Speakers

A new avenue was recently developed for analyzing large-scale structure data which does not depend on assumptions about the power spectrum shape, the specific background expansion, or the growth function. In this talk I discuss how this model-independent methodology can be applied to answer three fundamental questions: a) is space curve?; b) is gravity Einsteinian?; c) is the equivalence principle violated?

I argue on the possibility that compact extra dimensions obtain large size by higher dimensional inflation, relating the weakness of the actual gravitational force to the size of the observable universe. Although this can be realised for any number of extra dimensions, the requirement of (approximate) flat power spectrum of primordial density fluctuations consistent with present observations makes this simple proposal possible only for one extra dimension at around the micron scale. I will also discuss mechanisms for stabilising the radion modulus after the end of five-dimensional inflation. This scenario can then be nicely combined with the Dark Dimension proposal for the cosmological constant.

Theoretical estimates of the value of vacuum energy density lead to the results exceeding the observed value of dark energy by 50 – 100 orders of magnitude [1,2]. We propose the model of dynamical vacuum energy compensation down to the observed value due to interaction of massless scalar field \( \phi \) with curvature scalar [3]. The form of the nonlinear function \( f(\phi) \) is chosen to lead to the canonical cosmological evolution.

In supersymmetry the R-parity is a \( Z_2 \) symmetry, which gives a dark matter candidate, the lightest supersymmetric partner. In SO(10) and so also \( E_6 \) grand unification this same \( Z_2 \) is part of the gauge group so the existence of a dark matter candidate is not tied to supersymmetry anymore. I will show some consequences for a non supersymmetric \( E_6 \) theory following the requirement for a dark matter candidate.

The Szekeres solution to the field equations of General Relativity is known as a promising tool for the representation of the late universe. Besides being devoid of any symmetry, being able to represent the matter (cosmological constant) dominated region of the universe, to be matched at some redshift to a homogeneous FLRW space-time, and to exhibit a matter dipole, it can be used to reproduce the expansion multipoles which have been recently measured in supernova, quasar and radio-galaxy surveys. The latter property will be described here and methods for its implementation will be proposed.

We analyse an inhomogeneous cosmological model featuring a spherically symmetric bubble solution induced by a unified single perfect fluid, comprising spatially dependent Dark Energy and Dark Matter components.

Gravitational wave signals from coalescing binary black holes are detected, and analyzed, by using large banks of template waveforms. The construction of these templates makes an essential use of the analytical knowledge of the motion and radiation of gravitationally interacting binary systems. A new angle of attack on gravitational dynamics consists of considering (classical or quantum) scattering states [1]. Modern quantum amplitude techniques have recently given interesting novel results concerning both the dynamics and the gravitational wave emission of scattering black holes [2,3]. These results are reaching a level where subtle conceptual issues arise (quantum-classical transition, radiative effects versus conservative dynamics, zero-frequency gravitons, \( \epsilon/\epsilon \) effects, ...).

In \( SO(10) \)-inspired leptogenesis [1,2a,2b] the observed baryon asymmetry of the universe can be expressed in terms of the low energy neutrino parameters, resulting into various testable constraints [3,4]. It can also realise strong thermal leptogenesis, where the final asymmetry is independent of the initial conditions [5,6]. In this case one obtains various interesting predictions that are getting tested already by neutrino oscillation experiments and that will be tested during next years by absolute neutrino mass scale experiments, in particular neutrinoless double beta decay experiments. I will show how adding flavour coupling effects from spectator processes slightly corrects some of the results [7]. This is a first step of a systematic programme that should incorporate different effects in the calculation of the asymmetry that have been neglected so far. Finally, I will show how currently \( SO(10) \)-inspired leptogenesis can be realised within a realistic minimal \( SO(10) \) model [8].

Astronomical data of recent years strongly indicate that creation cosmic structures in the contemporary and early universe at the redshifts of order ten are seeded by primordial black holes (PBHs). The suggested mechanism of PBHs creation is confirmed by the agreement of the predicted mass spectrum of primordial black holes with observations as well as by the discovery in Milky Way of a considerable amount of antimatter including antistars. The talk is based on publications~[1,2]

After more than two years of scanning the sky during 2019–2022 the eROSITA X-ray telescope [1] aboard SRG orbital observatory [2] produced the best ever X-ray maps of the sky and discovered more than three million X-ray sources, of which about 20% are stars with active coronas in the Milky Way, and most of the rest are galaxies with active nuclei, quasars and clusters of galaxies. eROSITA detected over \( \sim 10^3 \) sources that changed their luminosity by more than an order of magnitude [3], including about a hundred tidal disruption events [4, 5]. SRG/eROSITA samples of quasars and galaxy clusters will make it possible to study the large-scale structure of the Universe at \( z\sim 1 \) and measure its cosmological parameters [6]. I will review some of the SRG/eROSITA results in the Eastern Galactic hemisphere and future prospects.

