Wormhole with varying cosmological constant

It has been suggested that the cosmological constant is a variable dynamical quantity. A class of solution has been presented for the spherically symmetric space time describing wormholes by assuming the erstwhile cosmological constant Λ to be a space variable scalar, viz., Λ = Λ (r) . It is shown that the averaged null energy condition (ANEC) violating exotic matter can be made arbitrarily small.     

Exact solution of phantom dark energy model

We investigate the phantom dark energy model derived from the scalar field with a negative kinetic term.By assuming a particular relation between the time derivative of the phantom field and the Hubble function,an exact solution of the model is constructed.Absence of the ’big rip’ singularity is shown explicitly.We then derive special features of phantom dark energy model and show that its predictions are consistent with all astrophysical observations.     

The Hubble parameter in the early universe with viscous QCD matter and finite cosmological constant

The evolution of a flat, isotropic and homogeneous universe is studied. The background geometry in the early phases of the universe is conjectured to be filled with causal bulk viscous fluid and dark energy. The energy density relations obtained from the assumption of covariant conservation of energy‐momentum tensor of the background matter in the early universe are used to derive the basic equation for the Hubble parameter H. The viscous properties described by ultra‐relativistic equations of state and bulk viscosity taken from recent heavy‐ion collisions and lattice QCD calculations have been utilized to give an approximate solution of the field equations. The cosmological constant is conjectured to be related to the energy density of the vacuum. In this treatment, there is a clear evidence for singularity at vanishing cosmic time t indicating the dominant contribution from the dark energy. The time evolution of H seems to last for much longer time than the ideal case, where both cosmological constant and viscosity coefficient are entirely vanishing.     

A neutrino universe with viscosity

A solution of the Einstein field equation corresponding to a distribution of fluid with equation of stateρ = 3p but with a nonvanishing shear viscosity is presented. The solution is spherical symmetric and the flow lines are geodetic.     

Bianchi-I Cosmologies with Electromagnetic and Spinor Fields. Isotropization Problem

We consider Bianchi type I cosmologies with unidirectional magnetic and electric fields, assuming as well the existence of a global spinor field ψ(t) as one more possible source of gravity able to suppress the inevitable anisotropy accompanying a nonzero vector field. The field ψ(t) is assumed to contain a nonlinearity in the form s ⁿ , where and n=const (the special case n=1 corresponds to a Dirac massive field). The structure of the stress-energy tensor of the spinor field is shown to be the same as that of a perfect fluid with the equation of state p=w ρ where w=n−1. The Dirac massive spinor field and nonlinear fields with n<4/3 are shown to be able to provide isotropization. A numerical estimate shows that this isotropization could occur early enough to be compatible with observations.     

Letter: Anisotropic Born-Infeld Cosmologies

Anisotropic cosmological spacetimes are constructed from spherically symmetric solutions to Einstein's equations coupled to nonlinear electrodynamics and a positive cosmological constant. This is accomplished by finding solutions in which the roles of r and t are interchanged for all r > 0 (i.e. r becomes timelike and t becomes spacelike). Constant time hypersurfaces have topology R× S ² and in all the spacetimes considered the radius of the two sphere vanishes as t goes to zero. The scale factor of the other dimension diverges as t goes to zero in some solutions and vanishes (or goes to a constant) in other solutions. At late times local observers would see the universe to be homogeneous and isotropic.     

Dimensionality and the cosmological constant

In the Kaluza-Klein model with a cosmological constant Λ and a flux, the external spacetime of the created universe from aS ^s × S ^n−s seed instanton can be identified in quantum cosmology. One can also show that in the internal space theeffective cosmological constant is most probably zero.     

Anisotropic cosmological models and generalized scalar tensor theory

In this paper generalized scalar tensor theory has been considered in the background of anisotropic cosmological models, namely, axially symmetric Bianchi-I, Bianchi-III and Kortowski-Sachs space-time. For bulk viscous fluid, both exponential and power-law solutions have been studied and some assumptions among the physical parameters and solutions have been discussed.     

Effective cosmological constant within the expanding axion universe

We show that the value of an effective cosmological constant, _Λeff${\Lambda }_{\mathit{eff}}$, is influenced by the dimensionality of the space. Results were obtained in the framework of the axion model describing expansion of the inhomogeneous universe. _Λeff${\Lambda }_{\mathit{eff}}$ determines the tension of the space (i.e. elasticity), and is relaxed when extra dimensions are accessible. We demonstrate that the effective value of the cosmological constant may be tuned to be consistent with experimental observation. Inhomogeneities considered are representative of temperature fluctuations observed within the cosmic microwave background radiation.     

The expansion of the universe and the cosmological constant problem

The discovery that the expansion of the universe is accelerating in time is a major discovery which still awaits adequate explanation. It is generally agreed that this implies a cosmic repulsion as a result of the existence of a cosmological constant ∧>0. However, estimates of ∧, based on calculations of the zero-point fluctuations of quantum fields are too large by over a hundred orders of magnitude. This result is obtained by summing the zero-point energies up to a large cutoff energy Ω, based on the Planck scale. Since there is no compelling reason for this choice, we argue that since all known quantum electrodynamic (QED) effects involves interaction with matter, a preferred choice should be based on causality and other considerations, leading to a much lower value for ∧.     

