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Jackson Levi Said

Publications and source records attributed to Jackson Levi Said.

At least 55 records · Page 3Linked to original sources
arXiv2023

$f(T)$ cosmology in the regime of quasar observations

The open problems related to cosmological tensions in current times have opened new paths to study new probes to constrain cosmological parameters in standard and extended cosmologies, in particular, to determine at a local level the value of the Hubble constant $H_0$, through independent techniques. However, while standard Cosmological Constant Cold Dark Matter ($Λ$CDM) model has been well constrained and parts of extended cosmology have been intensively studied, the physics behind them aspects restrains our possibilities of selecting the best cosmological model that can show a significant difference from the first model. Therefore, to explore a possible deviation from a such model that can explain the current discrepancy on the $H_0$ value, in this work we consider adding the current local observables, e.g. Supernovae Type Ia (SNIa), $H(z)$ measurements, and Baryon Acoustic Observations (BAO) combined with two new calibrated Quasars (QSO) datasets using ultraviolet, x-ray and optical plane techniques. While these can be identified as part of the high-redshift standard candle objects, the main characteristics of these are based on fluxes distributions calibrated up to $z \sim 7 $. We consider five $H_0$ prior scenarios to develop these calibrations. Furthermore, we found that our estimations provide the possibility to relax the $H_0$ tension at 2$σ$ using a QSO ultraviolet sample in combination with late measurements showing higher values of $H_0$. Our results can be an initial start for more serious treatments in the quasars physics from ultraviolet, x-ray, and optical plane techniques behind the local observations as cosmological probes to relax the cosmological tensions problems.

astro-ph.CO↗
arXiv2023

Stable bouncing solutions in Teleparallel Horndeski gravity: violations of the no-go theorem

In order to have singularity-free solutions at the beginning of the Universe, we need to violate the null energy condition. In the general class of Horndeski gravity, there are healthy NEC-violating solutions, which however are plagued with instabilities or some kind of pathologies in the history of the Universe; this is known as the no-go theorem. In this paper, we study the possibility of stable bouncing solutions in the Teleparallel analog of Horndeski gravity and we show explicitly that there exist healthy violations of the no-go theorem.

gr-qc↗
arXiv2023

Neural network reconstruction of cosmology using the Pantheon compilation

In this work, we reconstruct the Hubble diagram using various data sets, including correlated ones, in Artificial Neural Networks (ANN). Using ReFANN, that was built for data sets with independent uncertainties, we expand it to include non-Guassian data points, as well as data sets with covariance matrices among others. Furthermore, we compare our results with the existing ones derived from Gaussian processes and we also perform null tests in order to test the validity of the concordance model of cosmology.

gr-qc↗
arXiv2023

Cosmological constraints of Palatini $f(\mathcal{R})$ gravity

In this study, we investigate a Palatini $f(R)$ gravity model featuring a quadratic term correction, aligning it with the most recent expansion rate data, with a particular focus on the latest SNIa and BAO data. Our analysis employs CC data as the fundamental dataset, complemented by contributions from the SN sample and a combination of non-overlapping transversal BAO datasets. We conduct a comprehensive MCMC analysis for each data set combination, yielding constraints on all cosmological parameters within the model. Additionally, we incorporate the latest Hubble constant value from the SH0ES Team. Finally, we present a statistical comparison between the Palatini quadratic model and $Λ$CDM using the AIC and BIC metrics, ultimately obtaining the constraint $|α| \leq 10^{49}\,\text{m}^2$. We also stress the significance of studying stellar and substellar objects for obtaining more precise constraints on modified gravity compared to those derived from cosmological observations.

gr-qc↗
arXiv2023

Cosmic Growth in $f(T)$ Teleparallel Gravity

Physical evolution of cosmological models can be tested by using expansion data, while growth history of these models is capable of testing dynamics of the inhomogeneous parts of energy density. The growth factor, as well as its growth index, gives a clear indication of the performance of cosmological models in the regime of structure formation of early Universe. In this work, we explore the growth index in several leading $f(T)$ cosmological models, based on a specific class of teleparallel gravity theories. These have become prominent in the literature and lead to other formulations of teleparallel gravity. Here we adopt a generalized approach by obtaining the Mészáros equation without immediately imposing the subhorizon limit, because this assumption could lead to over-simplification. This approach gives avenue to study at which $k$ modes the subhorizon limit starts to apply. We obtain numerical results for the growth factor and growth index for a variety of data set combinations for each $f(T)$ model.

