Speakers

Quantum gravity can be defined by modifying conventional diagrammatic techniques to incorporate “purely virtual” particles [1]. This approach results in a unitary and renormalizable theory [2], with key testable predictions in the realm of primordial cosmology [3]. The inflation mechanism aligns with the Starobinsky \( R+R^{2} \) model [4], though the spectra include certain corrections. Specifically, the predicted value of the tensor-to-scalar ratio \( r \) for primordial fluctuations falls within a narrow range \( 4 \cdot 10^{-4} < r < 3 \cdot 10^{-3} \) [3]. Initial observational results are likely to have errors exceeding this range, effectively confirming Starobinsky's prediction.

The origin of cosmic rays with energies above \( 10^{19} \) eV encounters serious problems with attempts of explanation by conventional astrophysical mechanisms. This difficulty gave rise to a sequence of papers where ultrahigh energy cosmic rays were created by annihilation or decay of superheavy particles. However, the existence of such particles was postulated without any fundamental justification. In our previous papers (see review [1]) we have shown that dark matter particles with sufficiently high masses are naturally created in \( R^2 \)-gravity suggested by Alexei Starobinsky. We have calculated the flux of cosmic rays produced by these superheavy particles and demonstrated that with a natural choice of parameters the origin of extremely high energy cosmic rays could be explained [2,3].

Static black holes in general relativity modified by a linear scalar coupling to the Gauss-Bonnet invariant always carry hair. I will explain that the same mechanism that creates the hair makes it incompatible with a cosmological horizon. Other scalar-tensor models do not have this problem when time-dependence of the scalar provides a natural matching to cosmology. Scalar-Gauss-Bonnet is particularly rigid and such a scenario does not help. An extra operator makes the theory behave like the other models and the cosmological horizon can be accommodated. The hair, however, is drastically altered.

Thirty years after Alcubierre introduced the warp drive spacetime concept in General Relativity (GR) [1], a substantial body of literature has developed on his model and its extensions. These models, which we term “restricted warp drive” models, are limited within the context of GR. In contrast, we introduce a novel approach called the “tilted warp drive,” which integrates crucial elements absent from previous models, such as covariant descriptions of motion, including tilted, accelerated, and vortical motions.

In this talk, I will highlight the significant impact of the tilt in progressing towards a feasible physical warp drive model. I will outline the fundamental concepts and present an example illustrating the potential of this new proposal, which paves the way for a new line of research. This approach, not previously explored in the literature, may lead to a deeper understanding of a physical warp drive. If time allows, I will also connect these spacetimes to cosmological models.\\ This is joint work with Thomas Buchert and is based on [2].

The huge amount of cosmological data already available and the upcoming stage 4 surveys designed to multiply tremendously the available sets imply that we have entered an era of precision cosmology calling for a precise theoretical framework to analyze those data. The rightly celebrated FLRW homogeneous and isotropic model coupled to its linear pertubation theory, after having contributed to a splendid increase in our understanding of the Universe, is currently confronted to a number of tensions, some of which are developing into actual anomalies. These tensions are essentially encountered in the late Universe where the amplitude and width of the inhomogeneities might superseed any linear perturbative scheme at the future - and even present - required accuracies. Since GR is not a linear theory, we should use models which provide nonlinear perturbations of FLRW. A general class of exact inhomogeneous solutions exhibiting dust as a gravitational source and a possible cosmological constant seems promising to do the job: the Szekeres solution which is devoided of any pre-set symmetry is a tool properly designed for our purpose. Since this solution has FLRW as a homogeneous limit, it can be smoothly matched to the standard model at the inhomogeneity-homogeneity transition. In this talk, I will present the Szekeres solutions and their main properties as well as some among the most meaningful preliminary results already obtained by using them in a cosmological context. Then I will sketch out the prospects for a future broader use of their abilities.

