arXiv subjects

Hermano Velten

Publications and source records attributed to Hermano Velten.

At least 19 recordsLinked to original sources

arXiv2026

Perturbative aspects of analogue FLRW spacetime Jellium models

We study electro-acoustic perturbative modes in homogeneous, isotropic and expanding -- dubbed as Friedmann-Lemaitre-Robertson-Walker (FLRW) -- Jellium models, thereby mimicking density perturbations in analogous Newtonian cosmological expansions. We present both novel analytic solutions for linear perturbations in specific analogue cosmological expansions and full numerical evaluations that characterize the temporal evolution of the electro-acoustic modes capturing their full dynamical behavior across the linear and the nonlinear regimes. For both the case of pressure supported evolution or modes sourced by nonadiabatic contributions, we also characterize the temporal evolution of such perturbations by introducing their scale dependent particle number fluctuation power spectrum which can act as a tool to connect theory and experiments. The dependence of the latter on the physical parameters of the model is demonstrated in detail.

gr-qc↗

arXiv2026

Lagrangian Extensions of Newtonian Gravity constrained by Solar System tests

We explore an extension to Newtonian gravity through a generalised Lagrangian function with the introduction of a second dynamical scalar field. Building on previous research into gravity with variable gravitational coupling, the work derives the complete field equations and applies a weak-field approximation. This leads to an effective post-Newtonian gravitational potential that includes key aspects of relativistic theories. The resulting N-body equations of motion highlight differences among inertial and gravitational masses, which can constrain the theory's free parameter through data from the Nordtvedt effect. By employing the method of osculating orbits for a two-body system, the study calculates the secular variation of the orbital pericenter and aligns this with the latest data on Mercury's perihelion shift, for another observational constraint on the model. Furthermore, a few examples of theories are discussed.

gr-qc↗

arXiv2026

Galactic core-tail structure in BEC dark matter with Kapitza potential

Recently, the experimental realization of a Kapitza potential in a Bose-Einstein Condensate (BEC) has been reported for the first time in literature, motivating further theoretical investigations of such system. At the same time, in the astrophysical context, BEC dark matter models have been widely studied as a possible phenomenological explanation for the dark matter phenomena. We model the galactic structure with an inner cored profile obtained from the ground state equilibrium solution of the Schroedinger-Poisson together with a Kapitza-BEC like interaction for the tail region. We find reasonable agreement of the model with representative galaxy rotation curves available in the SPARC catalogue.

astro-ph.GA↗

arXiv2025

Bulk viscosity from early-time thermalization of cosmic fluids in light of DESI DR2 data

If nonrelativistic dark matter and radiation are allowed to interact, reaching an approximate thermal equilibrium, this interaction induces a bulk viscous pressure changing the effective one-fluid description of the universe dynamics, permitted by the existence of a common temperature. It has been shown that by modelling such components as perfect fluids, a cosmologically relevant bulk viscous pressure, expressed in terms of the Eckart formalism, emerges for dark matter particle masses in the range of $1\,\text{eV} - 10\,\text{eV}$ keeping thermal equilibrium with the radiation. Such a transient bulk viscosity introduces significant effects in the expansion rate near the matter-radiation equality redshift ($z_\rm{eq}\sim 3400$), impacting also late times leading to a higher inferred value of the Hubble constant $H_0$. Since this mechanism also impacts the sound speed of the baryon-photon fluid, we use the recent DESI DR2 BAO measurements, reported relative to a fiducial $\Lambda$CDM cosmology, to place an upper bound on the logarithm of the free parameter of the model $\tau_\rm{eq}$ which represents the time scale in which each component follows its own internal perfect fluid dynamics until thermalization occurs. Our main result is encoded in the bound $\log_{10}(\tau_\rm{eq}\,[\rm{s}]) \lesssim -9.76$ (2$\sigma$), with the corresponding dimensionless bulk coefficient $\tilde{\xi} H_0/H_\rm{eq}\lesssim5.94\times10^{-4}$ (2$\sigma$). The obtained constraints show that DESI DR2 data do not support such an interaction between radiation and dark matter prior to the recombination epoch, precluding the model from solving the cosmic tensions.

