Search

In a recent paper (Lacombe, Mukohyama, and Seitz, JCAP {\bf 2024}, 05, 064 (2024)), the authors provided an in-depth analysis of a class of modified gravity theories, generally called $f(R,{\rm Matter})$ theories, which assume the existence of a non-minimal coupling between geometry and matter. It was argued that if the matter sector consists of Standard Model particles, then these theories suffer from the presence of ghosts, or are just scalar/vector-tensor theories. Hence, the relevance of these theories for cosmology was questioned. It is the goal of the present work to carefully analyze, discuss, and assess the line of arguments proposed in Lacombe et al. JCAP {\bf 2024}, 05, 064 (2024). After a qualitative critical discussion of the five general arguments proposed for the validity of a gravitational theory, we present the theoretical foundations of the $f(R,{\rm Matter})$ theories, including their possible relations with quantum gravity, and discuss in detail the role of matter. The matter source discussed in Lacombe et al., consisting predominantly of a massless scalar field, is extremely restrictive, and rather irrelevant for cosmology and the description of the observational data. We also devote a detailed discussion of the problem of the energy scales of the $f(R,{\rm Matter})$ theories. To test the observational relevance of this type of theories we present the comparison of a simple theoretical model with a small set of observational data and with the $\Lambda$CDM paradigm. We conclude by pointing out that the analysis of Lacombe et al., JCAP {\bf 2024}, 05, 064 (2024), even very useful for the understanding of some limited aspects of the $f(R,{\rm Matter})$ theories, and of their theoretical foundations, cannot be considered as a valid or definite criticism of these approaches to gravity., Comment: 20 pages, 3 figures

The Cosmic Distance Duality Relation (CDDR) defines a fundamental link in modern cosmology between geometric distance measurements and redshift. Any observed deviations from the CDDR could suggest the need for new physics beyond the $\Lambda$CDM model. In this study, we model geometric distances (both luminosity and angular) using a cosmographic expansion with the Pade method, which stabilizes the behavior of the cosmographic series at high redshifts. This approach allows us to determine geometric distances independently of any specific model. By incorporating updated data from supernovae (SN), baryon acoustic oscillations (BAO), and cosmic chronometers (CC), we obtain observational constraints for parametric models that quantify the CDDR, alongside all baseline cosmographic parameters. We find that potential CDDR violations introduce new statistical correlations in the cosmographic parameters ($H_0$, $q_0$, and $j_0$). However, within this framework, we do not observe significant deviations from the CDDR, and our results remain consistent with the predictions of the $\Lambda$CDM model. This work offers a novel and straightforward method for testing the CDDR by fixing background evolution through cosmographic techniques, thereby paving the way for new geometric observational tests of possible deviations from standard cosmology., Comment: 9 pages, 5 figures

It has been proposed that the accelerated expansion of the universe can be explained by the merging of our universe with baby universes, resulting in dark energy with a phantom-like equation of state. However, the evidence in favor of it did not include the full set of cosmological observables. Here we examine the implications of this model for both early and late universe cosmology using data from Planck collaboration, DESI 2024 and other experiments. We find that the pure baby universe model gives a poor fit to current data. Extending it to include a contribution from the cosmological constant, we find two allowed regions of parameter space: one close to $\Lambda$CDM, and another with $\Lambda <0$ plus the exotic dark energy component. The two regions can be significantly favored over $\Lambda$CDM, depending on the choice of supernova datasets, and they can ameliorate the Hubble tension to the level of $2\sigma$, depending on the supernova dataset. The model with $\Lambda<0$ features an equation of state $w(a)$ with a pole singularity at early times., Comment: 7 pages, 7 figures

The low-energy limit of string theory contains additional gravitational degrees of freedom, a skew-symmetric tensor $B$-field and a scalar dilaton, that are not present in general relativity. Together with the metric, these three fields are naturally embedded in the $\mathbf{O}(D,D)$-symmetric framework of double field theory. The $\mathbf{O}(D,D)$ symmetry uniquely prescribes the interactions between the extended gravitational sector and other matter, leading to novel features beyond conventional string cosmology. In this work we present the equations of motion for linear perturbations around $\mathbf{O}(D,D)$ string cosmological backgrounds in $D=4$ under a scalar-vector-tensor decomposition. We obtain analytic solutions in the superhorizon limit for scalar perturbations around various homogeneous and isotropic background solutions, including some candidate models for bouncing cosmologies. The generalized energy-momentum tensor includes source terms for the $B$-field and dilaton, and we show how the resulting generalized conservation laws modify the conditions for conservation of curvature perturbations., Comment: 41 + 16 pages (main body + title, appendices, and references)

Although fast radio bursts (FRBs) were discovered more than a decade ago, and they have been one of the active fields in astronomy and cosmology, their origins are still unknown. An interesting topic closely related to the origins of FRBs is their classifications. On the other hand, FRBs are actually a promising probe to study cosmology. In the literature, some new classifications of FRBs different from repeaters and non-repeaters were suggested, and some tight empirical relations have been found for them. In particular, Guo and Wei suggested to classify FRBs into the ones associated with old or young populations, which have also some new empirical relations. They also proposed to use one of the empirical relations without dispersion measure (DM) to calibrate FRBs as standard candles for cosmology. This shows the potential of the new classification and the empirical relations for FRBs. Nowadays, more than 50 FRBs have been well localized, and hence their redshifts $z$ are observationally known. Thus, it is time to check the empirical relations with the current localized FRBs. We find that many empirical relations still hold, and in particular the one used to calibrate FRBs as standard candles for cosmology stands firm., Comment: 15 pages, 2 tables, 7 figures, revtex4

We have discussed in detail how neutrinos produced from inflaton solely through gravitational interaction can successfully reheat the universe. For this, we have introduced the well-known Type-I seesaw neutrino model. Depending on seesaw model parameters, two distinct reheating histories have been realized and dubbed as i) Neutrino dominating: Following the inflaton domination, the universe becomes neutrino dominated, and their subsequent decay concludes the reheating process, and ii) Neutrino heating: Despite being sub-dominant compared to inflaton energy, neutrinos efficiently heat the thermal bath and produce the radiation dominated universe. Imposing baryon asymmetric yield, the $\Delta N_{\rm eff}$ constraint at Big Bang Nucleosynthesis (BBN) considering primordial gravitational waves (PGW), we have arrived at the following constraints on reheating equation of state to lie within $0.5\lesssim w_\phi\lesssim1.0$. In these neutrino-driven reheating backgrounds, we further performed a detailed analysis of both thermal and non-thermal production of dark matter (DM), invoking two minimal models, namely the Higgs portal DM and classical QCD pseudo scalar axion. An interesting correlation between seemingly uncorrelated DM and Type-I seesaw parameters has emerged when confronting various direct and indirect observations. When DMs are set to freeze-in, freeze-out, or oscillate during reheating, new parameter spaces open, which could be potentially detectable in future experiments, paving an indirect way to look into the early universe in the laboratory., Comment: 24 pages,13 figures, 3 tables