Primordial black holes (PBHs) with log-normal mass spectrum with masses up to \( \sim 10^4-10^5 M_\odot \) can be created after QCD phase transition in the early Universe at \( z\sim 10^{12} \) by the modified Affleck-Dine baryogenesis. Using a model binary PBH formation, the expected detection rate of such binary intermediate-mass PBHs by the TianQin space laser interferometer is calculated to be from a few to hundreds events per year for the assumed parameters of the PBH log-normal mass spectrum and abundance consistent with LIGO-Virgo-KAGRA results. Distinctive features of such primordial IMBH mergings are vanishingly small effective spins, possible high redshifts \( z>20 \) and lack of association with gas-rich regions or galaxies.

The cosmological data on in contemporary and early universe are analyzed. It is argued that the tension between the observations and the conventional theory are neatly resolved if cosmic structures are seeded by massive primordial black holes (PBH). The proposed model is supported by the agreement of the observed mass spectrum of PBH with data from LIGO/Virgo/KAGRA and the discovery of antimatter in the Galaxy.

We evaluate the higher-order moments for the distribution of a given observable and explicitly discuss the case of the Hubble-Lemaître diagram to quantify its skewness at the leading order in the cosmological perturbative expansion of the gravitational potential. In particular, we focus on perturbations of the luminosity distance due to gravitational lensing. When compared with numerical relativistic simulations (e.g. gevolution), our findings confirm that the skewness in the Hubble-Lemaître diagram primarily originates from the late-time matter bispectrum, with other line-of-sight projection effects being sub-dominant. These results possibly open a new window where to detect non-linear features in the late Universe.

The cosmic birefringence effect, i.e., the in-vacuum rotation of the linear polarization plane of photons during propagation, probes new parity-violating physics beyond standard electromagnetism. Recent investigations of Planck data provide hints of the detection of the isotropic part of this phenomenon at a 3\( \sigma \) confidence level; see, e.g., [Minami:2020odp,Diego-Palazuelos:2022dsq]. I will review the status of these analyses performed with CMB data and describe the main difficulties in constraining such an effect [Planck:2016soo]. Additionally, I will comment on a related phenomenon, the anisotropic cosmic birefringence effect, which can provide further insights into the new physics that might be responsible for this effect. Forecasts for future CMB missions regarding this effect will be given.

The Sun may capture asymmetric dark matter (DM), which can subsequently form bound-states through the radiative emission of a sub-GeV scalar. This process enables generation of scalars without requiring DM annihilation. In addition to DM capture on nucleons, the DM-scalar coupling responsible for bound-state formation also induces capture from self-scatterings of ambient DM particles with DM particles already captured, as well as with DM bound-states formed in-situ within the Sun. This scenario is studied in detail by solving Boltzmann equations numerically and analytically. In particular, we take into consideration that the DM self-capture rates require a treatment beyond the conventional Born approximation. We show that, thanks to DM scatterings on bound-states, the number of DM particles captured increases exponentially, leading to enhanced emission of relativistic scalars through bound-state formation, whose final decay products could be observable. We explore phenomenological signatures with the example that the scalar mediator decays to neutrinos. We find that the neutrino flux emitted can be comparable to atmospheric neutrino fluxes within the range of energies below one hundred MeV. Future facilities like Hyper-K, and direct DM detection experiments can further test such scenario.

I will discuss dark matter neutron stars in a class of models [1] where the dark sector is a copy of the Minimal Supersymmetric Standard Model, but with the important difference that the dark supersymmetry-breaking scale is much higher than in the visible sector. This scenario allows the quark mass hierarchy to be different from the visible sector, resulting in a stable dark neutron which can play the role of cold dark matter on large scales, while dark dissipative cooling allows additional collapse of overdensities on small scales. At late times these become either small black holes or dark neutron stars. Possibilities for detection include microlensing observations, and a visible luminosity due to kinetic mixing between the dark and visible photons.