astro-ph.CO↗

arXiv2025

A note on Gravitational radiation in generalized Brans-Dicke theory: compact binary systems

This note revisits and corrects a previous analysis on gravitational radiation in compact binary systems within the framework of Brans-Dicke-f(R) theories-models featuring both massless and effectively massive scalar fields. We correct the lower bound on the Brans-Dicke coupling parameter $\omega_0$ presented in prior work, identifying teh issue that led to inverted constraints. By reanalyzing data from the binary system PSR J1012+5307, we present revised bounds on $\omega_0$ as a function of the geometrical scalar field mass $m_f$, emphasizing that the role of $\omega_0$ differs from its traditional interpretation in standard BD theory.

gr-qc↗

arXiv2025

Anisotropic quantum polytropes

The structure of astrophysical objects is usually modeled under the assumption of hydrostatic equilibrium. However, actual configurations may deviate from perfect spherical or isotropic properties. Consequently, cosmic objects are expected to exhibit some degree of anisotropy. This consideration also extends to hypothetical dark structures, such as dark stars and dark matter halos. Although the nature of dark matter remains unknown, axion-like particles (ALPs) are strong candidates, suggesting that dark matter halos may have originated from bosonic configurations undergoing gravitational collapse, sustained by boson-boson interactions in the condensate state. This system is described by the Gross-Pitaevskii-Poisson equation. Furthermore, within the framework of the Bohm-de Broglie approach, quantum effects,encapsulated in the so-called quantum potential, may play a significant role in equilibrium astrophysical configurations. In this study, we examine a class of static anisotropic boson stars which are non-minimally coupled to gravity. By including all these factors, we derive a generalized Lane-Emden-like equation and conduct a detailed analysis of the maximum degree of anisotropy that such systems can sustain, thereby identifying physically viable equilibrium configurations. Apart from focusing on the impact of anisotropic contributions, we find that for the so-called Quantum Polytropes (when the quantum potential is the main responsible for the equilibrium condition), the anisotropic factor and the gravitational field have opposite roles compared to the classical case. This leads to a new class of hydrostatic equilibrium objects.

gr-qc↗

arXiv2024

Cosmological eras with the neutrino non-relativistic transition in RTB gravity

Theories based on the Ricci and the trace of the energy-momentum tensor, or the short name Ricci-trace-based (RTB) theories, represent a gravitational approach based on the Ricci scalar $R$, and the trace of the energy-momentum tensor $g^{\mu\nu}T_{\mu\nu} = T$. This theory fits into gravitational models of the type $f(R, T) = R + f(T)$, where $f(T)$ is an arbitrary function of $T$. In this study, we explore a cosmological scenario within the context of RTB models, investigating in detail the cosmological consequences of the coupling between matter and geometry. In order to address this issue, we propose a toy model in which the non-relativistic cosmological neutrino transition plays a role in the cosmic evolution since the effective total energy-momentum tensor trace is affected in this process. We raise questions about the coupling of neutrinos with geometry during this transition, providing a detailed analysis of how RTB gravity deals with this phenomenon and the impact of neutrinos on cosmological dynamics. In summary, we show that the coupling of cosmological neutrinos with $T$ dependent cosmologies is severely challenged.

gr-qc↗

arXiv2024

A new framework for cosmological expansion in a reformulated Newtonian-Like gravity with variable G

A Newtonian-like theory inspired by the Brans-Dicke gravitational Lagrangian has been recently proposed in Ref. arXiv:2009.04434(v4). We propose here a new variant of this theory such that the usual Newtonian second law is preserved. The cosmological solutions are analysed and accelerated background expansion can be obtained even in a pure matter dominated universe. This happens due to the dynamic character of the effective gravitational coupling which is sourced by a time evolving scalar field . We also analyse the matter density perturbations and find they exhibit an enhanced growth in comparison with the usual Newtonian like behavior in Einstein-de Sitter model.

gr-qc↗

arXiv2024

Extended error threshold mechanism in {\it quasispecies} theory via population dynamics