In Cosmology, when dissipative effects are minimal, the energy content of the Universe can be effectively described as a sum of perfect fluids. Perfect fluid descriptions ensure thermal equilibrium since they equilibrate immediately. However, interactions among different energy content of the Universe might prevent such rapid equilibration. This limitation calls for a more fundamental framework that incorporates these interactions directly at the level of the action. In earlier work, two of the authors demonstrated that an interacting dark energy (DE)-dark matter (DM) field theory action could be derived from a modified gravity action via a conformal transformation, establishing a one-to-one correspondence between the field theory action and fluid for a unique interaction term [arXiv:2006.04618]. In this work, we extend that analysis by considering quadratic order Horndeski gravity, identifying two classes of models: field coupling and field-kinetic coupling. Our approach generalizes the coupling function for DE-DM interactions by incorporating an additional dependence on kinetic terms. We establish a field-to-fluid mapping for dark matter and find that this mapping only holds for a specific form of the interaction strength. Interestingly, we show that this interaction strength excludes non-gravitational interactions between dark energy and dark radiation. Numerical analysis reveals that purely kinetic interactions within the dark sector can significantly alter cosmological evolution compared to non-interacting scenarios, highlighting the strong dependence of cosmological dynamics on coupling strength. A preliminary examination of linear scalar perturbations indicates that the field-kinetic coupling results in a non-zero gravitational slip parameter and momentum exchange., Comment: 48 pages, 15 figures, 3 tables

In this manuscript, we investigate the possibility of constructing anisotropic dark matter compact stars motivated by the Einasto density profile. This work develops analytical solutions for an anisotropic fluid sphere within the framework of the well-known Adler-Finch-Skea metric. This toy model incorporates an anisotropic fluid distribution that includes a dark matter component. We use the minimal geometric deformation scheme within the framework of gravitational decoupling to incorporate anisotropy into the pressure profile of the stellar system. In this context, we model the temporal constituent of the $\Theta$-field sector to characterize the contribution of dark matter within the gravitational matter source. We present an alternative approach to studying anisotropic self-gravitating structures. This approach incorporates additional field sources arising from gravitational decoupling, which act as the dark component. We explicitly verify whether the proposed model satisfies all the requirements for describing realistic compact structures in detail. We conclude that the modeling of the Einasto density model with the Adler-Finch-Skea metric gives rise to the formation of well-behaved and viable astrophysical results that can be employed to model the dark matter stellar configurations., Comment: 25 pages, 13 figures, version accepted for publication in Physics of the Dark Universe

The prospect of detecting/constraining deviations from general relativity by studying gravitational waves (GWs) from merging black holes has been one of the primary motivations of GW interferometers like LIGO/Virgo. Within pure gravity, the only possible way deviations can arise is from the existence of higher order derivative corrections, namely higher powers of the Riemann curvature tensor, in the effective action. Any observational bounds imply constraints on the corresponding Wilson coefficients. At the level of the action, one can imagine the coefficients are sufficiently large so as to be in principle detectable. However, from the point of view of some fundamental principles, namely causality and unitarity, this is much less clear, as we examine here. We begin by reviewing certain known bounds on these coefficients, which together imply a low cut off on the effective theory. We then consider a possible mechanism to generate such terms, namely in the form of many minimally coupled light scalars that can be integrated out to give these higher order operators. We show that a by product of this is the generation of quantum corrections to Newton's potential, whose observable consequences are already ruled out by solar system tests. We point out that over 7 orders of magnitude of improvement in interferometer sensitivity would be required to avoid such solar system constraints., Comment: 23 pages, 2 figures

In this work we consider a dark matter candidate described by an ultralight vector field, whose mass is in principle in the range $H_{\rm{eq}}\sim 10^{-28}\rm{eV}\ll m< \rm{eV}$. The homogeneous background vector field is assumed to point in a given direction. We present a numerical implementation of cosmological perturbations in a Bianchi type I geometry with vector field dark matter in a modified version of the Cosmic Linear Anisotropy Solving System (CLASS). We study the evolution of large-scale cosmological perturbations in the linear regime. We compute the matter power spectrums defined for Fourier modes pointing in a given direction. We obtain interesting features in the power spectrums whose observational significance depends on the field mass. We compare the results with the standard $\rm{\Lambda CDM}$ and with the corresponding well-studied ultralight scalar field dark matter case. As for the scalar case we obtain a suppression in the power spectrums at small scales characterized by the same scale, namely the Jeans scale. The main characteristic feature of the vector field model we notice here for first time is that the amplitude of the suppression effect depends on the direction of the Fourier modes with respect to the background vector field, leaving eventually a possible anisotropic imprint in structure formation at small scales., Comment: 19 pages, 9 figures

In this study, we analyse constraints on the growth index of matter perturbations, $\gamma$, within the framework of $f(Q)$ gravity, using recent cosmological observations, at the background and the perturbation levels, including Pantheon$^{+}$, Cosmic Chronometer (CC), and Redshift Space Distortion (RSD) datasets. Our analysis focuses on quantifying the distortion parameter, which measures the deviation of the $f(Q)$ gravity model from the concordance $\Lambda$CDM cosmology at the background level. Specifically, we investigate two cases of the growth index parameter: a constant $\gamma$ and a time-varying $\gamma(z)$. We investigate various parametrizations of the growth index $\gamma$, expressed as $\gamma = \gamma_{0} +\gamma_{1} y(z)$, where the function $y(z)$ assumes different forms, including constant ($\Gamma_{0}$), Taylor expansion around $z = 0$ ($\Gamma_{1}$), Taylor expansion around the scale factor ($\Gamma_{2}$), and an exponential form ($\Gamma_{3}$). By employing the Akaike Information Criterion and Bayesian Information Criterion, we find that the combined Pantheon$^{+}$+ CC+ RSD datasets impose stringent constraints on the value of the growth index. For the $\Gamma_{0}$ model, our results indicate that within the concordance $\Lambda$CDM model, $\gamma$ is constrained to $0.545 \pm 0.096$, showing strong agreement with the theoretical expectation of $\gamma_{\Lambda} = \frac{6}{11}$. However, within the framework of $f(Q)$ gravity, we observe $\gamma = 0.571^{+0.095}_{-0.110}$, slightly exceeding the $\Lambda$CDM value by 4.66 $\%$. Furthermore, when considering a time-varying growth index, our analysis reveals that the range of $\gamma_{0}$ spans from $0.596$ to $0.62$ across the $\Gamma_{1-3}$ models., Comment: 15 pages, 6 figures

