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Two notable anomalies in radio observations -- the excess radiation in the Rayleigh-Jeans tail of the cosmic microwave background, revealed by ARCADE2, and the twice-deeper absorption trough of the global 21cm line, identified by EDGES -- remain unresolved. These phenomena may have a shared origin, as the enhancement of the 21cm absorption trough could arise from excess heating. We investigate this scenario through the framework of axion-like particles (ALPs), showing that the resonant conversion of ALPs into photons can produce a photon abundance sufficient to resolve both anomalies simultaneously. Our model naturally explains the observed radio excess between 0.4 and 10GHz while also enhances the 21cm absorption feature at 78MHz. Furthermore, it predicts a novel power-law scaling of the radio spectrum above 0.5GHz and an additional absorption trough below 30MHz, which could be verified through cross-detection in upcoming experiments., Comment: 11 pages, 4 figures

We explore wormhole solutions sourced by Casimir energy density involving generalized uncertainty principle corrections within the framework of Rastall-Rainbow gravity. The questions of traversability and stability, as well as the presence of exotic matter, are carefully investigated. In particular, the stability issue is addressed via an approach that has not been previously employed in the context of wormholes. This method, which represents an improved version of the so-called Herrera cracking technique, has the potential to yield novel insights in the field of wormhole geometries., Comment: 20 pages, 4 figures

The gravitational interaction, as described by the Einstein-Cartan theory, is shown to emerge as the by-product of the spontaneous symmetry breaking of a gauge symmetry in a pre-geometric four-dimensional spacetime. Starting from a formulation \`a la Yang-Mills on an SO(1,4) or SO(3,2) principal bundle and not accounting for a spacetime metric, the Einstein-Hilbert action is recovered after the identification of the effective spacetime metric and spin connection for the residual SO(1,3) gauge symmetry of the spontaneously broken phase - i.e. the stabiliser of the SO(1,4) or SO(3,2) gauge group. Thus, the two fundamental tenets of General Relativity, i.e. diffeomorphism invariance and the equivalence principle, can arise from a more fundamental gauge principle. The two mass parameters that characterise Einstein gravity, namely the Planck mass and the cosmological constant, are likewise shown to be emergent, with the correct sign for the cosmological constant depending on whether the fundamental gauge group is taken to be either the de Sitter or the anti-de Sitter group. The phase transition from the unbroken to the spontaneously broken phase is expected to happen close to the Planck temperature. This is conjectured to be dynamically driven by a scalar field that implements a Higgs mechanism, hence providing mass to new particles, with consequences for cosmology and high-energy physics. The couplings of gravity to matter are discussed after drawing up a dictionary that interconnects pre-geometric and effective geometric quantities. In the unbroken phase where the fundamental gauge symmetry is restored, the theory is potentially power-counting renormalisable without matter, offering a novel path towards a UV completion of Einstein gravity.

We study slow-roll inflation as a common feature in Metric-affine Theories of Gravity. In particular, we take into account extended metric, teleparallel and symmetric-teleparallel theories of gravity, based on different geometric invariants, discussing analogies and differences. The analysis for each model is performed in two approaches. First, we focus on the {\it potential-slow-roll approach} by studying the reconstructed potentials for different forms of the extended models related to the considered gravitational theory in the Einstein frame. Secondly, we investigate the {\it Hubble-slow-roll approach} for some conventional inflationary potentials related to the specific extended model in the Jordan frame. We compare all results with cosmic microwave background anisotropy observations coming from Planck 2018 and BICEP2/Keck array satellites in order to find the observational constraints on the parameters space of the models as well as their prediction from the spectral parameters. Eventually, we attempt to present a qualitative comparison between three classes of considered modified gravities using the obtained inflationary results. The aim is to select, in principle, cosmological signatures capable of discriminating among concurrent models in view to point out the representation of gravity, according with geometric invariants, which better addresses the early universe dynamics., Comment: 56 pages, 23 figures, 2 tables

Gravitational waves (GWs) can undoubtedly serve as a messenger from the early Universe acting as well as a novel probe of the underlying gravity theory. In this work, motivated by one-loop vacuum-polarization effects on curved spacetime, we investigate a gravitational theory with non-minimal curvature-electromagnetic coupling terms of the form $\xi R F_{\mu\nu}F^{\mu\nu}$, where $R$ is the scalar curvature and $F_{\mu\nu}$ the Faraday tensor, which can be responsible for the generation of primordial electromagnetic fields. We study then the GW signatures of such coupling terms deriving in particular for the first time to the best of our knowledge the modified tensor modes equation of motion. Remarkably, we find a universal infrared (IR) frequency scaling $f^5$ of the electromagnetically induced GW (EMIGW) signal, which, depending on the energy scale of inflation, the duration of inflation and reheating as well as the dynamical behaviour of the gauge coupling function $\xi$, can be well within the detection sensitivity bands of GW experiments such as SKA, LISA, ET and BBO, being thus potentially detectable in the future by GW observatories., Comment: 16 pages plus appendices (19 pages in total), 3 figures

