Swampland dS conjecture in mimetic f(R, T) gravity

Journal of Physics: Conference Series, 2019

We intend to study a new class of cosmological models in f(R, T) modified theories of gravity, hence define the cosmological constant as a function of the trace of the stress energy-momentum-tensor T and the Ricci scalar R, and name such a model Λ(R, T) gravity where we have specified a certain form of Λ(R, T). Λ(R, T) is also defined in the perfect fluid and dust case. Some physical and geometric properties of the model are also discussed. The pressure, density and energy conditions are studied both when Λ is a positive constant and when Λ = Λ(t), i.e a function of cosmological time, t. We study behavior of some cosmological quantities such as Hubble and deceleration parameters. The model is innovative in the sense that it has been described in terms of both R and T and display better understanding of the cosmological observations.

International Journal of Modern Physics D, 2020

In this work, we present a few simple cosmological models under the modified theory of gravity in the particular form of [Formula: see text], where [Formula: see text] is the Ricci Scalar and [Formula: see text] is the trace of the energy–momentum tensor. Two special cosmological models are studied with (i) hyperbolic scale factor and (ii) specific form of the Hubble parameter. The models are observed to predict relevant cosmological parameters closer to the observational values. Both the models reduce to overlap with the [Formula: see text]CDM model at late times. We have discussed some interesting results related to wormhole solutions as evolved from our model. The possible occurrence of Big Trip in wormholes for the models are also discussed.

arXiv (Cornell University), 2022

In the recent years, a host of modified gravity models have been proposed as alternatives to the dark energy. A quantum theory of gravity also requires to modify 'General Theory of Relativity'. In the present article, we consider five different modified theories of gravity, and compare inflationary parameters with recent data sets released by two Planck collaboration teams. Our analysis reveals that the scalar-tensor theory of gravity is the best alternative.

European Physical Journal C, 2015

We investigate inflation within f (R, φ)-theories, where a dynamical scalar field is coupled to gravity. A class of models which can support early-time acceleration with the emerging of an effective cosmological constant at high curvature is studied. The dynamics of the field allow for exit from inflation leading to the correct amount of inflation in agreement with cosmological data. Furthermore, the spectral index and tensor-to-scalar ratio of the models are carefully analyzed. A generalization of the theory to incorporate dark matter in the context of mimetic gravity, and further extensions of the latter, are also discussed.

International Journal of Theoretical Physics, 2014

A new class of cosmological models in f (R, T ) modified theories of gravity proposed by Harko et al. (Phys. Rev. D 84:024020, 2011), where the gravitational Lagrangian is given by an arbitrary function of Ricci scalar R and the trace of the stress-energy tensor T , have been investigated for a specific choice of f (R, T ) = f 1 (R) + f 2 (T ) by considering time dependent deceleration parameter. The concept of time dependent deceleration parameter (DP) with some proper assumptions yield the average scale factor a(t) = sinh 1 n (αt), where n and α are positive constants. For 0 < n ≤ 1, this generates a class of accelerating models while for n > 1, the models of universe exhibit phase transition from early decelerating phase to present accelerating phase which is in good agreement with the results from recent astrophysical observations. Our intention is to reconstruct f (R, T ) models inspired by this special law for the deceleration parameter in connection with the theories of modified gravity. In the present study we consider the cosmological constant Λ as a function of the trace of the stress energy-momentum-tensor, and dub such a model "Λ(T ) gravity" where we have specified a certain form of Λ(T ). Such models may display better uniformity with the cosmological observations. The statefinder diagnostic pair {r, s} parameter has been embraced to characterize different phases of the universe. We also discuss the physical consequences of the derived models.

2022

In this piece of work, we studied the inflation in the context of f(R,T) theory of gravity. We assumed the functional form of f(R,T) to be R + λT , where R is the Ricci scalar, T is the trace of the EnergyMomentum tensor and λ is the model parameter. The cosmological observable parameters like scalar spectral index ns and tensor-to-scalar ratio r are estimated for Double-Well potential. We found that for λ = 115, ns and r are in good agreement with PLANCK 2018 data. Further, considering the vacuum expectation value in Double-Well potential to be Planckian, we observed the admissible range of model parameter to be 101 &lt; λ &lt; 130 for which this model remains consistent with PLANCK 2018 data.

We consider a gravitational theory in which matter is nonminimally coupled to geometry, with the effective Lagrangian of the gravitational field being given by an arbitrary function of the Ricci scalar, the trace of the matter energy-momentum tensor, and the contraction of the Ricci tensor with the matter energy-momentum tensor. The field equations of the theory are obtained in the metric formalism, and the equation of motion of a massive test particle is derived. In this type of theory the matter energymomentum tensor is generally not conserved, and this nonconservation determines the appearance of an extra force acting on the particles in motion in the gravitational field. It is interesting to note that in the present gravitational theory, the extra force explicitly depends on the Ricci tensor, which entails a relevant deviation from the geodesic motion, especially for strong gravitational fields, thus rendering the possibility of a space-time curvature enhancement by the R T coupling. The Newtonian limit of the theory is also considered, and an explicit expression for the extra acceleration that depends on the matter density is obtained in the small velocity limit for dust particles. We also analyze in detail the socalled Dolgov-Kawasaki instability and obtain the stability conditions of the theory with respect to local perturbations. A particular class of gravitational field equations can be obtained by imposing the conservation of the energy-momentum tensor. We derive the corresponding field equations for the conservative case by using a Lagrange multiplier method, from a gravitational action that explicitly contains an independent parameter multiplying the divergence of the energy-momentum tensor. The cosmological implications of the theory are investigated in detail for both the conservative and the nonconservative cases, and several classes of exact analytical and approximate solutions are obtained.