We investigate Eigen's model for the evolution of the genetic code of microorganisms using a novel method based on population dynamics analysis. This model, for a given number of offspring, determines long-term survival as a function of the "genetic" information length and copy error probability. There exists a maximum threshold for the quantity of information that can be consistently preserved through the process of evolution within a population of perfectly replicating sequences, meaning no errors are allowed. With our formula, we expand upon the traditional error threshold formula of Eigen's theory and introduce a new expression for general cases where the self-reproduction process allows up to any integer number of copying errors per digit per replication step.

q-bio.PE↗

arXiv2024

Selecting energy-momentum trace dependent gravity theories with LSS

We study scalar cosmological perturbations in $f(R, T)$ modified gravity theories being $T$ the trace of the energy-momentum tensor. We provide detailed equations for the matter energy density contrast. We solve then numerically to promote a comparison with available large scale structure (LSS) formation observational data on $f \sigma_8$ and also addressing the $S_8$ tension. We identify $f(R,T)$ models that lead either to growth enhancement or suppression. Since recent results in the literature indicate a preference for the latter feature, this type of analysis is quite useful to select viable modifications of gravity. We studied class of such $f(R,T)$ models are either ruled out or severely restricted.

gr-qc↗

arXiv2024

Holographic Ricci dark energy in Nonconservative Unimodular Gravity

The structure of unimodular gravity (UG) is invariant to a subclass of diffeomorphism, the transverse diffeomorphism, due to the unimodular condition ($\sqrt{-g}=\epsilon=cte$). Consequently, there is a freedom to define how the conservation laws of the energy-momentum tensor in unimodular gravity in the cosmological context. One of the main characteristics of the complete system of equations that describe cosmological dynamics in UG is that they form an underdetermined system if the usual conservation law of the energy-momentum tensor is not used in your structure, that is, it is necessary to insert extra information into the system to solve the complete set of equations. In this article, we propose the construction of a background cosmological model based on the description of a holographic dark energy component with a cutoff of the order of Ricci scalar in non-conservative UG. Although this choice is indeed a new addition of information to the cosmological system, the complete set of equations remains underdetermined, however, the new feature of this cosmological model is the appearance of an interaction between matter and dark energy. Indeed, this is a well-known characteristic of cosmological models in which we have holographic dark energy density. Consequently, we propose an ansatz to the interaction term $Q=\beta H \rho_{m}$, and obtain the cosmological parameters of our model. We found a viable universe model with similar characteristics to the $\Lambda \mathrm{CDM}$ model. We performed statistical analysis of the background model using the "Cosmic Chronometer" (CC) data for $H(z)$, and obtain as a result using Akaike Information Criterion (AIC), and the Bayesian Information Criterion (BIC) as model selection criteria that $\Lambda \mathrm{CDM}$ prevails as the best model. However, the proposed model is competitive when compared to the cosmological model $\omega\mathrm{CDM}$.

gr-qc↗

arXiv2023

Spherically Symmetric Configurations in Unimodular Gravity

Unimodular gravity (UG) is considered, under many aspects, equivalent to General Relativity (GR), even if the theory is invariant under a more restricted diffeomorphic class of transformations. We discuss the conditions for the equivalence between the two formulations by applying the UG to the static and spherically symmetric configurations being the energy-momentum tensor sourced by a scalar field or by the electromagnetic field. We argue that the equivalence between UG and GR may be broken when analyzing the stability of the solutions at perturbative level.

gr-qc↗

arXiv2023

Celestial mechanics in Newtonian-like gravity with variable $G$

A Newtonian-like theory inspired by the Brans-Dicke gravitational Lagrangian has been recently proposed in Ref. arXiv:2009.04434(v4). This work demonstrates that the modified gravitational force acting on a test particle is analogous to that derived from the Manev potential. Specifically, an additional term $\propto r^{-3}$ emerges alongside the conventional Newtonian component. We analyse the predicted expression for the pericenter advance and the Roche limit and use them to constraint the theory's single free parameter $\omega$ which is analogous to the Brans-Dicke parameter. At the same time this theory is able to solve the advance of Mercury's perihelion, we also show that there is no relevant impact on the Roche limit in comparison to the well known Newtonian results.