Several correlations among Gamma-Ray Bursts (GRBs) quantities, both in the prompt and afterglow emissions, have been established during the last decades, thus enabling the standardization of GRBs as cosmological probes. Since GRBs are observed up to redshift $z \sim 9$, they represent a valuable tool to fill in the gap of information on the Universe evolution between the farthest type Ia supernovae and the Cosmic Microwave Background Radiation and to shed new light on the current challenging cosmological tensions. Without claiming for completeness, here we describe the state of the art of GRB correlations, their theoretical interpretations, and their cosmological applications both as standalone probes and in combination with other probes. In this framework, we pinpoint the importance of correcting the correlations for selection biases and redshift evolution to derive intrinsic relations, the assets of combining probes at different scales, and the need for the employment of the appropriate cosmological likelihood to precisely constrain cosmological parameters. Furthermore, we emphasize the benefits of the cosmographic approach to avoid any cosmological assumptions and the valuable applications of machine learning techniques to reconstruct GRB light curves and predict unknown GRB redshifts. Finally, we stress the relevance of all these factors, along with future observations, to definitely boost the power of GRBs in cosmology., Comment: 82 pages, 91 figures, accepted as a solicited review in New Astronomy Reviews

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

We explores the Pantheon+SH0ES dataset to identify patterns that can discriminate between different cosmological models. We focus on determining whether the behaviour of dark energy is consistent with the standard $\Lambda$CDM model or suggests novel cosmological features. The central goal is to evaluate the robustness of the $\Lambda$CDM model compared with other dark energy models, and to investigate whether there are deviations that might indicate new cosmological insights. The study takes into account a data-driven approach, using both traditional statistical methods and machine learning techniques. Initially, we evaluate six different dark energy models using traditional statistical methods like Markov Chain Monte Carlo (MCMC), Static and Dynamic Nested Sampling to infer the cosmological parameters. Subsequently, we adopt a machine learning approach, developing a regression model to compute the distance modulus of each supernova, expanding the feature set to 74 statistical features. Traditional statistical analysis confirms that the $\Lambda$CDM model is robust, yielding expected parameter values. Other models show deviations, with the Generalised and Modified Chaplygin Gas models performing poorly. In the machine learning analysis, feature selection techniques, particularly Boruta, significantly improve model performance. In particular, models initially considered weak (Generalised/Modified Chaplygin Gas) show significant improvement after feature selection. The study demonstrates the effectiveness of a data-driven approach to cosmological model evaluation. The $\Lambda$CDM model remains robust, while machine learning techniques, in particular feature selection, reveal potential improvements in alternative models which could be relevant for new observational campaigns like the recent DESI survey., Comment: 30 pages, 20 figures. arXiv admin note: text overlap with arXiv:1904.02180 by other authors

The emergence of the Ultra-Diffuse Galaxies in recent years has posed a severe challenge to the galaxy formation models as well as the Extended Theories of Gravity. The existence of both dark matter lacking and dark matter dominated systems within the same family of astrophysical objects indeed requires the gravity models to be versatile enough to describe very different gravitational regimes. In this work, we study a non-local extension of the theory of General Relativity that has drawn increasing attention in recent years due to its capability to account for the late time cosmic acceleration without introducing any dark energy fluid. We leverage the kinematic data of three Ultra-Diffuse Galaxies: NGC 1052-DF2 and NGC 1052-DF4, which are dark matter lacking, and Dragonfly 44, which exhibits a highly dominant dark matter component. Our analysis shows that the non-local corrections to the Newtonian potential do not affect the kinematic predictions, hence no spoiling effects emerge when the Non-local Gravity model serves as a dark energy model. We additionally provide the minimum value that the characteristic non-local radii can reach at these mass scales.

We investigate gravitational waves generated in $f(Q,B)$ non-metric gravity, i.e., a theory of gravity described by a non-metric compatible connection, free of torsion and curvature. It is an extension of symmetric teleparallel gravity, equipped with a boundary term $B$. This theory exhibits gravitational waves regardless of the gauge adopted: they are the standard massless tensors plus a massive scalar gravitational wave like in the case of $f(R)$ gravity. It is precisely the boundary term $B$ that generates the massive scalar mode with an effective mass $m_{B}$ associated to a Klein-Gordon equation in the linearized boundary term. As in $f(Q)$ gravity also in $f(Q,B)$ non-metric gravity, a free test particle follows a geodesic motion due to the covariant conservation with respect to the Levi-Civita connection of the energy and momentum densities on shell. Therefore, in $f(Q,B)$ gravity, the proper acceleration between two neighboring worldlines traveled by two free point-like particle is governed by a first-order geodesic deviation equation in the metric perturbation $h_{\mu\nu}$. Thanks to this approximate linear equation, $f(Q,B)$ non-metric gravity shows three polarization modes: two massless transverse tensor radiation modes, with helicity equal to 2, reproducing the standard plus and cross modes, exactly as in General Relativity, and an additional massive scalar wave mode with transverse polarization of zero helicity. We obtain the same result both by considering the coincidence gauge and by leaving the gauge free. In summary, three degrees of freedom propagate in the $f(Q,B)$ linearized theory with amplitudes $\tilde{h}^{(+)}$ and $\tilde{h}^{(\times)}$ for tensor modes and amplitude $\tilde{h}^{(s)}$ for the scalar mode. Specifically, both $f(Q,B)$ and $f(R)$ gravity involve the same massive transverse scalar perturbation., Comment: 33 pages

