arXiv subjects

J. C. Fabris

Publications and source records attributed to J. C. Fabris.

At least 37 records · Page 2Linked to original sources

arXiv2018

Modified gravity versus shear viscosity: imprints on the scalar matter perturbations

Cosmological scalar perturbation theory studied in the Newtonian gauge depends on two potentials $Φ$ and $Ψ$. In General Relativity (GR) they must coincide ($Φ=Ψ$) in the absence of anisotropic stresses sourced by the energy momentum tensor. On the other hand, it is widely accepted in the literature that potential deviations from GR can be parameterized by $Φ\neq Ψ$. The latter feature is therefore present in both GR cosmologies equipped with shear viscous fluids or modified gravity. We study the evolution of scalar matter density perturbations using the redshift-space-distortion based $f(z)σ_8(z)$ data as a tool to differentiate and characterize the imprints of both scenarios. We show that in the $f(z)σ_8(z)$ evolution both scenarios yields to completely different imprints in comparison to the standard cosmology. While the current available data is not sensitive to distinguish modified gravity from viscous shear cosmologies, future precise data can be used to break this indistinguishability.

astro-ph.CO↗

arXiv2018

Dark energy with zero pressure: Accelerated expansion and large scale structure in action-dependent Lagrangian theories

We develop a cosmological model based on action-dependent Lagrangian theories. The main feature here is the nonconservation of the energy momentum tensor due to the nontrivial geometrical construction of the theory. We provide the basic set of equations necessary to study both the cosmological background expansion as well as the linear matter perturbation growth. We show that the simplest realization of the Universe as described by only one component is not viable as expected from the existing correspondence between this model and the case of viscous cosmological fluids. However, modeling the energy content of the Universe as composed by two pressureless fluids, i.e., one a typical cold dark matter fluid and the other a pressureless dark energy fluid which is responsible for driving the late-time acceleration expansion, is qualitatively compatible with observational data.

gr-qc↗

arXiv2018

Scalar field black holes

With a suitable decomposition of its energy-momentum tensor into pressureless matter and a vacuum type term, we investigate the spherical gravitational collapse of a minimally coupled, self-interacting scalar field, showing that it collapses to a singularity. The formed blackhole has a mass $M \sim 1/m$ (in Planck units), where $m$ is the mass of the scalar field. If the latter has the axion mass, $m \sim 10^{-5}$ eV, the former has a mass $M \sim 10^{-5} M_{\odot}$.

gr-qc↗

arXiv2018

Braneworld gravity within non-conservative gravitational theory

We investigate the braneworld gravity starting from the non-conservative gravitational field equations in a five-dimensional bulk. The approach is based on the Gauss-Codazzi formalism along with the study of the braneworld consistency conditions. The effective gravitational equations on the brane are obtained and the constraint leading to a brane energy-momentum conservation is analyzed.

gr-qc↗

arXiv2018

Dirac $δ$-function potential in quasiposition representation of a minimal-length scenario

A minimal-length scenario can be considered as an effective description of quantum gravity effects. In quantum mechanics the introduction of a minimal length can be accomplished through a generalization of Heisenberg's uncertainty principle. In this scenario, state eigenvectors of the position operator are no longer physical states and the representation in momentum space or a representation in a quasiposition space must be used. In this work, we solve the Schroedinger equation with Dirac $δ$-function potential in quasiposition space. We calculate the bound state energy and the coefficients of reflection and transmission for scattering states. We show that leading corrections are of order of the minimal length $({\sl O}(\sqrtβ))$ and the coefficients of reflection and transmission are no longer the same for the Dirac delta well and barrier as in ordinary quantum mechanics. Furthermore, assuming that the equivalence of the 1s state energy of the hydrogen atom and the bound state energy of the Dirac $δ$-function potential in 1-dim is kept in a minimal-length scenario, we also find that the leading correction term for the ground state energy of the hydrogen atom is of order of the minimal length and $Δx_{min} \le 10^{-25}$ m.

