Scalar field power-law cosmology with spatial curvature and dark energy-dark matter interaction

We consider a late closed universe of which scale factor is a power function of time using observational data from combined WMAP5+BAO+SN Ia dataset and WMAP5 dataset. The WMAP5 data give power-law exponent, α=1.01 agreeing with the previous study of H(z) data while combined data gives α=0.985. Considering a scalar field dark energy and dust fluid evolving in the power-law universe, we find field potential, field solution and equation of state parameters. Decaying from dark matter into dark energy is allowed in addition to the non-interaction case. Time scale characterizing domination of the kinematic expansion terms over the dust and curvature terms in the scalar field potential are found to be approximately 5.3 to 5.5 Gyr. The interaction affects in slightly lowering the height of scalar potential and slightly shifting potential curves rightwards to later time. Mass potential function of the interacting Lagrangian term is found to be exponentially decay function.     

Tachyonic teleparallel dark energy

Teleparallel gravity is an equivalent formulation of general relativity in which instead of the Ricci scalar R, one uses the torsion scalar T for the Lagrangian density. Recently teleparallel dark energy has been proposed by Geng et al. (in Phys. Lett. B 704, 384, 2011). They have added quintessence scalar field, allowing also a non-minimal coupling with gravity in the Lagrangian of teleparallel gravity and found that such a non-minimally coupled quintessence theory has a richer structure than the same one in the frame work of general relativity. In the present work we are interested in tachyonic teleparallel dark energy in which scalar field is responsible for dark energy in the frame work of torsion gravity. We find that such a non-minimally coupled tachyon gravity can realize the crossing of the phantom divide line for the effective equation of state. Using the numerical calculations we display such a behavior of the model explicitly.     

Cosmological observations in non-local F(R) cosmology

In this article in a generalization of our previous work (Farajollahi and Milani in Mod. Phys. Lett. A 25:2349–2362, 2010), we investigate the dynamics of the non-local F(R) gravity after casting it into local form. The non-singular bouncing behavior and quintom model of dark energy are achieved without involving negative kinetic energy fields. Two cosmological tests are performed to constrain the model parameters. In case of phantom crossing the distance modulus predicted by the model best-fits the observational data. In comparison with the CPL parametrization for drift velocity, the model in some redshift intervals is in good agreement with the data.     

Phantom phase power-law solution in f(G) gravity

Power-law solutions for f(G) gravity coupled with perfect fluid have been studied for spatially flat universe. It is shown that despite the matter dominated and accelerating power-law solutions, the power-law solution exists for an special form of f(G) when this universe enters a Phantom phase.     

Energy density inhomogeneities with polynomial f(R) cosmology

In this paper, we study the effects of polynomial f(R) model on the stability of homogeneous energy density in self-gravitating spherical stellar object. For this purpose, we construct couple of evolution equations which relate the Weyl tensor with matter parameters. We explore different factors responsible for density inhomogeneities with non-dissipative dust, isotropic as well as anisotropic fluids and dissipative dust cloud. We find that shear, pressure, dissipative parameters and f(R) terms affect the existence of inhomogeneous energy density.     

Pilgrim dark energy in f(T,T G ) cosmology

We work on the reconstruction scenario of pilgrim dark energy (PDE) in f(T,T _G ). In PDE model it is assumed that a repulsive force that is accelerating the Universe is phantom type with (w _DE f(T,T _G ) models and correspondingly evaluate equation of state parameter for various choices of scale factor. Also, we assume polynomial form of f(T,T _G ) in terms of cosmic time and reconstruct H and w _DE in this manner. Through discussion, it is concluded that PDE shows aggressive phantom-like behavior for s=?2 in f(T,T _G ) gravity.     