The effects of cosmological expansion on quantum bosonic states are investigated, using quantum information theory. In particular, a generic Bogoliubov transformation of bosonic field modes is considered and the state change on a single mode is regarded as the effect of a quantum channel. Properties and capacities of this channel are thus explored in the framework of $f(Q)$ theories. As immediate result, we obtain that the information on a single-mode state appears better preserved, whenever the number of particles produced by the cosmological expansion is small. Hence, similarly to general relativity, we show that analogous particle productions result even if we consider symmetric teleparallel gravity theories. Thus, we investigate a power law $f(Q)$ model, leaving unaltered the effective gravitational coupling, and minimise the corresponding particle production. We thus show how to optimise the preservation of classical and quantum information, stored in a bosonic mode states in the remote past. Finally, we compare our findings with those obtained in general relativity., Comment: 16 pages, 3 figures, 2 tables

We investigate the polarization modes of gravitational waves in $f(Q)$ non-metric gravity without gauge fixing. The main result of this study is that no further scalar mode appears more than the two standard plus and cross transverse polarizations of massless tensor gravitational radiation, typical of General Relativity. This is because the first-order perturbation of connection does not modify the linearized field equations in vacuum which remain gauge invariant. Then, the world line equations of free point particles, as well as the equations of their deviations, are obtained using only the symmetric teleparallel connection. In $f(Q)$ gravity, test masses follow timelike geodesics and not autoparallel curves. In the proper reference frame, thanks to the geodesic deviation equation of the structure-less bodies in free fall, we prove that, in any gauge, only the metric perturbations $h_{\mu\nu}$, related to tensor modes, survive by exploiting the gauge invariance. Besides, scalar modes disappear. This allows us to conclude that only two degrees of freedom of linearized $f(Q)$ non-metric gravity propagate as in General Relativity and in $f(T)$ teleparallel gravity. The situation is different with respect to $f(R)$ gravity (with $f(R)\neq R$) where a further scalar mode is found., Comment: 22 pages, to appear in Phys.Dark Univ

We discuss spherically symmetric dynamical systems in the framework of a general model of $f({\cal R})$ gravity, i.e. $f({\cal R})={\cal R}e^{\zeta {\cal R}}$, where $\zeta$ is a dimensional quantity in squared length units [L$^2$]. We initially assume that the internal structure of such systems is governed by the Krori-Barua ansatz, alongside the presence of fluid anisotropy. By employing astrophysical observations obtained from the pulsar {\textit SAX J1748.9-2021}, derived from bursting X-ray binaries located within globular clusters, we determine that $\zeta$ is approximately equal to $\pm 5$ km$^2$. In particular, the model can create a stable configuration for {\textit SAX J1748.9-2021}, encompassing its geometric and physical characteristics. In $f({\cal R})$ gravity, the Krori-Barua approach links $p_r$ and $p_t$, which represent the components of the pressures, to ($\rho$), representing the density, semi-analytically. These relations are described as $p_r\approx v_r^2 (\rho-\rho_{I})$ and $p_t\approx v_t^2 (\rho-\rho_{II})$. Here, the expression $v_r$ and $v_t$ represent the radial and tangential sound speeds, respectively. Meanwhile, $\rho_I$ pertains to the surface density and $\rho_{II}$ is derived using the parameters of the model. Notably, within the frame of $f({\cal R})$ gravity where $\zeta$ is negative, the maximum compactness, denoted as $C$, is inherently limited to values that do not exceed the Buchdahl limit. This contrasts with general relativity or with $f({\cal R})$ with positive $\zeta$, where $C$ has the potential to reach the limit of the black hole asymptotically. The predictions of such model suggest a central energy density which largely exceeds the saturation of nuclear density, which has the value $\rho_{\text{nuc}} = 3\times 10^{14}$ g/cm$^3$. Also, the density at the surface $\rho_I$ surpasses $\rho_{\text{nuc}}$., Comment: 28 pages, 8 figures, Will appear in EPJC