In this study, we investigate the impact of modified gravity on the merger rate of compact binaries within dark-matter spikes surrounding super-massive black holes (SMBHs). Specifically, we calculate binary merger rates involving primordial black holes (PBHs) and/or neutron stars (NSs) in Hu-Sawicki $f(R)$ gravity and the normal branch of Dvali-Gabadadze-Porrati (nDGP) gravity, with three SMBH mass functions, Benson, Vika, and Shankar. The results show consistently higher merger rates predicted for PBH-PBH and PBH-NS binaries in these gravity models compared to general relativity (GR), in particular at lower SMBH masses and for steeper dark-matter spike density profiles. The predicted merger rates are compared to the LIGO-Virgo observations to constrain the parameters of the theory. In particular we find steeper dark-matter spike density profiles in the modified gravity scenarios compared to GR. When compared to current observational constraints on PBH abundance, the mass ranges allowed by Hu-Sawicki $f(R)$ models are found to be wider than by nDGP models, for given merger rates. The results are highly dependent on the choice of SMBH mass function, with Vika and Shankar mass functions predicting lower abundances. The considerable sensitivity of the results on the assumed gravity scenario and SMBH mass function demonstrates the necessity of incorporating the corresponding theoretical uncertainties in making relatively robust predictions on compact binary merger rates and, as a result on PBH properties., Comment: 18 pages, 9 figures, refs added

We investigate the gravitational production of superheavy scalar fields with nonminimal coupling during and after inflation. We derive analytical approximations using the mode function solution in a de Sitter background and also apply the steepest descent method. We study both positive and negative nonminimal couplings and show that their comoving number density spectra behave differently in the short and long wavelength regimes. We numerically compute the comoving number density spectra and dark matter abundance for a broad range of superheavy spectator fields exceeding the Hubble scale during inflation, with $m_{\chi} > H_I$, and nonminimal couplings ranging from $-300 \leq \xi \leq 300$. When computing the allowed dark matter parameter space, we impose the maximum reheating temperature constraint, the Big Bang nucleosynthesis constraint, and the isocurvature constraint. We show that the presence of a positive or negative coupling $\xi$ can expand the parameter space up to $3$ orders of magnitude above the Hubble inflationary scale, allowing such dark matter candidates to be as heavy as $\sim 10^{16} \, \rm{GeV}$ in the Starobinsky model of inflation., Comment: 37 pages, 10 Figures

We consider the effects of relaxing the assumption that gravitational waves composing the stochastic gravitational wave background (SGWB) are uncorrelated between frequencies in analyses of the data from Pulsar Timing Arrays (PTAs). While individual monochromatic plane waves are often a good approximation, a background composed of astrophysical sources cannot be monochromatic since an infinite plane wave carries no signal. We consider how relaxing this assumption allows us to extract potential information about modified dispersion relations and other fundamental physics questions, as both the group and phase velocity of waves become relevant. After developing the formalism we carry out simple Gaussian wavepacket examples and then consider more realistic waveforms, such as that from binary inspirals. When the frequency evolves only slowly across the PTA temporal baseline, the monochromatic assumption at an effective mean frequency remains a good approximation and we provide scaling relations that characterize its accuracy., Comment: 30 pages, 6 figures

It is well known that a coarse-grained scalar field living on a de~Sitter (dS) background exhibits classical stochastic behavior, driven by a noise whose amplitude is set by the Hubble constant $H$. The coarse-graining is achieved by discarding wave numbers larger than a cutoff $\sigma H$ and demanding that $\sigma \ll 1$. Similar results hold for quasi-dS space, where the equation of state parameter $w$ is close to $-1$. Here we present exact expressions for the noise amplitude of a free massless field on an inflationary background with constant $w < -1/3$. We find that a classical stochastic behavior can emerge for $-5/3 < w < -1/3$. Furthermore, as we move away from $w = -1$ and approach $w = -1/3$, the constraint $\sigma \ll 1$ is relaxed and larger cutoffs ($\sigma \sim 1$) become feasible, too. However, in general the amplitude of the noise depends on $\sigma$, except in the quasi-dS regime $w \approx -1$., Comment: 18 pages, 4 figures

We investigate the effects of non-thermally produced dark matter on large-scale structure formation, focusing on the gravitino dark matter. Our analysis shows that large-scale structure measurements from the SDSS LRG data offer a promising approach to probing non-thermally produced gravitinos. Specifically, if gravitinos resulting from neutralino decay constitute a fraction $F_\text{dec}=0.35~(0.1)$ of the dark matter component, the data exclude bino-like neutralino with a mass $m_{\chi_1^0}\lesssim25~(11)\text{ GeV}$. Furthermore, this constraint appears to be largely independent of the gravitino mass., Comment: 7 pages, 3 figures

We have performed a parametrized post-Newtonian (PPN) test of a black hole spacetime using observational data of the star S0-2/S2 orbiting the massive black hole at our galactic center Sgr A*. After introducing our PPN model of black hole spacetime, we report the result of $\chi^2$ fitting of the PPN model with the observational data. A new finding through our PPN model is a detectability of the gravitational lens effect on the null geodesics connecting S0-2 and observer under the present observational uncertainties, if a PPN parameter is about one order larger than the value for general relativity case. On the other hand, the effect of black hole spin on the S0-2's motion is not detectable. Thus our present PPN test is performed with spherically symmetric vacuum black hole spacetime. The resultant value of the PPN parameter, which corresponds to the minimum $\chi^2$, implies that the gravitational field of Sgr A* is not of Schwarzschild metric or that there exists a sufficient amount of dark matters around Sgr A* to be detected by present telescopes. However, the difference between the minimum $\chi^2$ and the $\chi^2$ of Schwarzschild case is not large enough to ensure a statistical significance of non-Schwarzschild result. A more precise statistical analysis than $\chi^2$ statistics is necessary to extract a statistically significant information of the gravitational field of Sgr A* from present observational data. We will report a result by a Bayesian analysis in next paper., Comment: 43 pages, 4 figures, Accepted for Publication in Prog.Theor.Exp.Phys / Typos are corrected

