arXiv subjects

P. Moniz

Publications and source records attributed to P. Moniz.

6 recordsLinked to original sources

arXiv2025

Gravitational Foundations and Exact Solutions in $n$-Dimensional Fractional Cosmology

Three theoretically plausible techniques to developing a fractional scalar field cosmological model are pointed in this paper; the time-dependent kernel weighted action being then selected. Upon this choice, we proceed to establish (i) a time weighted action associated with the generalized scalar field cosmology; and (ii) a fractional cosmological model in $n$ dimensions considering the FLRW metric and a generalized version of the S\'{a}ez-Ballester (SB) theory. Our study focuses on the following purposes. Firstly, to investigate the fundamental gravitational structural features of the model, we analyze the dynamical behavior of the field equations, the fulfillment of the Bianchi identities, the associated conservation laws, and the application of the second Noether theorem at the background and first-order perturbation levels. Moreover, the model's distinguishing characteristics and theoretical differences from the corresponding standard scenarios are also investigated. Secondly, we aim to obtain exact analytical solutions and analyze the time evolution of key cosmological quantities, considering the fractional parameter effects. Furthermore, the model's predictions are compared with those of the corresponding standard models. Lastly, we propose new ideas to further generalize our model, with a focus on constructing an effective potential and investigating the conditions under which bounce solutions may emerge.

gr-qc↗

arXiv2025

Quantum Creation of a FRW Universe: applying the Riesz fractional derivative

In this work, we apply fractional calculus to study quantum cosmology. Specifically, our Wheeler-DeWitt equation includes a FRW geometry, a radiation fluid, a positive cosmological constant, and an ad hoc potential; we employ the Riesz fractional derivative, which brings a parameter $\alpha$, where $1 < \alpha \leq 2$, appearing explicitly in the mentioned equation. We investigate numerically the tunnelling probability for the Universe to emerge using a suitable WKB approximation. Our findings are as follows. When we decrease the value for $\alpha$, the tunnelling probability also decreases, suggesting that if fractional features could be considered to ascertain among different early universe scenarios, then the value $\alpha=2$ (meaning strict locality and standard cosmology) would be the most likely. Finally, our results also allow for an interesting discussion between selecting values for $\Lambda$ (in a non-fractional conventional set-up) versus balancing, e.g., both $\Lambda$ and $\alpha$ in the fractional framework. Concretely, the probability transition in the former if, e.g., $\Lambda=0.7$, is very close to the value computed if in the latter we employ instead, e.g., $\Lambda=1.5$ and $\alpha=1.9397961$.

gr-qc↗

arXiv2018

Inflationary Universe in Deformed Phase Space Scenario

We consider a noncommutative (NC) inflationary model with a homogeneous scalar field minimally coupled to gravity. The particular NC inflationary setting produces entirely new consequences. We first analyze the free field case and subsequently examine the situation where the scalar field is subjected to a polynomial and exponential potentials. We propose to use a canonical deformation between momenta, in a spatially flat FLRW universe, and while the Friedmann equation remains unaffected the Friedmann acceleration equation (and thus the Klein-Gordon equation) is modified by an extra term linear in the NC parameter. This concrete noncommutativity on the momenta allows interesting dynamics that other NC models seem not to allow. Let us be more precise. This extra term behaves as the sole explicit pressure that under the right circumstances implies a period of accelerated expansion of the universe. We find that in the absence of the scalar field potential, and in contrast with the commutative case, in which the scale factor always decelerates, we obtain an inflationary phase for small negative values of the NC parameter. Subsequently, the period of accelerated expansion is smoothly replaced by an appropriate deceleration phase providing an interesting model regarding the graceful exit problem in inflationary models. Moreover, in the case of the free scalar field, we show that not only the horizon problem is solved but also there is some resemblance between the evolution equation of the scale factor associated to our model and that for the $R^2$ (Starobinsky) inflationary model. Therefore, our herein NC model not only can be taken as an appropriate scenario to get a successful kinetic inflation, but also is a convenient setting to obtain inflationary universe possessing the graceful exit when scalar field potentials are present.

gr-qc↗

arXiv2003

Nonlinear multidimensional cosmological models with form fields: stabilization of extra dimensions and the cosmological constant problem

We consider multidimensional gravitational models with a nonlinear scalar curvature term and form fields in the action functional. In our scenario it is assumed that the higher dimensional spacetime undergoes a spontaneous compactification to a warped product manifold. Particular attention is paid to models with quadratic scalar curvature terms and a Freund-Rubin-like ansatz for solitonic form fields. It is shown that for certain parameter ranges the extra dimensions are stabilized. In particular, stabilization is possible for any sign of the internal space curvature, the bulk cosmological constant and of the effective four-dimensional cosmological constant. Moreover, the effective cosmological constant can satisfy the observable limit on the dark energy density. Finally, we discuss the restrictions on the parameters of the considered nonlinear models and how they follow from the connection between the D-dimensional and the four-dimensional fundamental mass scales.

hep-th↗

arXiv2002

Multidimensional cosmology and asymptotical AdS

A non-linear gravitational model with a multidimensional geometry and quadratic scalar curvature is considered. For certain parameter ranges, the extra dimensions are stabilized if the internal spaces have negative curvature. As a consequence, the 4-dimensional effective cosmological constant as well as the bulk cosmological constant become negative. The homogeneous and isotropic external space is asymptotically AdS. The connection between the D-dimensional and the 4-dimensional fundamental mass scales sets an additional restriction on the parameters of the considered non-linear models.

gr-qc↗

arXiv2002

Asymptotical AdS from non-linear gravitational models with stabilized extra dimensions

We consider non-linear gravitational models with a multidimensional warped product geometry. Particular attention is payed to models with quadratic scalar curvature terms. It is shown that for certain parameter ranges, the extra dimensions are stabilized if the internal spaces have negative constant curvature. In this case, the 4-dimensional effective cosmological constant as well as the bulk cosmological constant become negative. As a consequence, the homogeneous and isotropic external space is asymptotically AdS. The connection between the D-dimensional and the 4-dimensional fundamental mass scales sets a restriction on the parameters of the considered non-linear models.

hep-th↗