We investigate a Schwarzschild metric exhibiting a signature change across the event horizon, which gives rise to what we term a Lorentzian-Euclidean black hole. The resulting geometry is regularized by employing the Hadamard partie finie technique, which allows us to prove that the metric represents a solution of vacuum Einstein equations. In this framework, we introduce the concept of atemporality as the dynamical mechanism responsible for the transition from a regime with a real-valued time variable to a new one featuring an imaginary time. We show that this mechanism prevents the occurrence of the singularity and, by means of the regularized Kretschmann invariant, we discuss in which terms atemporality can be considered as the characteristic feature of this black hole., Comment: 22 pages, 5 figures, accepted for publication in Phys. Rev. D

We demonstrate that a $\sim 2 \sigma$ discrepancy with the Planck-$\Lambda$CDM cosmology in DESI Luminous Red Galaxy (LRG) data at $z_{\textrm{eff}} = 0.51$ translates into an unexpectedly large $\Omega_m$ value, $\Omega_m = 0.668^{+0.180}_{-0.169}$. We independently confirm that this anomaly drives the preference for $w_0 > -1$ in DESI data confronted to the $w_0 w_a$CDM model. We show that redshift bins of DESI constraints allow $\Omega_m$ to wiggle at the $\sim 2 \sigma$ level with increasing effective redshift in the $\Lambda$CDM model. Given that LRG data at $z_{\textrm{eff}} = 0.51$ is at odds with Type Ia supernovae in overlapping redshifts, we expect that this anomaly will decrease in statistical significance with future DESI data releases leaving an increasing $\Omega_m$ trend with effective redshift at higher redshifts. We estimate the significance of the latter in DESI data at $\sim 1.8 \sigma$ and comment on how it dovetails with independent observations. It is imperative to understand what makes DESI LRG data at $z_{\textrm{eff}} = 0.51$ an outlier when it comes to $\Omega_m$ determinations., Comment: 5 pages, 3 figures; v2 comments added, references updated

We present a novel model-independent generic mechanism for primordial black hole formation within the context of non-singular matter bouncing cosmology. In particular, considering a short duration transition from the matter contracting phase to the Hot Big Bang expanding Universe, we find naturally enhanced curvature perturbations on very small scales which can collapse and form primordial black holes. Interestingly, the primordial black hole masses that we find can lie within the observationally unconstrained asteroid-mass window, potentially explaining the totality of dark matter. Remarkably, the enhanced curvature perturbations, collapsing to primordial black holes, can induce as well a stochastic gravitational-wave background, being potentially detectable by future experiments, in particular by SKA, PTAs, LISA and ET, hence serving as a new portal to probe the potential bouncing nature of the initial conditions prevailed in the early Universe., Comment: Accepted at JCAP

We address the galaxy rotation curves through the Yukawa gravitational potential emerging as a correction of the Newtonian potential in extended theories of gravity. On the one hand, we consider the contribution of the galactic bulge, galactic disk, and the dark matter halo of the Navarro-Frenk-White profile, in the framework of the standard $\Lambda$CDM model. On the other hand, we use modified Yukawa gravity to show that the rotational velocity of galaxies can be addressed successfully without the need for dark matter. In Yukawa gravity, we recover MOND and show that dark matter might be seen as an apparent effect due to the modification of the law of gravitation in terms of two parameters: the coupling constant $\alpha $ and the characteristic length $\lambda$. We thus test our theoretical scenario using the Milky Way and M31 rotation velocity curves. In particular, we place observational constraints on the free parameters of Yukawa cosmology through the Monte Carlo method and then compare our results with the predictions of the $\Lambda$CDM paradigm by making use of Bayesian information criteria. Specifically, we find that $\lambda$ is constrained to be of the order of kpc, while cosmological data suggest $\lambda$ of the order of Gpc. To explain this discrepancy, we argue that there is a fundamental limitation in measuring $\lambda$ due to the role of quantum mechanics on cosmological scales., Comment: 18 pages, 8 figures. Accepted for publication in EPJC

In view to scrutinize the idea that nonlocal modifications of General Relativity could dynamically address the dark energy problem, we investigate the evolution of the Universe at infrared scales as an Infinite Derivative Gravity model of the Ricci scalar, without introducing the cosmological constant $\Lambda$ or any scalar field. The accelerated expansion of the late Universe is shown to be compatible with the emergence of nonlocal gravitational effects at sufficiently low energies. A technique for circumventing the mathematical complexity of the nonlocal cosmological equations is developed and, after drawing a connection with the Starobinsky gravity, verifiable predictions are considered, like a possible decreasing in the strength of the effective gravitational constant. In conclusion, the emergence of nonlocal gravity corrections at given scales could be an efficient mechanism to address the dark energy problem., Comment: 16 pages, 2 figures, accepted for publication in Physics of the Dark Universe

A critical discussion on the $H_0$ Hubble constant tension is presented by considering both early and late-type observations. From recent precise measurements, discrepancies emerge when comparing results for some cosmological quantities obtained at different redshifts. We highlight the most relevant measurements of $H_0$ and propose potential ideas to solve its tension. These solutions concern the exploration of new physics beyond the $\Lambda$CDM model or the evaluation of $H_0$ by other methods. In particular, we focus on the role of the look-back time., Comment: 33 pages, 7 figures