To clarify a holographic modified thermodynamic relation, the present study applies a general formulation for cosmological equations in a flat FLRW universe to the first law of thermodynamics, using the Bekenstein-Hawking entropy $S_{\rm{BH}}$ and a dynamical Kodama-Hayward temperature $T_{\rm{KH}}$. For the general formulation, both an effective pressure $p_{e}$ of cosmological fluids for dissipative universes (e.g., bulk viscous cosmology) and an extra driving term $f_{\Lambda}(t)$ for non-dissipative universes (e.g., time-varying $\Lambda (t)$ cosmology) are phenomenologically assumed. When $f_{\Lambda}(t)$ is constant, the modified thermodynamic relation is equivalent to the formulation of the first law in standard cosmology. One side of this modified relation describes thermodynamic quantities in the bulk and can be divided into two time-derivative terms, namely $\dot{\rho}$ and $\dot{V}$ terms, where $\rho$ is the mass density of cosmological fluids and $V$ is the Hubble volume. Using the Gibbons-Hawking temperature $T_{\rm{GH}}$, the other side of this relation, $T_{\rm{KH}} \dot{S}_{\rm{BH}}$, can be formulated as the sum of $T_{\rm{GH}} \dot{S}_{\rm{BH}}$ and $[(T_{\rm{KH}}/T_{\rm{GH}}) -1] T_{\rm{GH}} \dot{S}_{\rm{BH}}$, which are equivalent to the $\dot{\rho}$ and $\dot{V}$ terms, respectively, with the magnitude of the $\dot{V}$ term being proportional to the square of the $\dot{\rho}$ term. In addition, the modified thermodynamic relation for constant $f_{\Lambda}(t)$ is examined by applying the equipartition law of energy on the horizon. This modified thermodynamic relation reduces to a kind of extended holographic-like connection when a constant $T_{\rm{KH}}$ universe is considered. The evolution of thermodynamic quantities is also discussed, using a constant $T_{\rm{KH}}$ model, extending a previous analysis [Phys. Rev. D 108, 083515 (2023) (arXiv:2306.11285)]., Comment: 17 pages, 3 figures

We present an up-to-date compilation of published Hubble constant ($H_0$) measurements that are independent of the CMB sound horizon scale. This compilation is split in two distinct groups: A. Distance Ladder Measurements sample comprising of 20 recent measurements, primarily from the past four years, utilizing various rung 2 calibrators and rung 3 cosmic distance indicators. B.One-Step Measurements sample including 33 measurements of $H_0$ that are independent of both the CMB sound horizon scale and the distance ladder approach. These 33 measurements are derived from diverse probes such as Cosmic Chronometers, gamma-ray attenuation, strong lensing, megamasers etc. Statistical analysis reveals a significant distinction between the two samples. The distance ladder-based sample yields a best fit $H_0 = 72.8 \pm 0.5$ km s$^{-1}$ Mpc$^{-1}$ with $\chi^2/dof=0.51$ indicating some correlations. The one-step measurements result in $H_0 = 69.0 \pm 0.48$ km s$^{-1}$ Mpc$^{-1}$ with $\chi^2/dof=1.37$ indicating some internal tension. If two outlier measurements are removed (TDCOSMO.I-2019 known to have systematics and MCP-2020) the best fit of the one step sample reduces to $H_0 = 68.3 \pm 0.5$ km s$^{-1}$ Mpc$^{-1}$ with $\chi^2/dof=0.95$, fully self-consistent and consistent with sound horizon based measurements. A Kolmogorov-Smirnov test yields a p-value of 0.0001 suggesting that the two samples are fundamentally distinct, with a probability of less than 0.01\% that they are drawn from the same underlying distribution. These findings suggest that the core of the Hubble tension lies not between early and late-time measurements, but between distance ladder measurements and all other $H_0$ determinations. This discrepancy points to either a systematic effect influencing all distance ladder measurements or a fundamental physics anomaly affecting at least one rung of the distance ladder., Comment: 44 pages, 4 figures 3 tables. Updated data with three additional H0 measurements (total 53). Expanded Table 3 and the $\chi^2$ considering additional subsamples further testing the robustness of results and conclusions. Comments added, typos corrected, references added. The numerical analysis files used in this work are publicly available at https://github.com/leandros11/distladder/

Primordial black holes (PBHs) with initial mass $\sim5\times10^{14}$ g are evaporating due to Hawking radiation, leading to bursts of very-high-energy gamma rays. In this work, we investigate the prospective sensitivity of the Large High Altitude Air Shower Observatory (LHAASO) to measure the local burst rate density of PBHs. Our findings reveal that LHAASO is capable of searching for the PBH bursts within a distance $\sim0.1$ pc from the sun and thereby measure the local burst rate density $\sim$ 1164 (or 699)$\,\mathrm{pc}^{-3}\,\mathrm{yr}^{-1}$ at $99\%$ confidence level during a 3 (or 5) year observing run. This stands for a sensitivity that is one order of magnitude stronger than the strongest observational constraint from the High Altitude Water Cherenkov Observatory (HAWC). In addition, we further suggest an observing strategy to search for the PBH bursts during upcoming observing runs of LHAASO., Comment: 12 pages, 2 figures

Axio-dilaton models are among the simplest scalar-tensor theories that contain the two-derivative interactions that naturally compete at low energies with the two-derivative inter-actions of General Relativity. Such models are well-motivated as the low energy fields arising from string theory compactification. We summarize these motivations and compute their cosmological evolution, in which the dilaton acts as dark energy and its evolution provides a framework for dynamically evolving particle masses. The derivative axion-dilaton couplings play an important role in the success of these cosmologies. We derive the equations for fluctuations needed to study their implications for the CMB anisotropy, matter spectra and structure growth. We use a modified Boltzmann code to study in detail four benchmark parameter choices, including the vanilla Yoga model, and identify couplings that give viable cosmologies, including some with surprisingly large matter-scalar interactions. The axion has negligible potential for most of the cosmologies we consider but we also examine a simplified model for which the axion potential plays a role, using axion-matter couplings motivated by phenomenological screening considerations. We find such choices can also lead to viable cosmologies., Comment: 38 pages, 8 figures

Current cosmological constraints allow primordial black holes (PBHs) to constitute dark matter in the mass range of $10^{18}$-$10^{22}$g. We show that a major portion of this logarithmic window can be ruled-out based on the Solar System ephemeris, given that the external mass enclosed within 50 au from the Sun did not change by more than $\sim 5\times 10^{-14}$ solar masses per year in recent decades., Comment: Accepted for publication in Research Notes of the American Astronomical Society (August 2024), 4 pages