第一讲微波背景辐射的各向异性、偏振及宇宙电离的历史

文章对微波背景辐射的各向异性、偏振及宇宙电离的历史给出了评述性介绍．从大爆炸理论的预言，到观测的发现，到其各向异性及偏振的探测，微波背景辐射(CMB)向人们揭示了丰富的宇宙学信息．文章在对基本理论作了简单介绍后，着重讲述了最新的CMB的观测结果及其物理意义．特别对微波背景各向异性探测器(Wilkinson Microwave Anisotropy Probe，WMAP)的偏振观测及其对宇宙重新电离的限制给出了较详细的叙述．     

第一讲 微波背景辐射的各向异性、偏振及宇宙电离的历史

文章对微波背景辐射的各向异性、偏振及宇宙电离的历史给出了评述性介绍.从大爆炸理论的预言,到观测的发现,到其各向异性及偏振的探测,微波背景辐射（CMB）向人们揭示了丰富的宇宙学信息.文章在对基本理论作了简单介绍后,着重讲述了最新的CMB的观测结果及其物理意义.特别对微波背景各向异性探测器（Wilkinson Microwave Anisotropy Probe, WMAP）的偏振观测及其对宇宙重新电离的限制给出了较详细的叙述.     

Non-existence of a massive scalar field for the Marder universe in Lyra and Riemannian geometries

In this paper, we have studied a massive scalar field for a Marder type universe in the context of Lyra and Riemannian geometries. From the exact solutions obtained we show that the massive scalar field does not survive in Lyra and Riemannian geometries for an anisotropic Marder type universe. Therefore we have solved the massless scalar field problem in Lyra and Riemann geometries for two cases. Also we have obtained vacuum solutions for homogeneous and anisotropic Marder space-time in Lyra geometry and the solutions obtained are compared by considering Lyra and Riemann geometries. Finally, some physical and kinematical properties are discussed by using graphics.     

The Planck era with a negative cosmological constant and cosmic strings

In the present Letter, we consider the DeBroglie-Bohm interpretation of quantum Friedmann-Robertson-Walker (FRW) models in the presence of a negative cosmological constant and cosmic strings. We compute Bohm's trajectories and quantum potentials for a quantity related to the scale factor.     

Evolving Horava cosmological horizons

Several sets of radially propagating null congruence generators are exploited in order to form 3-dimensional marginally trapped surfaces, referred to as black hole and cosmological apparent horizons in a Ho?ava universe. Based on this method, we deal with the characteristics of the 2-dimensional space-like spheres of symmetry and the peculiarities of having trapping horizons. Moreover, we apply this method in standard expanding and contracting FLRW cosmological models of a Ho?ava universe to investigate the conditions under which the extra parameters of the theory may lead to trapped/anti-trapped surfaces both in the future and in the past. We also include the cases of negative time, referred to as the finite past, and discuss the formation of anti-trapped surfaces inside the cosmological apparent horizons.     

Gravitational coupling,dynamical changes of units and the cosmological constant problem

A spacetime interval connecting two neighbouring points can be measured in different unit systems.For instance,it can be measured in atomic unit defined in terms of fundamental constants existing in quantum theories.It is also possible to use a gravitational unit which is defined by the use of properties of macroscopic objects.These two unit systems are usually regarded as indistinguishable up to a constant conversion factor.Here we consider the possibility that these two units are related by an epoch-dependent conversion factor.This is a dynamical changes of units.Regarding a conformal transformation as a local unit transformation,we use a gravitational model in which the gravitational and the matter sectors are given in different conformal frames(or unit systems).It is relevant to the cosmological constant problem,namely the huge discrepancy between the estimated and the observational values of the cosmological constant in particle physics and cosmology,respectively.We argue that the problem arises when one ignores evolution of the conversion factor relating the two units during expansion of the Universe.Connection of the model with violation of equivalence principle and possible variation of fundamental constants are also discussed.     

Gauging universe expansion via scalar fields

In this study, we investigate the expansion of the FRLW universe in the open, closed, and flat geometries. The universe is dominated by a scalar field (spatially homogeneous) as a source of dark energy. We consider the three different classes of scalar fields – quintessence, tachyonic, and phantom field – for our analysis. A mathematical analysis is carried out by considering these three scalar fields with exponential and power-law potentials. Both potentials give exponential expansion in the open, closed, and flat FRLW universes. It is found that quintessence, tachyonic, and phantom scalar fields are indistinguishable under the slow roll approximation.     

A cosmological model with negative energy photons

According to recent investigations of states of quantum fields, we postulate that there exist negative energy photons in the universe. With this assumption, we find a solution of Einstein's equation without introducing the cosmological constant. A new and sizable type Ia supernovae sample is employed to perform a fit with our model and the conventional model. Both models can well account for the current type Ia supernovae observation and they are not distinguishable. With the new model, the cause of the accelerated expansion of the universe and the mechanism of the negative pressure existing in outer space can be explained in ordinary physical terms.