In order to shed light on the quantum to classical transition of the primordial perturbations during inflation (see, e.g.,[2]), we investigated, in [1], the decoherence of a system of scalar curvature perturbations induced by an unobservable environment of deep subhorizon tensorial modes. We computed the associated corrections to the cosmological correlation functions, looking for distinguishable signatures which could, in future observations, prove the quantum origin of primordial perturbations. In doing this, we commented on proposed techniques to deal with non-Markovianity (i.e. memory effects in the environment, which drastically complicates calculations), which seems to be ubiquitous in an inflationary framework.

I will discuss a relativistic framework to investigate the evolution of cosmological structures from the initial density perturbations to the highly non-linear regime. Our approach involves proposing a procedure to match 'best-fit', exact Bianchi IX (BIX) spacetimes to finite regions within the perturbed Friedmann-Lemaitre-Robertson-Walker universe characterized by a positive averaged spatial curvature. This method enables us to approximately track the non-linear evolution of the initial perturbation using an exact solution. By employing the BIX symmetries, we can systematically incorporate the approximate effects of shear and curvature into the process of collapse.

Many cosmological observables derive from primordial vacuum fluctuations evolved to late times. These observables represent statistical draws from an underlying quantum or statistical field-theoretic framework where infinities arise and require renormalization. We review this process on backgrounds that transition from finite duration inflation to radiation domination, and show how, in spite of the ubiquity of scaleless integrals, ultra-violet (UV) divergences can still be meaningfully extracted from quantities that nominally vanish when dimensionally regularized. Furthermore, we contextualize calculations with hard cutoffs, distinguishing between physical UV and infra-red (IR) scales and UV and IR scales corresponding to the unknown completion of the theory.

Focusing then on vacuum tensor perturbations, we stress the need to regularize stress tensors that do not presume a prior scale separation in their definition (as with the standard Isaacson form), deriving an improved stress tensor fit for this purpose. We then present the details for the covariant renormalization of the stress tensor for vacuum tensor perturbations at the level of the effective action, adopting Hadamard regularization techniques to isolate short-distance divergences and gauge fixing via the Faddeev-Popov procedure.\\ With these ingredients at hand, we question the possibility of constraining vacuum tensor perturbations with upper bounds on the effective number of species \( N_{\rm eff} \) derived from CMB data. This talk is based on:[1,2]

I will discuss how the new BAO measurements by the DESI collaboration have changed the status of the so-called Hubble tension, i.e. the mismatch between global cosmological measurements and local measurements of H_0 (by the SH0ES Collaboration). In particular, in models with Dark Radiation the tension decreases to a moderate level, between 2 and 3 sigma, depending on the specific realization. This allows a combination of cosmological data with the local SH0ES measurement, leading to a 4-5 sigma evidence for a new Dark Radiation component. I will also discuss Dark Energy and neutrino mass fits with the new DESI dataset.

Using a fundamental relation, as suggested by Ernst Mach, between the masses of elementary particles and the cosmological constant, we obtain the value of the mass of the electron from the observable value of the cosmological constant and by using a length constructed with the constant of interaction of Fermi process.

This talk is based on articles [1,2]. System of equations for gravitational wave (GW) conversion into electromagnetic waves under influence of external magnetic field is derived over arbitrary curved spacetime. After that the system is simplifyed for the case of FLRW metric and evaluation of the effect influense on relic GW amplitude is conducted up to the end of radiation dominance epoch. To simplify estimates, we use a homogeneous cosmological field model. Finally the conclusion is done that due to photon interaction with the primary plasma the effect becomes negligible for the low-frequensy relic GWs.

We revisit Weyl's unified field theory, which arose in 1918, shortly after general relativity was discovered. As is well known, in order to extend the program of geometrization of physics started by Einstein to include the electromagnetic field, H. Weyl developed a new geometry which constitutes a kind of generalization of Riemannian geometry. In this paper, our aim is to discuss Weyl's proposal anew and examine its consistency and completeness as a physical theory. We propose new directions and possible conceptual changes in the original work. Among these, we investigate with some details the propagation of gravitational waves, and the new features arising in this recent modified gravity theory, in which the presence of a massive vector field appears somewhat unexpectedly. We also speculate whether the results could be examined in the context of primordial gravitational waves.

