A test of the standard dark matter density evolution law using galaxy clusters and cosmic chronometers

In this paper, we implement a test of the standard law for the dark matter density evolution as a function of redshift. For this purpose, only a flat universe and the validity of the FRW metric are assumed. A deformed dark matter density evolution law is considered, given by $$\rho _c(z) \propto (1+z)^{3+\epsilon }$$ ρ c ( z ) ∝ ( 1 + z ) 3 + ϵ , and constraints on $$\epsilon $$ ϵ are obtained by combining the galaxy cluster gas mass fractions with cosmic chronometers measurements. We find that $$\epsilon =0$$ ϵ = 0 within 2$$\sigma $$ σ c.l., in full agreement with other recent analyses.

Noether symmetries of Einstein-aether scalar field cosmology

In this paper, we explore an Einstein-aether cosmological model by adding the scalar field in which it has an interaction with the aether field. For the cosmological implications of the model, we consider that the universe can be described by the spatially flat FRW metric together with the matter dominated universe. Applying Noether symmetry approach to the point-like Lagrangian we determine the explicit forms of unknown functions i.e. the potential and coupling function. We solve the analytical cosmological solutions of the field equations admitting the Noether symmetry, basically divided into two parts. Our results show that the obtained solutions lead to an accelerated expansion of the universe. We also discuss the tensor perturbations within the framework of this model in order to get information about the mass of gravitational waves.

Quasi-cosmological traversable wormholes in f(R) gravity

In this paper, we study traversable wormholes in the context of f(R) gravity. Exact solutions of traversable wormholes are found by imposing the nonconstant Ricci scalar. These solutions asymptotically match spherical, flat and hyperbolic FRW metric. By choosing some static f(R) gravity models, we verify the standard energy conditions for the asymptotically spherical, flat and hyperbolic wormhole solutions. Unlike the Einstein gravity, we find that in the context of f(R) modified gravity, the asymptotically spherical, flat and hyperbolic wormhole solutions can respect the null energy condition (NEC) at the wormhole throat and near that. We find that in some static f(R) models, asymptotically flat and hyperbolic wormholes respect the weak energy condition (WEC) through the whole space.

Exact solutions for a scalar-tensor theory through symmetries

In this paper, we study how to determine the unknown functions for the scalar tensor model [Formula: see text] where the Ricci scalar is allowed to appear in a nonlinear way. The methods followed to determine these functions are: the matter collineation approach, the Lie group method and the Lagrangian collineation approach. We find several exact analytical solutions for a cosmological model with a FRW metric. We determine that some of the results are also valid for some anisotropic metric (e.g. the self-similar ones).

Quantum Cosmology of Quadratic f(R) Theories with a FRW Metric

We study the quantum cosmology of a quadratic fR theory with a FRW metric, via one of its equivalent Horndeski type actions, where the dynamic of the scalar field is induced. The classical equations of motion and the Wheeler-DeWitt equation, in their exact versions, are solved numerically. There is a free parameter in the action from which two cases follow: inflation + exit and inflation alone. The numerical solution of the Wheeler-DeWitt equation depends strongly on the boundary conditions, which can be chosen so that the resulting wave function of the universe is normalizable and consistent with Hermitian operators.

Kantowski–Sachs cosmological solutions in the generalized teleparallel gravity via Noether symmetry

We study the f(T) theory as an extension of teleparallel gravity and consider the Noether symmetry of Kantowski–Sachs (KS) anisotropic model for this theory. We specify the explicit teleparallel form of f(T) and find the corresponding exact cosmological solutions under the assumption that the Lagrangian admits the Noether symmetry. It is found that the universe experiences a power law expansion for the scale factors in the context of f(T) theory. By deriving equation of state (EOS) parameter, we show that the universe passes through the phantom and [Formula: see text]CDM theoretical scenarios. In this way, we estimate a lower limit age for the universe in excellent agreement with the value reported from recent observations. When KS model reduces to the flat Friedmann–Robertson–Walker (FRW) metric, our results are properly transformed into the corresponding values.

When can an "Expanding Universe" look "Static" and vice versa: A comprehensive study

The Friedmann–Robertson–Walker (FRW) metric expressed, in terms of comoving coordinates (r, t), always looks nonstatic. But by employing the recently derived curvature/Schwarzschild form, (R, T), of FRW metric (A. Mitra, Gravit. Cosmol. 19 (2013) 134), we show here that FRW metric can assume static forms when the net energy density (ρe) is solely due to the vacuum contribution. Earlier this question was explored by Florides (Gen. Relativ. Gravit. 12 (1980) 563) whose approach was complex and of purely mathematical nature. Also, unlike Florides, we do not assume any a priori separability of T(r, t) = F(r)G(t) and thus our treatment is truly general and yet simpler. More interestingly, even if the net energy density involved in a certain FRW model may appear to be nonzero from its algebric appearance, it may still be possible that tacitly ρe = 0 and the model actually corresponds to a vacuum Minkowski metric. For instance, it has been found that FRW universes which appear to be expanding with a fixed speed in comoving coordinates are intrinsically static universes. While such a linearly expanding universe having k = -1 is well-known as the Milne universe, the corresponding k = 0 case has recently been shown to be vacuum in disguise (A. Mitra, Mon. Not. Roy. Astron. Soc. 442 (2014) 382). In addition, here we show that even the k = +1 linearly "expanding" universe (in comoving coordinates) tacitly corresponds to Einstein's static universe.

Lattice gauge theory and gluon color-confinement in curved spacetime

The lattice gauge theory (LGT) for curved spacetime is formulated. A discretized action is derived for both gluon and quark fields which reduces to the generally covariant form in the continuum limit. Using the Wilson action, it is shown analytically that for a general curved spacetime background, two propagating gluons are always color-confined. The fermion-doubling problem is discussed in the specific case of Friedman–Robertson–Walker (FRW) metric. Last, we discussed possible future numerical implementation of lattice QCD in curved spacetime.

Signature change in p-adic and noncommutative FRW cosmology

The significant matter for the construction of the so-called no-boundary proposal is the assumption of signature transition, which has been a way to deal with the problem of initial conditions of the universe. On the other hand, results of Loop Quantum Gravity indicate that the signature change is related to the discrete nature of space at the Planck scale. Motivated by possibility of non-Archimedean and/or noncommutative structure of space–time at the Planck scale, in this work we consider the classical, p-adic and (spatial) noncommutative form of a cosmological model with Friedmann–Robertson–Walker (FRW) metric coupled with a self-interacting scalar field.