Charged-particle decay and suppression of primordial power on small scales

We study the suppression of the small-scale power spectrum due to the decay of charged matter to dark matter prior to recombination. Prior to decay, the charged particles couple to the photon-baryon fluid and participate in its acoustic oscillations. However, after these charged particles decay to neutral dark matter, the photon-baryon fluid is coupled only gravitationally to the newly created dark matter. This generically leads to suppression of power on length scales that enter the horizon prior to decay. For decay times of approximately 3.5 yr this leads to suppression of power on subgalactic scales, bringing the observed number of galactic substructures in line with observation. Decay times of a few years are possible if the dark matter is purely gravitationally interacting, such as the gravitino in supersymmetric models or a massive Kaluza-Klein graviton in models with universal extra dimensions.     

Two-Fluid Cosmological Models in Kaluza-Klein Space Time

In this paper we presented a class of solutions of Einstein’s field equations describing two-fluid models of the universe in a five dimensional spherical symmetric space time. In these models one fluid is the radiation distribution which represents the cosmic microwave background and the other fluid is the perfect fluid representing the matter content of the universe. The radiation and matter content of the universe are in interactive phase. Also we have discussed physical and kinematical behaviors of the model.     

Cosmological shock waves in general relativity

The problem of finding spherically symmetric self-similar solutions of Einstein's field equations with a barotropic perfect fluid, which can be joined through a shock wave to some cosmological models, is considered. It is found that such solutions comprise an expanding shell of matter surrounding a horizon with an interior singularity.     

Was the big bang a whimper?

In many cases the spatially homogeneous cosmological models of General Relativity begin or end at a “big bang” where the density and temperature of the matter in the universe diverge. However in certain cases the spatially homogeneous development of these universes terminates at a singularity where all physical quantities are well—behaved (a “whimper”) and an associated Cauchy horizon. We examine the existence and nature of these singularities, and the possible fate of matter which crosses the Cauchy horizon in such a universe. The nature of both kinds of singularity is illustrated by simple models based on two-dimensional Minkowski space-time; and the possibility of other types of singularity occuring is considered.     

Couplings between holographic dark energy and dark matter

We consider the interaction between dark matter and dark energy in the framework of holographic dark energy, and propose a natural and physically plausible form of interaction, in which the interacting term is proportional to the product of the powers of the dark matter and dark energy densities. We investigate the cosmic evolution in such models. The impact of the coupling on the dark matter and dark energy components may be asymmetric. While the dark energy decouples from the dark matter at late time, just as other components of the cosmic fluid become decoupled as the universe expands, interestingly, the dark matter may actually become coupled to the dark energy at late time. We shall call such a phenomenon incoupling. We use the latest type Ia supernovae data from the SCP team, baryon acoustics oscillation data from SDSS and 2dF surveys, and the position of the first peak of the CMB angular power spectrum to constrain the model. We find that the interaction term which is proportional to the first power product of the dark energy and dark matter densities gives an excellent fit to the current data.     

Bianchi cosmologies with anisotropic matter: Locally rotationally symmetric models

The dynamics of cosmological models with isotropic matter sources (perfect fluids) is extensively studied in the literature; in comparison, the dynamics of cosmological models with anisotropic matter sources is not. In this paper we consider spatially homogeneous locally rotationally symmetric solutions of the Einstein equations with a large class of anisotropic matter models including collisionless matter (Vlasov), elastic matter, and magnetic fields. The dynamics of models of Bianchi types I, II, and IX are completely described; the two most striking results are the following. (i) There exist matter models, compatible with the standard energy conditions, such that solutions of Bianchi type IX (closed cosmologies) need not necessarily recollapse; there is an open set of forever expanding solutions. (ii) Generic type IX solutions associated with a matter model like Vlasov matter exhibit oscillatory behavior toward the initial singularity. This behavior differs significantly from that of vacuum/perfect fluid cosmologies; hence “matter matters”. Finally, we indicate that our methods can probably be extended to treat a number of open problems—in particular, the dynamics of Bianchi type VIII and Kantowski-Sachs solutions.     

Supersymmetry breaking in warped geometry

We examine the soft supersymmetry breaking parameters in supersymmetric theories on a slice of AdS₅ which generate the hierarchical Yukawa couplings by dynamically quasi-localizing the bulk matter fields in an extra dimension. Such models can be regarded as the AdS dual of the recently studied 4-dimensional models which contain a supersymmetric CFT to generate the hierarchical Yukawa couplings. In such models, if supersymmetry breaking is mediated by the bulk radion superfield and/or some brane chiral superfields, potentially dangerous flavor-violating soft parameters are suppressed with an appropriate correlation with the Yukawa coupling suppression, thereby avoiding the SUSY flavor problem in a natural manner. We present some models of radion-dominated supersymmetry breaking which yield a highly predictive form of soft parameters in this framework, and discuss the constraints from flavor-changing rare processes. Most of the discussions in this paper can be applied also to models with a flat extra dimension in which the Yukawa hierarchy is generated by quasi-localizing the bulk matter fields in the extra dimension.Received: 21 October 2003, Revised: 12 January 2004, Published online: 5 May 2004     

Two-Fluid Cosmological Models in Bianchi Type-V Space-Time

We have presented anisotropic, homogeneous two-fluid cosmological models using Bianchi type-V space-time. Here one fluid represents the matter content of the universe and another fluid is chosen to model the CMB radiation. The radiation and matter content of the universe are in interactive phase. Also we have discussed the behavior of fluid parameters and kinematical parameters.     