In the \( \Lambda \)CDM cosmological model, the Universe is assumed to be isotropicand homogeneous when averaged on large scales. That the CMB has a dipole anisotropy is interpreted as due to our peculiar (non-Hubble) motion because of local inhomogeneity [1]. There must then be a corresponding dipole in the sky distribution of sources at high redshift [2]. Using catalogues of radio sources and quasars we find that this expectation is rejected at \( >5\sigma \) [3], i.e. the distribution of distant matter is not isotropic in the `CMB frame'. This calls into question the standard practice of boosting to this frame to analyse cosmological data, in particular to infer acceleration of the Hubble expansion rate using Type Ia supernovae, which is interpreted as due to a Cosmological Constant \( \Lambda \).

This talk is based on Refs. [1] and [2], which respectively studied gravitational waves and non-relativistic stars in higher-curvature theories of gravity. Our gravity theory is specified by the Einstein–Hilbert Lagrangian \( R \) plus extra \( R^2 \) and \( C_{\mu\nu\rho\sigma}\,C^{\mu\nu\rho\sigma} \) terms. We first prove that the gravitational waves in such a theory exhibit six additional massive helicity (polarization) modes on top of the ordinary massless helicity-\( 2 \) mode. We then derive the governing equation in the same theory for a non-relativistic polytropic fluid in spherically symmetric static configurations and study the resulting stellar structures using the analytic solutions for the polytropic indices \( n = 0, 1 \).

Spectral distortions in the Cosmic Microwave Background (CMB) could have been induced by multiple astrophysical and cosmological processes. One such cosmological mechanism that could source the CMB spectral distortions is the acoustic dissipation of inflationary fluctuations. Owing to the possibility of large enhancement in the primordial scalar power spectrum at small scales thereby leading to the formation of Primordial Black Holes, the plausibility of Gaussian initial conditions is highly questionable. The talk will focus on how spectral distortions vary when one incorporates primordial non-Gaussianity either in perturbative or non-perturbative limits. This will be accompanied by a detailed comparison of various approximation schemes to compute the distortions. Further discussion will show how this picture has direct implications for the constraints on Supermassive Black Holes if they were to be of primordial origin.

Measuring tensor to scalar ratio \( r \) is considered to be an evidence favouring inflation since popular alternatives predicts r well below the limits of future experiments. Previous works have shown that a bouncing Universe with sourced fluctuations allows for nearly scale-invariant spectra of both scalar and tensor perturbations [1,2]. Past works have analyzed the model until the bounce, under the assumption that the bounce will not change the final predictions. In our latest work we follow the evolution of the Universe and fluctuations across the bounce until reheating [3]. The bounce enhances scalar spectrum while leaving tensor spectrum unchanged. The enhancement depends on the duration of the bounce - a shorter bounce implies a larger enhancement. The model matches current observations and predicts any viable tensor-to-scalar ratio \( r\leq10^{-2} \), which may be observed in upcoming CMB experiments.

This talk is based on [1,2,3]. We consider the decay of a particle with some energy \( E_{0 } > 0 \) inside the ergosphere of a black hole. After the first decay, one of particles with the energy \( E_{1 } < 0 \) falls towards a black hole while the second one with \( E_{2 } > E_{0} \) moves in the outward direction. It bounces back from a reflectin shell and, afterwards, the process repeats. For radial motion of charged particles in the Reissner-Nordstom metric, the result depends strongly on a concrete scenario. In particular, an indefinitely large growth of energy inside a shell is possible that gives rise to a black-hole bomb. We also consider a similar multiple process with neutral particles in the background of a rotating axially symmetric stationary black hole. We demonstrate that, if particle decay occurs in the turning point, a black-hole bomb in this case is impossible at all. For a generic point inside the ergoregion, there is a condition for a black-hole bomb to exist. It relates the ratio of masses before and after decay and the velocity of a fragment in the center of mass frame. One more process (decay near naked singuarity( is also considered).