Presence Of Cosmological Constant Research Articles

Trace-anomaly driven inflation in f(T) gravity with a cosmological constant

In this paper, we have investigated trace-anomaly (TA) driven inflation in Generalized Teleparallel gravity (namely f(T)). A quasi de Sitter (dS) scenario for inflation is proposed like in the case of R 2 gravity. It has been found that in f(T) gravity, the dS solution for inflation is unstable due to the conformal anomaly (CA). As an illustrative example, we have analyzed trace anomaly driven inflation in a T 2+f(T)-model. Furthermore, a model with a non zero cosmological constant is studied. We have shown that dS solution always remains unstable also in the presence of cosmological constant, but for Anti dS (AdS) case the situation dramatically changes.

Hairy black holes sourced by a conformally coupled scalar field inDdimensions

There exist well-known no-hair theorems forbidding the existence of hairy black hole solutions in general relativity coupled to a scalar conformal field theory in asymptotically flat space. Even in the presence of cosmological constant, where no-hair theorems can usually be circumvented and black holes with conformal scalar hair were shown to exist in dimensions three and four, no-go results were reported for D>4. In this paper we prove that these obstructions can be evaded and we answer in the affirmative a question that remained open: Whether hairy black holes do exist in general relativity sourced by a conformally coupled scalar field in arbitrary dimensions. We find the analytic black hole solution in arbitrary dimension D>4, which exhibits a backreacting scalar hair that is regular everywhere outside and on the horizon. The metric asymptotes to (Anti-)de Sitter spacetime at large distance and admits spherical horizon as well as horizon of a different topology. We also find analytic solutions when higher-curvature corrections O(R^n) of arbitrary order n are included in the gravity action.

Interacting Viscous Modified Chaplygin Gas Cosmology in Presence of Cosmological Constant

Here, we consider interacting viscous modified Chaplygin gas in presence of cosmological constant. We assumed bulk viscosity as a function of density. We consider interaction between modified Chaplygin gas and baryonic matter. Then, the effects of viscosities on the cosmological parameters such as energy, density, Hubble expansion parameter, scale factor and deceleration parameter investigated. This model may be considered as a toy model of our universe.

Slowly Rotating Black Holes with Nonlinear Electrodynamics

We study charged slowly rotating black hole with a nonlinear electrodynamics (NED) in the presence of cosmological constant. Starting from the static solutions of Einstein-NED gravity as seed solutions, we use the angular momentum as the perturbative parameter to obtain slowly rotating black holes. We perform the perturbations up to the linear order for black holes in 4 dimensions. These solutions are asymptotically AdS and their horizon has spherical topology. We calculate the physical properties of these black holes and study their dependence on the rotation parameteraas well as the nonlinearity parameterβ. In the limitβ→∞, the solution describes slowly rotating AdS type black holes.

Stability of viscous fluid in Bianchi type-VI model with cosmological constant

In this paper, we investigate Bianchi type-$VI$ cosmological model for the universe filled with dark energy and viscous fluid in the presence of cosmological constant. Also, we show accelerating expansion of the universe by drawing volume scale, pressure and energy density versus cosmic time. In order to solve the Einstein field equations, we assume the expansion scalar is proportional to a component of the shear tensor. Therefore, we obtain the directional scale factors and show the EOS parameter crosses over phantom divided-line.

Role of electric charge and cosmological constant in structure scalars

The physical meaning of structure scalars is analyzed for charged dissipative spherical fluids and for neutral dust in the presence of cosmological constant. The role played by such factors in the structure scalars is clearly brought out and physical consequences are discussed. Particular attention needs to be paid to the changes introduced by the above mentioned factors in the inhomogeneity factor and the evolution of the expansion scalar and the shear tensor.

Dark energy and matter in 4 dimensions from an empty Kaluza–Klein spacetime

We consider the third order Lovelock equations without the cosmological constant term in an empty n(⩾8)-dimensional Kaluza–Klein spacetime M4×Kn−4, where Kn−4 is a constant curvature space. We show that the emptiness of the higher-dimensional spacetime imposes a constraint on the metric function(s) of 4-dimensional spacetime M4. We consider the effects of this constraint equation in the context of black hole physics, and find a black hole solution in 4 dimensions in the absence of matter field and the cosmological constant (dark energy). This solution has the same form as the 4-dimensional solution introduced in [H. Maeda, N. Dadhich, Phys. Rev. D 74 (2006) 021501(R)] for Gauss–Bonnet gravity in the presence of cosmological constant, and therefore the metric of M4 which satisfies the vacuum Lovelock equations in higher-dimensional Kaluza–Klein spacetime is unique. This black hole solution shows that the curvature of an empty higher-dimensional Kaluza–Klein spacetime creates dark energy and matter with non-traceless energy–momentum tensor in 4 dimensions.

The Decay of Massive Scalar Field in Non-Static Gödel Type Universe with Viscous Fluid and Heat Flow

In this study, we have investigated the dynamics of non-static Godel type rotating universe with massive scalar field, viscous fluid and heat flow in the presence of cosmological constant. For various cosmic matter forms, the behavior of the cosmological constant (Λ), shear (η) and bulk (ξ) viscosity coefficients and other kinematic quantities have studied in the early universe. We have showed the decay of massive scalar field in the non-static rotating Godel type universe and we have obtained constant scalar field with and without source density. Also, we have investigated the effects of massive scalar field on the matter density and pressure. From solutions of the field equations, we have a cosmological model with non-zero expansion, shear, heat flux and rotation. Also some physical and geometrical aspects of the model discussed.

