arXiv subjects

R. Gonzalez Quaglia

Publications and source records attributed to R. Gonzalez Quaglia.

5 recordsLinked to original sources

arXiv2023

Local scale symmetry in non-riemannian geometry based gravitational theories and the role of the noether current

In this paper, we delve into the significance of local scale symmetry and the role of the associated noether current, within gravitational theories which are based in non-riemannian background space. Our focus is in Weyl and in Riemann-Cartan geometry based gravitational theories. We show that local scale symmetry is associated with vanishing noether current whenever there are not new propagating gravitational degrees of freedom beyond the two polarizations of the massless graviton. In contrast, in local scale invariant theories where there are more than two propagating gravitational degrees of freedom, local scale symmetry is associated with nonvanishing noether current. The known result that Weyl symmetry has vanishing noether current, is generalized to non-riemannian gravitational theories. An exception are the local scale invariant gravitational theories with vectorial nonmetricity, where the associated noether current is nonvanishing.

gr-qc↗

arXiv2023

Reheating constraints and consistency relations of the Starobinsky model and some of its generalizations

Building on the success of the Starobinsky model in describing the inflationary period of the universe, we investigate two simple generalizations of this model and their constraints imposed by the reheating epoch. The first generalization takes the form $R^{2p}$, while the second is the $α$-Starobinsky model. We first focus on the case where $p=1$ or equivalently, $α=1$, which corresponds to the original Starobinsky model. We derive exact consistency relations between observables and cosmological quantities, without neglecting any terms, and impose the reheating condition $0 < ω_{re} < 0.25$, where $ω_{re}$ is the equation of state parameter at the end of reheating. This allows us to obtain new bounds for $n_s$ and $r$ that satisfy this condition and apply them to other observables and cosmological quantities. We repeat this process for the cases where $p \neq 1$ and $α\neq 1$ and find that these generalizations only result in minor modifications of the Starobinsky model, including the potential and the bounds on observables and cosmological quantities.

astro-ph.CO↗

arXiv2023

A comparison between the Jordan and Einstein Frames in Brans-Dicke theories with torsion

In recent years, gravitational models motivated by quantum corrections to gravity which introduce higher order terms like $R^{2}$ or terms in which the Riemann tensor is not symmetric have been studied by several authors in the form of a general Brans-Dicke type model containing the Ricci scalar, the Holst term and the Nieh-Yan invariant. In this paper we focus on the less explored Jordan frame of such theories and in the comparison between both this frame and the Einstein one. Furthermore, we discuss the role of the transformation of the torsion under conformal transformations and show that the transformation proposed in this paper (extended conformal transformation) contains a special case of the projective transformation of the connection used in some of the papers that motivated this work. We discuss the role and advantages of the extended conformal transformation and show that this new approach can have interesting consequences by working with different variables such as the metric and torsion. Moreover, we study the stability of the system via a dynamical analysis in the Jordan frame, this in order to analyze whether or not we have the fixed points that can be later identified as the inflationary attractor and the unstable fixed point where inflation could take place. Finally we study the scale invariant case of the general model in the Jordan frame. We find out that both the scalar spectral index and the tensor-to-scalar ratio are in agreement with the latest Planck results.

gr-qc↗

arXiv2023

Model independent bounds for the number of $e$-folds during the evolution of the universe

We present a simple procedure to obtain universal bounds for quantities of cosmological interest, such as the number of $e$-folds during inflation, reheating, and radiation, as well as the reheating temperature. The main assumption is to represent each of the various epochs of evolution of the universe as being due to a single substance changing instantaneously into the next, describing a new era of evolution of the universe. This assumption, commonly used to obtain solutions of the Friedmann equations for simple cosmological models, is implemented here to find model-independent bounds on cosmological quantities of interest. In particular, we find that the bound $N_k\approx 56$ for $-\frac{1}{3} < ω_{re} < \frac{1}{3}$ is very robust as an upper bound on the number of $e$-folds during inflation and also as a lower bound when $ω_{re} > \frac{1}{3}$, where $ω_{re}$ is the effective equation of state parameter during reheating. These are model-independent results that any single-field model of inflation should satisfy. As an example, we illustrate the two approaches with the basic $α$ attractor model and show how they complement each other.

gr-qc↗

arXiv2023

An early and late times dynamical analysis of a scale invariant gravitational model with a vector scalar interaction: the isotropic case

Scalar fields are widely and popularly used in cosmology in order to explain different phenomena among which, inflation and dark energy are two of the most popular ones. Specifically, in recent years, scale invariance in the gravitational sector has gained interest due to its simplicity, ability to model inflation and the dynamical generation of the Planck scale. In this paper, motivated by a non minimally coupled scale invariant $R^{2}$ gravitational model originally proposed by M.Rinaldi and L. Vanzo, we investigate how the inclusion of a vector scalar interaction that respects the scale invariance of the original model may affect the early and late time dynamics. We employ dynamical analysis tools in order to find the fixed points of the system and the local solutions for each variable around each fixed point finding out that the early universe solution of the extended model is compatible with that found in the original model with the exception of a new unstable fixed point appearing in the extended model. This new fixed point however has the same linealized solutions as the unstable fixed point found for the original model. We later employ numerical calculations in order to check that the analytical approach holds. Furthermore we show how the dynamical generation of the Planck mass is unaffected by the new field content of the model. Finally, we motivate and investigate a reduced version of the extended model finding out that, at late times, the extended model has only stable de Sitter fixed points where the scalar field becomes constant taking the role of the cosmological constant and the vector field is washed out.

gr-qc↗