The redshift $z_t$ and the jerk parameter $j_t$ of the transition epoch are constrained by using two model-independent approaches involving the direct expansion of the Hubble rate and the expansion of the deceleration parameter around $z=z_t$. To extend our analysis to high-redshifts, we employ the \emph{Amati}, \emph{Combo}, \emph{Yonetoku} and \emph{Dainotti} gamma-ray burst correlations. The \textit{circularity problem} is prevented by calibrating these correlations through the B\'ezier interpolation of the updated observational Hubble data. Each gamma-ray burst data set is jointly fit with type Ia supernovae and baryonic acoustic oscillations through a Monte Carlo analysis, based on the Metropolis-Hastings algorithm, to obtain $z_t$, $j_t$ and the correlation parameters. The overall results are compatible with the concordance model with some exceptions. We also focus on the behaviors of the dark energy, verifying its compatibility with a cosmological constant, and the matter density $\Omega_m$ and compare them with the expectations of the concordance paradigm., Comment: 20 pages, 9 figures, 4 tables

In this work, we investigate the dissociation energy of the North (N) and South (S) poles of a quantum magnetic particle, incorporated within both classical and quantum mechanical perspectives. A simple model of a harmonic oscillator is employed to estimate the dissociation energy of the N-S poles, as well as the corresponding breakdown temperature and internal pressure. The results indicate that the separation of magnetic poles occurs in two states: (a) in an ultra-hot plasma medium with extremely high temperatures, such as in the core of a hot star, and (b) at extremely high pressures, such as between internal plates in complex superlattices of layered solids. The breakdown temperature is found to be of the order of $10^7$ to $10^8$ Kelvin, which is only achievable in an ultra-hot plasma environment, known as the fifth phase of matter. Based on this model, the possibility of dissociation of bonds between N and S magnetic poles for solid superlattices under very high pressures between crystal plates is also calculated. The results suggest that the presence of isolated magnetic monopoles in superlattices of solids under ultra-high-pressure conditions is possible. Consequently, the model proposes that the conductivity of magnetic monopole carriers can be applied to the manipulation of nanomaterials for the production of advanced devices, such as new generations of superconductors, new spin devices, and magnetic-electronics, advanced materials with magnetic monopoles, as well as super-dielectrics., Comment: 15 pages, 6 figures. Accepted for publication in the International Journal of Modern Physics D

We investigate graviton-photon oscillations sourced by cosmological magnetic fields from Gertsenshtein effect. We adopt a robust perturbative approach and we find that the conversion probability from graviton to photon can be resonantly enhanced in monochromatic, multi-chromatic and scale invariant spectrum models of stochastic magnetic field fluctuations. In addition, the expansion of the Universe acts as a decoherence factor, which demands a natural discretization scheme along the line of sight. Including also decoherence from cosmic acceleration, we find that conversion probabilities for stochastic magnetic fields are completely different than results predicted from existing magnetic domain-like models in a wide range of magnetic strengths and coherence lengths. Resonances can be tested by radio telescopes as a probe of high frequency gravitational wave sources and primordial magnetogenesis mechanisms., Comment: 29 pages, 5 figures

Gravitational radiation-reaction phenomena occurring in the dynamics of inspiralling compact binary systems are investigated at the first post-Newtonian order beyond the quadrupole approximation in the context of Einstein-Cartan theory, where quantum spin effects are modeled via the Weyssenhoff fluid. We exploit balance equations for the energy and angular momentum to determine the binary orbital decay until the two bodies collide. Our framework deals with both quasi-elliptic and quasi-circular trajectories, which are then smoothly connected. Key observables like the laws of variation of the orbital phase and frequency characterizing the quasi-circular motion are derived analytically. We conclude our analysis with an estimation of the spin contributions at the merger, which are examined both in the time domain and the Fourier frequency space through the stationary wave approximation., Comment: 19 pages, 3 figures, 1 table; accepted for publication on EPJ C

Although the $\Lambda$ Cold Dark Matter model is the most accredited cosmological model, information at high redshifts ($z$) between type Ia supernovae ($z=2.26$) and the Cosmic Microwave Background ($z=1100$) is crucial to validate this model further. To this end, we have discovered a sample of 1132 quasars up to $z=7.54$ exhibiting a reduced intrinsic dispersion of the relation between ultraviolet and X-ray fluxes, $\delta_\mathrm{F}=0.22$ vs. $\delta_\mathrm{F}=0.29$ ($24\%$ less), than the original sample. This gold sample, once we correct the luminosities for selection biases and redshift evolution, enables us to determine the matter density parameter $\Omega_M$ with a precision of 0.09. Unprecedentedly, this quasar sample is the only one that, as a standalone cosmological probe, yields such tight constraints on $\Omega_M$ while being drawn from the same parent population of the initial sample., Comment: 36 pages, 19 figures,2 table. Comments are welcome. Accepted in Galaxies

