arXiv subjects

Daniele Malafarina

Publications and source records attributed to Daniele Malafarina.

At least 19 recordsLinked to original sources

arXiv2026

A novel phenomenological parametrization of dynamical dark energy via matter-gravity coupling

We develop a new parametrization for generic departures from the $Λ$CDM model based on the Markov-Mukhanov action with non-minimal matter-gravity coupling. In this approach the cosmological constant $Λ$ and the gravitational coupling $G$ become dynamical with their values determined by the explicit form of the matter-gravity coupling. We use the framework to build a simple phenomenological model where dynamical dark energy is naturally obtained as a consequence of the departure of the theory from classical General Relativity at low densities. We show how the model's free parameters can be constrained from observational data and interpret the tentative observations of `phantom crossing' by the DESI collaboration in view of the proposed formalism.

gr-qc↗

arXiv2026

Can UV meet IR in the Swiss cheese?

We consider the embedding of regular black holes in an expanding universe and study how the ultraviolet modifications to the Schwarzschild geometry that regularize the black hole singularity affect the exterior universe's expansion rate. We consider several proposals for the regular black hole geometry and obtain the corresponding Friedmann equations for a universe filled only with dust and black holes. We show that different proposals have different implications which may be distinguished. We then test the hypothesis that the UV corrections to the black hole geometry may be responsible for the current phase of accelerated expansion. To this aim we constrain the value of the regular black hole UV cutoff parameter from observations. Interestingly we find that the best fit is obtained by values of the parameter corresponding to regular horizonless compact objects.

gr-qc↗

arXiv2026

Dust collapse in asymptotic safety: a path to regular black holes

Regular black hole spacetimes are obtained from an effective Lagrangian for Quantum Einstein Gravity. The interior matter is modeled as a dust fluid, which interacts with the geometry through a multiplicative coupling function denoted as $χ$. The specific functional form of $χ$ is deduced from Asymptotically Safe gravity, under the key assumption that the Reuter fixed point remains minimally affected by the presence of matter. As a consequence the gravitational coupling vanishes at high energies. The static exterior geometry of the black hole is entirely determined by the junction conditions at the boundary surface. Consequently, the resulting global spacetime geometry remains devoid of singularities at all times. This outcome offers a new perspective on how regular black holes are formed through gravitational collapse.

gr-qc↗

arXiv2026

How to obtain slow roll inflation driven by non-linear electrodynamics

We establish for the first time the conditions that must be imposed on the action for a magnetic universe in a theory of non-linear electrodynamics in order to have an asymptotically de Sitter initial state followed by a slow roll inflationary phase. We show that models so far proposed in the literature do not allow for a prolonged inflationary phase consistent with observations. We construct a Lagrangian that reduces to the Maxwell one in weak field; this is the only class of models that satisfies the required conditions for slow roll in the early Universe.

gr-qc↗

arXiv2025

A unified model of dark energy and inflation from the Markov-Mukhanov action

We propose a unified model of dark energy and inflation through the Markov-Mukhanov modification of the Einstein-Hilbert action, where the matter sector is coupled to gravity via a scalar coupling function depending only on the energy density of the matter content. We assume that the coupling function encodes the UV corrections to the standard model of cosmology and we determine the form of the coupling that allows for the dark energy component to be dynamical and act as the inflaton field in the early universe. Interestingly we show that our model, in order to account for inflation, prefers a dark energy equation of state with $w$ close but not equal to $-1$ in agreement with the latest DESI data.

gr-qc↗

arXiv2025

Observational properties of regular black holes in Asymptotic Safety

We consider the observational properties of a spherically symmetric, static regular black hole within the framework of asymptotic safety (AS) as proposed by Bonanno et al. The metric resembles the Schwarzschild solution in the classical limit. The departure from Schwarzschild at small scales is controlled by a single free parameter related to the ultraviolet (UV) cutoff of the theory. We investigated null and time-like geodesics around the AS metric, including circular orbits, photon rings and lensing effects. In particular we focused on the optical properties of thin accretion disks in the equatorial plane of the object and compared them with those of accretion disks in the Schwarzschild metric. We found that the radiation flux, luminosity, and efficiency of the accretion disk increase with the value of the free parameter. Using a spacetime generic open-source relativistic ray-tracing code, we simulate the K$α$ iron line profiles emitted by the disk and analyze their deviation from that of the Schwarzschild geometry.

