arXiv subjects

S. R. Pinto

Publications and source records attributed to S. R. Pinto.

5 recordsLinked to original sources

arXiv2026

Cosmological Averaging in Nonminimally Coupled Gravity

We address the challenge, commonly referred to as the cosmological averaging problem, of relating the large-scale evolution of an inhomogeneous universe to that predicted by a homogeneous matter distribution in theories of gravity with nonminimal matter-gravity couplings. To this end, we focus on the class of $f(R,T)$ models given by $f(R,T) = R + F(T)$, where $R$ denotes the Ricci scalar and $T$ the trace of the energy-momentum tensor. This framework provides a simple yet theoretically consistent realization of nonminimal coupled gravity and can be recast as General Relativity minimally coupled to a modified matter Lagrangian. Using global K-monopoles as an illustrative toy model, we show that, when $F$ is a nonlinear function of $T$, the ratio between the spatial average of $F$ and $F$ evaluated at the spatial average of $T$ can deviate significantly from unity and depends on the particle number density. We demonstrate that the common assumption that this ratio is equal to unity generally leads to an inaccurate description of cosmological dynamics. We further show that dust in these theories generally exhibits a non-vanishing proper pressure. Our results highlight the importance of properly accounting for spatial averaging in cosmological models with nonminimal matter-gravity couplings.

astro-ph.CO↗

arXiv2026

Lagrangian Identity and Mass Evolution of Particle-like Objects in Nonminimally Coupled Gravity

We show that the Lagrangian of a Nambu-Goto $p$-brane satisfies the identity $\mathcal{L}_{\rm [\it p \rm]}=T_{\rm [\it p \rm]}/(p+1)$, with $T_{\rm [\it p \rm]}$ denoting the trace of the corresponding energy-momentum tensor, independently of the properties of the gravitational field. While for $p=0$ this reduces to the standard $\mathcal{L}_{\rm [0]}=T_{\rm [0]}$ relation, which determines the on-shell Lagrangian of point particles and their fluids, more generally it depends explicitly on the $p$-brane dimensionality. We explore the implications of this Lagrangian identity for the dynamics of non-self-intersecting cosmic string loops in a homogeneous and isotropic universe within nonminimally coupled scalar-tensor gravity, showing that, unlike in general relativity, their rest mass can evolve in response to the cosmological evolution of the background spacetime, regardless of their small size or tension. We further generalize this analysis to closed $p$-branes in $(N+1)$-dimensional Friedmann-Lema\^itre-Robertson-Walker spacetimes, showing that the evolution of the rest mass depends explicitly on the dimensionality of the brane, and therefore that the cosmological evolution of particle-like objects in theories of gravity with nonminimal matter couplings is sensitive to their internal structure.

gr-qc↗

arXiv2025

Updated constraints on Regge-Teitelboim gravity

In the Regge-Teitelboim model, gravity is described by embedding the space-time manifold in a (usually flat) fixed higher-dimensional background, where the embedding coordinates, rather than the metric tensor, are the dynamical degrees of freedom. Stern \& Xu extended the Regge-Teitelboim framework to encompass scenarios where the background embedding space is not flat, noting that when the background is a five-dimensional de Sitter space, the Robertson-Walker manifold undergoes a transition from a decelerating phase to an accelerating one. Previously, we constrained this model using only low-redshift observations. Here we further explore the observational constraints on this scenario, and report significantly more stringent constraints by including high-redshift data, specifically from the cosmic microwave background. Our results are consistent with $Λ$CDM, with the putative model-specific energy component responsible for the recent acceleration being constrained to $Ω_{RT}<0.006$ and the de Sitter curvature radius in units of the Hubble constant being constrained to $LH_0>1.45$, both at the 95 percent confidence level.

astro-ph.CO↗

arXiv2025

Deviations from the von Laue condition: Implications for the on-shell Lagrangian of particles and fluids

According to the von Laue condition, the volume integral of the proper pressure inside isolated particles with a fixed structure and finite mass vanishes in the Minkowski limit of general relativity. In this work, we consider a simple illustrative example: nonstandard static global monopoles with finite energy, for which the von Laue condition is satisfied when the proper pressure is integrated over the whole space. We demonstrate, however, that the absolute value of this integral, when calculated up to a finite distance from the center of the global monopole, generally deviates from zero by no more than the energy located outside the specified volume (under the assumption of the dominant energy condition). Furthermore, we find that the maximum deviation from unity of the ratio between the volume averages of the on-shell Lagrangian and the trace of the energy-momentum tensor cannot exceed three times the outer energy fraction. Extending these results to real particles, we demonstrate that these constraints generally hold for finite-mass systems with fixed structure, including stable atomic nuclei, provided the dominant energy condition is satisfied. Specifically, we show that, except in extremely dense environments with energy densities comparable to that of the particles themselves, the volume average of the aforementioned ratio must be extremely close to unity. Finally, we discuss the broader implications of our findings for the form of the on-shell Lagrangian of real fluids, which is often a crucial element for accurately modeling the dynamics of the gravity and matter, especially in scenarios involving nonminimal couplings to other matter fields or gravity. We find that, in general, the ideal gas on-shell Lagrangian provides an accurate approximation of the true on-shell Lagrangian, even for nonideal gases with significant interparticle interactions.

gr-qc↗

arXiv2023

Quantitative constraints on modified gravity paradigms

We use low-redshift background cosmology data to place quantitative constraints on three separate modified gravity models, each of which aims to explain the low-redshift acceleration through a different physical mechanism. The Lifshitz cosmology is effectively a parametric extension of the canonical $Λ$CDM model, where a time-dependent cosmological constant originates from vacuum energy. The Infinite Statistics model is also a parametric extension of $Λ$CDM, where the dark energy is dynamic and originates from the curvature of a dual space-time. We show that the data restricts the additional parameters in these models to be consistent with their $Λ$CDM values, and in particular that it implies that the theoretically predicted value for a dimensionless coupling parameter in the Lifshitz model is ruled out at more than six standard deviations. In the Regge-Teitelboim model, gravity is described by embedding the usual space-time manifold in a fixed higher-dimensional background, and there is no parametric $Λ$CDM limit. We study several separate realizations of the model, respectively introduced by Davidson, by Fabi \textit{et al.}, and by Stern \& Xu, and show that the first two are ruled out by the low-redshift data we use, while the latter is consistent with this data but requires a non-standard value of the matter density. Overall, our analysis highlights the tight constraints imposed by current data on the allowed low-redshift deviations from the standard $Λ$CDM background evolution.

astro-ph.CO↗