arXiv subjects

Rodrigo R. Cuzinatto

Publications and source records attributed to Rodrigo R. Cuzinatto.

4 recordsLinked to original sources

arXiv2023

Dynamical analysis of the covarying coupling constants in scalar-tensor gravity

A scalar-tensor theory of gravity is considered wherein the gravitational coupling $G$ and the speed of light $c$ are admitted as space-time functions and combine to form the definition of the scalar field $ϕ$. The varying $c$ participates in the definition of the variation of the matter part of the action; it is related to the effective stress-energy tensor which is a result of the requirement of symmetry under general coordinate transformations. The effect of the cosmological coupling $Λ$ is accommodated within a possible behaviour of $ϕ$. We analyze the dynamics of $ϕ$ in the phase space, thereby showing the existence of an attractor point for reasonable hypotheses on the potential $V(ϕ)$ and no particular assumption on the Hubble function. The phase space analysis is performed both with the linear stability theory and via the more general Lyapunov's method. Either method lead to the conclusion that the condition $\dot{G}/G=σ\left(\dot{c}/c\right)$ where $σ=3$ must hold for the rest of cosmic evolution after the system gets to the globally asymptotically stable fixed point and the dynamics of $ϕ$ ceases. This result provides a physical foundation for the phenomenological model admitting $\left(G/G_{0}\right)=\left(c/c_{0}\right)^{3}$ used recently to interpret cosmological and astrophysical data. The thus co-varying couplings $G$ and $c$ impact the cosmic evolution after the dynamical system settles to equilibrium. This impact is investigated by constructing the generalized continuity equation in our scalar-tensor model and considering two possible regimes for the varying speed of light -- decreasing $c(a)$ and increasing $c(a)$ -- while solving our modified Friedmann equations. The solutions to the latter equations make room for radiation- and matter-dominated eras that progress to a dark-energy-type of accelerated expansion.

gr-qc↗

arXiv2019

Covariant c-flation: a variational approach

We develop an action principle to construct the dynamics that give rise to a minimal generalization of Einstein's equations, where the speed of light ($c$), the gravitational constant ($G$) and the cosmological constant ($Λ$) are allowed to vary. Our construction preserves general covariance of the theory, which yields a general dynamical constraint on $c$, $G$ and $Λ$. This action is general and can be applied to describe different cosmological solutions. We apply this formulation to the initial condition puzzles of the early universe and show that it generates a dynamical mechanism to obtain the homogeneous and flat universe we observe today. We rewrite the conditions necessary to solve the horizon and flatness problems in this framework, which does not necessarily lead to an accelerated expansion as in inflation. Then, we show how the dynamics of the scalar field that represents $c$ or $G$ (and $Λ$) can be used to solve the problems of the early universe cosmology by means of different ways to c-inflate the horizon in the early universe. By taking $Λ= 0$, we show that the dynamics of the scalar field representing $c$ can be described once a potential is given.

gr-qc↗

arXiv2019

New Scalar Field Quartessence

We propose a cosmological scenario involving a scalar field, $φ$, that is a source of Dark Matter as well as of Dark Energy. Besides $φ$, the Lagrangian of the field theory envisaged in our scenario contains a second field $χ$, for simplicity assumed to be a scalar, too. For fixed values of $χ$, the potential term decays exponentially at large positive values of $φ$. While $φ$ is not coupled to Standard Model fields, $χ$ is assumed to be coupled to them, and the Green functions of $χ$ depend on the cosmological redshift in the expanding universe. We assume that the term in the Lagrangian coupling $χ$ to $φ$ is such that, at redshifts $z$ larger than some critical redshift $z_c$, $φ$ is trapped near $φ=0$, and oscillations of $φ$ about $φ=0$ describing massive scalar particles give rise to Dark Matter. At redshifts below $z_c$, the field $φ$ is no longer trapped near the origin and starts to `roll' towards large field values. A homogenous component of $φ$ emerges that acts as Dark Energy. Within over-dense regions, such as galaxies and galaxy clusters, the redshifting of $χ$ stops, and $φ$ therefore remains trapped near $φ=0$ as long as $z_c$ is smaller than the redshift when structures on galactic scales decouple from the Hubble flow. Thus, at the present time, $φ$ describes both Dark Energy and Dark Matter.

gr-qc↗

arXiv2018

Analytic Study of Cosmological Perturbations in a Unified Model of Dark Matter and Dark Energy with a Sharp Transition

We study cosmological perturbations in a model of unified dark matter and dark energy with a sharp transition in the late-time universe. The dark sector is described by a dark fluid which evolves from an early stage at redshifts $z > z_C$ when it behaves as cold dark matter (CDM) to a late time dark energy (DE) phase ($z < z_C$) when the equation of state parameter is $w = -1 + ε$, with a constant $ε$ which must be in the range $0 < ε< 2/3$. We show that fluctuations in the dark energy phase suffer from an exponential instability, the mode functions growing both as a function of comoving momentum $k$ and of conformal time $η$. In order that this exponential instability does not lead to distortions of the energy density power spectrum on scales for which we have good observational results, the redshift $z_C$ of transition between the two phases is constrained to be so close to zero that the model is unable to explain the supernova data.

astro-ph.CO↗