CyberSec Research

The Cosmological Principle posits that the Universe is isotropic on the largest scales. While widely supported, this foundational assumption remains testable. We analyse the angular distribution of over one million quasars from the Gaia-unWISE catalogue using Renyi entropy, a multiscale statistical measure sensitive to higher-order clustering. Dividing the sample into three redshift bins, we find that both the low- and high-redshift distributions are statistically consistent with isotropy. However, at intermediate redshift ($1 \leq z < 2.2$), we detect a statistically significant and scale-dependent anisotropy that persists under stringent masking, suggesting a physical origin. We interpret this as evidence for a transitional epoch in cosmic history, during which large-scale structures such as superclusters became prominent before their growth was gradually damped by the onset of accelerated expansion. These findings position Renyi entropy as a powerful probe of cosmic evolution and highlight the potential thermodynamic links between structure formation, entropy dissipation, and the emergence of large-scale isotropy.

We examine the joint effects of relativistic accretion and memory burdened evaporation on the evolution of primordial black holes (PBHs). The memory burden effect, which delays the evaporation by inducing a backreaction and making the evaporation rate scale as an inverse power law of the PBH entropy, opens up a new window that allows PBHs with $M \lesssim 10^{15}~\mathrm{g}$ to survive until the present epoch. Meanwhile, accretion increases the mass of PBHs, thereby enhancing their chances of survival for a given initial mass. We consider two main scenarios: one where PBHs evaporate completely before big bang nucleosynthesis, and another where PBHs persist until today. In the case of evaporation, we analyse the emission of dark matter (DM) and dark radiation (DR) during the process of evaporation. Conversely, in the other case, the surviving PBHs themselves can contribute as DM. We further investigate how relativistic and non-relativistic accretion, together with memory burdened evaporation, impact the parameter space of the emitted DM, the abundance of stable PBHs as DM, and the contribution of DR to the effective number of relativistic degrees of freedom, $\Delta N_{\mathrm{eff}}$.

Holography and entropy bounds suggest that the ultraviolet (UV) and infrared (IR) cutoffs of gravitational effective theories are related to one another as a form of UV/IR mixing. Motivated by this, we derive a bound on the allowed scalar field range in theories with cosmic horizons. We show how this bound challenges several inflationary scenarios, such as $\alpha$-attractors and modular-invariant inflation. Besides, we find a relation between the number of extra spatial dimensions and the tensor-to-scalar ratio.

Parity-violation leaves tell-tale trails in many cosmological observables. We illustrate parity-odd primordial scalar trispectra, that despite being of modest strength, impart detectable chirality to scalar-induced gravitational waves (SIGW). This allows us to impose strong bounds on the parity-odd part of trispectrum. Over certain scales, we find SIGW directly quantify parity-violation in primordial non-Gaussianity, unobscured by the Gaussian contribution. Our results call for treatment of SIGW and parity-odd trispectrum as complementary predictions of parity-violating theories.

We investigate cosmic structure formation in the framework of a path-integral formulation of an $N$-particle ensemble in phase space, dubbed Resummed Kinetic Field Theory (RKFT), up to one-loop perturbative order. In particular, we compute power spectra of the density contrast, the divergence and curl of the momentum density and arbitrary $n$-point cumulants of the stress tensor. In contrast to earlier works, we propose a different method of sampling initial conditions, with a Gaussian initial phase-space density. Doing so, we exactly reproduce the corresponding results from Eulerian standard perturbation theory (SPT) at one-loop order, showing that formerly found deviations can be fully attributed to inconsistencies in the previous sampling method. Since, in contrast to SPT, the full phase-space description does not assume a truncation of the Vlasov hierarchy, our findings suggest that non-perturbative techniques are required to accurately capture the physics of cosmic structure formation.

