Jackson Levi Said

University of Malta

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Celia Escamilla‐Rivera

Universidad Nacional Autónoma de México

Jurgen Mifsud

University of Malta

Konstantinos F. Dialektopoulos

Transylvania University of Brașov

Gabriel Farrugia

University of Malta

Sebastián Bahamonde

Tokyo Institute of Technology

Emmanuel N. Saridakis

National Observatory of Athens

Rebecca Briffa

University of Malta

Joseph Sultana

University of Malta

Maria Caruana

University of Malta

Salvatore Capozziello

University of Naples Federico II

Cooperative Institutions

Centre National de la Recherche Scientifique

140

Université Paris Cité

Istituto Nazionale di Fisica Nucleare

University of Lisbon

University of Cambridge

Sorbonne Université

Max Planck Society

Institut National de Physique Nucléaire et de Physique des Particules

Istituto Nazionale di Fisica Nucleare, Sezione di Napoli

European Organization for Nuclear Research

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Reviving Horndeski theory using teleparallel gravity after GW170817

Physical review. D/Physical review. D. (2020)

Sebastián Bahamonde Konstantinos F. Dialektopoulos Viktor Gakis Jackson Levi Said

Horndeski gravity was highly constrained from the recent gravitational wave observations by the LIGO Collaboration down to $|{c}_{g}/c\ensuremath{-}1|\ensuremath{\gtrsim}{10}^{\ensuremath{-}15}$. In this paper, we study the propagation of gravitational waves in a recently proposed model of Horndeski gravity in which its teleparallel gravity analog is formulated. As usually done in these analyses, we consider a flat cosmological background in which curvature is replaced by torsion as the expression of gravitation. It is found that in this approach, one can construct a more general Horndeski theory satisfying ${c}_{T}={c}_{g}/c=1$ without eliminating the coupling functions ${G}_{5}(\ensuremath{\phi},X)$ and ${G}_{4}(\ensuremath{\phi},X)$ that were highly constrained in standard Horndeski theory. Hence, in the teleparallel approach one is able to restore these terms, creating an interesting way to revive Horndeski gravity. In this way, we retain the original spirit of Horndeski gravity (unlike beyond Horndeski theories) while only changing the form in which the geometry of gravitation is expressed.

Massive gravity

Formalism (music)

10.1103/physrevd.101.084060

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Cosmological Effects on Black Hole Formation

arXiv (Cornell University) (2012)

Jackson Levi Said Kristian Zarb Adami

The formation of cosmological black holes is investigated using the functional Schroedinger equation as observed by an asymptotic observer, assuming a spherical domain wall collapse process. The mass formula of the Sultana-Dyer black hole is derived using Israel's domain wall mathematical framework. This is used to examine the semi-classical and quantum nature of the collapsing domain wall in the general scale of an arbitrary scale factor, while ignoring evaporation and back reaction mass losses. Particular FLRW scale factors are then explored for black hole horizon formation times, all yielding the same over-all classical result, namely that an arbitrary amount of time is required for an asymptotic observer to register the formation of this surface.

Apparent horizon

Black hole (networking)

Friedmann–Lemaître–Robertson–Walker metric

Observer (physics)

10.48550/arxiv.1201.0750

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

arXiv (Cornell University) (2022)

S. A. Kadam Jackson Levi Said B. Mishra

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.

Formalism (music)

10.48550/arxiv.2210.17075

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White Dwarf Envelops and Temperature Corrections in Exponential $f(T)$ Gravity

arXiv (Cornell University) (2024)

Gabriel Farrugia Carlos Gafa’Jackson Levi Said

Compact stars have long served as a test bed of gravitational models and their coupling with stellar matter. In this work, we explore the behavior of an exponential model in f(T) gravity through the Tolman-Oppenheimer-Volkoff equation. This is performed for different envelope thicknesses. Finally, constraints on the models parameters are obtained, which are comparable to the results obtained using cosmological survey data. This consistency across the strong astrophysical and weak cosmological scales shows reasonable viability of the underlying model.

White (mutation)

10.48550/arxiv.2406.05771

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The cosmic string in Horndeski-like gravity

arXiv (Cornell University) (2024)

Fabiano F. Santos Jackson Levi Said

Scalar-tensor theories have shown promise in many sectors of cosmology. However, recent constraints from the speed of gravitational waves have put severe limits on the breadth of models such classes of theories can realize. In this work, we explore the possibility of a string-like Horndeski Lagrangian that is equipped with two dilaton fields. This is an interesting low-energy effective string theory that has a healthy general relativity limit. The evolution of a two-dilation coupled cosmology is not well-known in the literature. We explore the tensor perturbations in order to assess the behavior of the model again the speed of gravitational wave constraint. Our main result is that this model exhibits of a class of cosmological theories that is consistent with this observational constraint.