This talk is based on [1]. We construct non-singular cosmological models for flat Friedmann universes filled with minimally coupled scalar fields or by tachyon Born-Infeld-type fields. The regularity of the cosmological evolution, i.e. the existence of bounce, implies the necessity of the transition between scalar fields with standard kinetic terms to those with phantom ones. In both cases, the potentials in the vicinity of the point of the transition have a non-analyticity of the cusp form that is characterized by the same exponent and is equal to 2/3. We also undertake an analysis of the stability of the cosmological evolution in our models.

The detection of primordial tensor modes, i.e. gravitational waves, through primordial CMB B-modes is considered the smoking gun signal for inflation. However, in order to solidify this conclusion, we need to insure that other primordial mechanisms do not produce the same signal. To that end, primordial gravitational vector modes (V-modes) and their sourcing of primordial magnetic fields (PMF), i.e. magnetogenesis, is revisited in this talk, based on. As the adiabatic V-mode generically decays with expansion, we consider three exotic initial conditions, involving both the neutrino and dark sectors, which circumvent this issue and lead to observational imprints. The best fitting parameters in these three cases to CMB and BAO data are found, and their resulting B-mode spectra are compared to data from BICEP/Keck and SPTpol. The outcome is that none of the proposed initial conditions can produce large enough PMFs to seed every type of magnetic fields observed. However, the resultant V-modes are still consistent with the data and ought to be constrained for a better understanding of the primordial Universe before its hot big-bang phase.

Primordial black holes are black holes that may have formed in the early Universe. Their masses potentially span a range from as low as the Planck mass up to many orders of magnitude above the solar mass. This, in particular, includes those black holes recently discovered through gravitational waves, and (part of) these may conceivably be of primordial origin. After a general introduction on primordial black holes, I review the observational hints for their existence— from a variety of lensing, dynamical, accretion and gravitational-wave effects. As I will show, all of these (over \( 20 \)) may be explained by a single and simple unified model, naturally shaped by the thermal history of the Universe. If time permits, I discuss how recent advances in our understanding of quantum effects in black holes impact PBHs. On the one hand, this concerns deviations from Hawking radiation in the form of the memory-burden effect. On the other hand, I will discuss vorticity, which we recently conjectured to be a new characteristic of (near-extremally rotating) black holes. In the second part of my talk, I will present novel results on large-scale simulations of spatially-correlated random fields, being able to resolve events as rare as one in \( 10^{13} \), and discuss their application to PBHs. This talk is mainly based on References [1, 2, 3].

Primordial magnetic fields present since before decoupling have effects on the temperature anisotropies and polarization of the cosmic microwave background (CMB) as well as large scale structure. Their effect on the linear matter power spectrum has implications for the 21 cm line signal.  New possibilities to constrain primordial magnetic fields arise with current and upcoming observations of the 21 cm line of neutral hydrogen such as with the Square Kilometre Array Observatory (SKAO). 

Using 21 cm intensity maps as well as cross correlations of the CMB Doppler mode and the 21 cm signal the prospects of constraining primordial cosmic magnetic fields are considered for LOFAR and SKAO.  In particular the latter in combination with SKA1-mid shows promising signal-over-noise ratios [1].

Primordial black holes (PBHs) are hypothetical black holes that may have formed in the early Universe. They offer a rich phenomenology and are attracting attention as potential dark matter candidates. In this talk, I will discuss constraints on PBHs through gravitational wave (GW) observations, focusing on data from the LIGO-Virgo-KAGRA collaboration. One approach sets an upper bound on the stochastic GW background that could be produced in connection with PBH formation, while another examines the background generated by the superposition of numerous PBH binary events. I will also mention searches for individual GW events from PBH binaries, along with current challenges and future prospects.