The Einstein polarization interferometer for cosmology (EPIC) and the millimeter-wave bolometric interferometer (MBI)

We provide an overview of a mission concept study underway for the Einstein Inflation Probe (EIP). Our study investigates the advantages and tradeoffs of using an interferometer (EPIC) for the mission. We also report on the status of the millimeter-wave bolometric interferometer (MBI), a ground-based pathfinder optimized for degree-scale CMB polarization measurements at 90 GHz.     

Functional form of f(R) with power-law expansion in anisotropic model

In this paper, we study an anisotropic Bianchi-I space-time model in f(R) theory of gravity in the presence of perfect fluid as a matter contains. The aim of this paper is to find the functional form of f(R) from the field equations and hence the solution of various cosmological parameters. We assume that the deceleration parameter to be a constant, and the shear scalar proportional to the expansion scalar to obtain the power-law form of the scale factors. We find that the model describes the decelerated phases of the universe under the choice of certain constraints on the parameters. The model does not show the acceleration expansion and also transition from past deceleration to present accelerating epoch. We discuss the stability of the functional form of f(R) and find that it is completely stable for describing the decelerating phase of the universe.     

Dimensionless cosmology

Although it is well known that any consideration of the variations of fundamental constants should be restricted to their dimensionless combinations, the literature on variations of the gravitational constant G is entirely dimensionfull. To illustrate applications of this to cosmology, we explicitly give a dimensionless version of the parameters of the standard cosmological model, and describe the physics of both Big Bang Nucleosynthesis and recombination in a dimensionless manner. Rigorously determining how to talk about the model in a way which avoids physical dimensions is a requirement for proceeding with a calculation to constrain time-varying fundamental constants. The issue that appears to have been missed in many studies is that in cosmology the strength of gravity is bound up in the cosmological equations, and the epoch at which we live is a crucial part of the model. We argue that it is useful to consider the hypothetical situation of communicating with another civilization (with entirely different units), comparing only dimensionless constants, in order to decide if we live in a Universe governed by precisely the same physical laws. In this thought experiment, we would also have to compare epochs, which can be defined by giving the value of any one of the evolving cosmological parameters. By setting things up carefully in this way one can avoid inconsistent results when considering variable constants, caused by effectively fixing more than one parameter today. We show examples of this effect by considering microwave background anisotropies, being careful to maintain dimensionlessness throughout. We present Fisher matrix calculations to estimate how well the fine structure constants for electromagnetism and gravity can be determined with future microwave background experiments. We highlight how one can be misled by simply adding G to the usual cosmological parameter set.     

High-energy cosmology

Our knowledge of the high-energy universe is undergoing a period of rapid change as new astronomical detectors of high-energy radiation start to operate at their design sensitivities. Now is a boomtime for high-energy astrophysics, with new discoveries from Swift and HESS, results from MAGIC and VERITAS starting to be reported, the upcoming launches of the γ-ray space telescopes GLAST and AGILE, and anticipated data releases from IceCube and Auger. A formalism for calculating statistical properties of cosmological γ-ray sources is presented. Application is made to model calculations of the statistical distributions of γ-ray and neutrino emission from (i) beamed sources, specifically, long-duration GRBs, blazars, and extragalactic microquasars, and (ii) unbeamed sources, including normal galaxies, starburst galaxies and clusters. Expressions for the integrated intensities of faint beamed and unbeamed high-energy radiation sources are also derived. A toy model for the background intensity of radiation from dark-matter annihilation taking place in the early universe is constructed. Estimates for the γ-ray fluxes of local group galaxies, starburst, and infrared luminous galaxies are briefly reviewed. Because the brightest extragalactic γ-ray sources are flaring sources, and these are the best targets for sources of PeV–EeV neutrinos and ultra-high energy cosmic rays, rapidly slewing all-sky telescopes like MAGIC and an all-sky γ-ray observatory beyond Milagro will be crucial for optimal science return in the multi-messenger age.     

