispersion relation, but that it is removed by a proper boundary condition for the retarded kernel in the linear response. In Ref.~[Ota:2026yzp], we further extended the analysis to conformal fields of spin 0, 1/2, and 1, and found that the linear response effects reduce to Weinberg's damping tensor kernel in the high-temperature limit.

In this talk I will discuss the structure of inflationary \( f(R) \) theories and the limitations of a perturbative treatment of higher-curvature corrections to Starobinsky model. I will first review a classification of \( f(R) \) models based on a mathematical analogy between slow-roll inflation and renormalization-group flow. This framework determines the expressions of the corresponding \( f(R) \) functions and allows the inflationary observables to be studied class by class. A particularly interesting feature is that polynomial \( f(R) \) theories with degree higher than 2, i.e. higher-curvature correction to Starobinsky model, belong to a separate class. Although such operators are expected to give small corrections from the effective-field-theory perspective, their contribution to the slow-roll expansion can be enhanced. In particular, a naive perturbative treatment may generate corrections involving inverse powers of the slow-roll parameter, raising questions about the validity of the expansion.

I will show that a smooth interpolation requires a nonperturbative resummation of the higher-curvature contributions. I will illustrate this explicitly in the case of a \( R^3 \) correction and compare it with recent attempts to account for ACT data through higher powers of \( R \). I will conclude with some remarks on the role that renormalizable gravity may play in this context.

Theories with radiative symmetry breaking (RSB) lead to first-order phase transitions and the production of gravitational waves as well as primordial black holes if the supercooling period lasted long enough. Here we explain how to efficiently reheat the universe after such period in the above-mentioned class of theories. Two cases are possible, depending on whether the RSB scale is much larger than the electroweak symmetry breaking scale or not. When it is, the dominant reheating mechanism can be the decays of the field responsible for RSB in the Standard Model sector, which can produce also DM abundance (e.g. sterile neutrino). Otherwise, reheating must occur thorough parametric resonant production of particles. The talk is based on [1].

The idea that compact objects are coupled to the expansion of the universe was put forward almost a century ago by McVittie, who introduced a specific metric that, however, exhibits naked singularities. Since then, many authors have attempted to improve the McVittie solution to eliminate the singularity, with limited success. In recent years, the cosmological coupling of compact objects has resurfaced prominently, driven by new theoretical arguments and observations [1]. In this talk, I will offer a brief survey of this topic and present some recent insights [2,3].

In this talk, I will present recent results from the fifth release of the Gravitational-Wave Transient Catalog (GWTC-5)~[1,2] of the LIGO, Virgo, and KAGRA (LVK) collaborations, together with the latest results from the search for an isotropic gravitational-wave background (GWB) using data from the second and third segments of LVK's fourth observing run~[[placeholder]]. Although no evidence for a stochastic gravitational-wave background has been found, the analysis yields improved constraints on several theoretical models and places new limits on non-standard polarization modes. I will discuss these results in the context of the GWTC-5 compact-binary population models and assess the prospects for detecting the compact-binary background with future A+ detector sensitivity.

Collisions of ultra-relativistic bubbles during cosmological phase transitions can leave signatures producing particles much heavier than the transition scale. We show that the standard formalism gives unphysical results and parametrically over-estimates the hard production rate. We propose a new formalism, analogous to the partonic approximation in high-energy collisions. We apply it to heavy scalar, fermion and vector production, and study its implications for the dark-matter abundance and gravitational-waves.

Gravitational waves (GWs) have emerged as a powerful tool for probing the universe, offering novel insights into fundamental physics, astrophysics, and cosmology. This talk will review the methodologies employed in GW cosmology, including standard sirens as distance indicators, multi-messenger observations, and novel approaches leveraging population statistics. I will summarise the latest results from LIGO-Virgo-KAGRA observations, highlighting the state-of-the-art GW constraints on the Hubble constant. Looking ahead, I will discuss the transformative potential for GW cosmology of upcoming detectors, including the space mission LISA and third-generation ground-based observatories such as the Einstein Telescope and Cosmic Explorer.

The morphology of the spatial distribution of galaxies at large-scale resembles a spongy structure with densely populated regions (filaments, planes, clusters, etc.). It also reveals nearly empty regions, some of which can be quite large. This talk focuses on studying the dynamics of these regions in an expanding universe. A preliminary wavelet analysis shows that the evolution of these structures is not linear at any time and is dominant over the dense ones [1]. The property that empty regions are swept from their interiors to their edges is confirmed by a covariant model embedded in a Friedmann-Lemaître universe [2]. Another relativistic model, consisting of the junction of two homogeneous spaces bounded by a sphere, yields similar results [3]. The properties thus described can be used to estimate the values ​​of cosmological parameters from observation.

When primordial inhomogeneities are produced with sufficiently large amplitude in the early universe, they may subsequently collapse into primordial black holes. I will explain why the effect of quantum diffusion during inflation needs to be taken into account in such a case, and how the statistics of cosmological fluctuations can be predicted within the formalism of stochastic inflation. Quantum diffusion leads to a peculiar type of non-Gaussianity that cannot be captured by perturbative parameterizations. This leaves specific imprints on the statistics of collapsed structures that I will show can be reconstructed using stochastic trees.

During last decade and a half many results are obtained that show when (i) particle collisions in a strong gravitational field lead to an unbounded energy in the center of mass \( E_{c.m.} \) of two colliding particles, (ii) when the process leads also to significant energy extraction measured at infinity. The set of possible scenarios can be divided to three main cases: (i) collisions near black holes, (ii) near wormholes, (iii) near singularities Case (i) is mainly connected with what is called the Ba\~{n}% ados-Silk-West (BSW) effect [1]. We suggest a brief review of such processes. In doing so, we make an accent on recent developments and quite unexpected results [2]. We include into consideration the presence of a force [3]. Some general statement on possible or forbidden scenarios are formulated. In particular, it is shown that such collisions are possible even near the Schwarzschild horizon, provided a white hole or mirror region is included in consideration. It turns out that for collisions near black holes an unbounded energy extraction (so-called super-Penrose process) is impossible. However, this happens for collisions near wormholes.