The Equivalence Principle is considered in the framework of metric-affine gravity. We show that it naturally emerges as a Noether symmetry starting from a general non-metric theory. In particular, we discuss the Einstein Equivalence Principle and the Strong Equivalence Principle showing their relations with the non-metricity tensor. Possible violations are also discussed pointing out the role of non-metricity in this debate., Comment: 10 pages, to appear in International Journal of Geometric Methods in Modern Physics

We investigate gravitational waves in the $f(Q)$ gravity, i.e., a geometric theory of gravity described by a non-metric compatible connection, free from torsion and curvature, known as symmetric-teleparallel gravity. We show that $f(Q)$ gravity exhibits only two massless and tensor modes. Their polarizations are transverse with helicity equal to two, exactly reproducing the plus and cross tensor modes typical of General Relativity. In order to analyze these gravitational waves, we first obtain the deviation equation of two trajectories followed by nearby freely falling point-like particles and we find it to coincide with the geodesic deviation of General Relativity. This is because the energy-momentum tensor of matter and field equations are Levi-Civita covariantly conserved and, therefore, free structure-less particles follow, also in $f(Q)$ gravity, the General Relativity geodesics. Equivalently, it is possible to show that the curves are solutions of a force equation, where an extra force term of geometric origin, due to non-metricity, modifies the autoparallel curves with respect to the non-metric connection. In summary, gravitational waves produced in non-metricity-based $f(Q)$ gravity behave as those in torsion-based $f(T)$ gravity and it is not possible to distinguish them from those of General Relativity only by wave polarization measurements. This shows that the situation is different with respect to the curvature-based $f(R)$ gravity where an additional scalar mode is always present for $f(R)\neq R$., Comment: 21 pages, accepted for publication in Phys. Lett. B

We investigate supermassive black holes (SMBH), surrounded by a dark matter (DM) spike, that can be found at the centers of Milky Way and M87 galaxies and are accompanied by a specific kind of topological defect. The investigation is developed within the framework of Bumblebee Gravity (BG) with a global monopole (GM). The power-law density profile, describing the DM spike, is expected to follow a power-law distribution starting from a few times the BH horizon radius and extending outward to an external radius Rsp. We determine the normalization parameter, \r{ho}0, for the density profile to ensure its compatibility with both the galaxy total mass and the enclosed DM mass within the radius of influence of Sagittarius A (Sgr A) and M87 SMBHs, approximately of order 105Rs. Our main objective is to assess how the background arising from spontaneous Lorentz symmetry (LS) breaking and the presence of a GM influence the properties of the Kerr BH within the region affected by the spike. The scarcity of non-rotating and rotating BH models embedded in a DM spike and subject to BG and GM (BGGM) effects significantly hinders the progress in testing BGGM through observations. To address this issue, we initiate our investigation by considering a spherically symmetric static BH with BGGM properties as the seed metric. From there, we construct a non-rotating spacetime with a DM spike (similar to the Schwarzschild spacetime), leading to the emergence of the BGGM-motivated Schwarzschild-like BH (BGMSLBH). To construct the BGMSLBH spacetime, we employ two different methods: one involves solving the modified Tolman-Oppenheimer-Volkoff (TOV) equations, while the other entails solving the modified Einstein field equations. In the latter approach, we consider the energy-momentum tensor incorporating the contributions of DM spike and BH counterpart of the BGGM.

We consider non-local modifications of General Relativity given by a distortion function in terms of the inverse of the d'Alembert operator. The inclusion of these terms is motivated by the possibility of reproducing the current accelerated expansion of the Universe starting from non-local gravity. In particular, we propose a model-independent method, based on current observations, to reconstruct the shape of the distortion function, without resorting to a specific cosmological history. We describe a numerical procedure based on Pad\'e polynomials that allows us to study the time evolution of the auxiliary scalar fields introduced to localize non-local gravity action. Thus, adopting suitable boundary conditions, we reconstruct the form of the non-local Lagrangian and infer the best analytical approximation of the numerical outcome. Furthermore, the distortion function turns on during the matter-dominated era, and its effects are delayed to the most recent cosmic times. This provides a natural explanation for the late cosmic acceleration, avoiding any fine-tuning problem of the cosmological constant. Finally, we compare our predictions with previous findings based on enforcing the standard $\Lambda$CDM background in the reconstruction process., Comment: 8 pages, 3 figures. Accepted for publication in Physics of the Dark Universe

