arXiv subjects

Federico Re

Publications and source records attributed to Federico Re.

13 recordsLinked to original sources

arXiv2026

Non-trivial boundary conditions in general-relativistic models

We propose an alternative interpretation of dark matter effects within the framework of General Relativity. In particular, we suggest that, in astrophysical and cosmological contexts, different initial assumptions about a system inevitably lead to different interpretations of the same phenomena. As a concrete example, we examine self-gravitating systems composed of an axially symmetric rotating dust fluid and show that effects typically attributed to the presence of additional matter, can instead be reproduced through an appropriate choice of initial and boundary conditions for the equations governing the system.

gr-qc↗

arXiv2025

Dark Coriolis Fields

We argue that the standard post-Newtonian expansion scheme used in General Relativity leaves room for time-space components $g_{ti}$ of the metric to be of the same order of the usual gravitational potential. We explore this possibility and find that such leading order contributions to $g_{ti}$ are related to the Coriolis field of Newton-Cartan gravity. We investigate the possibility that Coriolis fields mimic dark matter effects in disk galaxies. We find solutions from their field equations that sustain the velocity rotation curves in the bulge region and beyond it, notably describing flattish velocity profiles. We dub such solutions Dark Coriolis Fields.

gr-qc↗

arXiv2025

Non zero Coriolis field in Ehlers' Frame Theory

Ehlers' Frame Theory is a class of geometric theories parameterized by $\lambda := 1/c^2$ and identical to the General Theory of Relativity for $\lambda \neq 0$. The limit $\lambda \to 0$ does not recover Newtonian gravity, as one might expect, but yields the so-called Newton-Cartan theory of gravity, which is characterized by a second gravitational field $\boldsymbol{\omega}$, called the Coriolis field. Such a field encodes at a non-relativistic level the dragging feature of general spacetimes, as we show explicitly for the case of the $(\eta,H)$ geometries. Taking advantage of the Coriolis field, we apply Ehlers' theory to an axially symmetric distribution of matter, mimicking, for example, a disc galaxy, and show how its dynamics might reproduce a flattish rotation curve. In the same setting, we further exploit the formal simplicity of Ehlers' formalism in addressing non-stationary cases, which are remarkably difficult to be treated in the General Theory of Relativity. We show that the time derivative of the Coriolis field gives rise to a tangential acceleration which allows to study a possible formation in time of the rotation curve's flattish feature.

gr-qc↗

arXiv2024

Dark Matter

In this article we address the mystery of dark matter. We expound the various evidences, astrophysical and cosmological, leading to hypothesize the existence of an invisible form of matter, whose attempts at detecting it have so far all failed. We also discuss some alternative suggestions that replace the hypothesis of exotic matter with the assumption of modifications of the gravitational dynamics. For each of the various proposals we also discuss the strong and weak points.

gr-qc↗

arXiv2024

Quasilocal Newtonian limit of general relativity and galactic dynamics

We present a new self-consistent perturbative expansion for realistic isolated differentially rotating systems -- disc galaxies. At leading order it is formally equivalent to Ehlers' Newton-Cartan limit, which we reinterpret in terms of quasilocal energy and angular momentum. The self-consistent coupling of these quasilocal terms leads to first-order differences from the conventional Newtonian limit. A modified Poisson equation is obtained, along with modifications to the equations of motion for the effective fluid elements. By fitting to astrophysical data, we show that the phenomenology of collisionless dark matter for disc galaxies can be reproduced. Potential important consequences for gravitational physics on galactic and cosmological scales are briefly discussed.

gr-qc↗

arXiv2024

Dynamical friction in the quasi-linear formulation of modified Newtonian dynamics (QuMOND)

Aims. We explore the dynamical friction on a test mass in gravitational systems in the Quasi linear formulation of Modified Newtonian Dynamics (QuMOND). Methods. Exploiting the quasi linearity of QuMOND we derive a simple expression for the dynamical friction in akin to its Newtonian counterpart in the standard Chandrasekhar derivation. Moreover, adopting a mean field approach based on the Liouville equation we obtain a more rigorous (though in integral form) dynamical friction formula that can be evaluated numerically for a given choice of the QuMOND interpolation function. Results. Consistently with previous work, we observe that dynamical friction is stronger in MOND with respect to a baryon only Newtonian system with the same mass distribution. This amounts to a correction of the Coulomb logarithmic factor via extra terms proportional to the MOND radius of the system. Moreover, with the aid of simple numerical experiments we confirm our theoretical predictions and those of previous work on MOND.

astro-ph.GA↗

arXiv2024

Effective galactic dark matter: first order general relativistic corrections

Stationary, axisymmetric, dust sourced solutions of Einstein's equations have been proposed as fully general relativistic models for disc galaxies. These models introduce a novel physical element, i.e., a non-negligible dragging vortex emerging from a full consideration of the essential self-interaction of matter and geometry in general relativity, which might demand a profound recalibration of the inferred amount of dark matter in disc galaxies. Within this framework, we identify the correct observables for redshift-inferred rotation curves of distant galaxies, correcting previously overlooked mistakes in the literature. We find that the presence of the dragging vortex introduces non-negligible corrective terms for the matter density required to maintain a stable physical system. We present the first estimate of the dragging speed which is required to explain a non-negligible fraction of dark matter in disc galaxies. In particular, we show that a sub-relativistic dragging velocity of tens of kilometers per second in the neighbourhood of the Sun is sufficient to reduce the need of dark matter by 50% in the Milky Way. Finally, we find that the presence of such a dragging vortex also returns a net contribution to the strong gravitational lensing generated by the galaxy. Thus, we show that the considered class of general relativistic galaxy models, is not only physically viable, but suggests the need for recalibration of the estimated dark matter content in disc galaxies, with far reaching consequences for astrophysics and cosmology.