Gravitational waves (GWs) from gravitational three-body decay (graviton Bremsstrahlung process) can leave an indelible signal at ultrahigh frequencies. We focus on a scenario where superheavy particles are produced gravitationally at a transition between the inflationary and kination phases and analyze the detectability of the signal in the presence of GWs generated from the vacuum fluctuations during inflation. We find that, in many cases, GWs from the graviton Bremsstrahlung are buried in the stochastic gravitational wave background originating from inflation. However, if the superheavy particles are as heavy as the Planck scale, the graviton Bremsstrahlung can produce a sizable amount of GWs, surpassing the inflationary ones., Comment: 17 pages, 3 figures

Cosmological first-order phase transition (FOPT) sources the stochastic gravitational wave background (SWGB) through bubble collisions, sound waves, and turbulence. So far, most studies on the fluid profile of an expanding bubble are limited to transitions that complete in a much shorter time scale than the cosmic expansion. In this study, we investigate gravitational effects on the fluid profile beyond the self-similar regime. For this purpose we combine a hydrodynamic scheme in the presence of gravity with a fluid computation scheme under energy injection from the bubble wall. By performing (1+1)d simulations of spherical bubble for constant wall velocities, we find that the fluid generally develops a thinner shell in our cosmological setup, which qualitatively agrees with previous studies discussing the late-time behavior of fluid in expanding spacetime. We also observe reduction in the energy budget for the fluid kinetic energy. Furthermore, we find that the fluid profile develops sub-structure for accelerating bubble walls. We also comment on the possible broadening of the SGWB spectral plateau., Comment: 32 pages, 12 figures, v2: grant information updated

In this study, we present an analysis of the standard flat-$\Lambda$CDM model using a cosmographic approach, incorporating recent DESI BAO observations and Supernovae Type Ia catalogues (SNIa), including the DES-SN5YR and Pantheon+ compilations. We find full consistency between the standard model and the cosmographic approach when considering DESI BAO and SNIa catalogues independently. When combining DESI BAO with SNIa data, we examine the impact of the Planck prior on the sound horizon at the drag epoch, $r_d$, and the Cepheid prior on the absolute magnitude, $M$. Applying the Planck prior on $r_d$ alone yields an $H_0$ value consistent with the Planck measurement, while applying the Cepheid prior on $M$ alone results in an $H_0$ value consistent with the SH0ES measurement. Without any priors, the $H_0$ value obtained has a large error margin, reconciling the Planck and SH0ES measurements. In all cases where individual priors are applied, we observe no significant tension between the flat-$\Lambda$CDM model and the cosmographic approach. However, when both Planck and Cepheid priors are applied simultaneously, significant tensions arise between the model and cosmography. This tension is even more pronounced when excluding LRG1 and LRG2 from the DESI measurements. These results suggest that the standard model cannot simultaneously align with both high-redshift Planck CMB observations and local Cepheid measurements.

Persistent tensions in the $\Lambda$CDM cosmological model underline the importance of tests of its basic assumptions. One such potential test arises from the fact that the surface of zero expansion around the collapsing object with spherical symmetry is strictly related to the object's mass and the value of the cosmological constant. We propose a complementary probe relating the averaged zero-expansion volume to the mass and the background cosmological Hubble parameter. Using the relativistic Zel'dovich approximation we are able to relax the spherical symmetry assumption and hence obtain a more general test of cosmological dynamics. Alternatively, our method can serve as a test of compatibility of relativistic N-body simulations and the scalar, averaged Einstein's equations with the relativistic Zel'dovich approximation serving as a closure condition.

Gravitational-wave (GW) events can serve as standard sirens for cosmology, as the luminosity distance to source can be directly measured from the waveform amplitude. Specifically, the ``dark'' siren method involves inferring cosmological parameters, e.g. the Hubble constant, by comparing the luminosity distance distribution and that of the redshift, typically obtained through a combination of galaxy survey catalog and theoretical models. Especially with the prospect of third-generation GW detectors, the statistical uncertainty of the Hubble measurement can be suppressed to a percent level. However, incorrect assumption in galaxy population models can lead to systematic bias, which becomes increasingly relevant as third-generation GW detectors can detect large-redshift sources beyond the reach of currently available galaxy catalogs. In this work, we adopt a Fisher information formalism and study the maximum model error tolerance given specific total error budget. We find that, to achieve a total error budget of 1% in the Hubble constant, the galaxy mass function redshift evolution should be known to O(1%). We find that galaxy redshift uncertainty, survey magnitude limit and GW angular localization error are important factors. We find galaxy clustering also improves model error tolerance, thus our results provide a conservative benchmark to future analysis using real galaxy catalogs. We also investigate the effective bright siren scenario and highlight that the dark siren selection strategy should be catered to measurement uncertainty and the target total error budget. This work thus highlights the challenge in the dark siren method and quantifies requirements on both the galaxy catalog and GW measurement to contribute to constraining cosmology., Comment: 22 pages, 12 figures, comments welcome

We present the Big-Bang Nucleosynthesis (BBN) simulation with a bubble universe scenario around a rotating black hole (BH) in Kerr-AdS$_5$ spacetime to explain recently updated observations of light elements such as the primordial helium abundance. In this scenario, the geometry of the 4D- early Universe is described as a vacuum bubble that undergoes quasi-de Sitter expansion in Kerr- AdS$_5$ spacetime. We find that the BH mass and spin parameter, which show an anti-correlation against the total radiation, are important to resolve the ${}^4$He anomaly. The present results provide clues to finding a connection between the observed results of light-element nucleosynthesis and the scenario of the 4D-bubble universe in AdS$_5$ spacetimes, Comment: 17 pages, 5 figures

Strong gravitational lensing occurs when photons pass through the vicinity of a black hole. We investigate this phenomenon in the context of a gravitational-wave event, specifically when a black hole is settling into its final state. The deflection angle of photons mimics the ringdown pattern of the gravitational wave at intermediate times. At late times it has an inverse cubic dependence on observation time. The deviation angle increases exponentially as photons approach the photon ring orbit, reflecting its unstable nature. Our findings are directly applicable to imaging scenarios involving stars against the background of compact binaries, or circumbinary accretion disks, particularly during the merger of two black holes., Comment: 16 pages, 4 figures

The NANOGrav 15-year data provides compelling evidence for a stochastic gravitational-wave (GW) background at nanohertz frequencies. The simplest model-independent approach to characterizing the frequency spectrum of this signal consists in a simple power-law fit involving two parameters: an amplitude A and a spectral index \gamma. In this paper, we consider the next logical step beyond this minimal spectral model, allowing for a running (i.e., logarithmic frequency dependence) of the spectral index, \gamma_run(f) = \gamma + \beta \ln(f/f_ref). We fit this running-power-law (RPL) model to the NANOGrav 15-year data and perform a Bayesian model comparison with the minimal constant-power-law (CPL) model, which results in a 95% credible interval for the parameter \beta consistent with no running, \beta \in [-0.80,2.96], and an inconclusive Bayes factor, B(RPL vs. CPL) = 0.69 +- 0.01. We thus conclude that, at present, the minimal CPL model still suffices to adequately describe the NANOGrav signal; however, future data sets may well lead to a measurement of nonzero \beta. Finally, we interpret the RPL model as a description of primordial GWs generated during cosmic inflation, which allows us to combine our results with upper limits from big-bang nucleosynthesis, the cosmic microwave background, and LIGO-Virgo-KAGRA., Comment: 17 pages, 4 figures, 2 tables

