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We construct a cosmological model from the inception of the Friedmann-Lem\^aitre-Robertson-Walker metric into the field equations of the f(R,L_m) gravity theory, with R being the Ricci scalar and L_m being the matter lagrangian density. The formalism is developed for a particular f(R,L_m) function, namely R/16\pi +(1+\sigma R)L_{m}, with \sigma being a constant that carries the geometry-matter coupling. Our solutions are remarkably capable of evading the Big-Bang singularity as well as predict the cosmic acceleration with no need for the cosmological constant, but simply as a consequence of the geometry-matter coupling terms in the Friedmann-like equations, Comment: 5 pages, 3 figures, Accepted version Fortschritte der physik

Currently, in order to explain the accelerated expansion phase of the universe, several alternative approaches have been proposed, among which the most common are dark energy models and alternative theories of gravity. Although these approaches rest on very different physical aspects, it has been shown that both are in agreement with the data in the current status of cosmological observations, thus leading to an enormous degeneration between these models. So until evidences of higher experimental accuracy are available, more conservative model independent approaches are a useful tool for breaking this degenerated cosmological models picture. Cosmography as a kinematic study of the universe is the most popular candidate on this regard. Here we show how to construct the cosmographic equations for the f (R, T ) theory of gravity within a conservative scenario of this theory, where R is the Ricci curvature scalar and T is the trace of the energy-moment tensor. Such equations relate f(R,T) and its derivatives at the current time t0 to the cosmographic parameters q0, j0 and s0. In addition, we show how these equations can be written within different dark energy scenarios, thus helping to discriminate between them. We also show how different f(R,T) gravity models can be constrained using these cosmographic equations., Comment: 20 pages, no figures

We revisit in this work a model for repeating Fast Radio Bursts based of the release of energy provoked by the magnetic field dynamics affecting a magnetar's crust. We address the basics of such a model by solving the propagation of the perturbation approximately, and quantify the energetics and the radiation by bunches of charges in the so-called {\it charge starved} region in the magnetosphere. The (almost) simultaneous emission of newly detected X-rays from SGR 1935+2154 is tentatively associated to a reconnection behind the propagation. The strength of $f$-mode gravitational radiation excited by the event is quantified, and more detailed studies of the non-linear (spiky) soliton solutions suggested., Comment: Submitted, discussion on GW enlarged, references added and physical pictured clarified

In this paper, we present the cosmological scenario obtained from $f(R,T)$ gravity by using an exponential dependence on the trace of the energy-momentum tensor. With a numerical approach applied to the equations of motion, we show several precise fits and the respective cosmological consequences. As a matter of completeness, we also analyzed cosmological scenarios where this new version of $f(R,T)$ is coupled with a real scalar field. In order to find analytical cosmological parameters, we used a slow-roll approximation for the evolution of the scalar field. This approximation allowed us to derived the Hubble and the deceleration parameters whose time evolutions describe the actual phase of accelerated expansion, and corroborate with our numerical investigations. Therefore, the analytical parameters unveil the viability of this proposal for $f(R,T)$ in the presence of an inflaton field., Comment: 7 pages, 9 figures

This article presents cosmological models that arise in a subclass of $f(R,T)=f(R)+f(T)$ gravity models, with different $f(R)$ functions and fixed $T$-dependence. That is, the gravitational lagrangian is considered as $f(R,T)=f(R)+��T$, with constant $��$. Here $R$ and $T$ represent the Ricci scalar and trace of the stress-energy tensor, respectively. The modified gravitational field equations are obtained through the metric formalism for the Friedmann-Lema��tre-Robertson-Walker metric with signature $(+,-,-,-)$. We work with $f(R)=R+��R^2-\frac{��^4}{R}$, $f(R)=R+k\ln(��R)$ and $f(R)=R+me^{[-nR]}$, with $��, ��, k, ��, m$ and $n$ all free parameters, which lead to three different cosmological models for our Universe. For the choice of $��=0$, this reduces to widely discussed $f(R)$ gravity models. This manuscript clearly describes the effects of adding the trace of the energy-momentum tensor in the $f(R)$ lagrangian. The exact solution of the modified field equations are obtained under the hybrid expansion law. Also we present the Om diagnostic analysis for the discussed models., 11 pages, 20 figures, Accepted version in EPJ C

We present a new approach for the $f(R,T)$ formalism, by exploring the extra terms of its effective energy-momentum tensor $T_{\mu\nu}^{eff}$, namely $\tilde{T}_{\mu\nu}$. Those arise from the consideration of quantum effects, which are usually neglected in general relativity and $f(R)$ theories, and are summed to the usual matter energy-momentum tensor, yielding $T_{\mu\nu}^{eff}=T_{\mu\nu}+\tilde{T}_{\mu\nu}$. Purely from the Bianchi identities, the conservation of both parts of the effective energy-momentum tensor is obtained, rather than the non-conservation of the matter one, originally approached in the $f(R,T)$ theories. In this way, the intriguing scenario of matter-creation, which still lacks observational evidences, is evaded. One is left, then, with two sets of cosmological equations to be solved: the Friedmann-like equations along with the conservation of $T_{\mu\nu}$ and along with the conservation of $\tilde{T}_{\mu\nu}$. We present a physical interpretation for the conservation of the extra terms of the effective energy-momentum tensor, which is related to the presence of stiff matter in the universe. The cosmological features of this approach are presented and discussed as well as the benefits of evading the matter energy-momentum tensor non-conservation.