We study the consequences of Degenerate Higher Order Scalar Tensor (DHOST) theories in the early universe. By considering deviations from a pure de Sitter background induced by an axion-like potential that breaks the shift symmetry, we show how these theories can be used to build inflationary models that are compatible with the latest Planck measurements of the Cosmic Microwave Background of the power spectrum [1] and the bispectrum [2].

We show how DHOST theories can provide a strong violation of the consistency relation for slow-roll single-field inflationary models and a distinct signature in the gravitational wave sector, that would allow future gravitational wave experiments to distinguish between the two scenarios [3].

Recent observations such as \( H_0 \) and cosmic birefringence among others are challenging the Standard cosmological model of \( \Lambda \)CDM. We introduce the Ultralight Axion-like model of Early Dark Energy (EDE). With the potential of \( n=3 \) we work the observations of the EB mode of the cosmic microwave background (CMB) radiation, and local expansion rate measurements. Our results show that the shape of the CMB EB angular power spectrum is sensitive to the background cosmological parameters. We find that the EDE model with \( n=3 \) can provide a good fit to the observed CMB EB spectra, consistent with the locally measured value of the Hubble constant. Our result [1] is the first to show that axion-like EDE can provide a unified explanation for the observed cosmic birefringence and the Hubble tension.

We explore cosmological particle production during the inflationary epoch, focusing on the possibility to create particles from purely gravitational interactions [1,2]. We review how an unperturbed spacetime expansion generates particle-antiparticle pairs from an initial Bunch-Davies vacuum state and we quantify perturbative corrections arising from the quantum fluctuations of a scalar inflaton field [3]. In particular, we discuss how dark matter may be traced back to such gravitational mechanisms via spectator fields in inflation or through the emergence of geometric quasi-particles from the inflaton fluctuations. We then highlight how perturbative particle production also results in entanglement generation across the Hubble horizon, which can be quantified via the von Neumann entropy of the reduced density operator limited to sub or super-Hubble modes. We compute such entanglement spectrum within different single-field inflationary scenarios, comparing large and small-field approaches and addressing the quantum-to-classical transition of inflationary fluctuations from a particle production perspective [4,5].

I will argue that the application of the rules of quantum mechanics to cosmological systems generically results in the so-called multiverse states in which neither the homogeneous background spacetime nor the inhomogeneous perturbation are in definite states. The multiverse states can be expressed as a sum of products of the background and perturbation states. First, I will consider a simple model with biverse states made of two branches of distinct background spacetimes [1]. I will investigate how the interaction between the branches influences the forming perturbation states within each of them. Then, by applying perturbation theory to general multiverse states [2], I will show that fully quantum cosmological dynamics imprints specific non-Gaussian features in primordial fluctuations and thus could in principle be tested experimentally.

I will discuss how to construct the quantum version of the Ray Choudhury equation in its simplest form, where matter and curvature are entangled. I will discuss some of the applications in the quantum domain.

We study static tidal Love numbers (TLNs) of a static and spherically symmetric black hole for odd-parity metric perturbations. We describe black hole perturbations using the effective field theory (EFT), formulated on an arbitrary background with a timelike scalar profile in the context of scalar-tensor theories. In particular, we obtain a static solution for the generalized Regge-Wheeler equation order by order in a modified-gravity parameter and extract the TLNs uniquely by analytic continuation of the multipole index \( \ell \) to non-integer values. For a stealth Schwarzschild black hole, the TLNs are vanishing as in the case of Schwarzschild solution in general relativity. We also study the case of Hayward black hole as an example of non-stealth background, where we find that the TLNs are non-zero (or there is a logarithmic running). This result suggests that our EFT allows for non-vanishing TLNs and can in principle leave a detectable imprint on gravitational waves from inspiralling binary systems, which opens a new window for testing gravity in the strong-field regime.

The strongest experimental evidence for dark matter is the Galactic Center \( \gamma \)-ray excess observed by the Fermi telescope and interpreted as a potential signature of WIMP self-annihilations [1]. However, an equally compelling explanation of the excess \( \gamma \)-ray flux appeals to a population of old millisecond pulsars that also accounts for the observed boxy morphology inferred from the bulge old star population [2]. We employ a set of Milky Way-like galaxies found in the \textsc{Hestia} constrained simulations of the local universe to explore the morphology of the central dark matter distribution. We predict a significantly non-spherical \( \gamma \)-ray morphology from the WIMP interpretation in a Milky Way-like galaxy.