Cosmic string

Dilaton

String cosmology

10.48550/arxiv.2401.03558

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Existence of non-singular stellar solutions within the context of electromagnetic field: a comparison between minimal and non-minimal gravity models

The European Physical Journal C (2024)

Tayyab Naseer Jackson Levi Said

Abstract In this paper, we explore the existence of various non-singular compact stellar solutions influenced by the Maxwell field within the matter-geometry coupling based modified gravity. We start this analysis by considering a static spherically symmetric spacetime which is associated with the isotropic matter distribution. We then determine the field equations corresponding to two specific functions of this modified theory. Along with these models, we also adopt different forms of the matter Lagrangian. We observe several unknowns in these equations such as the metric potentials, charge and fluid parameters. Thus, the embedding class-one condition and a particular realistic equation of state is used to construct their corresponding solutions. The former condition provides the metric components possessing three constants, and we calculate them through junction conditions. Further, four developed models are graphically analyzed under different parametric values. Finally, we find all our developed solutions well-agreeing with the physical requirements, offering valuable insights for future explorations of the stellar compositions in this theory.

10.1140/epjc/s10052-024-13172-z

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Conformal gravity and transformations in the symmetric teleparallel framework

Physical review. D/Physical review. D. (2020)

Viktor Gakis Martin Krššák Jackson Levi Said Emmanuel N. Saridakis

Conformal symmetries appear in many parts of physics and play a unique role in exploring the Universe. We consider the possibility of constructing conformal theories of gravity in the symmetric teleparallel gravity framework, where gravitation is expressed through nonmetricity rather than curvature or torsion. We demonstrate that it is possible to formulate a family of conformal theories with second-order field equations and have the metric tensor as the fundamental variable. Moreover, we consider the addition of a scalar field to the theory, and compare the results to the teleparallel gravity setting. Finally, we present the scalar-tensor representation of modified symmetric teleparallel theories, and among others we show that there is an interesting subcase that has the possibility of playing the role of dark energy.

Conformal gravity

10.1103/physrevd.101.064024

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Noether symmetry approach in scalar-torsion $$f(T,\phi )$$ gravity

The European Physical Journal C (2023)

L. K. Duchaniya B. Mishra Jackson Levi Said

Abstract The Noether Symmetry approach is applied to study an extended teleparallel $$f(T,\phi )$$ f ( T , ϕ ) gravity that contains the torsion scalar T and the scalar field $$\phi $$ ϕ in the context of an Friedmann–Lemaître–Robertson–Walker space-time. We investigate the Noether symmetry approach in $$f(T,\phi )$$ f ( T , ϕ ) gravity formalism with the specific form of $$f(T,\phi )$$ 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 $$f(T,\phi )$$ 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.

10.1140/epjc/s10052-023-11792-5

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Neural network reconstruction of H'(z) and its application in teleparallel gravity

Journal of Cosmology and Astroparticle Physics (2022)

Purba Mukherjee Jackson Levi Said Jurgen Mifsud

In this work, we explore the possibility of using artificial neural networks to impose constraints on teleparallel gravity and its $f(T)$ extensions. We use the available Hubble parameter observations from cosmic chronometers and baryon acoustic oscillations from different galaxy surveys. We discuss the procedure for training a network model to reconstruct the Hubble diagram. Further, we describe the procedure to obtain $H'(z)$, the first order derivative of $H(z)$, using artificial neural networks which is a novel approach to this method of reconstruction. These analyses are complemented with further studies on the impact of two priors which we put on $H_0$ to assess their impact on the analysis, which are the local measurements by the SH0ES team ($H_0^{\text{R20}} = 73.2 \pm 1.3$ km Mpc$^{-1}$ s$^{-1}$) and the updated TRGB calibration from the Carnegie Supernova Project ($H_0^{\text{TRGB}} = 69.8 \pm 1.9$ km Mpc$^{-1}$ s$^{-1}$), respectively. Additionally, we investigate the validity of the concordance model, through some cosmological null tests with these reconstructed data sets. Finally, we reconstruct the allowed $f(T)$ functions for different combinations of the observational Hubble data sets. Results show that the $\Lambda$CDM model lies comfortably included at the 1$\sigma$ confidence level for all the examined cases.

Baryon Acoustic Oscillations

10.1088/1475-7516/2022/12/029

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On reconstruction of extended teleparallel gravity from the cosmological jerk parameter

The European Physical Journal C (2019)

Soumya Chakrabarti Jackson Levi Said Kazuharu Bamba

Teleparallel gravity offers a path to resolve a number of longstanding issues in general relativity by re-interpreting gravitation as an artifact of torsion rather than curvature. The present work deals with cosmological solutions in an extension of teleparallel gravity. A reconstruction scheme of the theory has been proposed based on the cosmological jerk parameter. The work contains analysis of ensuing cosmological parameters for different viable models and the stability of the models against cosmic time through an investigation of perturbation of matter overdensity and the hubble parameter.

Jerk

10.1140/epjc/s10052-019-6975-1

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Citations (21)