Recent elaborated by T. Harko and collaborators, the $f(R,T)$ theories of gravity contemplate an optimistic alternative to dark energy, for which $R$ and $T$ stand for the Ricci scalar and the trace of the energy-momentum tensor, respectively. Although the literature has shown that the $T$ dependence on the gravitational part of the action - which is due to the consideration of quantum effects - may induce some novel features in the scope of late-time cosmological dynamics, in the radiation-dominated universe, when $T=0$, no contributions seem to rise from such theories. Apparently, $f(R,T)$ contributions to a radiation-dominated universe may rise only from the $f(R,T^��)$ approach, which is nothing but the $f(R,T)$ gravity in the case of a self-interacting scalar field whose trace of the energy-momentum tensor is $T^��$. We intend, in this article, to show how $f(R,T^��)$ theories of gravity can contribute to the study of the primordial stages of the universe. Our results predict a graceful exit from inflationary stage to a radiation-dominated era. They also predict a late-time cosmic acceleration after a matter-dominated phase, making the $f(R,T^��)$ theories able to describe, in a self-consistent way, all the different stages of the universe dynamics., 10 pages, 6 figures

There is a host of alternative theories of gravitation in the literature, among them the $f(R,T)$ recently elaborated by Harko and collaborators. In these theories the $R$ and $T$ are respectively the Ricci scalar and the trace of the energy momentum tensor. There is already in literature a series of studies of different forms of the $f(R,T)$ functions as well as their cosmological consequences. However, there is not so far in the literature studies related to the gravitational waves in $f(R,T)$ gravity. Here we consider such an issue, in particular studying the putative extra polarization models that can well appear in such theories. To do that, we consider different functional forms for $f(R,T)$., Comment: 7 pages, no figures, accepted for publication in Physical Review D

In this article we study the hydrostatic equilibrium configuration of neutron stars and strange stars, whose fluid pressure is computed from the equations of state $p=\omega\rho^{5/3}$ and $p=0.28(\rho-4{\cal B})$, respectively, with $\omega$ and ${\cal B}$ being constants and $\rho$ the energy density of the fluid. We start by deriving the hydrostatic equilibrium equation for the $f(R,T)$ theory of gravity, with $R$ and $T$ standing for the Ricci scalar and trace of the energy-momentum tensor, respectively. Such an equation is a generalization of the one obtained from general relativity, and the latter can be retrieved for a certain limit of the theory. For the $f(R,T)=R+2\lambda T$ functional form, with $\lambda$ being a constant, we find that some physical properties of the stars, such as pressure, energy density, mass and radius, are affected when $\lambda$ is changed. We show that for a fixed central star energy density, the mass of neutron and strange stars can increase with $\lambda$. Concerning the star radius, it increases for neutron stars and it decreases for strange stars with the increment of $\lambda$. Thus, in $f(R,T)$ theory of gravity we can push the maximum mass above the observational limits. This implies that the equation of state cannot be eliminated if the maximum mass within General Relativity lies below the limit given by observed pulsars., Comment: 8 pages, 12 figures

One common approach for cosmic inflation consists in couple Einstein's gravity with a scalar field, often referred to inflaton field. In order to derive analytic simple scenarios, we usually work in the {\it slow-roll} regime. In such an approximation one considers the scalar field potentials to be nearly flat. It is possible to directly generalize such an approach for hybrid inflationary models, where the inflaton sector is composed of two or more scalar fields. However, the Friedmann equations and the equations of motion for such hybrid models are often hard to be analytically solved. Another recent path to describe cosmic inflation is through the so-called {\it fast-roll} regime, where one considers exactly flat potentials. Our purpose in this work is to obtain solutions for a hybrid inflaton model in the fast-roll regime. Cosmological interpretations through the behavior of Hubble, slow-roll, and equation of state parameters are also presented., Comment: 8 pages, 8 figures, new updates

In this work we investigate generalized theories of gravity in the so-called configurational entropy (CE) context. We show, by means of this information-theoretical measure, that a stricter bound on the parameter of f ( R , T ) brane models arises from the CE. We find that these bounds are characterized by a valley region in the CE profile, where the entropy is minimal. We argue that the CE measure can play a new role and might be an important additional approach to selecting parameters in modified theories of gravitation.