This talk introduces the polarization of Cosmic Microwave Background (CMB) as a powerful tool to investigate the occurrence of a parity violating process in the Universe, whose effect is the in-vacuum rotation of the linear polarization plane of photons during propagation, usually referred to as Cosmic Birefringence (CB). The CMB represents the earliest source of polarized radiation available in Nature, hence it offers a unique opportunity to prove new parity-violating physics occurring in the Universe.

Recent analyses of \texttt{Planck} data yield hints of the detection of an isotropic CB signal at \( \sim \! 3\sigma \) confidence level [1,2,3,4]; such constraints are limited by the uncertainty on the instrumental polarization angle, and by the polarized foreground emission. Exploiting the enhanced sensitivity and resolution of future CMB experiments (Simmons Observatory, CMB-S4, LiteBird) we will be able to impose more stringent constraints on CB, and investigate the possibility of an anisotropic CB signal.

I would like to discuss recent results in well-known (and not so much) techniques to calculate correlation functions in de Sitter space and beyond. The main focus of my talk is the connection of the stochastic formalism to perturbative QFT's results in curved spacetime.

Based on [1] and work in progress.

Over the last decade, measurements of the expansion rate of the universe today, \( H_0 \), made with cepheid-calibrated SN1a have become increasingly discrepant with the value predicted from the \( \Lambda \)CDM model when fit to CMB data. After a brief review of the experimental situation, I will show the potential implications of this “Hubble tension” for new physics. I will argue that it points to some new mechanism at play in the pre-recombination universe (i.e., \( z>1000 \)), rather than a new dynamical effect at late-time (i.e. \( 0

Interior(s) of Schwarzschild (S), Reissner-Nordstr\"{o}m (RN) and Kerr (K) black holes (BH) may be regarded as cosmological models [1]. The spatial part of SBH and RNBH is homogeneous but anisotropic and has the topology of a hyper-cylinder [2,3,4]. Such a hyper-cylinder expands along its (homogeneity) axis in the former case and the initial expansion is stopped and contraction until exit instant occurs in the latter case; it contracts perpendicularly to its axis in both cases [5,6]. In this study the details of the dynamics are analysed by means of an approach in which the Resting Observers are exchanging the (null) signals [7]. In that way the two time-dependent scales factors are determined and two \textquotedblleft Hubble\textquotedblright\ expansion rates are given in both cases.

There are very few results concerning the dynamical properties of the interior of KBH [8]. We argue that also in this case there exists a special class of observers who, contrary to other test objects (massive and/or massless), are not dragged by rotating interior of KBH. Applying the signals exchanged among those un-dragged observers one can analyse the properties of the interior of KBH – the dynamics of that cosmological model is described.

Some interesting features in common for the dynamics of S, RN and K BHs are indicated. Namely, the critical Doppler, red/blue shifts as the result of the presence of the outer [9,10] and inner (RN, K) horizons are found.

First-order phase transitions, which take place when the symmetries are predominantly broken (and masses are then generated) through radiative corrections, produce observable gravitational waves and primordial black holes. I discuss a model-independent approach that is valid for large-enough supercooling to quantitatively describe these physical phenomena in terms of few parameters, which are computable once the model is specified. This talk is based on Refs.~[1,2,3].

We present a covariant framework for studying metric signature transitions using a Lorentzian metric constructed with a hypersurface-orthogonal timelike vector field and an interpolation function \( \Theta(\lambda) \) that smoothly connects Euclidean and Lorentzian regimes across a codimension-one hypersurface \( \Sigma_0 \). Within this formalism, we first derive the Euclidean action for a general space(time). Next, we compute entropy corrections for spacetimes with horizons in the context of Lanczos-Lovelock gravity, thereby refining the Bekenstein-Hawking entropy relation. Finally, extending previous works [1,2,3] to cosmology, we show that the signature transition naturally induces an inflationary phase under no-boundary conditions [4], without invoking additional matter fields.

Long-lived heavy particles present during the big bang could have a decay channel opened by gravitons. Such decays can produce gravitational waves with large enough abundance to be detectable, and a peculiar narrow spectrum peaked today around optical frequencies. We identify which particles can decay in one or two gravitons. The maximal gravitational wave abundance arises from theories with extra hidden strong gauge dynamics, such as a confining pure-glue group. An interesting abundance also arises in theories with perturbative couplings. Future observation might shed light on early cosmology and allow some spectroscopy of sub-Planckian gravitationally-decaying particles, plausibly present in a variety of theories such as gauge unification, supersymmetry, extra dimensions, strings.

