arXiv subjects

Nicolas R. Bertini

Publications and source records attributed to Nicolas R. Bertini.

9 recordsLinked to original sources

arXiv2026

Renormalization group corrections to $Λ$CDM model and observational consequences for $H_0$ tension

We explore the renormalization group-based extension of the $Λ$CDM model as a potential solution to the current cosmological tensions. In this approach, both the cosmological constant density and Newton's constant are allowed to vary with the energy scale, as a consequence of the remnant effects of massive quantum fields in the low-energy regime. The corresponding cosmological model is consistent with the principles of quantum field theory, based on the covariance of the vacuum effective action, and is characterized by an unique extra parameter $ν$. Our analysis yields a best-fit value of $ν= - (2.5 \pm 1.3)\times 10^{-4}$, placing the $Λ$CDM limit at the $2σ$ region of the $ν$ posterior. This narrow range is consistent with data from CMB (Planck), BAO (DESI), and SN Ia (DES Y5). Our result also alleviates the $H_0$ tension and is consistent with the previously established constraints from large-scale structure. In these kind of models, there is a link between cosmology and particle physics. Our results point to possibility of a new physics, characterized by a mass spectrum lying below the Planck scale but above the values typically associated with Grand Unified Theories (GUTs).

astro-ph.CO↗

arXiv2026

Low-energy limit in the anomaly-induced action and the semiclassical cosmological bounce

In the recently proposed scenario, the cosmological bounce occurs because the initially contracting Universe is not empty. In the region close to singularity, matter contents of the Universe heat up and effectively become radiation. Then, the trace anomaly automatically provides bounce if the overall beta function in the matter sector is positive. Independent of the remaining open questions on the quantum field theory side, it is interesting to consider this model from the cosmological perspective. In the present work, we develop the general formalism which is a necessary step for exploring the primordial cosmological perturbations. The main technical development is the formulation of the low-energy version for the nonlocal part of the effective action. The complete form of this action can be done local using two auxiliary scalars. In our new version, there are more scalars, but this enables one to avoid higher derivatives.

gr-qc↗

arXiv2026

Post-Newtonian expansion of scale-dependent gravity

Scale-dependent gravity is an extension of general relativity in which the Newton and cosmological constants may vary slightly with the energy scale due to remnant low-energy quantum effects. A fundamental feature of this approach is the scale-setting procedure. In a previous work, a covariant expression for the scale, consistent with conservation laws, was established. Here, we apply the full Will-Nordtvedt version of the parameterized post-Newtonian (PPN) formalism to this framework, assuming that the constants can be expressed as a power series in the scale. We find that a new potential arises at the first post-Newtonian order, which is absent from the standard PPN formalism. This potential modifies the definitions of pressure and internal energy but does not affect center-of-mass orbits and is, therefore, not constrained by Solar System tests.

gr-qc↗

arXiv2025

Scale-dependent gravity and covariant scale-setting

A fundamental element of scale-dependent gravity is the scale-setting procedure. We present a new covariant expression to set the scale that arises when examining the field equations. Considering the renormalization group equations and imposing energy-momentum tensor conservation, we arrive at two models of running of the gravitational and cosmological constants. In the cosmological setting, we found that in one model the Big Bang singularity is avoided, while in the other the Hubble tension can be alleviated. At the level of cosmological perturbations, we derived the basic solutions and qualitatively discussed the impacts of this scenario on structure formation.

gr-qc↗

arXiv2024

Scale-dependent cosmology from effective quantum gravity in the invariant framework

We explore the possibility of a consistent cosmology based on the gauge-fixing independent running of the gravitational and cosmological constants ($G$ and $Λ$) in the framework of effective quantum gravity. In particular, their running in this framework was found to satisfy $G \propto Λ^4$. In the cosmological setting, the covariance of the theory provides energy conservation relations, which are impossible to satisfy with the unique scale parameter. However, by introducing the second sub-dominant scale corresponding to the higher-loop corrections and higher-derivative terms, one can close the system of equations for the running of parameters and arrive at the consistent cosmological solutions. This approach yields a change in the cosmological expansion history that affects the ratio of the Hubble parameter today to the Hubble parameter at high redshift.

gr-qc↗

arXiv2023

Fully conservative $f(R,T)$ gravity and Solar System constraints

The $f(R,T)$ gravity is a model whose action contains an arbitrary function of the Ricci scalar $R$ and the trace of the energy-momentum tensor $T$. We consider the separable model $f (R, T ) = χ(R) + φ(T )$ and shown that, for perfect fluids, the dynamical equations are sufficient to determine how $φ$ depends on $T$, independently of the matter state equation and the geometry of space-time. Imposing the energy-momentum tensor conservation we obtain that $φ$ must be linear in $T$. However, the $T$ dependence is severely constrained using the full Will-Nordtvedt version of the parameterized post-Newtonian (PPN) formalism. The result of the PPN analysis is discussed and in addition it is shown that the diffeomorphism invariance of the matter action imposes strong constraints on conservative versions of $f(R,T)$ gravity.

gr-qc↗

arXiv2022

Primordial perturbations and inflation in holographic cosmology

We consider an inflationary scenario in the holographic braneworld with a cosmological fluid occupying the 3+1 dimensional brane located at the holographic boundary of an asymptotic ADS$_5$ bulk. The contribution of the boundary conformal field can be represented as a modification of Einstein's equations on the boundary. Using these effective Einstein equations we calculate the cosmological perturbations and derive the corresponding power spectra assuming a general $k$-essence type of inflaton. We find that the braneworld scenario affects the scalar power spectrum only in the speed of sound dependence on the slow-roll parameters whereas there is no change in the tensor power spectrum. This implies that the changes in the spectral indices appear at the second order in the slow-roll parameter expansion.

gr-qc↗

arXiv2022

Primordial perturbations and inflation in a holography inspired Gauss-Bonnet cosmology

We consider an action for gravity that, in addition to the Einstein-Hilbert term, contains a function of the Ricci scalar and the Gauss-Bonnet invariant. The specific form of the function considered is motivated by holographic cosmology. At background level the field equations imply modified Friedmann equations of the same form as those in the holographic cosmology. We calculate the cosmological perturbations and derive the corresponding power spectra assuming a general $k$-inflation. We find that the resulting power spectra differ substantially from those obtained in both holographic and standard cosmology. The estimated spectral index and tensor-to-scalar ratio are confronted with the Planck results.

gr-qc↗

arXiv2020

Cosmological framework for renormalization group extended gravity at the action level

General relativity (GR) extensions based on renormalization group (RG) flows may lead to scale-dependent couplings with nontrivial effects at large distance scales. Here we develop further the approach in which RG effects at large distance scales are fully encoded in an effective action and we apply it to cosmology. In order to evaluate the cosmological consequences, our main assumption is the use of a RG scale such that the (infrared) RG effects only appear at perturbative order (not at the background level). The emphasis here is on analytical results and qualitative understanding of the implied cosmology. We employ commonly used parametrizations for describing modified gravity in cosmology (as the slip parameter). From them, we describe the dynamics of the first order perturbations and estimate bounds on the single dimensionless parameter ($ν$) introduced by this framework. Possible impacts on dark matter and dark energy are discussed. It is also shown here that the $ν$ parameter effects to $fσ_8$ are stronger at low redshifts ($z<1.5$), while different values for $ν$ do not appreciably change $fσ_8$ at higher redshifts, thus opening a window to alleviate an issue that is currently faced by $Λ$CDM.

gr-qc↗