gr-qc↗

arXiv2023

Structure of the equivalent Newtonian systems in MOND N-body simulations. Density profiles and the core-cusp problem

We investigate the core-cusp problem of the $\Lambda$ cold dark matter ($\Lambda$CDM) scenario in the context of Modified Newtonian Dynamics (MOND) paradigm exploiting the concept of equivalent Newtonian system (ENS). By means of particle-mesh $N-$body simulations in MOND we explore processes of galaxy formation via cold dissipationless collapse or merging of smaller substructures. From the end states of our simulations we recover the associated ENS and study the properties of their dark matter halos. We compare the simulation results with simple analytical estimates with a family of $\gamma-$models. We find that the dark matter density of ENSs of most spherical cold collapses ha a markedly cored structure, in particular for the lowest values of the initial virial ratios. End states of some simulations with clumpy initial conditions have more complex profiles and some of their ENSs exhibit a moderate cusp, with logarithmic density slope always shallower than 1. These results seem to point towards the fact that the absence in most observed galaxies of a central DM cusp, at variance with what one would expect from theoretical and numerical arguments in $\Lambda$CDM, would be totally consistent in a MONDian description.

astro-ph.GA↗

arXiv2022

Re-weighting dark matter in disc galaxies: a new general relativistic observational test

A recent analysis of data from the ESA Gaia mission demonstrated that the kinematics of stars in the Milky Way can be modelled without invoking the presence of dark matter whatsoever. Indeed, the higher-than-Keplerian velocities observed in outer stars can be ascribed to the properties of the general relativistic (GR) metric assumed to describe the Galaxy. Here we generalize the concept, and derive the most general exact GR model for a stationary axi-symmetric dynamically cold dust structure. We explicitly show how deviations from the commonly adopted Newtonian dynamics are indeed manifest even at low velocities and low densities. We provide for the first time a detailed description of the frequency shift experienced by photons travelling from any emission site within an external disc galaxy to the observer, relating the outcome of the shift measurement to the gravitational properties of the galaxy. Finally, we devise a novel, groundbreaking observational test potentially able to fully characterize the GR metric under the minimal set of assumptions mentioned above. The proposed experiment exploits the effects non-diagonal GR terms have on the frequency shift of photons, ultimately providing a test to evaluate whether dark matter is actually required by disc galaxy kinematics.

astro-ph.GA↗

arXiv2021

Towards a full general relativistic approach to galaxies

We analyze the dynamics of a single spiral galaxy from a general relativistic viewpoint. We employ the known family of stationary axially-symmetric solutions to Einstein gravity coupled with dust in order to model the halo external to the bulge. In particular, we generalize the known results of Balasin and Grumiller, relaxing the condition of co-rotation, thus including non co-rotating dust. This further highlights the discrepancy between Newtonian theory of gravity and general relativity at low velocities and energy densities. We investigate the role of dragging in simulating dark matter effects. In particular, we show that non co-rotance further reduce the amount of energy density required to explain the rotation curves for spiral galaxies.

gr-qc↗

arXiv2020

On Generalized Lemaitre-Tolman-Bondi Metric. Fractal Matter at the end of Matter-Antimatter Recombination

Many recent researches have investigated the deviations from the Friedmannian cosmological model, as well as their consequences on unexplained cosmological phenomena, such as dark matter and the acceleration of the Universe. On the one hand, a first order perturbative study of matter inhomogeneity returned a partial explanation of dark matter and dark energy, as relativistic effects due to the retarded potentials of far objects. On the other hand, the fractal cosmology, now modeled with a Lemaitre-Tolman-Bondi (LTB) metric, results in distortions of the luminosity distances of SNe Ia, explaining the acceleration as apparent. In this work we extend the LTB metric to ancient times. The origin of the fractal distribution of matter is explained as the matter remnant after the matter-antimatter recombination epoch. We show that the evolution of such a inhomogeneity necessarily requires a dynamical generalization of LTB, and we propose a particular solution.

gr-qc↗

arXiv2020

Fake dark matter from retarded distortions

We push ahead the idea developed in [24], that some fraction of the dark matter and the dark energy can be explained as a relativistic effect. The inhomogeneity matter generates gravitational distortions, which are general relativistically retarded. These combine in a magnification effect since the past matter density, which generated the distortion we feel now, is greater than the present one. The non negligible effect on the averaged expansion of the universe contributes both to the estimations of the dark matter and to the dark energy, so that the parameters of the Cosmological Standard Model need some corrections. In this second work we apply the previously developed framework to relativistic models of the universe. It results that one parameter remain free, so that more solutions are possible, as function of inhomogeneity. One of these fully explains the dark energy, but requires more dark matter than the Cosmological Standard Model (91% of the total matter). Another solution fully explains the dark matter, but requires more dark energy than the Cosmological Standard Model (15% more). A third noteworthy solution explains a consistent part of the dark matter (it would be 63% of the total matter) and also some of the dark energy (4%).

gr-qc↗

arXiv2019

Distortions of Robertson-Walker metric in perturbative cosmology and interpretation as dark matter and cosmological constant

In the last years, we saw more and more attempts to explain dark matter as a general relativistic effect, at least for some fraction. Following this philosophy, we considered the gravitational distortions due to the inhomogeneous distribution of matter in the universe, which we know from general relativity to be retarded distortions. This provides a magnification effect, since the distortions we feel now depend not on the present matter density, but on the past one, which is greater. The expansion rate of the universe is perturbed as well, in a not negligible way, despite matter inhomogeneities are small, because of the same magnification effect. The deceleration parameter, which is a way to evaluate the quantity of dark matter in the universe, is perturbed in turn, so that the real amount of dark matter is less than what is usually believed.

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