Dynamical mass generation for the gravitino in N = 1 supergravity

The Volkov-Akulov field is coupled to supergravity and it is gauged away through a field redefinition, remaining with a negative cosmological constant plus N = 1 supergravity lagrangian. Then the gravitino sector is quantized and a positive cosmological constant is obtained along with a mass-like term for the gravitino. Imposing the effective cosmological constant to be zero, consequently a genuine mass term for the gravitino is obtained. The corresponding energy-gap equation shows that this mass turns out to be of the order of the Planck mass.     

Cosmologies based on Lyra's geometry

It is pointed out that the cosmologies based on Lyra's manifold, with constant gauge-vector which have been studied in the literature, will either include a creation field and be equal to Hoyle's creation field cosmology, or contain a special vacuum fluid which together with the gauge-vector term may be considered as a cosmological term. In the latter case the solutions are equal to the general relativistic cosmologies with a cosmological term.     

Locally Rotationally Symmetric Bianchi Type-I Model with Time Varying Λ Term

We investigate the locally rotationally symmetric (LRS) Bianchi type-I cosmological model for stiff matter and a vacuum solution with a cosmological term proportional to R^-m (R is the scale factor and m is a positive constant). The cosmological term decreases with time. We obtain that for both the cases the present universe is accelerating with a large fraction of cosmological density in the form of a cosmological term.     

Bulk Viscous Bianchi Type V Cosmological Models with Decaying Cosmological Term Λ

We present bulk viscous Bianchi type V cosmological models with time-dependent cosmological term Λ. Exact solutions of Einstein field equations have been obtained by assuming shear scalar σ proportional to volume expansion θ. The coefficient of bulk viscosity is taken to be power function of energy density ρ or volume expansion θ. In these models cosmological term Λ come out to be negative. It is found that models obtained are expanding, shearing and non-rotating. They do not approach isotropy for large values of time t. Some observational parameters for the model have also been discussed.     

An Anisotropic Homogeneous Cosmological Model in a Modified Brans-Dicke Cosmology

Adding the cosmological term, which is assumed to be variable in Brans-Dicke theory we have discussed about a spatially homogeneous and anisotropic cosmological model corresponding to Bianchi type-I solution. The physical and geometrical properties of this model has been discussed. Finally this model has been transformed to the original form (1961) of Brans-Dicke theory (including a variable cosmological term).     

Bianchi Type-I Cosmological Model Filled with Viscous Fluid in a Modified Brans-Dicke Cosmology

Adding the cosmological term, which is assumed to be variable in Brans-Dicke theory we have discussed about a Bianchi type-I cosmological model filled with viscous fluid with free gravitational field of Petrov type-D. The effect of viscousity on various kinematical parameters has been discussed. Finally, this model has been transformed to the original form (1961) of Brans-Dicke theory including a variable cosmological term.     

Accelerating Flat Universe and Analytic Study of the m-z Relation

An accelerating flat universe with a variable cosmological term is obtained in the Robertson-Walker metric. The variable cosmological term is defined by the correction terms of the metric tensor field. Simple solutions of the scale factor and the cosmological term are shown. In this model of the universe, the magnitude-redshift relation is analytically studied to see if the model reproduces the tendency of the present observational data. The equation of state parameter is touched.     

Bianchi V Cosmology with a Specific Hubble?Parameter

In the present paper, we investigate the possibility of a variation law for Hubble’s parameter H in the background of spatially homogeneous, anisotropic Bianchi type V space-time with perfect fluid source and time-dependent cosmological term. The model obtained presents a cosmological scenario which describes an early deceleration and late time acceleration. The model approaches isotropy and tends to a de Sitter universe at late times. The cosmological term Λ asymptotically tends to a genuine cosmological constant. It is observed that the solution is consistent with the results of recent observations.     

Hamiltonian cosmology of bigravity

This article is written as a review of the Hamiltonian formalism for the bigravity with de Rham–Gabadadze–Tolley (dRGT) potential, and also of applications of this formalism to the derivation of the background cosmological equations. It is demonstrated that the cosmological scenarios are close to the standard ΛCDM model, but they also uncover the dynamical behavior of the cosmological term. This term arises in bigravity regardless on the choice of the dRGT potential parameters, and its scale is given by the graviton mass. Various matter couplings are considered.     

The problems of quantum gravity

The effect of unitary inequivalence of different versions of the formalism of quantum gravity, according to the arbitrary choice of coordinate conditions, does not mean a violation of general covariance but a furcation of a single classical theory into several inequivalent, but also generally covariant quantum theories. We choose a version closely analogous to the theory of Proca, with a strong supplementary condition, leading to a quantum gravity with a cosmological term. However, it is only the bare but not necessarily the dressed cosmological term that has to be different from zero. Ordinary theory without the cosmological term may be obtained by renormalization.     

Dynamical approach to the cosmological constant

We consider a dynamical approach to the cosmological constant. There is a scalar field with a potential whose minimum occurs at a generic, but negative, value for the vacuum energy, and it has a nonstandard kinetic term whose coefficient diverges at zero curvature as well as the standard kinetic term. Because of the divergent coefficient of the kinetic term, the lowest energy state is never achieved. Instead, the cosmological constant automatically stalls at or near zero. The merit of this model is that it is stable under radiative corrections and leads to stable dynamics, despite the singular kinetic term. The model is not complete, however, in that some reheating is required. Nonetheless, our approach can at the very least reduce fine-tuning by 60 orders of magnitude or provide a new mechanism for sampling possible cosmological constants and implementing the anthropic principle.     