Symmetry

The latest released data from Planck in 2018 put up tighter constraints on inflationary parameters. In the present article, the in-built symmetry of the non-minimally coupled scalar-tensor theory of gravity is used to fix the coupling parameter, the functional Brans–Dicke parameter, and the potential of the theory. It is found that all the three different power-law potentials and one exponential pass these constraints comfortably, and also gracefully exit from inflation.

Phys.Rev. D105 (2022) 043514, 2022

We consider a modified gravity framework for inflation by adding to the Einstein-Hilbert action a direct f(ϕ)T term, where ϕ is identified as the inflaton and T is the trace of the energy-momentum tensor. The framework goes to Einstein gravity naturally when the inflaton decays out. We investigate inflation dynamics in this f(ϕ)T gravity (not to be confused with torsion-scalar coupled theories) on a general basis and then apply it to three well-motivated inflationary models. We find that the predictions for the spectral tilt and the tensor-to-scalar ratio are sensitive to this new f(ϕ)T term. This f(ϕ)T gravity brings chaotic and natural inflation into better agreement with data and allows a larger tensor-to-scalar ratio in the Starobinsky model.

Axioms

This research is an extension of our earlier published (2+1) dimensional cosmological models in f(R,T) gravity with Λ(R,T) (IOP Conf. Ser. J. Phys. Conf. Ser. 2019, 1258, 012026). A different class of cosmological space model is studied under modified theories of f(R,T) gravity, where the cosmological constant Λ is expressed as a function of the Ricci scalar R and the trace of the stress-energy momentum tensor T. We call such a model as “Λ(R,T) gravity”. Such a specific form of Λ(R,T) has been defined in the dust as well as in the perfect fluid case. We intend to search for a combined model that satisfies the equation of state for dark energy matter or quintessence matter or perfect matter fluid. Some geometric and intrinsic physical properties of the model are also described. The energy conditions, pressure and density are discussed both when Λ=Λ(r) is a function of the radial parameter r, as well as when Λ is zero. We study the effective mass function and also the gravitational re...

2019

A novel function for modified gravity is proposed, f(R, T)=R+λ R^2+2βln(T), with λ and β constants, R the Ricci scalar, and the energy-momentum tensor trace, T, satisfying T=ρ-3p&gt;0. Subsequently, two equation of state parameters, namely ω and a parametric form of the Hubble parameter H, are employed in order to study the accelerated expansion and initial cosmological bounce of the corresponding universe. Hubble telescope experimental data for redshift z within the range 0.07≤ z ≤ 2.34 are used to compare the theoretical and observational values of the Hubble parameter. Moreover, it is observed that all the energy conditions are fulfilled within a neighborhood of the bouncing point t=0, what shows that the necessary condition for violation of the null energy condition, within the neighborhood of a bouncing point in general relativity, could be avoided by modifying the theory in a reasonable way. Furthermore, a large amount of negative pressure is found, which helps to understand t...

2013

A generic homogenous and isotropic cosmology is investigated based on the scalar-tensor theory of gravitation involving general metric coupling and scalar potential functions. We show that for a broad class of such functions, the scalar gravitational field can be dynamically trapped using a recently suggested mechanism. The corresponding scalar potential can drive inflation, accelerating expansion in the early and late universe respectively, with features consistent with standard requirements. Remarkably, the inflationary phase admits a natural exit with a well-defined value of the Hubble parameter dictated by the duration of inflation in a parameter independent manner, regardless of the detailed forms of the metric coupling and scalar potential. For an inflation duration consistent with the GUT description of the early universe, the resulting Hubble parameter is found to be consistent with its observed value.

Modern Physics Letters A, 2018

We consider the cosmological inflation with scalar–tensor gravity and compare it with standard inflation based on General Relativity. The difference is determined by the value of the parameter [Formula: see text]. This approach is associated with using the special ansatz which links a function that defines a type of gravity with a scale factor of the universe.

arXiv (Cornell University), 2021

In this paper, we are going to investigate a new perspective of the two-field inflation model with respect to the swampland dS conjecture. At the first step, we study the two-fields inflation model, and apply the swampland conjecture to our model. Then, we calculate some cosmological parameters such as scalar spectrum index, tensor-to-scalar ratio, and compare our results with the recent observational data. Also, we give numerical analysis to show agreement with observational data.

arXiv (Cornell University), 2017

Among many alternative gravitational theories to General Relativity (GR), $f(R,T)$ gravity (where $R$ is the Ricci scalar and $T$ the trace of the energy-momentum tensor) has been widely studied recently. By adding a matter contribution to the gravitational Lagrangian, $f(R,T)$ theories have become an interesting extension to GR displaying a broad phenomenology in astrophysics and cosmology. In this paper, we discuss however the difficulties appearing in explaining a viable and realistic cosmology within the $f(R,T)$ class of theories. Our results challenge the viability of $f(R,T)$ as an alternative modification of gravity.