hep-th↗

arXiv2018

On general features of warm dark matter with reduced relativistic gas

Reduced Relativistic Gas (RRG) is a useful approach to describe the warm dark matter (WDM) or the warmness of baryonic matter in the approximation when the interaction between the particles is irrelevant. The use of Maxwell distribution leads to the complicated equation of state of the Jüttner model of relativistic ideal gas. The RRG enables one to reproduce the same physical situation but in a much simpler form. For this reason RRG can be a useful tool for the theories with some sort of a "new Physics". On the other hand, even without the qualitatively new physical implementations, the RRG can be useful to describe the general features of WDM in a model-independent way. In this sense one can see, in particular, to which extent the cosmological manifestations of WDM may be dependent on its Particle Physics background. In the present work RRG is used as a complementary approach to derive the main observational exponents for the WDM in a model-independent way. The only assumption concerns a non-negligible velocity $v$ for dark matter particles which is parameterized by the warmness parameter $b$. The relatively high values of $b$ ( $b^2\gtrsim 10^{-6}$) erase the radiation (photons and neutrinos) dominated epoch and cause an early warm matter domination after inflation. Furthermore, RRG approach enables one to quantify the lack of power in linear matter spectrum at small scales and in particular, reproduces the relative transfer function commonly used in context of WDM with accuracy of $\lesssim 1\%$. A warmness with $b^2\lesssim 10^{-6}$ (equivalent to $v\lesssim 300 km/s$) does not alter significantly the CMB power spectrum and is in agreement with the background observational tests.

astro-ph.CO↗

arXiv2017

Reconstructing a $f(R)$ theory from the $α$-Attractors

We show an analogy at high curvature between a $f(R) = R + aR^{n - 1} + bR^2$ theory and the $α$-Attractors. We calculate the expressions of the parameters $a$, $b$ and $n$ as functions of $α$ and the predictions of the model $f(R) = R + aR^{n - 1} + bR^2$ on the scalar spectral index $n_{\rm s}$ and the tensor-to-scalar ratio $r$. We find that the power law correction $R^{n - 1}$ allows for a production of gravitational waves enhanced with respect to the one in the Starobinsky model, while maintaining a viable prediction on $n_{\rm s}$. We numerically reconstruct the full $α$-Attractors class of models testing the goodness of our high-energy approximation $f(R) = R + aR^{n - 1} + bR^2$. Moreover, we also investigate the case of a single power law $f(R) = γR^{2 - δ}$ theory, with $γ$ and $δ$ free parameters. We calculate analytically the predictions of this model on the scalar spectral index $n_{\rm s}$ and the tensor-to-scalar ratio $r$ and the values of $δ$ which are allowed from the current observational results. We find that $-0.015 < δ< 0.016$, confirming once again the excellent agreement between the Starobinsky model and observation.

gr-qc↗

arXiv2017

Assessing the impact of bulk and shear viscosities on large scale structure formation

It is analyzed the effects of both bulk and shear viscosities on the perturbations, relevant for structure formation in late time cosmology. It is shown that shear viscosity can be as effective as the bulk viscosity on suppressing the growth of perturbations and delaying the nonlinear regime. A statistical analysis of the shear and bulk viscous effects is performed and some constraints on these viscous effects are given.

astro-ph.CO↗

arXiv2017

Is the cosmological dark sector better modeled by a generalized Chaplygin gas or by a scalar field?

Both scalar fields and (generalized) Chaplygin gases have been widely used separately to characterize the dark sector of the Universe. Here we investigate the cosmological background dynamics for a mixture of both these components and quantify the fractional abundances that are admitted by observational data from supernovae of type Ia and from the evolution of the Hubble rate. Moreover, we study how the growth rate of (baryonic) matter perturbations is affected by the dark-sector perturbations.

astro-ph.CO↗

arXiv2017

Static, spherically symmetric solutions with a scalar field in Rastall gravity

Rastall's theory belongs to the class of non-conservative theories of gravity. In vacuum, the only non-trivial static, spherically symmetric solution is the Schwarzschild one, except in a very special case. When a canonical scalar field is coupled to the gravity sector in this theory, new exact solutions appear for some values of the Rastall parameter $a$. Some of these solutions describe the same space-time geometry as the recently found solutions in the $k$-essence theory with a power function for the kinetic term of the scalar field. There is a large class of solutions (in particular, those describing wormholes and regular black holes) whose geometry coincides with that of solutions of GR coupled to scalar fields with nontrivial self-interaction potentials; the form of these potentials, however, depends on the Rastall parameter $a$. We also note that all solutions of GR with a zero trace of the energy-momentum tensor, including black-hole and wormhole ones, may be re-interpreted as solutions of Rastall's theory.

gr-qc↗

arXiv2016

Quantum cosmology with k-Essence theory

A class of $k$-Essence cosmological models, with a power law kinetic term, is quantised in the mini-superspace. It is shown that for a specific configuration, corresponding to a pressureless fluid, a Schrödinger-type equation is obtained with the scalar field playing the rôle of time. The resulting quantum scenario reveals a bounce, the classical behaviour being recovered asymptotically.