We consider a model of inflation in which the inflaton field is a rolling axion with a potential which is flat enough to support an intermediate phase of USR inflation. Because of the Chern-Simons interaction, one polarization of the gauge field experiences the tachyonic growth during the first slow-roll stage, inducing large corrections in curvature perturbations via the inverse decay effect. A non-trivial feature of our setup is that once the system enters the USR phase, the instability parameter falls off rapidly, terminating the gauge field production efficiently. Consequently, the power spectrum involves two separate peaks, the first peak is induced by the gauge field particles production while the second peak is due to standard USR mechanism. We show that the power spectrum at the intermediate scales develops strong scale-dependence $\propto k^m$ with the index $m >4$. Calculating the bispectrum, we demonstrate that non-Gaussianities with non-trivial shapes and multiple peaks are generated in this setup.

Probing the nature of dark matter (DM) remains an outstanding problem in modern cosmology. The 21 cm signal, as a sensitive tracer of neutral hydrogen during cosmic dawn, provides a unique means to investigate DM nature during this critical epoch. Annihilation and decay of DM particles, as well as Hawking radiation of primordial black holes (PBHs), can modify the thermal and ionization histories of the early universe, leaving distinctive imprints on the 21 cm power spectrum. Therefore, the redshifted 21 cm power spectrum serves as a powerful tool to investigate such DM processes. In this work, we systematically assess the potential of the upcoming Square Kilometre Array (SKA) to constrain DM and PBH parameters using the 21 cm power spectrum. Assuming $10,000$ hours of integration time, the SKA is projected to reach sensitivities of $\langle\sigma v\rangle \leq 10^{-28}\,{\rm cm}^{3}\,{\rm s}^{-1}$ and $\tau\geq 10^{28}\,{\rm seconds}$, for $10\,{\rm GeV}$ DM particles. It can also probe PBHs with masses of $10^{16}\,\mathrm{g}$ and abundances $f_{\mathrm{PBH}} \leq 10^{-6}$. These results indicate that the SKA could place constraints on DM annihilation, decay, and PBH Hawking radiation that are up to two to three orders of magnitude stronger than current limits. Furthermore, the SKA is expected to exceed existing bounds on sub-GeV DM and to probe Hawking radiation from PBHs with masses above $10^{17}\,{\rm g}$, which are otherwise inaccessible by conventional cosmological probes. Overall, the SKA holds great promise for advancing our understanding of both DM particles and PBHs, potentially offering new insights into the fundamental nature of DM.

We classify $f(R)$ theories using a mathematical analogy between slow-roll inflation and the renormalization-group flow. We derive the power spectra and spectral indices class by class and compare them with the latest data. The framework used for the classification allows us to determine the general structure of the $f(R)$ functions that belong to each class. Our main result is that only two classes survive. Moreover, we show that the Starobinsky model is the only polynomial $f(R)$ that can realize slow-roll inflation. In fact, all other polynomials belong to a special class that can only realize constant-roll inflation, at least far enough in the past. We point out some of the issues involved in considering a smooth transition between constant-roll and slow-roll inflation in this class of models. Finally, we derive the map that transforms the results from the Jordan frame to the Einstein frame.

A fast-rolling axion can transfer its kinetic energy to a gauge field via the Chern-Simons coupling, leading to copious production of gauge quanta, which can act as a source of gravitational waves (GWs) with potentially observable amplitudes. In this work, we investigate GW production in a spectator axion model when strong backreaction is taken into account. We find that decreasing the decay constant of the axion enhances GW production. Since the initial value of the axion is larger than its quantum fluctuations, such a condition imposes a lower bound on the axion dacay constant, which sets an upper bound on the amplitude of the energy spectrum of GWs. As a result, the amplitude of the predicted GW energy spectrum is lower than $10^{-10}$ in the nHz to mHz frequency range.

Most theories that attempt to reproduce the Modified Newtonian Dynamics (MOND) phenomenology for dark matter at galactic scales rely on ad hoc free functions, preventing them from being regarded as fundamental. In this work, we present a new theory that reproduces MOND, built on a supposed to be fundamental Yang-Mills gauge field based on SU(2), with a gravitational coupling constant, and emerging in a low-acceleration regime, below the MOND acceleration scale. The gauge field plays the role of the internal force in the dipolar dark matter (DDM) model. We discuss how certain solutions of this theory recover the deep MOND regime without introducing arbitrary functions in the action. Within this framework, the MOND phenomenology appears to be due to the existence of a new sector of particle physics.