Recently, cosmography emerged as a valuable tool to effectively describe the vast amount of astrophysical observations without relying on a specific cosmological model. Its model-independent nature ensures a faithful representation of data, free from theoretical biases. Indeed, the commonly assumed fiducial model, the $\Lambda$CDM, shows some shortcomings and tensions between data at late and early times that need to be further investigated. In this paper, we explore an extension of the standard cosmological model by adopting the $f(z)$CDM approach, where $f(z)$ represents the cosmographic series characterizing the evolution of recent universe driven by dark energy. To construct $f(z)$, we take into account the Pad\'e series, since this rational polynomial approximation offers a better convergence at high redshifts than the standard Taylor series expansion. Several orders of such an approximant have been proposed in previous works, here we want to answer the questions: What is the impact of the cosmographic series choice on the parameter constraints? Which series is the best for the analysis? So, we analyse the most promising ones by identifying which order is preferred in terms of stability and goodness of fit. Theoretical predictions of the $f(z)$CDM model are obtained by the Boltzmann solver code and the posterior distributions of the cosmological and cosmographic parameters are constrained by a Monte Carlo Markov Chains analysis. We consider a joint data set of cosmic microwave background temperature measurements from the Planck collaboration, type Ia supernovae data from the latest Pantheon+ sample, baryonic acoustic oscillations and cosmic chronometers data. In conclusions, we state which series can be used when only late time data are used, while which orders has to be considered in order to achieve the necessary stability when large redshifts are considered., Comment: 27 pages, 6 figures, 2 tables

We derive a novel class of four-dimensional black hole solutions in Gauss-Bonnet gravity coupled with a scalar field in presence of Maxwell electrodynamics. In order to derive such solutions, we assume the ansatz $ g_{tt}\neq g_{rr}{}^{-1}$ for metric potentials. Due to the ansatz for the metric, the Reissner Nordstr\"om gauge potential cannot be recovered because of the presence of higher-order terms ${\cal O}\left(\frac{1}{r}\right)$ which are not allowed to be vanishing. Moreover, the scalar field is not allowed to vanish. If it vanishes, a function of the solution results undefined. For this reason, the solution cannot be reduced to a Reissner Nordstr\"om space-time in any limit. Furthermore, it is possible to show that the electric field is of higher-order in the monopole expansion: this fact explicitly comes from the contribution of the scalar field. Therefore, we can conclude that the Gauss-Bonnet scalar field acts as non-linear electrodynamics creating monopoles, quadrupoles, etc. in the metric potentials. We compute the invariants associated with the black holes and show that, when compared to Schwarzschild or Reissner-Nordstr\"om space-times, they have a soft singularity. Also, it is possible to demonstrate that these black holes give rise to three horizons in AdS space-time and two horizons in dS space-time. Finally, thermodynamic quantities can be derived and we show that the solution can be stable or unstable depending on a critical value of the temperature., Comment: 28 pages three figures, will appear in Class.& Quant. Grav

Physical evolution of cosmological models can be tested by using expansion data, while growth history of these models is capable of testing dynamics of the inhomogeneous parts of energy density. The growth factor, as well as its growth index, gives a clear indication of the performance of cosmological models in the regime of structure formation of early Universe. In this work, we explore the growth index in several leading $f(T)$ cosmological models, based on a specific class of teleparallel gravity theories. These have become prominent in the literature and lead to other formulations of teleparallel gravity. Here we adopt a generalized approach by obtaining the M\'{e}sz\'{a}ros equation without immediately imposing the subhorizon limit, because this assumption could lead to over-simplification. This approach gives avenue to study at which $k$ modes the subhorizon limit starts to apply. We obtain numerical results for the growth factor and growth index for a variety of data set combinations for each $f(T)$ model.

We consider static and spherically symmetric wormhole solutions in extended metric-affine theories of gravity supposing that stability and traversability of these objects can be achieved by means of the geometric degrees of freedom. In particular, we consider $f(R)$ metric, $f(T)$ teleparallel, and $f(Q)$ symmetric teleparallel models where curvature, torsion, and non-metricity rule entirely the background geometry without invoking any exotic energy-momentum tensor as matter field source. Starting from the flaring out and null energy conditions, we gather together a series of constraints which allow us to state that stable and traversable wormholes can be derived in a purely geometric approach resorting to modified gravity theories with more degrees of freedom than general relativity., Comment: 10 pages, 1 table; accepted for publication on Phys. Rev. D

The Gauss-Bonnet topological scalar is presented in metric-teleparallel formalism as well as in the symmetric and general teleparallel formulations. In all of the aforementioned frameworks, the full expressions are provided explicitly in terms of torsion, non-metricity and Levi-Civita covariant derivative. The number of invariant terms of this form is counted and compared with the number which can appear in the corresponding effective field theory. Although the difference in this number is not very large, it is found that the Gauss-Bonnet invariant excludes some of the effective field theory terms. This result sheds new light on how General Relativity symmetries can be maintained at higher order in teleparallel theories: this fact appears to be highly nontrivial in the teleparallel formulation. The importance of the so-called ``pseudo-invariant'' theories like $f(T)$- and $f(T,T_\mathcal{G})$-gravity is further discussed in the context of teleparallel Gauss-Bonnet gravity., Comment: 18 pages; Accepted for publication in IJGMMP