We generalise Phinney's 'practical theorem' to account for modified graviton dispersion relations motivated by certain cosmological scenarios. Focusing on specific examples, we show how such modifications can induce characteristic localised distortions, bumps, in the frequency profile of the stochastic gravitational wave background emitted from distant binary sources. We concentrate on gravitational waves at nano-Hertz frequencies probed by pulsar timing arrays, and we forecast the capabilities of future experiments to accurately probe parameters controlling modified dispersion relations. Our predictions are based on properties of gravitational waves emitted in the first inspiral phase of the binary process, and do not rely on assumptions of non-linear effects occurring during the binary merging phase, Comment: 19 pages, 5 figures, 2 tables

The $\delta N$ formalism is a powerful approach to compute non-linearly the large-scale evolution of the comoving curvature perturbation $\zeta$. It assumes a set of FLRW patches that evolve independently, but in doing so, all the gradient terms are discarded, which are not negligibly small in models beyond slow-roll. In this paper, we extend the formalism to capture these gradient corrections by encoding them in a homogeneous-spatial-curvature contribution assigned to each FLRW patch. For a concrete example, we apply this formalism to the ultra-slow-roll inflation, and find that it can correctly describe the large-scale evolution of the comoving curvature perturbation from the horizon exit. We also briefly discuss non-Gaussianities in this context., Comment: 5 pages main text, 6 pages appendix, 2 figures

This brief note presents standard computations of primordial black hole mass $M$ given perturbations of scale $k$, and their late-time abundance $\Omega_\text{PBH}$ given their initial density fraction $\beta$. I recap the assumptions made in these computations and present formulas and reference charts useful for a working cosmologist., Comment: Brief note. 7 pages, 1 table, 2 figures

The observation that accelerated cosmic expansion is dominant since the Mega-parsec cosmic structure became nonlinear seems like an extraordinary coincidence, unless the acceleration is somehow driven by the emergence of the structure. That has given rise to the controversial concept of a gravitational backreaction through which inhomogeneity becomes a driver of accelerated expansion. The standard route when studying strongly inhomogeneous cosmological models is to take either a perturbative approach or a spatial averaging approach. Here we argue that because backreaction is in fact a nonlinear multiscale phenomenon, perturbative approaches may have a limited validity. The alternative is the proposed averaging approach. In this paper we demonstrate that the implied backreaction terms are artificial, that is gauge dependent, which may easily cause ambiguous estimates of its significance. In the current study, we forward a formal fully geometric framework of cosmic foliations in the context of relativistic cosmology. Here we show that fixing a foliation of spacetime determines a choice of gauge. Addressing the correspondence between the metric tensor and the foliation allows us to clarify the theoretical implications of choosing a foliation. Within the context of backreaction, this formalism allows us to discuss the complications of averaging. It reveals that spatial averaging can induce artificial backreaction terms that arise from any specific choice of gauge. Averaging methods presented so far all encounter this problem. Within our foliation framework, we can produce a gauge invariant method of averaging by considering a group of proper time foliations which any cosmic observe can agree upon. We demonstrate that this implies the gauge invariance of the averaging procedure. This makes it applicable to standard cosmological simulations., Comment: arXiv admin note: text overlap with arXiv:2310.19451

We explore Noether symmetries of Horndeski gravity, extending the classification of general scalar-tensor theories. Starting from the minimally coupled scalar field and the first-generation scalar-tensor gravity, the discussion is generalised to kinetic gravity braiding and Horndeski gravity. We highlight the main findings by focusing on the non-minimally coupled Gauss-Bonnet term and the extended cuscuton model. Finally, we discuss how the presence of matter can influence Noether symmetries. It turns out that the selected Horndeski functions are unchanged with respect to the vacuum case., Comment: 18 pages. Matches published version in EPJC

The interplay between cosmology and strongly coupled dynamics can yield transient spectral features that vanish at late times, but which may leave behind phenomenological signatures in the spectrum of primordial fluctuations. Of particular interest are strongly coupled extensions of the standard model featuring approximate conformal invariance. In flat space, the spectral density for a scalar operator in a conformal field theory is characterized by a continuum with scaling law governed by the dimension of the operator, and is otherwise featureless. AdS/CFT arguments suggest that for large $N$, in an inflationary background with Hubble rate $H$, this continuum is gapped. We demonstrate that there can be additional peak structures that become sharp and particle-like at phenomenologically interesting regions in parameter space, and we estimate their contribution to cosmological observables. We find phenomena that are potentially observable in future experiments that are unique to these models, including displaced oscillatory features in the squeezed limit of the bi-spectrum. These particles can be either fundamental, and localized to a UV brane, or composite at the Hubble scale, $H$, and bound to a horizon in the bulk of the 5D geometry. We comment on how stabilization of conformal symmetry breaking vacua can be correlated with these spectral features and their phenomenology., Comment: 31 pages, 11 figures

The main challenge of 21 cm cosmology experiments is astrophysical foregrounds which are difficult to separate from the signal due to telescope systematics. An earlier study has shown that foreground residuals induced by antenna gain errors can be estimated and subtracted using the hybrid foreground residual subtraction (HyFoReS) technique which relies on cross-correlating linearly filtered data. In this paper, we apply a similar technique to the CHIME stacking analysis to subtract beam-induced foreground contamination. Using a linear high-pass delay filter for foreground suppression, the CHIME collaboration reported a $11.1\sigma$ detection in the 21 cm signal stacked on eBOSS quasar locations, despite foreground residual contamination mostly due to the instrument chromatic transfer function. We cross-correlate the foreground-dominated data at low delay with the contaminated signal at high delay to estimate residual foregrounds and subtract them from the signal. We find foreground residual subtraction can improve the signal-to-noise ratio of the stacked 21 cm signal by $ 10 - 20\%$ after the delay foreground filter, although some of the improvement can also be achieved with an alternative flagging technique. We have shown that it is possible to use HyFoReS to reduce beam-induced foreground contamination, benefiting the analysis of the HI auto power spectrum with CHIME and enabling the recovery of large scale modes.