Correspondence between f(G) gravity and holographic dark energy via power-law solution

In this paper, we discuss cosmological application of holographic Dark Energy (HDE) in the framework of f(G) modified gravity. For this purpose, we construct f(G) model with the inclusion of HDE and a well-known power law form of the scale factor a(t). The reconstructed f(G) is found to satisfy a sufficient condition for a realistic modified gravity model. We find quintessence behavior of effective equation of state (EoS) parameter ω _DE through energy conditions in this context. Moreover, we observe that the squared speed of sound $v_{s}^{2}$ remains negative, which indicates the instability of HDE f(G) model.     

Quantum cosmology

The newest developments in the investigation of redshift quantization and variability are summarized and several approaches to cosmological models consistent with aspects of the data are discussed. The periodicities present when redshifts are referred to the Cosmic Background Radiation rest frame appear to be very precisely described by a series of period-doubling sequences based upon the ninth-root of 2 times the speed of light. Several examples of redshift quantization and variability are discussed to illustrate the periods, the CBR association and the dependence upon galaxy properties. Possible cosmological models involving properties of time, including a form of three-dimensional time and possible time networks, are introduced. Such models appear to have the potential to admit quantized and variable redshifts while remaining consistent with local continuous physical theory.     

Twin universes cosmology

Starting from the field equationS =(T -A(T)), presented in a former paper, we present a test result, based on numerical simulations, giving a new model applied to the very large structure of the Universe. A theory of inverse gravitational lensing is developed, in which the observed effects could be due mainly to the action of surrounding antipodal matter. This is an alternative to the explanation based on dark matter existence. We then develop a cosmological model. Because of the hypothesis of homogeneity, the metric must be a solution of the equationS = 0, although the total mass of the Universe is non-zero. In order to avoid the trivial solutionR = constant ×t, we consider a model with variable constants. Then we derive the laws linking the different constants of physics:G, c, h, m; in order to keep the basic equations of physics invariant, so that the variation of these constants is not measurable in the laboratory, the only effect of this process being the red shift, due to the secular variation of these constants. All the energies are conserved, but not the masses. We find that all of the characteristic lengths (Schwarzschild, Jeans, Compton, Planck) vary like the characteristic lengthR, from where all the characteristic times vary like the cosmic timet. As the energy of the photonh is conserved over its flight, the decrease of its frequency is due to the growth of the Planck constanth t. In such conditions the field equations have a single solution, corresponding to a negative curvature and to an evolution law:R t ^2/3.The model is no longer isentropic ands logt. The cosmologic horizon varies likeR, so that the homogeneity of the Universe is ensured at any time which constitues an alternative to the theory of inflation. We re-find, for moderate distances, Hubble's law. A new law: distance =f(z) is derived, very close to the classical one for moderate red shifts.     

Relativistic hierarchical cosmology

While Euclidean models with uniform matter density have a number of radio sources of flux density greater thanF at frequencyv that varies asN(>F, v)¹ F ^–3/2, hierarchical models with = ₀ r ^–2 haveN(>F,)F ^–1/2 (Section 1). Since the observed dependency isN(>F,)F ^–1.8, severe density and/or luminosity evolution must be present in a workable hierarchical cosmology (Section 2). The same argument applies (Section 3) to the number of sources of apparent luminosity greater thanl,N(>l) and (Section 4) to the number of sources within redshift distancez from the local origin. To give agreement with empirical data demandsq _o=+1 and large first and second derivatives with respect to time of the number source density (Section 5). The adoption ofq _o=+1 allows one to show (Section 6) that a Lemaitre-type hierarchical Universe with a long coasting or waiting time can give agreement with observations of the numbers of QSO's etc. if the age of the Universe is more than 10¹³ yr. The dependence of the effective Hubble parameter onk(t), (t) andR (Section 7) leads one to suggest that ak=0, =0 hierarchy with 0 might be the simplest acceptable form of model Universe. Section 8 (Conclusion) points out that further data on source count anisotropies should allow the component levels of the hierarchy to be delineated.