gr-qc↗
arXiv2023

Production of Primordial Gravitational Waves in Teleparallel Gravity

We study the production of primordial gravitational waves in the context of extended teleparallel gravity models and compare them with those of general relativity. Teleparallel gravity has been widely studied in the context of the late universe but not much in the early universe. Two sources of primordial gravitational waves are considered, vacuum fluctuations and tensor anisotropies within two inflation-compatible backgrounds: a perfect de Sitter and a quasi de Sitter background. We find that in the vacuum case with a perfect de Sitter background, the gravitational waves propagation equation is the same as that of general relativity, however, if the background is promoted to a quasi de Sitter background, the propagation equations are different resulting in an important difference on the tensor spectral index. When tensor anisotropies are included, we compute the most general solution for gravitational waves in terms of a retarded Green's function and analyze the contributions to the power spectrum from these anisotropies. Finally, we investigate the energy density of these gravitational waves.

gr-qc↗
arXiv2023

Noether Symmetry Approach in Scalar-Torsion $f(T,ϕ)$ Gravity

The Noether Symmetry approach is applied to study an extended teleparallel $f(T,ϕ)$ gravity that contains the torsion scalar $T$ and the scalar field $ϕ$ in the context of an Friedmann-Lemaître-Robertson-Walker space-time. We investigate the Noether symmetry approach in $f(T,ϕ)$ gravity formalism with the specific form of $f(T,ϕ)$ and analyze how to demonstrate a nontrivial Noether vector. The Noether symmetry method is a helpful resource for generating models and finding out the exact solution of the Lagrangian. In this article, we go through how the Noether symmetry approach enables us to define the form of the function $f(T,ϕ)$ and obtain exact cosmological solutions. We also find the analytical cosmological solutions to the field equations, that is consistent with the Noether symmetry. Our results demonstrate that the obtained solutions enable an accelerated expansion of the Universe. We have also obtained the present value of the Hubble parameter, deceleration parameter, and effective equation of state parameter, which is fit in the range of current cosmological observations.

gr-qc↗
arXiv2023

Cosmological Perturbations in the Teleparallel analog of Horndeski gravity

In this work we study the cosmological perturbations in Bahamonde-Dialektopoulos-Levi Said (BDLS) theory, i.e. the teleparallel analog of Horndeski gravity. In order to understand the evolution of structure in a cosmological model, it is necessary to study its cosmology not only in the background but also perturbatively. Both Horndeski and its teleparallel analog have been analyzed a lot in the literature, but in order to study them quantitatively, we need to know their cosmological perturbations. That is why, we study here the scalar-vector-tensor decomposition of the theory and we also express the so-called alpha parameters in terms of the arbitrary functions of the theory, that designate the deviation from the ΛCDM model. We have explored tensor, vector and scalar perturbation of the action up to second order, which drastically opens up new possibilities on searches in the parameter space of scalar-tensor theories in the context of observations.

gr-qc↗
arXiv2023

Accelerating Cosmological Models in $f(T, B)$ Gravitational Theory

In this paper, we have explored the field equations of f(T, B) gravity as an extension of teleparallel gravity in an isotropic and homogeneous space time. In the basic formalism developed, the dynamical parameters are derived by incorporating the power law and exponential scale factor function. The models are showing accelerating behaviour and approaches to $Λ$CDM at late time.The present value of the equation of state parameter for both the cases are obtained to be in accordance with the range provided by cosmological observations. The geometrical parameters and the scalar field reconstruction are performed to assess the viability of a late time accelerating Universe. Further the stability of both the models are presented. It has been observed that both the models are parameters dependent. Since most of the geometrically modified theories of gravity are favouring the violation of strong energy condition, we have derived the energy conditions both for the power law and exponential model. In both the models, the violation of strong energy condition established.