gr-qc↗

arXiv2016

Quantum cosmology of scalar-tensor theories and self-adjointness

In this paper, the problem of the self-adjointness for the case of a quantum minisuperspace Hamiltonian retrieved from a Brans-Dicke (BD) action is investigated. Our matter content is presented in terms of a perfect fluid, onto which the Schutz's formalism will be applied. We use the von Neumann theorem and the similarity with the Laplacian operator in one of the variables to determine the cases where the Hamiltonian is self-adjoint and if it admits self-adjoint extensions. For the latter, we study which extension is physically more suitable.

gr-qc↗

arXiv2016

Neutron Stars in Rastall Gravity

We calculate static and spherically symmetric solutions for the Rastall modification of gravity to describe Neutron Stars (NS). The key feature of the Rastall gravity is the non-conservation of the energy-momentum tensor proportionally to the space-time curvature. Using realistic equations of state for the NS interior we place a conservative bound on the non-GR behaviour of the Rastall theory which should be $\lesssim 1\%$ level. This work presents the more stringent constraints on the deviations of GR caused by the Rastall proposal.

gr-qc↗

arXiv2016

The Harmonic Oscillator in the Classical Limit of a Minimal-Length Scenario

In this work we explicitly solve the problem of the harmonic oscillator in the classical limit of a minimal-length scenario. We show that (i) the motion equation of the oscillator is not linear anymore because the presence of a minimal length introduces an anarmonic term and (ii) its motion is described by a Jacobi sine elliptic function. Therefore the motion is periodic with the same amplitude and with the new period depending on the minimal length. This result (the change in the period of oscillation) is very important since it enables us to find in a quite simple way the most relevant effect of the presence of a minimal length and consequently traces of the Planck-scale physics. We show applications of our results in spectroscopy and gravity.

hep-th↗

arXiv2016

Non-trivial static, spherically symmetric vacuum solution in a non-conservative theory of gravity

We analyse the vacuum static spherically symmetric space-time for a specific class of non-conservative theories of gravity based on the Rastall's theory. We obtain a new vacuum solution which has the same structure as the Schwarzschild-de Sitter solution in the General Relativity theory obtained with a cosmological constant playing the rôle of source. We further discuss the structure (in particular, the coupling to matter fields) and some cosmological aspects of the underline non-conservative theory

gr-qc↗

arXiv2016

Evidence for cosmological particle creation?

A joint analysis of the linear matter power spectrum, distance measurements from type Ia supernovae and the position of the first peak in the anisotropy spectrum of the cosmic microwave background indicates a cosmological, late-time dark matter creation at 95% confidence level.

astro-ph.CO↗

arXiv2016

Cosmology and stellar equilibrium using Newtonian hydrodynamics with general relativistic pressure

We revisit the analysis made by Hwang and Noh [JCAP 1310 (2013)] aiming the construction of a Newtonian set of equations incorporating pressure effects typical of the General Relativity theory. We explicitly derive the Hwang-Noh equations, comparing them with similar computations found in the literature. Then, we investigate $i)$ the cosmological expansion, $ii)$ linear cosmological perturbations theory and $iii)$ stellar equilibrium by using the new set of equations and comparing the results with those coming from the usual Newtonian theory, from the Neo-Newtonian theory and from the General Relativity theory. We show that the predictions for the background evolution of the Universe are deeply changed with respect to the General Relativity theory: the acceleration of the Universe is achieved with positive pressure. On the other hand, the behaviour of small cosmological perturbations reproduces the one found in the relativistic context, even if only at small scales. We argue that this last result may open new possibilities for numerical simulations for structure formation in the Universe. Finally, the properties of neutron stars are qualitatively reproduced by Hwang-Noh equations, but the upper mass limit is at least one order of magnitude higher than the one obtained in General Relativity.

gr-qc↗

arXiv2015

Dark matter velocity dispersion effects on CMB and matter power spectra

Effects of velocity dispersion of dark matter particles on the CMB TT power spectrum and on the matter linear power spectrum are investigated using a modified CAMB code. Cold dark matter originated from thermal equilibrium processes does not produce appreciable effects but this is not the case if particles have a non-thermal origin. A cut-off in the matter power spectrum at small scales, similar to that produced by warm dark matter or that produced in the late forming dark matter scenario, appears as a consequence of velocity dispersion effects, which act as a pressure perturbation.

astro-ph.CO↗