International Journal of Modern Physics D

Exact general solutions to the Einstein–Cartan equations are obtained for spatially flat Friedmann cosmologies with a nonminimally coupled ghost scalar field and perfect fluid. It is shown that both singular and bouncing models are possible. An analogous problem is investigated in general relativity. Some effects of torsion are elucidated. The role of perfect fluid in the Einstein–Cartan cosmology is discussed.

Late time cosmological solutions for scalar field model with kinetic and Gauss–Bonnet couplings are considered. The quintom scenario is realized with and without Big Rip singularity. We find that under specific choice of the Gauss–Bonnet coupling, the model considerably simplifies, giving rise to solutions where the kinetic term is proportional to the square of the Hubble parameter. This allows to reconstruct the model for a suitable cosmological evolution. We considered a solution that matches the observed behavior of the equation of state, while Big Rip singularity may be present or absent, depending on the parameters of the solution. Evolutionary scenarios known as Little Rip, have also been considered.

We investigate the cosmological reconstruction in modified f(R, T) gravity, where R is the Ricci scalar and T the trace of the stress–energy tensor. Special attention is attached to the case in which the function f is given by f(R, T) = f₁(R) + f₂(T). The use of auxiliary scalar field is considered with two known examples for the scale factor corresponding to an expanding universe. In the first example, where ordinary matter is usually neglected for obtaining the unification of matter dominated and accelerated phases with f(R) gravity, it is shown in this paper that this unification can be obtained without neglecting ordinary matter. In the second example, as in f(R) gravity, model of f(R, T) gravity with transition of matter dominated phase to the acceleration phase is obtained. In both cases, linear function of the trace is assumed for f₂(T) and it is obtained that f₁(R) is proportional to a power of R with exponents depending on the input parameters.

We show that the string gas — a perfect fluid with the equation of state possesses rather interesting properties. In Friedmann universes its presence can change the observable topology of the space; in the spherically symmetric spacetimes it produces rather bizzare geometries and in a way its influence on the rotation curves mimics the dark matter effects.

Time variable cosmological constant (TVCC) of holographic origin with interaction in Brans–Dicke theory is discussed in this paper. We investigate some characters for this model, and show the evolutions of deceleration parameter and equation of state (EOS) for dark energy. It is shown that in this scenario an accelerating universe can be obtained and the evolution of EOS for dark energy can cross over the boundary of phantom divide. In addition, a geometrical diagnostic method, jerk parameter, is applied to this model to distinguish it with cosmological constant.

We consider a model of non-local gravity with a large bare cosmological constant, Λ, and study its cosmological solutions. The model is characterized by a function f(ψ) = f₀e^αψ, where ψ = □^-1R and α is a real dimensionless parameter. In the absence of matter, we find an expanding universe solution a ∝ tⁿ with n < 1, that is, a universe with decelerated expansion without any fine-tuning of the parameter. Thus the effect of the cosmological constant is effectively shielded in this solution. It has been known that solutions in non-local gravity often suffer from the existence of ghost modes. In the present case, we find the solution is ghost-free if α > α_cr ≈ 0.17. This is quite a weak condition. We argue that the solution is stable against the inclusion of matter fields. Thus our solution opens up new possibilities for solution to the cosmological constant problem.

In this article, we explore the consequences in the calculation of deflection angle of photons, caused by a gravitational source of mass M, for the case of an energy dependent metric. We analyze the corrections to the standard Schwarzschild case for the weak and strong limits. In both cases, the corrections to the deflection angle are of the order ϵ/E_Pl, where E_Pl is the Planck energy and ϵ, the energy of the photon at spatial infinite. The corrections to the angular separation image is also of order ϵ/E_Pl, for the weak limit, while in the strong case there is a corrective term with the shape ϵ/E_Pl n e^2nπ for the relativistic image of order n. The amplification of the image is also discussed. Even if corrections are tiny, in both limits, we discuss the qualitative effects for different types of Lorentz invariance deformations.

Constraints on the Hubble parameter, H₀, via X-ray surface brightness and Sunyaev–Zel'dovich effect (SZE) observations of the galaxy clusters depend on the validity of the cosmic distance duality relation (DD relation), η = D_L(z)(1+z)^-2/D_A(z) = 1, where D_L and D_A are the luminosity distance and angular diameter distance (ADD), respectively. In this work, we argue that if the DD relation does not hold, the X-ray plus SZE technique furnishes a . We use 25 ADD of galaxy clusters to obtain simultaneous constraints on H₀ and possible violation of the DD relation in a flat ΛCDM model. Such a violation is parametrized by two functions: η(z) = 1 + η₀z and η(z) = 1 + η₀z/(1+z), where η₀ is a constant parameter quantifying possible departures from the strict validity. Finally, by marginalizing on the η₀ in both parametrizations, we obtain constraints on H₀ regardless of the validity of the DD relation. For the linear and nonlinear η(z) functions, we obtain km/s/Mpc and km/s/Mpc, respectively (without systematic errors). Our results support recent H₀ measurements by using X-ray and SZE observations of galaxy clusters which have taken the distance duality as valid.

The importance of nuclear reactions in low-density astrophysical plasmas with ion temperatures T ≥10¹⁰ K has been recognized for more than thirty years. However, the lack of comprehensive data banks of relevant nuclear reactions and the limited computational power have not previously allowed detailed theoretical studies. Recent developments in these areas make it timely to conduct comprehensive studies on the nuclear properties of very hot plasmas formed around compact relativistic objects such as black holes and neutron stars. Such studies are of great interest in the context of scientific programs of future low-energy cosmic γ-ray spectrometry. In this work, using the publicly available code TALYS, we have built a large nuclear network relevant for temperatures exceeding 10¹⁰ K. We have studied the evolution of the chemical composition and accompanying prompt gamma-ray emission of such high-temperature plasmas. We present the results on the abundances of light elements D, T, ³He, ⁴He, ⁶Li, ⁷Li, ⁹Be, ¹⁰B, ¹¹B, and briefly discuss their implications on the astrophysical abundances of these elements.