gr-qc↗
arXiv2023

Neural network reconstruction of scalar-tensor cosmology

Neural networks have shown great promise in providing a data-first approach to exploring new physics. In this work, we use the full implementation of late time cosmological data to reconstruct a number of scalar-tensor cosmological models within the context of neural network systems. In this pipeline, we incorporate covariances in the data in the neural network training algorithm, rather than a likelihood which is the approach taken in Markov chain Monte Carlo analyses. For general subclasses of classic scalar-tensor models, we find stricter bounds on functional models which may help in the understanding of which models are observationally viable.

gr-qc↗
arXiv2023

Modified Gravity and Cosmology: An Update by the CANTATA Network

General Relativity and the $Λ$CDM framework are currently the standard lore and constitute the concordance paradigm. Nevertheless, long-standing open theoretical issues, as well as possible new observational ones arising from the explosive development of cosmology the last two decades, offer the motivation and lead a large amount of research to be devoted in constructing various extensions and modifications. All extended theories and scenarios are first examined under the light of theoretical consistency, and then are applied to various geometrical backgrounds, such as the cosmological and the spherical symmetric ones. Their predictions at both the background and perturbation levels, and concerning cosmology at early, intermediate and late times, are then confronted with the huge amount of observational data that astrophysics and cosmology are able to offer recently. Theories, scenarios and models that successfully and efficiently pass the above steps are classified as viable and are candidates for the description of Nature. This work is a Review of the recent developments in the fields of gravity and cosmology, presenting the state of the art, high-lighting the open problems, and outlining the directions of future research. Its realization was performed in the framework of the COST European Action ``Cosmology and Astrophysics Network for Theoretical Advances and Training Actions''.

gr-qc↗
arXiv2023

General Effective Field Theory of Teleparallel Gravity

We construct the Effective Field Theory (EFT) of the teleparallel equivalent of general relativity (TEGR). Firstly, we present the necessary field redefinitions of the scalar field and the tetrads. Then we provide all the terms at next-to-leading-order, containing the torsion tensor and its derivatives, and derivatives of the scalar field, accompanied by generic scalar-field-dependent couplings, where all operators are suppressed by a scale $Λ$. Removing all redundant terms using the field redefinitions we result to the EFT of TEGR, which includes significantly more terms comparing to the EFT of General Relativity. Finally, we present an application in a cosmological framework. Interestingly enough, although GR and TEGR are completely equivalent at the level of classical equations, we find that their corresponding EFTs possess minor but non-zero differences. Hence, we do verify that at higher energies the excitation and the features of the extra degrees of freedom are slightly different in the two theories, thus making them theoretically distinguishable. Nevertheless, we mention that these differences are suppressed by the heavy mass scale $Λ$ and thus it is not guaranteed that they could be measured in future experiments and observations.

gr-qc↗
arXiv2023

Noether Symmetries in $f(T,T_G)$ Cosmology

All degrees of freedom related to the torsion scalar can be explored by analysing, the $f(T,T_G)$ gravity formalism where, $T$ is a torsion scalar and $T_G$ is the teleparallel counterpart of the Gauss-Bonnet topological invariant term. The well-known Noether symmetry approach is a useful tool for selecting models that are motivated at a fundamental level and determining the exact solution to a given Lagrangian, hence we explore Noether symmetry approach in $f(T,T_G)$ gravity formalism with three different forms of $f(T,T_G)$ and study how to establish nontrivial Noether vector form for each one of them. We extend the analysis made in \cite{capozziello2016noether} for the form $f(T,T_{G})=b_{0}T_{G}^{k}+t_{0}T^{m}$ and discussed the symmetry for this model with linear teleparallel equivalent of the Gauss-Bonnet term, followed by the study of two models containing exponential form of the teleparallel equivalent of the Gauss-Bonnet term. We have shown that all three cases will allow us to obtain non-trivial Noether vector which will play an important role to obtain the exact solutions for the cosmological equations.

gr-qc↗
arXiv2023

Constraints on $f(T)$ Cosmology with Pantheon+

$f(T)$ cosmology has shown promise in explaining aspects of cosmic evolution. In this work, we analyze constraints on leading models of $f(T)$ gravity in the context of the recently released Pantheon+ data set, together with comparisons with previous releases. We also consider other late-time data sets including cosmic chronometers and baryonic acoustic oscillation data. Our main result is that we find that the different $f(T)$ models under investigation connect to a variety of Hubble constant, which may help alleviate the cosmic tension on this parameter.