Conformal cosmological model and SNe Ia data

Now there is a huge scientific activity in astrophysical studies and cosmological ones in particular. Cosmology transforms from a pure theoretical branch of science into an observational one. All the cosmological models have to pass observational tests. The supernovae type Ia (SNe Ia) test is among the most important ones. If one applies the test to determine parameters of the standard Friedmann-Robertson-Walker cosmological model one can conclude that observations lead to the discovery of the dominance of the ?? term and as a result to an acceleration of the Universe. However, there are big mysteries connected with an origin and an essence of dark matter (DM) and the ?? term or dark energy (DE). Alternative theories of gravitation are treated as a possible solution of DM and DE puzzles. The conformal cosmological approach is one of possible alternatives to the standard ??CDM model. As it was noted several years ago, in the framework of the conformal cosmological approach an introduction of a rigid matter can explain observational data without ?? term (or dark energy). We confirm the claim with much larger set of observational data.     

Can electro-magnetic field,anisotropic source and varying Λ be sufficient to produce wormhole spacetime?

It is well known that solutions of general relativity which allow for traversable wormholes require the existence of exotic matter (matter that violates weak or null energy conditions (WEC or NEC)). In this article, we provide a class of exact solution for Einstein-Maxwell field equations describing wormholes assuming the erstwhile cosmological term Λ to be space variable, viz., Λ=Λ(r). The source considered here not only a matter entirely but a sum of matters i.e. anisotropic matter distribution, electromagnetic field and cosmological constant whose effective parts obey all energy conditions out side the wormhole throat. Here violation of energy conditions can be compensated by varying cosmological constant. The important feature of this article is that one can get wormhole structure, at least theoretically, comprising with physically acceptable matters.     

Fluctuations produced by the cosmological constant in the empty Bianchi type-IX model

We analyze the fluctuations of the gravitational field, produced by the cosmological term, in the empty Bianchi type-IX cosmological model. It is shown that such fluctuations do not change the process of alternation of Kasner eras very near to the singularity, and that a positive cosmological constant leads to an asymptotic isotropization of the model.     

A Cylindrically Symmetric Universe Filled with Viscous Fluid in a Modified Brans-Dicke Cosmology

Adding the cosmological term, which is assumed to be variable in Brans-Dicke theory we have discussed about a cylindrically symmetric model filled with viscous fluid with free gravitational field of Petrov type-D. The effect of viscosity on various kinematical parameters has been discussed. Finally, this model has been transformed to the original form (1961) of Brans-Dicke theory (including a variable cosmological term).     

Cosmological constant: a lesson from Bose-Einstein condensates

The cosmological constant is one of the most pressing problems in modern physics. We address this issue from an emergent gravity standpoint, by using an analogue gravity model. Indeed, the dynamics of the emergent metric in a Bose-Einstein condensate can be described by a Poisson-like equation with a vacuum source term reminiscent of a cosmological constant. The direct computation of this term shows that in emergent gravity scenarios this constant may be naturally much smaller than the naive ground-state energy of the emergent effective field theory. This suggests that a proper computation of the cosmological constant would require a detailed understanding about how Einstein equations emerge from the full microscopic quantum theory. In this light, the cosmological constant appears as a decisive test bench for any quantum or emergent gravity scenario.     

Cosmological constant from quarks and torsion

We present a simple and natural way to derive the observed small, positive cosmological constant from the gravitational interaction of condensing fermions. In the Riemann‐Cartan spacetime, torsion gives rise to the axial–axial vector four‐fermion interaction term in the Dirac Lagrangian for spinor fields. We show that this nonlinear term acts like a cosmological constant if these fields have a nonzero vacuum expectation value. For quark fields in QCD, such a torsion‐induced cosmological constant is positive and its energy scale is only about 8 times larger than the observed value. Adding leptons to this picture could lower this scale to the observed value.     

Gravitationally Non-Degenerate Petrov Type–I Cosmological Model Filled with Viscous Fluid in Modified Brans-Dicke Cosmology

Adding the Cosmological term, which is assumed to be variable in Brans-Dicke theory, we have discussed about a cylindrically symmetric cosmological model filled with viscous fluid with free gravitational field of a non-degenerate petrov type-I. The effect of viscosity on various kinematical parameters has been discussed. Finally, this model has been transformed to the original form (1961) of Brans-Dicke theory (including a variable cosmological term).     

The double complex Ashtekar gravitational theory and the corresponding constraint equations with the cosmological term

We establish the double complex Ashtekar gravitational theory with the cosmological term.In particular,by performing the 3 1 decomposition of the double Ashtekar action containing the cosmological term to pass on the Hamiltonian framework,the double Ashtekar constraint equations are derived,which respectively correspond to Lorentzian and Euclidean gravity.     

More supergravity cosmologies

We present new non-trivial cosmological d=11 supergravity Friedmann-Robertson-Walker-Bianchi solutions with a time-dependent cosmological term.