Cosmology in Eddington-Inspired Gravity

In this paper, we study the cosmological evolution of the universe filled with the perfect fluid in the Eddington-inspired Born-Infeld gravity, proposed recently by Banados and Ferreira. Applying a method in which the evolution of the scale factor is regarded as that of a particle moving in a potential, we show all possible cases of cosmological evolution.     

宇宙常数的Bulk流形的起源

关于宇宙常数问题是个至今没有解决的问题, 它的来源至今还没有一个共识. 从额外维的流形出发, 给出了宇宙常数的,bulk,流形起源的理论, 得到了不同情况下宇宙常数的取值和宇宙常数随时间演化的函数, 并且得到了可拟合现代天文观测的宇宙常数.     

Evolution of cosmological perturbations in a brane-universe

In our present Letter, we analyze the impact of the existence of extra dimensions on cosmology, in particular, on the evolution of cosmological perturbations. For a five-dimensional anti-de Sitter spacetime where ordinary matter is confined to a brane-universe, the equations governing the cosmological perturbations are presented in a form very close to the equations of standard cosmology. Two types of corrections appear: corrections due to the unconventional evolution of the homogeneous solution, which change the background-dependent coefficients of the equations, and corrections due to the curvature along the fifth dimension, which act as source terms in the evolution equations.     

Spatially Homogeneous Bianchi Type-I Universes with Variable G and Λ

In this paper, we have studied the cosmological models of Bianchi type-I universes in a different basic form filled with bulk viscous fluid, in the presence of time-dependent gravitational as well as cosmological constants. A set of new exact cosmological solutions have been obtained in both full and truncated causal theories. These solutions are suitable for describing the evolution of the universe in its early stages. The physical and dynamical consequences have been discussed in detail.     

Geometrized dynamics of multidimensional cosmological models

In the present work we reduce the dynamics of multidimensional cosmological models to the geodesics on a pseudo-Riemannian space. The significance of Killing vectors and tensors for the integrability problem is discussed. We also investigate geometric properties of the geodesics representing the evolution of cosmological models.     

Cosmological solutions with charged black holes

We consider the problem of constructing cosmological solutions of the Einstein–Maxwell equations that contain multiple charged black holes. By considering the field equations as a set of constraint and evolution equations, we construct exact initial data for N charged black holes on a hypersphere. This corresponds to the maximum of expansion of a cosmological solution, and provides sufficient information for a unique evolution. We then consider the specific example of a universe that contains eight charged black holes, and show that the existence of non-zero electric charge reduces the scale of the cosmological region of the space. These solutions generalize the Majumdar–Papapetrou solutions away from the extremal limit of charged black holes, and provide what we believe to be some of the first relativistic calculations of the effects of electric charge on cosmological backreaction.     

Linear coasting in cosmology and SNe Ia

A strictly linear evolution of the cosmological expansion scale factor is a characteristic feature in several classes of alternative gravity theories as also in the standard (Big-bang) model with specially chosen equations of state of matter. Such an evolution has no free parameters as far as the classical cosmological tests are concerned and should therefore be easily falsifiable. In this Letter we demonstrate how such models present very good fits to the current supernovae Ia data. We discuss the overall viability of such models.     

Evolution of Density Parameters on a Smooth Embedded Universe

In this paper we examine the evolution of cosmic density parameters in a four‐dimensional space‐time embedded in a five‐dimensional bulk space. We show that the extrinsic curvature is an independent spin‐2 field governed by the Gupta equations. Without evoking a dark energy fluid, the corresponding cosmological model is compared with the phenomenological XCDM model and shows a good concordance with recent cosmological datasets from Planck Collaboration and the latest Baryons Acoustic Oscillations/Cosmic Microwave Background (BAO/CMBR) + SNIa studying the evolution of density parameters. In addition, a discussion on the coincidence problem is also proposed.     

Decaying cosmological parameter in the early universe from NKK theory of gravity

Using a formalism recently introduced we study the decaying of the cosmological parameter during the early evolution of an universe, whose evolution is governed by a vacuum equation of state. We use a stochastic approach in a nonperturbative treatment of the inflaton field from a Noncompact Kaluza–Klein (NKK) theory, to study the evolution of energy density fluctuations in the early universe.     