We investigate the clustering of Lyman-break galaxies at redshifts of 3 $\lesssim z \lesssim$ 5 within the COSMOS field by measuring the angular two-point correlation function. Our robust sample of $\sim$60,000 bright ($m_{\rm UV}\lesssim 27$) Lyman-break galaxies was selected based on spectral energy distribution fitting across 14 photometric bands spanning optical and near-infrared wavelengths. We constrained both the 1- and 2-halo terms at separations up to 300 arcsec, finding an excess in the correlation function at scales corresponding to $<20$ kpc, consistent with enhancement due to clumps in the same galaxy or interactions on this scale. We then performed Bayesian model fits on the correlation functions to infer the Halo Occupation Distribution parameters, star formation duty cycle, and galaxy bias in three redshift bins. We examined several cases where different combinations of parameters were varied, showing that our data can constrain the slope of the satellite occupation function, which previous studies have fixed. For an $M_{\rm{UV}}$-limited sub-sample, we found galaxy bias values of $b_g=3.18^{+0.14}_{-0.14}$ at $z\simeq3$, $b_g=3.58^{+0.27}_{-0.29}$ at $z\simeq4$, $b_g=4.27^{+0.25}_{-0.26}$ at $z\simeq5$. The duty cycle values are $0.62^{+0.25}_{-0.26}$, $0.40^{+0.34}_{-0.22}$, and $0.39^{+0.31}_{-0.20}$, respectively. These results suggest that, as the redshift increases, there is a slight decrease in the host halo masses and a shorter timescale for star formation in bright galaxies, at a fixed rest-frame UV luminosity threshold.

The local primordial density fluctuations caused by quantum vacuum fluctuations during inflation grow into stars and galaxies in the late universe and, if they are large enough, also produce primordial black holes. We study the formation of the primordial black holes in $k$-essence inflation models with a potential characterized by an inflection point. The background and perturbation equations are integrated numerically for two specific models. Using the critical collapse and peaks formalism, we calculate the abundance of primordial black holes today.

We investigated the widths and profiles of cosmic filaments using the IllustrisTNG simulations. Filaments were identified with DisPerSE, using galaxy samples in simulations as input. Since the width of an individual filament can vary significantly along its spine, we divided each filament into segments with lengths between $1.5\,h^{-1}\,\mathrm{Mpc}$ and $2.5\,h^{-1}\,\mathrm{Mpc}$ and measure their properties. The typical width of these filament segments increases gradually from approximately $0.3\,\mathrm{Mpc}$ at redshift $z = 2.0$ to about $1.0-1.5\,\mathrm{Mpc}$ at $z = 0.0$. We find that the segment width correlates nearly linearly with the linear halo mass density, consistent with previous studies. A similar linear relation is observed between the segment width and the linear stellar mass density, providing a potential estimator for filament width. Furthermore, the density profiles of filaments with different widths exhibit self-similarity and can be described by a unified formula akin to the isothermal $\beta$-model. For segments with a given width, the rescaled density profiles show only mild evolution from $z = 2.0$ to $z = 0.0$. Within the filament width, the gas temperature decreases slowly from the center to the boundary, with thicker filaments generally containing hotter gas than thinner ones. These trends in filament width, density, and thermal profiles are consistently observed across the TNG50, TNG100, and TNG300 simulations, and align well with results from earlier studies. We briefly discuss the potential implications and applications of our findings.

Low-temperature detectors are a powerful technology for dark matter search, offering excellent energy resolution and low energy thresholds. COSINUS is the only experiment that combines scintillating sodium iodide (NaI) crystals with an additional phonon readout at cryogenic temperatures, using superconducting sensors (remoTES), alongside the conventional scintillation light signal. Via the simultaneous phonon and scintillation light detection, a unique event-by-event particle identification is enabled. This dual-channel approach allows for a model-independent cross-check of the long-standing DAMA/LIBRA signal with a moderate exposure of a few hundred kg d, while completely avoiding key systematic uncertainties inherent to scintillation-only NaI-based searches. COSINUS built and commissioned a dedicated low-background cryogenic facility at the LNGS underground laboratories. Data taking with eight NaI detector modules (COSINUS1$\pi$ Run1) is planned to begin in late 2025.