Effect of dynamical cosmological constant in presence of modified Chaplygin gas for accelerating universe

In this paper we have considered the Universe to be filled with Modified Gas and the Cosmological Constant Λ to be time-dependent with or without the Gravitational Constant G to be time-dependent. We have considered various phenomenological models for Λ, viz., $\Lambda\propto\rho,\Lambda\propto\frac{\dot{a}^{2}}{a^{2}}$ and $\Lambda\propto\frac{\ddot{a}}{a}$ . Using these models it is possible to show the accelerated expansion of the Universe at the present epoch. Also we have shown the natures of G and Λ over the total age of the Universe. Using the statefinder parameters we have shown the diagrammatical representation of the evolution of the Universe starting from radiation era to ΛCDM model.

Effective action, massive gravitons and the Cosmological Constant

The one loop effective action in a Schwarzschild background is here used to compute the cosmological constant in presence of massive gravitons. It is shown that the expression of the Zero Point Energy (ZPE) is equivalent to the one computed by means of a variational approach. To handle with ZPE divergences, we use the zeta function regularization. The regularization is closely related to the subtraction procedure appearing in the computation of Casimir energy in a curved background. A renormalization procedure is introduced to remove the infinities together with a renormalization group equation.

COVARIANT LINEAR PERTURBATIONS IN A CONCORDANCE MODEL

We present the complete solution of the first order metric and density perturbation equations in a spatially flat (K = 0), Friedmann–Robertson–Walker (FRW) universe filled with pressureless ideal fluid, in the presence of cosmological constant. We use covariant linear perturbation formalism and the comoving gauge condition to obtain the field and conservation equations. The solution contains all modes of the perturbations, i.e. scalar, vector and tensor modes, and we show that our results are in agreement with the Sachs and Wolfe metric perturbation formalism.

Dynamical instability of fluid spheres in the presence of a cosmological constant

The equations describing the adiabatic, small radial oscillations of general relativistic stars are generalized to include the effects of a cosmological constant. The generalized eigenvalue equation for the normal modes is used to study the changes in the stability of the homogeneous sphere induced by the presence of the cosmological constant. The variation of the critical adiabatic index as a function of the central pressure is studied numerically for different trial functions. The presence of a large cosmological constant significantly increases the value of the critical adiabatic index. The dynamical stability condition of the homogeneous star in the Schwarzschild –de Sitter geometry is obtained and several bounds on the maximum allowable value for a cosmological constant are derived from stability considerations.

Domain walls without cosmological constant in higher order gravity.

We consider a class of higher order corrections in the form of Euler densities of arbitrary rank n to the standard gravity action in D dimensions. We present a generating functional and an explicit form of the conserved energy-momentum tensors. We show that this class of corrections allows for domain-wall solutions despite the presence of higher powers of the curvature. The existence of such solutions no longer depends on the presence of cosmological constants. For example, the Randall-Sundrum-type scenario can be realized without bulk and/or brane cosmological constant.

CMB anisotropy in compact hyperbolic universes. II. COBE maps and limits

The measurements of CMB anisotropy have opened up a window for probing the global topology of the universe on length scales comparable to, and even beyond, the Hubble radius. For compact topologies, the two main effects on the CMB are: (1) the breaking of statistical isotropy in characteristic patterns determined by the photon geodesic structure of the manifold and (2) an infrared cutoff in the power spectrum of perturbations imposed by the finite spatial extent. We calculate the CMB anisotropy in compact hyperbolic universe models using the regularized method of images described in detail in the preceding paper, including the line-of-sight ``integrated Sachs-Wolfe'' effect, as well as the last-scattering surface terms. We calculate the Bayesian probabilities for a selection of models by confronting our theoretical pixel--pixel temperature correlation functions with the COBE-DMR data. Our results demonstrate that strong constraints on compactness arise: if the universe is small compared to the horizon size, correlations appear in the maps that are irreconcilable with the observations. This conclusion is qualitatively insensitive to the matter content of the universe, in particular, the presence of cosmological constant. If the universe is of comparable size to the ``horizon,'' the likelihood function is very dependent upon orientation of the manifold w.r.t. the sky. While most orientations may be strongly ruled out, it sometimes happens that for a specific orientation the predicted correlation patterns are preferred over those for the conventional infinite models. The full Bayesian analysis we use is the most complete statistical test that can be done on the cosmic background explorer maps, taking into account all possible signals and their variances in the theoretical skies, in particular the high degree of anisotropic correlation that can exist. We also show that standard visual measures for comparing theoretical predictions with the data such as the isotropized power spectrum ${C}_{l}$ are not so useful in small compact spaces because of enhanced cosmic variance associated with the breakdown of statistical isotropy.

On the compactification of type IIA string theory

The ten dimensional type IIA string effective action with cosmological constant term is dimensionally reduced on a d -dimensional torus to derive lower dimensional effective action. The symmetries of the reduced effective action are examined. It is shown that the resulting six dimensional theory does not remain invariant under SO (4,4) symmetry whereas the reduced action, in the absence of the cosmological constant respects the symmetry as was shown by Sen and Vafa. New class of black hole solutions are obtained in five and four dimensions in the presence of cosmological constant. For the six dimensional theory, a four brane solution is presented.

Robertson-Walker Lyttleton-Bondi universe with cosmological constant

The exterior field of the Robertson-Walker metric in the Lyttleton-Bondi universe with cosmological constant is considered. It is shown that in the presence of cosmological constant, the exterior solution of this Universe is simply the empty space-time of general relativity.

The nondiverging and nontwisting type D electrovac solutions with λ

Assuming the type D of the metric and the alignment of the EM field along the double D–P directions all solutions of Einstein–Maxwell equations free of complex expansion in the presence of cosmological constant are studied. All solutions of this type are found equivalent to Carter’s branch [B̄ (−)] in a previous paper and are derivable from the general 7-parametric D’s by contractions. Various special cases are examined, and at least a 4-parameter group of symmetries of these solutions is exhibited and studied.