Matter Epochs Research Articles

An accelerating cosmology without dark energy

The negative pressure accompanying gravitationally-induced particlecreation can lead to a cold dark matter (CDM) dominated, accelerating Universe (Lima et al. 1996 [1]) without requiring the presence of dark energy or a cosmological constant. In a recent study, Lima et al. 2008 [2] (LSS) demonstrated that particle creation driven cosmological models are capable of accounting for the SNIa observations [3] of the recent transition from a decelerating to an accelerating Universe, without the need for Dark Energy. Here we consider a class of such models where the particle creation rate is assumed to be of the form Γ = βH+γH0, where H is the Hubble parameter and H0 is its present value. The evolution of such models is tested at low redshift by the latest SNe Ia data provided by the Union compilation [4] and at high redshift using the value of zeq, the redshift of the epoch of matter — radiation equality, inferred from the WMAP constraints on the early Integrated Sachs-Wolfe (ISW) effect [5]. Since the contributions of baryons and radiation were ignored in the work of LSS, we include them in our study of this class of models. The parameters of these more realistic models with continuous creation of CDM are constrained at widely-separated epochs (zeq ≈ 3000 and z ≈ 0) in the evolution of the Universe. The comparison of the parameter values, {β, γ}, determined at these different epochs reveals a tension between the values favored by the high redshift CMB constraint on zeq from the ISW and those which follow from the low redshift SNIa data, posing a potential challenge to this class of models. While for β = 0 this conflict is only at ≲ 2σ, it worsens as β increases from zero.

Phantom crossing, equation-of-state singularities, and local gravity constraints in [formula omitted] models

We identify the class of f(R) dark energy models which have a viable cosmology, i.e. a matter dominated epoch followed by a late-time acceleration. The deviation from a ΛCDM model (f=R−Λ) is quantified by the function m=Rf,RR/f,R. The matter epoch corresponds to m(r=−1)≃+0 (where r=−Rf,R/f) while the accelerated attractor exists in the region 0⩽m<1. We find that the equation of state wDE of all such “viable”f(R) models exhibits two features: wDE diverges at some redshift zc and crosses the cosmological constant boundary (“phantom crossing”) at a redshift zb smaller than zc. Using the observational data of Supernova Ia and Cosmic Microwave Background, we obtain the constraint m<O(0.1) and we find that the phantom crossing could occur at zb≳1, i.e., within reach of observations. If we add local gravity constraints, the bound on m becomes very stringent, with m several orders of magnitude smaller than unity in the region whose density is much larger than the present cosmological density. The representative models that satisfy both cosmological and local gravity constraints take the asymptotic form m(r)=C(−r−1)p with p>1 as r approaches −1.

Conditions for the cosmological viability off(R)dark energy models

We derive the conditions under which dark energy models whose Lagrangian densities $f$ are written in terms of the Ricci scalar $R$ are cosmologically viable. We show that the cosmological behavior of $f(R)$ models can be understood by a geometrical approach consisting of studying the $m(r)$ curve on the $(r,m)$ plane, where $m\ensuremath{\equiv}R{f}_{,RR}/{f}_{,R}$ and $r\ensuremath{\equiv}\ensuremath{-}R{f}_{,R}/f$ with ${f}_{,R}\ensuremath{\equiv}\mathrm{d}f/\mathrm{d}R$. This allows us to classify the $f(R)$ models into four general classes, depending on the existence of a standard matter epoch and on the final accelerated stage. The existence of a viable matter-dominated epoch prior to a late-time acceleration requires that the variable $m$ satisfies the conditions $m(r)\ensuremath{\approx}+0$ and $\mathrm{d}m/\mathrm{d}r&gt;\ensuremath{-}1$ at $r\ensuremath{\approx}\ensuremath{-}1$. For the existence of a viable late-time acceleration we require instead either (i) $m=\ensuremath{-}r\ensuremath{-}1$, $(\sqrt{3}\ensuremath{-}1)/2&lt;m\ensuremath{\le}1$ and $\mathrm{d}m/\mathrm{d}r&lt;\ensuremath{-}1$ or (ii) $0&lt;m\ensuremath{\le}1$ at $r=\ensuremath{-}2$. These conditions identify two regions in the $(r,m)$ space, one for the matter era and the other for the acceleration. Only models with an $m(r)$ curve that connects these regions and satisfies the requirements above lead to an acceptable cosmology. The models of type $f(R)=\ensuremath{\alpha}{R}^{\ensuremath{-}n}$ and $f=R+\ensuremath{\alpha}{R}^{\ensuremath{-}n}$ do not satisfy these conditions for any $n&gt;0$ and $n&lt;\ensuremath{-}1$ and are thus cosmologically unacceptable. Similar conclusions can be reached for many other examples discussed in the text. In most cases the standard matter era is replaced by a cosmic expansion with scale factor $a\ensuremath{\propto}{t}^{1/2}$. We also find that $f(R)$ models can have a strongly phantom attractor but in this case there is no acceptable matter era.

Brans-Dicke Cosmology–A Generalization in Anisotropic Space-Time Models

The generalized Brans-Dicke (BD) Cosmology has been studied for Bianchi-I, Bianchi-III and Kantowski-Sachs anisotropic space-time models. Solutions have been obtained for radiation-dominated era, stiff matter epoch and other equations of state.