In earlier companion papers, we showed that non-singular primordial black holes (PBHs) could account for all the dark matter (DM) over a significantly wider mass range compared to Schwarzschild PBHs. Those studies, mostly based on phenomenological metrics, are now extended by considering the quantum-corrected space-time recently proposed by Zhang, Lewandowski, Ma and Yang (ZLMY), derived from an effective canonical (loop) quantum gravity approach explicitly enforcing general covariance. Unlike the BHs considered earlier, ZLMY BHs are free from Cauchy horizons, and are hotter than their Schwarzschild counterparts. We show that this higher temperature boosts the evaporation spectra of ZLMY PBHs, tightening limits on their abundance relative to Schwarzschild PBHs and shrinking the asteroid mass window where they can constitute all the DM, a result which reverses the earlier trend, but rests on firmer theoretical ground. While stressing the potential key role of quantum gravity effects in addressing the singularity and DM problems, our study shows that working within a consistent theoretical framework can strongly affect observational predictions.

The Milky Way Survey of the Dark Energy Spectroscopic Instrument (DESI) has so far observed three classical dwarf spheroidal galaxies (dSphs): Draco, Sextans and Ursa Minor. Based on the observed line-of-sight velocities and metallicities of their member stars, we apply the axisymmetric Jeans Anisotropic Multi-Gaussian Expansion modeling (JAM) approach to recover their inner dark matter distributions. In particular, both the traditional single-population Jeans model and the multiple population chemodynamical model are adopted. With the chemodynamical model, we divide member stars of each dSph into metal-rich and metal-poor populations. The metal-rich populations are more centrally concentrated and dynamically colder, featuring lower velocity dispersion profiles than the metal-poor populations. We find a diversity of the inner density slopes $\gamma$ of dark matter halos, with the best constraints by single-population or chemodynamical models consistent with each other. The inner density slopes are $0.71^{+0.34}_{-0.35}$, $0.26^{+0.22}_{-0.12}$ and $0.33^{+0.20}_{-0.16}$ for Draco, Sextans and Ursa Minor, respectively. We also present the measured astrophysical J and D factors of the three dSphs. Our results indicate that the study of the dark matter content of dSphs through stellar kinematics is still subject to uncertainties behind both the methodology and the observed data, through comparisons with previous measurements and data sets.

We propose a novel mechanism for realizing slow-roll inflation that is fully consistent with observational data, based on conformal transformations acting exclusively on a complex scalar field -- without coupling to the gravitational sector. These transformations generically produce a plateau in the inflaton potential, as guaranteed by the maximum modulus theorem, thereby naturally satisfying the slow-roll conditions. Our framework utilizes squeezing operations generated by the Virasoro algebra without central extension, as developed in our earlier work. The resulting inflationary potentials depend on the Virasoro mode $n$, the power $m$ of the original potential, and the squeezing parameter $\theta$. We present approximate analytical expressions at leading order for the special case $n=-2$, and perform numerical analyses for both $n=-2$ and other values of $n$. These reveal parameter regimes in which the predicted cosmological observables $(n_{s},r)$ align remarkably well with current CMB measurements.

We develop a new class of cosmological attractors which are compatible with the recent ACT results. They are based on two types of fractional Kaehler potentials, K, for a gauge-singlet inflaton phi which reduce, along the inflationary path, to the form N/(1-phi^qM)^p with qM=1, 2 and 0.1< p<10. The combination of these K's with the chaotic potentials phi^n (where n=2, 4) within a non-linear sigma model leads to inflationary observables which are consistent with the current data and largely independent from qM and n. Endowing these K's with a shift symmetry we also offer a supergravity realization of our models introducing two chiral superfields and a monomial superpotential, linear with respect to the inflaton-accompanying field. The attainment of inflation with subplanckian inflaton values and the large values for the tensor-to-scalar ratio, which increases with N, are two additional attractive features of our proposal.