gr-qc↗

arXiv2023

Revisiting $f(R,T)$ cosmologies

We review the status of $f(R,T)$ cosmological models, where $T$ is the trace of the energy momentum tensor $T^{\mu\nu}$. We start focusing on the modified Friedmann equations for the minimally coupled gravitational Lagrangian of the type $f(R,T)=R +\alpha e^{\beta T} + \gamma_{n} T^{n}$. We show that in such a minimally coupled case there exists a useful constraining relation between the effective fractionary total matter density with an arbitrary equation of state parameter and the modified gravity parameters. With this association the modified gravity sector can be independently constrained using estimations of the gas mass fraction in galaxy clusters. Using cosmological background cosmic chronometers data and demanding the universe is old enough to accommodate the existence of Galactic globular clusters with ages of at least $\sim 14$ Gyrs we find a narrow range of the modified gravity free parameter space in which this class of theories remains viable for the late time cosmological evolution. This preferred parameter space region accommodates the $\Lambda$CDM limit of $f(R,T)$ models. We also work out the non-minimally coupled case in the metric-affine formalism and find that there are no viable cosmologies in the latter situation. However, when analysing the cosmological dynamics including a radiation component, we find that this energy density interacts with the matter field and it does not scale according to the typical behavior. We conclude stating that $f(R,T)$ gravity is not able to provide a full cosmological scenario and should be ruled out as a modified gravity alternative to the dark energy phenomena.

astro-ph.CO↗

arXiv2023

Fully conservative $f(R,T)$ gravity and Solar System constraints

The $f(R,T)$ gravity is a model whose action contains an arbitrary function of the Ricci scalar $R$ and the trace of the energy-momentum tensor $T$. We consider the separable model $f (R, T ) = \chi(R) + \varphi(T )$ and shown that, for perfect fluids, the dynamical equations are sufficient to determine how $\varphi$ depends on $T$, independently of the matter state equation and the geometry of space-time. Imposing the energy-momentum tensor conservation we obtain that $\varphi$ must be linear in $T$. However, the $T$ dependence is severely constrained using the full Will-Nordtvedt version of the parameterized post-Newtonian (PPN) formalism. The result of the PPN analysis is discussed and in addition it is shown that the diffeomorphism invariance of the matter action imposes strong constraints on conservative versions of $f(R,T)$ gravity.

gr-qc↗

arXiv2023

Using cosmological perturbation theory to distinguish between General Relativity and Unimodular Gravity

Unimodular Gravity is one of the oldest geometric gravity theory alternative to General Relativity. Essentially, it is based on the Einstein-Hilbert Lagrangian with an additional constraint on the determinant of the metric. It can be explicitly shown that Unimodular Gravity can be recast as General Relativity in presence of a cosmological constant. This fact has led to many discussions on the equivalence of both theories at classical and quantum levels. Here we present an analysis focused on the classical scalar perturbations around a cosmological background. The discussion is extended for the case where a non-minimal coupled scalar field is introduced. Our results indicate that the equivalence is not verified completely at perturbative level.

gr-qc↗

arXiv2022

Black hole lensing in Kerr-de Sitter spacetimes

We have derived analytical solutions using Jacobi elliptic functions for bound and nearly bound photon orbits in Kerr-de Sitter (KdS) and Kerr-de Sitter Revisited (RKdS) spacetimes. Leveraging our obtained solutions, we have conducted an analytic ray-tracing in both spacetimes. We have obtained direct images, lensing rings and photon rings for equatorial disks considering inclined locally static observers. Images corresponding to n=(2,3) exhibit a significantly closer resemblance to the critical curve as compared to the n=1 image. This highlights the remarkable potential of these higher-order images as robust testing grounds for General Relativity (GR). Furthermore in both spacetimes, we have obtained analytical solutions for the critical parameters governing the structure of the photon ring and analyzed these parameters in details.

gr-qc↗

arXiv2022

Constraining the anomalous coupling of gravitational waves with double pulsar

We revisit the decay of the orbital period in binary systems that occurs due to the emission of gravitational waves in the context of modified gravity models where the coupling $G_{gw}$ between matter and on-shell gravitons is allowed to differ from the Newton constant $G_N$. Using the most precise orbital parameters of binary pulsars, those of the Double Pulsar, we constrain the ratio $G_{gw}/G_N$ to the level of $10^{-4}$, improving by two orders of magnitude the present bound on this quantity.

gr-qc↗