gr-qc↗
arXiv2023

Dark energy by natural evolution: Constraining dark energy using Approximate Bayesian Computation

We look at dark energy from a biology inspired viewpoint by means of the Approximate Bayesian Computation (ABC) and late time cosmological observations. We find that dynamical dark energy comes out on top, or in the ABC language naturally selected, over the standard $Λ$CDM cosmological scenario. We confirm this conclusion is robust to whether baryon acoustic oscillations and Hubble constant priors are considered. Our results show that the algorithm prefers low values of the Hubble constant, consistent or at least a few standard deviation away from the cosmic microwave background estimate, regardless of the priors taken initially in each model. This supports the result of the traditional MCMC analysis and could be viewed as strengthening evidence for dynamical dark energy being a more favorable model of late time cosmology.

astro-ph.CO↗
arXiv2023

Ghost and Laplacian Instabilities in the Teleparallel Horndeski Gravity

Teleparallel geometry offers a platform on which to build up theories of gravity where torsion rather than curvature mediates gravitational interaction. The teleparallel analogue of Horndeski gravity is an approach to teleparallel geometry where scalar-tensor theories are considered in this torsional framework. Being teleparallel gravity of lower order in dynamics, this turns out to be more general than metric Horndeski gravity. In other words, the class of teleparallel Horndeski gravity models is much broader than the standard metric one. In this work, we explore constraints on this wide range of models coming from ghost and Laplacian instabilities. The aim is to limit pathological branches of the theory by fundamental considerations. It is possible to conclude that a very large class of models results physically viable.

gr-qc↗
arXiv2023

Teleparallel Gravity: From Theory to Cosmology

Teleparallel gravity has significantly increased in popularity in recent decades, bringing attention to Einstein's other theory of gravity. In this Review, we relate this form of geometry to the broader metric-affine approach to forming gravitational theories where we describe a systematic way of constructing consistent teleparallel theories that respect certain physical conditions such as local Lorentz invariance. We first use teleparallel gravity to formulate a teleparallel equivalent of general relativity which is dynamically equivalent to general relativity but which may have different behaviors for other scenarios, such as quantum gravity. After setting this foundation, we describe the plethora of modified teleparallel theories of gravity that have been proposed in the literature. In the second part of the Review, we first survey works in teleparallel astrophysics literature where we focus on the open questions in this regime of physics. We then discuss the cosmological consequences for the various formulations of teleparallel gravity. We do this at background level by exploring works using various approaches ranging from dynamical systems to Noether symmetries, and more. Naturally, we then discuss perturbation theory, firstly by giving a concise approach in which this can be applied in teleparallel gravity theories and then apply it to a number of important theories in the literature. Finally, we examine works in observational and precision cosmology across the plethora of proposal theories. This is done using some of the latest observations and is used to tackle cosmological tensions which may be alleviated in teleparallel cosmology. We also introduce a number of recent works in the application of machine learning to gravity, we do this through deep learning and Gaussian processes, together with discussions about other approaches in the literature.

gr-qc↗
arXiv2023

Dynamical systems analysis in $f(T,ϕ)$ gravity

Teleparallel based cosmological models provide a description of gravity in which torsion is the mediator of gravitation. Several extensions have been made within the so-called Teleparallel equivalent of general relativity which is equivalent to general relativity at the level of the equations of motion where attempts are made to study the extensions of this form of gravity and to describe more general functions of the torsion scalar $T$. One of these extensions is $f(T,ϕ)$ gravity; $T$ and $ϕ$ respectively denote the torsion scalar and scalar field. In this work, the dynamical system analysis has been performed for this class of theories to obtain the cosmological behaviour of a number of models. Two models are presented here with some functional form of the torsion scalar and the critical points are obtained. For each critical point, the stability behaviour and the corresponding cosmology are shown. Through the graphical representation the equation of state parameter and the density parameters for matter-dominated, radiation-dominated and dark energy phase are also presented for both the models.

gr-qc↗