Viscous Cosmological Model with Variable Gravitational and Cosmological Constants

We have considered some cosmological solutions with variable gravitational and cosmological constants with bulk viscosity. It is found that the solutions are singularity free and the deceleration parameter is in general not a constant unless we assume perfect fluid with equation of state in the standard cosmologies. Moreover, the deceleration parameter is a function of the scale factor and changes sign with evolution, so our solution is a generalization of those obtained by Arbab I. Arbab. The introduction of viscosity not only free from singularity but also give the deceleration parameter a freedom to vary with scale factor. Thus, a viscous cosmological fluid gives a more general situation in the early universe.     

No evidence for the evolution of mass density power-law index γ from strong gravitational lensing observation

In this paper,we consider the singular isothermal sphere lensing model that has a spherically symmetric power-law mass distributionρ_(tot)(r)～r~(-γ).We investigate whether the mass density power-law indexγis cosmologically evolutionary by using the strong gravitational lensing(SGL)observation,in combination with other cosmological observations.We also check whether the constraint result ofγis affected by the cosmological model,by considering several simple dynamical dark energy models.We find that the constraint onγis mainly decided by the SGL observation and independent of the cosmological model,and we find no evidence for the evolution ofγfrom the SGL observation.     

Exact Solution in Chaotic Inflation Model with Potential Minima

Taking the cosmological expansion rate directly as a function of field φ, H=H(φ), we present a new exact solution to Einstein's equations that describe the evolution of cosmological chaotic inflation model. The inflation is driven by the evolution of scalar field with inflation potential V(φ)=(λ/8)(φ²-υ²)². The spectral indices of the scalar density n_s and gravitational wave fluctuations n_g are computed. The value of n_s lies well inside the limits set by the cosmic background explorer satellite.     

Absence of preclassical solutions in Bianchi I loop quantum cosmology

Loop quantum cosmology, the symmetry reduction of quantum geometry for the study of various cosmological situations, leads to a difference equation for its quantum evolution equation. To ensure that solutions of this equation act in the expected classical manner far from singularities, additional restrictions are imposed on the solution. In this Letter, we consider the Bianchi I model, both the vacuum case and the addition of a cosmological constant, and show using generating function techniques that only the zero solution satisfies these constraints. This implies either that there are technical difficulties with the current method of quantizing the evolution equation, or else loop quantum gravity imposes strong restrictions on the physically allowed solutions.     

Massive neutrinos and cosmology

The present experimental results on neutrino flavour oscillations provide evidence for non-zero neutrino masses, but give no hint on their absolute mass scale, which is the target of beta decay and neutrinoless double-beta decay experiments. Crucial complementary information on neutrino masses can be obtained from the analysis of data on cosmological observables, such as the anisotropies of the cosmic microwave background or the distribution of large-scale structure. In this review we describe in detail how free-streaming massive neutrinos affect the evolution of cosmological perturbations. We summarize the current bounds on the sum of neutrino masses that can be derived from various combinations of cosmological data, including the most recent analysis by the WMAP team. We also discuss how future cosmological experiments are expected to be sensitive to neutrino masses well into the sub-eV range.     

The Growth of Cosmological Perturbations in the Transition Eras

In this article we determine the dominating modes of the cosmological perturbations in different transition eras of the universe evolution (namely, radiation-dust, dust-vacuum, dust-K-matter, and K-matter-vacuum) in a covariant and gauge-invariant manner.     

Cosmological Model with Ω M -Dependent Cosmological Constant

We investigate the evolution of the scale factor in a cosmological model in which the cosmological constant is given by the scalar arisen by the contraction of the stress-energy tensor.     

Thermodynamics of the Bulk Viscous Cosmological Fluid in Presence of the Particle Creation Pressure

In the present work, we consider FRW metric and investigate some cosmological quantities in presence of bulk viscosity and particle creation pressure. The obtained results for a viscous cosmological fluid with particle creation show that the Hubble expansion parameter, energy density, bulk viscosity pressure, creation pressure and temperature depend on the particle creation rate and increase with increasing particle creation coefficient. It is found that the bulk viscosity and particle creation pressure seem to play important roles in the evolution of the early Universe.     

Q-balls: some analytical results

Motivated by the renewed interest in the role of Q-balls in cosmological evolution, we present a discussion of the main properties of Q-balls, including some new results. Received: 30 March 2001 / Published online: 29 June 2001