As of today, the LIGO-Virgo-KAGRA collaboration has cataloged nearly 200 GW detections from various compact object mergers [1]. These discoveries began the endeavors to search for other kinds of GW sources. Among these, the Gravitational-Wave Background (GWB), arising as the superposition of individually undetectable cosmological and/or astrophysical sources, is one of the potential sources to observe with the network of ground-based GW observatories in the coming years [2]. A cosmological GWB would carry unique signatures from the earliest epochs in the evolution of the Universe. Likewise, an astrophysical GWB would provide information about the population properties of the sources that generated it. To a first approximation, the GWB is assumed to be isotropic; one could determine its statistical properties by observing any part of the sky. However, these backgrounds can be anisotropic as well. Therefore, searches for both isotropic and anisotropic GWB have been conducted [3,4]. In this talk, I will explain the search methods and the results from the most up-to-date quests for the GWB. In addition, I will outline the new analysis and searches planned for the upcoming runs of these detectors and the exciting results expected from these probes.

In this talk we present our study [1] on the inventory of the gravitational force and tidal field induced by filaments, walls, cluster nodes and voids on Megaparsec scales and how they assemble and shape the Cosmic Web [2],[3],[4]. The study is based on a N\( _{\rm Part}=512^3 \) \( \Lambda \)CDM dark matter only N-body simulation in a (300\( h^{-1} \)~Mpc)\( ^3 \) box at \( z=0 \). We invoke the density field NEXUS+ multiscale morphological procedure to assign the appropriate morphological feature to each location. We then determine the contribution by each of the cosmic web components to the local gravitational and tidal forces. We find that filaments are, by far, the dominant dynamical component in the interior of filaments, in the majority of underdense void regions and in all wall regions. The gravitational influence of cluster nodes is limited, and they are only dominant in their immediate vicinity. The force field induced by voids is marked by divergent outflowing patterns, yielding the impression of a segmented volume in which voids push matter towards their boundaries. Voids manifest themselves strongly in the tidal field as a cellular tapestry that is closely linked to the multiscale cosmic web. However, even within the interior of voids, the dynamical influence of the surrounding filaments is stronger than the outward push by voids. Therefore, the dynamics of voids cannot be understood without taking into account the influence of the environment. We conclude that filaments constitute the overpowering gravitational agent of the cosmic web, while voids are responsible for the cosmic web's spatial organisation and hence of its spatial connectivity.

The influx of precision-cosmology data fuels a proliferation of phenomenological models, particularly in the search for an explanation of dark energy. A fundamental theoretical framework is needed to meet the oversupply of proposed solutions. Asymptotically safe quantum gravity, a leading candidate for quantum gravity, provides such a framework. I show how consistency with asymptotically safe quantum gravity severely constrains model space, using Horndeski gravity as an example. Only a few free parameters remain. Based on [1], I demonstrate that a simple model of dynamical dark energy is incompatible with asymptotically safe quantum gravity and discuss the implications for more elaborate models.

This talk is based on [1] - [4]. If two particles move towards a black hole and collide in the vicinity of the horizon, under certain conditions their energy Ec.m. in the center of mass frame can grow unbounded. This is the Banados-Silk-West (BSW) effect. Usually, this effect is considered for extremal horizons and geodesic (or electrogedesic) trajectories. We study this effect in a more general context, when both geometric and dynamic factors are taken into account. We consider generic axially symmetric rotating black holes. This includes nonextremal, extremal and ultraextremal horizons. We also give general classification of possible trajectories that include so-called usual, subcritical, critical and ultracritical ones depending on the near-horizon behavior of the radial component of the four-velocity. We assume that particles move not freely but under the action of some unspecified force. We find when the finiteness of a force and the BSW effect are compatible with each other. The BSW effect implies that one of two particles has fine-tuned parameters. We show that such a particle always requires an infinite proper time for reaching the horizon. Otherwise, either a force becomes infinite or a horizon fails to be regular. This realizes the so-called principle of kinematic censorship that forbids literally infinite Ec.m. in any act of collision. The obtained general results are illustrated for the Kerr-Newman-(anti-)de Sitter metric used as an example.