If a set of massive objects collide in space and the fragments disperse, possibly forming black holes, then this process will emit gravitational waves. Computing the detailed gravitational wave-form associated with this process is a complicated problem, not only due to the non-linearity of gravity but also due to the fact that during the collision and subsequent fragmentation the objects could undergo complicated non-gravitational interactions. Nevertheless the classical soft graviton theorem determines the power law fall-off of the wave-form at late and early times, including logarithmic corrections, in terms of only the momenta of the incoming and outgoing objects without any reference to what transpired during the collision. In this short review I shall explain the results and very briefly outline the derivation of these results., Comment: LaTeX; 25 pages

We investigate anisotropic equation of state parameterization of dark energy within the framework of an axisymmetric (planar) Bianchi-I universe. In addition to constraining the equation of state for anisotropic dark energy and other standard cosmological model parameters, we also constrain any underlying anisotropic axis using the latest Pantheon+ Type Ia Supernovae data set augmented by SH0ES Cepheid distance calibrators. The mean anisotropic dark energy equation of state and corresponding difference in the equation of states in and normal to the plane of our axisymmetric Bianchi-I space-time are found to be $\bar{w}=-0.90^{+0.16}_{-0.12}$ and $\delta_w=-0.140^{+0.11}_{-0.082}$ respectively. We also find an axis of anisotropy in this planar Bianchi-I model with anisotropic dark energy to be $\approx(279^{\circ} ,12^{\circ})$ in galactic coordinates. Our analysis of different cosmological models suggests that while a Bianchi-I universe with anisotropic dark energy (the $w_b$CDM model) shows some preference over other anisotropic models, it is still less likely than the standard $\Lambda$CDM model or a model with a constant dark energy equation of state ($w$CDM). Overall, the $\Lambda$CDM model remains the most probable model based on the Akaike Information Criterion., Comment: 12 pages, 6 figures, 2 tables; Comments are welcome

We propose a mechanism of preheating stage after inflation, using a new idea of parametrically amplified super-radiance. Highly coherent state, characterized by macro-coherence of scalar field coupled to produced massless particle in pairs, is created by parametric resonance effects associated with field oscillation around its potential minimum, within a Hubble volume. The state is described effectively by the simple Dicke-type of super-radiance model, and super-radiant pulse is emitted within a Hubble time, justifying neglect of cosmic expansion. Produced particles are shown to interact to change their energy and momentum distribution to realize thermal hot big bang universe. A long standing problem of heating after inflation may thus be solved. A new dark matter candidate produced at the emergence of thermalized universe is suggested as well., Comment: 6 pages, 2 figures

In a recent paper in Astronomy & Astrophysics, Alimi & Koskas (2024) have highlighted in wCDM models derived from general relativity (with Dark Energy Universe numerical simulation data), a cosmological invariance of the distribution of dark matter (DM) halo shapes when expressed in terms of the non-linear fluctuations of the cosmic matter field. We show in this paper that this invariance persists when tested on numerical simulations performed with a different code, i.e. a different N-body solver, DUSTGRAIN-pathfinder simulation (Giocoli et al. 2018) and that it is also robust to the addition of neutrinos to the cold component of dark matter. However, this discovery raises crucial questions about the validity of this invariance in modified gravity models. Thus, we examine whether the invariance observed by Alimi & Koskas (2024) remains robust in the case of Hu & Sawicki models of modified gravity using numerical simulations performed by Giocoli et al. (2018). By comparing the results of advanced numerical simulations in these different theoretical frameworks, we found significant deviations from the invariance observed in the framework of wCDM models of GR. These deviations suggest that the nature of the gravitation significantly influences the shape of the DM halos. We then interpret this deviation observed in the modified gravity models from the GR models as due to the scalar-field screening effect corresponding to such f(R)-type theories. This one modifies the sphericalization process of DM halos during their formation, precisely because the critical mass at which this scalar field becomes non-negligible is the mass at which the deviation appears. To this extent, the deviation from cosmological invariance in the shape of DM halos is a cosmological probe of the nature of gravity, and the mass scale at which it appears can be used to estimate the $f_{R0}$ parameter of such theories., Comment: 11 pages, 12 figures, comments are welcomed

Superradiant instability can form clouds around rotating black holes (BHs) composed of ultralight bosonic fields, such as axions. A BH with such a cloud in a binary system exhibits rich phenomena, and gravitational waves (GWs) from the BH merger provide a means to probe axions. For the first time, we study the evolution of axion clouds in a binary system during the inspiral phase, including axion self-interaction effects. When the self-interaction is significant, unlike in the negligible case, two types of clouds coexist through mode coupling. We examine the evolution of the system considering the effects of dissipation caused by both self-interaction and tidal interaction. For tidal interaction, in addition to the processes of emission to infinity and absorption by the BH, indirect emission via transitions (both resonant and off-resonant) is also considered as a second-order perturbation. Our results demonstrate that the signatures of axion self-interaction are imprinted in the modification of the GW phase. Furthermore, we find the possibility of a dynamical instability called bosenova during the binary inspiral phase., Comment: 19 pages, 13 figures, 1 table

Modern cosmology presents important challenges such as the \textit{Hubble tension}, \textit{El Gordo's collision} or the \textit{impossible galaxies} ($z > 10$). Slight modifications to the standard model propose new parameters (e.g. the early and dynamical dark energy). On the other hand, alternatives such as the coasting universes (e.g. the \textit{hyperconical model} and the spatially flat $R_h = ct$ universe) are statistically compatible with most of observational tests, but still present theoretical problems in matching the observed matter contents since they predict a ``zero active gravitational mass.'' To solve these open issues, we suggest that general relativity might be not valid at cosmic scales, but it would be valid at local scales. This proposal is addressed from two main features of the embedding hyperconical model: 1) the background metric would be independent of the matter content, and 2) the observed cosmic acceleration would be fictitious and because of a distorted stereographic projection of coordinates that produce an apparent radial inhomogeneity from homogeneous manifolds. Finally, to support the discussion, standard observational tests were updated here, showing that the hyperconical model is adequately fitted to Type Ia Supernovae, quasars, galaxy clusters, BAO, and cosmic chronometer data sets., Comment: 22 pages, 4 figures