gr-qc↗

arXiv2025

Intermediate mass ratio inspirals in dark matter halos

We study eccentric equatorial orbits of a stellar-mass black hole around an intermediate-mass slowly rotating Kerr black hole in the presence of gravitational radiation and a dark matter halo. The stellar-mass companion will inspiral towards the central black hole while emitting gravitational waves. The evolution is affected by the dynamical friction force caused by the presence of dark matter surrounding the intermediate-mass black hole at the core. Previous studies have shown that the presence of dark matter can be deduced from the gravitational waveforms. We explore the combined effects of slow-rotation of the central black hole and the presence of the dark matter halo around it on the system's gravitational wave emission.

gr-qc↗

arXiv2025

An emergent cosmological model from running Newton constant

We propose an emergent cosmological model rooted in the Asymptotically Safe antiscreening behavior of the Newton constant at Planckian energies. Distinguishing itself from prior approaches, our model encapsulates the variable nature of $G$ through a multiplicative coupling within the matter Lagrangian, characterized by a conserved energy-momentum tensor. The universe emerges from a quasi-de Sitter phase, transitioning to standard cosmological evolution post-Planck Era. Our analysis demonstrates the feasibility of constraining the transition scale to nearly classical cosmology using Cosmic Microwave Background (CMB) data and the potential to empirically probe the antiscreening trait of Newton's constant, as predicted by Asymptotic Safety.

gr-qc↗

arXiv2024

Running gravitational constant induced dark energy as a solution to $σ_8$ tension

We consider a modified gravity model with a running gravitational constant coupled to a varying dark energy fluid and test its imprint on the growth of structure in the universe. Using Redshift Space Distortion (RSD) measurement results, we show a tension at the $3 σ$ level between the best fit $Λ$CDM and the corresponding parameters obtained from the Planck data. Unlike many modified gravity-based solutions that overlook scale dependence and model-specific background evolution, we study this problem in the broadest possible context by incorporating both factors into our investigation. We performed a full perturbation analysis to demonstrate a scale dependence in the growth equation. Fixing the scale to $k = 0.1 h$ Mpc$^{-1}$ and introducing a phenomenological functional form for the varying Newton coupling $G$ with only one free parameter, we conduct a likelihood analysis of the RSD selected data. The analysis reveals that the model can bring the tension level within $1 σ$ while maintaining the deviation of $G$ from Newton's gravitational constant at the fifth order.

astro-ph.CO↗

arXiv2024

Gravitational lensing of neutrinos in the Rezzolla-Zhidenko spacetime

We consider gravitational lensing of neutrinos in the Rezzolla-Zhidenko spacetime in the weak-field limit with plane-wave approximation. We apply the analysis to an hypothetical system with a central object with its mass of the order of solar mass and a detector located at an Earth-like distance from the source. We find that the deformation parameters of the Rezzolla-Zhidenko metric can have significant impact on the oscillation probabilities of the neutrinos. We also investigate the role of decoherence on flavor oscillations of the lensed neutrinos and show that the parameters of the Rezzolla-Zhidenko metric does not have significant effects on the decoherence length.

gr-qc↗

arXiv2024

Observational properties of Coherent Quantum Black Holes

We consider null and time-like geodesics around a spherically symmetric, non-rotating Coherent Quantum Black Hole (CQBH). The classical limit of the geometry of CQBH departs from that of the Schwarzschild spacetime at short scales and depends on one parameter $R_s$ which can be interpreted as the physical radius of the 'quantum' core. We study circular orbits, photon rings, and lensing effects and compare them with the Schwarzschild metric. Using the relativistic ray-tracing code GYOTO, we produce a simulation of the shadow and show that thin accretion disks around a CQBH possess unique ring structures that distinguish them from other theoretical models.

gr-qc↗

arXiv2023

Testing the $δ$-Kerr metric with black hole X-ray data

The spacetime around astrophysical black holes is thought to be described by the Kerr solution. However, even within general relativity, there is not yet a proof that the final product of the complete collapse of an uncharged body can only be a Kerr black hole. We can thus speculate on the possibility that the spacetime around astrophysical black holes may be described by other solutions of the Einstein Equations and we can test such a hypothesis with observations. In this work, we consider the $δ$-Kerr metric, which is an exact solution of the field equations in vacuum and can be obtained from a non-linear superposition of the Kerr metric with a static axially symmetric solution, often referred to as the $δ$-metric. The parameter $δ=1+q$ quantifies the departure of the source from the Kerr metric and for $q=0$ we recover the Kerr solution. From the analysis of the reflection features in the X-ray spectrum of the Galactic black hole in EXO 1846-031, we find $-0.1 < q < 0.7$ (90% CL), which is consistent with the hypothesis that the spacetime around the compact object in EXO 1846-031 is a Kerr black hole but does not entirely rule out the $δ$-Kerr metric.