We sketch the main features of the Noether Symmetry Approach, a method to reduce and solve dynamics of physical systems by selecting Noether symmetries, which correspond to conserved quantities. Specifically, we take into account the vanishing Lie derivative condition for general canonical Lagrangians to select symmetries. Furthermore, we extend the prescription to the first prolongation of the Noether vector. It is possible to show that the latter application provides a general constraint on the infinitesimal generator $\xi$, related to the spacetime translations. This approach can be used for several applications. In the second part of the work, we consider a gravity theory including the coupling between a scalar field $\phi$ and the Gauss-Bonnet topological term $\mathcal{G}$. In particular, we study a gravitational action containing the function $F(\mathcal{G}, \phi)$ and select viable models by the existence of symmetries. Finally, we evaluate the selected models in a spatially-flat cosmological background and use symmetries to find exact solutions., Comment: 13 pages, 1 figure. Accepted for publication in Symmetry

In the framework of metric-affine gravity, we consider the role of the boundary term in Symmetric Teleparallel Gravity assuming $f(Q,B)$ models where $f$ is a smooth function of the non-metricity scalar $Q$ and the related boundary term $B$. Starting from a variational approach, we derive the field equations and compare them with respect to those of $f(Q)$ gravity in the limit of $B\to0$. It is possible to show that $f(Q,B)=f(Q-B)$ models are dynamically equivalent to $f(R)$ gravity as in the case of teleparallel $f(\tilde{B}-T)$ gravity (where $B\neq \tilde{B}$). Furtherrmore, conservation laws are derived. In this perspective, considering boundary terms in $ f(Q)$ gravity represents the last ingredient towards the Extended Geometric Trinity of Gravity, where $f(R)$, $f(T,\tilde{B})$, and $f(Q,B)$ can be dealt with under the same standard. We also compare and discuss about the Gibbons-Hawking-York boundary term of General Relativity and the boundary term $B$ in $f(Q,B)$ gravity., Comment: 8 pages, 1 figure, accepted for publication in EPJC

In the framework of scalar-tensor gravity, we consider non-flat interacting quintessence cosmology where a scalar field is interacting with dark matter. Such a scalar field can be a standard or a phantom one. We use the Noether Symmetry Approach to obtain general exact solutions for cosmological equations and to select scalar-field self-interaction potentials. It turns out that the found solutions can reproduce the accelerated expansion of the Universe, and are compatible with observational dataset, as the SNeIa Pantheon data, gamma ray bursts Hubble diagram, and direct measurements of the Hubble parameter., Comment: 30 pages, 7 figures, in press on: Phys.Dark Univ. (2023) 101274

Large frame Ring laser gyroscopes, based on the Sagnac effect, are top sensitivity instrumentation to measure angular velocity with respect to the fixed stars. GINGER (Gyroscopes IN GEneral Relativity) project foresees the construction of an array of three large dimension ring laser gyroscopes, rigidly connected to the Earth. GINGER has the potentiality to measure general relativity effects and Lorentz Violation in the gravity sector, once a sensitivity of $10^{-9}$, or better, of the Earth rotation rate is obtained. Being attached to the Earth crust, the array will also provide useful data for geophysical investigation. For this purpose, it is at present under construction as part of the multi-components observatory called Underground Geophysics at Gran Sasso (UGSS). Sensitivity is the key point to determine the relevance of this instrument for fundamental science. The most recent progress in the sensitivity measurement, obtained on a ring laser prototype called GINGERINO, indicates that GINGER should reach the level of 1 part in $10^{11}$ of the Earth rotation rate., Comment: 7 pages, 1 figure

We consider a model-independent approach to constrain the equivalence redshift, $z_{eq}$, at which dark energy and the total matter (cold dark matter and baryonic) equate their magnitudes. To this aim, in the context of a homogeneous and isotropic universe, we first consider a generic model where the dark energy contribution is provided by an unknown function of barotropic fluids. Afterwards, we compute the deceleration and jerk parameters, evaluating at our epoch, namely $z=0$, and at $z=z_{eq}$. Thus, by Taylor expanding around current time the Hubble rate, luminosity and angular distances, we substitute the theoretical expressions obtained from the aforementioned generic dark energy model, defining a correspondence between quantities evaluated at $z=0$ and $z=z_{eq}$. In so doing, we directly fit these quantities by means of current data sets, involving the most recent Pantheon type Ia supernovae, baryonic acoustic oscillation and Hubble rate points. We consider two hierarchies in our fitting procedures and compare our findings in the spatially-flat universe first and including spatial curvature, later. We assess constraints on the overall equation of state of the universe and its first derivative. We compare our results with those predicted by the standard $\Lambda$CDM paradigm. Specifically, our findings are in agreement at the 2$\sigma$ confidence level, assuming a constant dark energy term. However, our analysis does not rule out the possibility of a slight evolution of dark energy, indicating a small deviation from the scenario of a pure cosmological constant. In particular, the possible departures appear consistent with a phenomenological $\omega$CDM model, rather than more complicated dark energy parameterizations., Comment: 28 pages, 8 figures, 3 tables