Anisotropic open cosmological models of spin matter with magnetic moment

We have derived a set of field equations for a Weyssenhoff spin fluid including magnetic interaction among the spinning particles prevailing in spatially homogeneous, but anisotropically cosmological models of Bianchi type V based on Einstein-Cartan theory. We analyze the field equations in three different equations of states specified by p=(1/3)ρ, p-ρ and p=0. The analytical solutions found are non-singular provided that the combined energy arising from matter spin and magnetic interaction among particles overcomes the anisotropy energy in the Universe. We have also deduced that the minimum particle numbers for the radiation (p=(1/3)ρ) and matter (p=0) epochs are 1088 and 10108 respectively, the minimum particle number for the state p=ρ is 1096, leading to the conclusion that we must consider the existence of neutrinos and other creation of particles and anti-particles under torsion and strong gravitational field in the early Universe.

Statistical physics of cosmological networks of string loops

We solve numerically the Boltzmann equation describing the evolution of a cosmic string network which contains only loops. In Minkowski space-time the equilibrium solution predicted by statistical mechanics is recovered, and we prove that this solution is stable to nonlinear perturbations provided that their energy does not exceed the critical energy for the Hagedorn transition. In expanding Einstein--de Sitter universes we probe the distribution of loops with a length much smaller than the horizon. Under the assumption that the length scales characteristic of the loop network scale, we discover stable scaling solutions for the energy density in loops, both in the radiation and matter dominated epochs. The shape of these solutions is very different in the two eras, with a much higher energy density in the radiation epoch, and a larger average loop length in the matter epoch. These results suggest that if the conditions for the formation of loop networks are indeed satisfied, these could, in principle, be good candidates for structure formation.

Cosmological consequences of string-forming open inflation models

We present a study of open inflation cosmological scenarios in which cosmic strings form between the two inflationary epochs. It is shown that in these models, strings are stretched outside the horizon due to the inflationary expansion, but must necessarily re-enter the horizon before the epoch of equal matter and radiation densities. We determine the linear power spectrum of cold dark matter perturbations in these hybrid models, finding good agreement with observations for values of {Gamma}={Omega}{sub 0}h{approximately}0.3 and comparable contributions from the active and passive sources to the cosmic microwave background. Finally, we briefly discuss other cosmological consequences of these models. {copyright} {ital 1999} {ital The American Physical Society}

Anisotropic viscous universe with variableG and ?

An anisotropic model with variableG and Λ and bulk viscosity is considered. The model exhibits an inflationary behavior during which the coefficient of bulk viscosity varies lineraly with the energy density. This allows the anisotropy energy to decrease exponentially with time. Other results overlap with our earlier work with a different ansatz for Λ. The gravitational ‘constant’ was found to increase during the radiation and matter epochs.

Large-scale structure from cosmic string loops. I - Formation and linear evolution of perturbations

view Abstract Citations (18) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Large-Scale Structure from Cosmic String Loops. I. Formation and Linear Evolution of Perturbations Scherrer, Robert J. Abstract A numerical model for the formation of large-scale structure from cosmic string loops is presented. The configuration of cosmic strings at the end of the epoch of equal matter and radiation is discussed, giving a numerical model for this configuration. A model is presented for the linear evolution of the perturbations induced by these loops, and the perturbation spectrum which exists before the perturbations go nonlinear is analyzed. The model produces a non-Gaussian fluctuation spectrum with nonrandom phases. The power at large scales is suppressed by the loop perturbations being isocurvature fluctuations. There are no long-range Brownian correlations between loops which fragment from the same infinite string. These factors inhibit the formation of structure on large scales. Publication: The Astrophysical Journal Pub Date: September 1987 DOI: 10.1086/165519 Bibcode: 1987ApJ...320....1S Keywords: Astronomical Models; Cosmology; Galactic Evolution; Linear Evolution Equations; Loops; Perturbation Theory; Computational Astrophysics; Domains; Early Stars; Gauge Theory; Power Spectra; Universe; Astrophysics; COSMOLOGY; EARLY UNIVERSE; GALAXIES: FORMATION full text sources ADS |

On the Generatin of Sound from the Primordial Turbulence in an Expanding Universe

In order that galaxies may be formed from the primordial turbulence in a big-bang universe, the cosmic turbulence with suitable scales must survive up to the decoupling epoch of matter from radiation against its decay due to the generation of sound as envisaged by Lighthill and Crighton. To attack the problem with due consideration of the effect of cosmic expansion neglected in the Lighthill-- Crighton theory, the generation of sound from the cosmic turbulence is dealt with by constructing the appropriate retarded Oreen's function in which all significant influences of the cosmic expansion upon the propagation of sound waves to be generated are woven. rt is shown that the expanding turbulent fluid is inevitably of a diffusive nature in such a way that its physical radius is equal to the acoustical event horizon. The latter situation means that the retardation effect is essential in the evaluation of the intensity of sound at the center of the turbulent fluid sphere, while the effect was discarded in the Crighton formula for a static diffusive medium. In other words, the formalism is superior to Crighton's for the examination of the aforementioned problem. (auth)