We prove explicitly the absence of one-loop corrections to large scales from small scales in transient non-slow-roll dynamics. Specifically, we address loop corrections to the primordial power spectrum, relative to tree-level, that are independent of the ratio between the two scales. We review all the necessary components, adapted to our context, to express one-loop diagrams as three-point functions, emphasizing the crucial role played by quartic interactions. Notably, we include the quartic Hamiltonian induced by the cubic Lagrangian and quartic interactions that are ensured by diffeomorphism invariance. We then explicitly prove consistency relations for an arbitrary transient non-slow-roll phase involving operators with (time) derivatives. Finally, we calculate one-loop corrections by including contributions from the relevant cubic and quartic interactions, and express the final result as a total derivative term over comoving momenta, utilizing the consistency relations we established. This leads us to conclude that one-loop corrections to long-wavelength modes are unaffected by the physics of short and enhanced modes in non-slow-roll dynamics., Comment: 46 pages

Modified Newtonian Dynamics (MOND) was originally proposed to model galaxy rotation curves without dark matter. However, MOND presents difficulties in explaining the Radial Acceleration Relation (RAR) observed in galaxy clusters, and moreover, it does not completely eliminate the need for dark matter, since it requires using sterile neutrinos to explain the observed hydrostatic equilibrium of the hot gas. Hyperconical Modified Gravity (HMG) offers a relativistic framework that recovers the success of MOND in galaxy rotation curves as a natural particular case, and it could potentially reconcile the above discrepancies without invoking any type of dark matter. This Letter analyses the performance of the HMG model for hydrostatic equilibrium in four representative systems: groups of NGC 533 and NGC 5044 galaxies, and clusters of Abell 2717 and Abell 2029. Specifically, we used high-resolution X-ray data with which gas density, temperature, and mass profiles were previously derived to constrain modified gravity models. Our results show that the hydrostatic equilibrium in the four systems is naturally adjusted to the HMG model without the need for fitting parameters or adding sterile neutrinos, which are required for MOND theories., Comment: 5 pages, 1 figure

In this paper, we investigate the impact of scalar fluctuations ($\chi$) non-minimally coupled to gravity, $\xi\chi^2 R$, as a potential source of secondary gravitational waves (SGWs). Our study reveals that when reheating EoS $\wre > 1/3$ and $\xi \gtrsim 1/6$, the super-horizon modes of scalar field experience a \textit{Tachyonic instability} during the reheating phase. Particularly for $\xi >1$ such instability causes substantial growth in the scalar field amplitude leading to pronounced production of SGWs in an intermediate-frequency range that is strong enough to be detected by future gravitational wave detectors. Such growth in super-horizon modes of the scalar field and associated GW production may have a significant effect on the strength of the tensor fluctuation at the Cosmic Microwave Background (CMB) scales (parametrized by $r$) and the number of relativistic degrees of freedom (parametrized by $\dneff$) at the time of CMB decoupling. To prevent such overproduction, the PLANCK constraints on tensor-to-scalar ratio $r \leq 0.036$ and $\dneff \leq 0.284$ yield a strong upper bound on the value of $\xi$ depending upon the value of $\wre$ and reheating temperature $\Tre$. Taking into account all the observational constraints we found the value of $\xi$ should be $ \lesssim 4.0$ for any value of reheating temperature within $10^{-2} \lesssim \Tre \lesssim 10^{14}$ GeV, reheating equation of state within $ 1/2 \leq\wre \leq 1$, and for a wide range of inflationary energy scales. However, for $\wre < 1/3$, $\xi$ is found to be unconstrained by any known observation. Finally, we identify the parameter regions in $(\Tre,\xi)$ plane which can be probed by the upcoming GW experiments namely BBO, DECIGO, LISA, and ET., Comment: 30 pages, 8 figures, 1 table

At finite temperature, the coupling of an axion to non-Abelian gauge fields causes dissipation due to sphaleron heating. This mechanism is ideally suited for realizing warm inflation since it can lead to large thermal friction while preserving the flatness of the potential. We show, however, that requiring standard properties of an axion $-$ in particular a discrete shift symmetry and a potential generated non-perturbatively by instanton effects $-$ excludes the strong regime of warm slow-roll inflation, in which thermal friction dominates. The derivation of this result does not require any phenomenological input. While non-minimal (axion-like) models provide a way out, sphaleron heating with a standard axion is capable of enabling the weak regime of warm inflation., Comment: 6 pages

This set of notes was designed to accompany two hours of lectures and practical exercises at the New Horizons for Psi workshop in Lisbon in July 2024 entitled "Studying fundamental fields with numerical relativity". Numerical relativity is a tool used to help understand the behaviours of metric and matter fields in dynamical, strong gravity situations. It has been used to study a range of situations involving fundamental fields, including superradiance, modified gravity, dynamical friction, dark matter accretion and early universe cosmology. The purpose of this course is to provide some background and hands-on experience in numerical relativity that will help students to better understand the possibilities provided by this tool, as well as its limitations., Comment: Comments welcome. 20 pages, 5 figures. See https://github.com/GRTLCollaboration/engrenage/wiki/New-Horizons-for-Psi-workshop for supporting material

We show that black holes can be made of light by adding gravity to the discussion of Alvarez-Dominguez et al., arXiv:2405.02389 [PRL 130, 041401 (2024)]., Comment: Comment on arXiv:2405.02389 [PRL 130, 041401 (2024)], 2 pages; submitted for publication as a comment to Physical Review Letters, with added references

We review recent developments in physical implications of Weyl conformal geometry. The associated Weyl quadratic gravity is a gauge theory of the Weyl group of dilatations and Poincar\'e symmetry. Weyl geometry is defined by equivalence classes of the metric and Weyl gauge field ($\omega_\mu$), related by Weyl gauge transformations. Weyl geometry can be seen as a covariantised version of Riemannian geometry with respect to Weyl gauge symmetry (of dilatations). This Weyl gauge-covariant formulation of Weyl geometry is metric, which avoids long-held criticisms on the physical relevance of this geometry. Weyl quadratic gravity and its geometry have interesting properties: Weyl gauge symmetry spontaneously broken to Einstein-Hilbert gravity (Riemannian geometry) respectively, with $\Lambda>0$; this is the only true gauge theory of a space-time symmetry i.e. with a physical Weyl gauge boson and is Weyl-anomaly free; all fields and masses have geometric origin, with no added scalar compensators; the theory is the leading order of a Weyl gauge invariant DBI action of conformal geometry that gives a natural analytical continuation of the former to $d$ dimensions; Weyl anomaly is recovered in the broken phase; in the limit of vanishing Weyl gauge current, one obtains conformal gravity and Riemannian geometry; SM has a natural embedding in Weyl conformal geometry with no new degrees of freedom, with a successful Starobinsky-Higgs inflation. Briefly, Weyl conformal geometry alone generates a (quantum) gauge theory of gravity, leading to a unified description by the gauge principle of Einstein-Hilbert gravity and SM., Comment: 39 pages, LaTeX