gr-qc↗

arXiv2023

Effects of gravitational lensing by Kaluza-Klein black holes on neutrino oscillations

We study gravitational lensing of neutrinos in a Kaluza-Klein black hole spacetime and compare the oscillation probabilities of neutrinos with the case of lensing by black holes in General Relativity. We show that measuring neutrino oscillations in curved spacetimes may allow us to distinguish the two kinds of black holes. This promises to become an useful tool for future measurements of the properties of black hole candidates and possibly help to constrain the validity of alternative theories of gravity.

gr-qc↗

arXiv2023

Gravitational Collapse in pure Gauss-Bonnet gravity

We study the process of gravitational collapse in pure Gauss-Bonnet gravity. In the homogeneous dust collapse, we show that the $D=7$ pure Gauss-Bonnet theory has gravitational dynamics indistinguishable from Einstein's theory in $D=4$, meaning that collapsing particle feel the same potential as in the classical 4-dimensional general relativistic case. In $D<7$ pure Gauss-Bonnet gravity becomes weaker, while in $D>7$ it becomes stronger, with respect to General Relativity. In the inhomogeneous dust collapse we find the mass modes in the expansion of the energy density in any dimensions that lead to either naked singularities or black holes as final states of collapse.

gr-qc↗

arXiv2022

Semi-classical dust collapse and regular black holes

Semi-classical corrections at large curvature are employed in toy models of spherically symmetric gravitational collapse in order to avoid the formation of singularities. The resulting spacetimes may produce bounces, compact remnants or regular black holes in place of the usual Schwarzschild black hole. Within these models, a whole class of collapse scenarios leading to the formation of regular black holes may be obtained from General Relativity coupled to some theory of non-linear electrodynamics. In the present chapter we provide a thorough exposition of semi-classical dust collapse with particular attention to the conditions for the formation of regular black holes as the endstate of collapse.

gr-qc↗

arXiv2022

Constraining the deformation of a black hole mimicker from the shadow

We consider a black hole mimicker given by an exact solution of the stationary and axially symmetric field equations in vacuum known as the $δ$-Kerr metric. We study its optical properties based on a ray-tracing code for photon motion and characterize the apparent shape of the shadow of the compact object and compare it with the Kerr black hole. For the purpose of obtaining qualitative estimates related to the observed shadow of the supermassive compact object in the galaxy M87 we focus on values of the object's spin $a$ and inclination angle of observation $θ_0$ close to the measured values. We then apply the model to the shadow of the $δ$-Kerr metric to obtain onstraints on the allowed values of the deformation parameter. We show that based uniquely on one set of observations of the shadow's boundary it is not possible to exclude the $δ$-Kerr solution as a viable source for the geometry in the exterior of the compact object.

gr-qc↗

arXiv2022

Accretion disk luminosity for black holes surrounded by dark matter with tangential pressure

We study the motion of test particles in the gravitational field of a Schwarzschild black hole surrounded by a spherical dark matter cloud with non-zero tangential pressure and compute the luminosity of the accretion disk. The presence of non vanishing tangential pressures allows to mimic the dark matter's angular momentum while still considering a static model, which simplifies the mathematical framework. We compare the numerical results about the influence of dark matter on the luminosity of accretion disks around static supermassive black holes with the previously studied cases of isotropic and anisotropic pressures. We show that the flux and luminosity of the accretion disk in the presence of dark matter are different from the case of a Schwarzschild black hole in vacuum and highlight the impact of the presence of tangential pressures.

gr-qc↗

arXiv2022

The connection between regular black holes in nonlinear electrodynamics and semi-classical dust collapse

There exist a correspondence between black holes in non linear electrodynamics (NLED) and gravitational collapse of homogeneous dust with semi-classical corrections in the strong curvature regime that to our knowledge has not been noticed until now. We discuss the nature of such correspondence and explore what insights may be gained from considering black holes in NLED in the context of semi-classical dust collapse and vice-versa.

gr-qc↗