Higher Dimensional Plane Symmetric Cosmological Models with Two-Fluid Source in General Relativity

We consider anisotropic, homogeneous two-fluid plane symmetric cosmological models in higher dimensions. Here one fluid represents the matter content of the universe and another fluid is chosen to model the CMB (cosmic microwave background) radiation. The radiation and matter content of the universe are in interactive phase. Also we have discussed the behaviour of fluid parameters and kinematical parameters.     

Kasner Cosmological Models with Two-Fluid Source in General Relativity

In this paper we present Bianchi type-I metric of the Kasner form describing two-fluid source of the universe in general relativity. In Kasner cosmological models one fluid is a radiation field modeling the cosmic microwave background, while the other is a matter field, modeling material content of the universe. The radiation and matter content of the universe are in interactive phase. We have also presented anisotropic, homogeneous nature of Kasner cosmological models with two-fluid. The behavior of fluid parameters and kinematical parameters of the models are also discussed.     

Evolution of Thin-wall Configurations ofTexture Matter

We consider the matter of global textures withinthe frameworks of a perfect fluid model in generalrelativity. We examine thermodynamical properties oftexture matter in comparison with radiation fluid and bubble matter. Then we study dynamics ofthin-wall selfgravitating texture objects, and show thatclassical motion can be elliptical (finite), parabolicalor hyperbolical. It is shown that total gravitational mass of neutral textures in equilibrium equalszero, as was expected. Finally, we carry out theWheeler-DeWitt minisuperspace quantization of thetheory, obtain exact wave functions and discrete spectra of bound states with provision for spatialtopology.     

The Scalar Fields with Negative Kinetic Energy, Dark Matter and Dark Energy

The inhomogeneous cosmological model with generalized nonstatic Majumdar-Papapetrou metric is considered. The scalar field with negative kinetic energy and some usual matter sources of the gravitational field such as two-component nonlinear sigma model and perfect fluid are presented. Some exact solutions in these models are obtained and analyzed. In particular it is shown that the latent mass effect and effect of accelerating expansion (quintessence) of the Universe exist in these models. The 5-dimensional generalization of the model is presented, too.     

Electromagnetic Bianchi cosmologies

All exact solutions of the Einstein-Maxwell equations of Bianchi type-I which are of physical importance have been found. The solutions represent non-locally rotationally symmetric universes with source-free electromagnetic fields and the matter content is a perfect fluid, with equation of state p=(γ?1)?(1?γ?2). Non-titled Bianchi type-II models are integrated for perfect fluid matter for all values of γ.     

Bianchi Type V models with a matter distribution admitting anisotropic pressure and heat flow

This paper discusses the evolution of the nontilted, diagonal, nonlocally rotationally symmetric Bianchi Type V models with a matter distribution that allows anisotropic pressure and heat flow. It is found that if the entropy of such models is assumed to be increasing, the anisotropy density necessarily decreases faster than the case with perfect fluid as the source. The factor that accelerates this decrease, in viscosity approximation, is shown to be fully characterizing the effect of viscosity. The initial value equation and the energy conservation equation for such models are derived and discussed in order to find the solution of the Einstein field equations. A new approach is devised to tackle these two equations and used in obtaining an exact solution.     

Tilt and phantom cosmology

We show that in tilting perfect fluid cosmological models with an ultra-radiative equation of state, generically the tilt becomes extreme at late times and, as the tilt instability sets in, observers moving with the tilting fluid will experience singular behaviour in which infinite expansion is reached within a finite proper time, similar to that of phantom cosmology (but without the need for exotic forms of matter).     

Anisotropic dark energy models with constant deceleration parameter

We consider a spatially homogeneous and totally anisotropic Bianchi-I space-time with perfect fluid (dark matter and standard visible matter) and anisotropic dark energy, which has dynamical energy density. The two sources are assumed to interact minimally and therefore their energy momentum tensors are conserved separately. Using suitable physical assumptions, the field equations are solved exactly. Various dark energy models are studied and it is found that quintessence model is suitable for describing the present evolution of the universe. The geometrical and kinematical features of the models and the behavior of the anisotropy of the dark energy, are examined in detail.     

Slowly rotating universes with radiating perfect fluid distribution coupled with a scalar field in general relativity

Along with the presentation of some interesting new analytic solutions, the dynamics of slowly rotating radiating perfect fluid universes coupled with a scalar field are investigated, and their physical and geometrical properties are studied from various angles. The rotational perturbations of such models are examined in detail in order to substantiate the possibility that the universe is endowed with some rotation. The nature and role of the metric rotation which is related to the local dragging of inertial frames and that of the matter rotation are studied. The effects of the radiation and the scalar fields on the rotation are discussed. The periods of physical validity for some of the models and the restrictions on the radii of the models for real astrophysical situations are found. Most of the rotating models obtained here turn out to be expanding ones as well, and may be taken as good examples of real astrophysical objects in this universe.     

Discrete isotropies in a class of cosmological models

It is shown that a certain class of cosmological models admits discrete isotropies. These models are solutions of Einsteins field equations, characterised by: (1) the matter is described as a perfect fluid, and (2) there exists a group of motions simply transitive on three-surfaces orthogonal to the fluid flow vector.     

Cosmological models with fluid matter undergoing velocity diffusion

A new type of fluid matter model in general relativity is introduced, in which the fluid particles are subject to velocity diffusion without friction. In order to compensate for the energy gained by the fluid particles due to diffusion, a cosmological scalar field term is added to the left hand side of the Einstein equations. This hypothesis promotes diffusion to a new mechanism for accelerated expansion in cosmology. It is shown that diffusion alters not only quantitatively, but also qualitatively the global dynamical properties of the standard cosmological models.