Closed inflationary universe in patch cosmology

In this paper, we study closed inflationary universe models using the Gauss–Bonnet Brane. We determine and characterize the existence of a universe with Ω>1, with an appropriate period of inflation. We have found that this model is less restrictive in comparison with the standard approach where a scalar field is considered. We use recent astronomical observations to constrain the parameters appearing in the model.     

Anisotropic Bianchi Type-II Massive String Cosmological Models with Time-Decaying Λ Term and Thermodynamic Parameters

The present paper envisages a spatially homogeneous and anisotropic Bianchi II massive string cosmological models with time-decaying Λ term in general relativity. By using the variation law of Hubble’s parameter, the Einstein’s field equations have been solved for two general cases. The first case involving a power law solution describes the dynamics of universe from big bang to present epoch while the second case admit an exponential solution seems reasonable to project dynamics of future universe. We observed that massive strings dominate in early universe and eventually disappear at late time, which is consistent with the current astronomical observations. It has been found that the cosmological constant (Λ) is a decreasing function of time and it approaches to small positive value at sufficiently large time. The thermodynamic properties of anisotropic Bianchi II universe are studied and also the absolute temperature and entropy distribution are given explicitly. The relations between thermodynamic parameters and cosmological constant Λ has been established. Physical behavior of the derived model is elaborated in detail.     

A physical model for dimensional reduction and its effects on the observable parameters of the universe

We construct a physical model to study the effects of dimensional reduction that might have taken place during the inflationary phase of the universe. The model we propose is a (1 + D)-dimensional (D > 3), nonsingular, spatially homogeneous and isotropic Friedmann model. We consider dimensional reduction to take place in a stepwise manner and interpret each step as a phase transition. Independent of the details of the process of dimensional reduction, we impose suitable boundary conditions across the transitions and trace the effects of dimensional reduction to the currently observable parameters of the universe. In order to exhibit the cosmological features of the proposed model, we construct a (1 + 4)-dimensional toy model for both closed and open cases of Friedmann geometries. It is shown that in these models the universe makes transition into the lower dimension when the critical length parameter l _4,3, which signals dimensional reduction, reaches the Planck length in D = 3. The numerical models we present in this paper have the capability of making definite predictions about the cosmological parameters of the universe such as the Hubble parameter, age and density.     

Closed inflationary universe with tachyonic field

In this article we study closed inflationary universe models by using a tachyonic field theory. We determine and characterize the existence of a universe which has Ω>1 and which describes a period of inflation. We find that the considered models are less restrictive compared to the standard ones with a scalar field. We use recent astronomical observations to constrain the parameters appearing in the model. The results obtained are compared to those found in the standard scalar field inflationary universes. PACS 98.80.Bp; 98.80.Cq     

Effects of the quantization ambiguities on the Big Bounce dynamics

In this paper, we investigate dynamics of the modified loop quantum cosmology models using dynamical systems methods. Modifications considered come from the choice of the different field strength operator F̂ and result in different forms of the effective Hamiltonian. Such an ambiguity of the choice of this expression from some class of functions is allowed in the framework of loop quantization. Our main goal is to show how such modifications can influence the bouncing universe scenario in the loop quantum cosmology. In effective models considered we classify all evolutional paths for all admissible initial conditions. The dynamics is reduced to the form of a dynamical system of the Newtonian type on a two-dimensional phase plane. These models are equivalent dynamically to the FRW models with the decaying effective cosmological term parameterized by the canonical variable p (or by the scale factor a). We demonstrate that the evolutional scenario depends on the geometrical constant parameter Λ as well as the model parameter n. We find that for the positive cosmological constant there is a class of oscillating models without the initial and final singularities. The new phenomenon is the appearance of curvature singularities for the finite values of the scale factor, but we find that for the positive cosmological constant these singularities can be avoided. The values of the parameter n and the cosmological constant differentiate asymptotic states of the evolution. For the positive cosmological constant the evolution begins at the asymptotic state in the past represented by the de Sitter contracting (deS₋) spacetime or the static Einstein universe H = 0 or H =  − ∞ state and reaches the de Sitter expanding state (deS₊), the state H = 0 or H =  + ∞ state. In the case of the negative cosmological constant we obtain the past and future asymptotic states as the Einstein static universes.     

Cosmological lore—and some heresies

Cosmology is based on a number of well-founded assumptions, which include Hubble's law and the cosmological principle. Most cosmologists and astronomers also tacitly accept a number of other assumptions and beliefs which constitute a sort of traditional cosmological lore. Among these are the notions that the universe is finite, that if it is not, then there must be an observational horizon which renders it finite for all practical purposes, that it is valid to employ the special relativistic Doppler formula to interpret large cosmological redshifts, and that the expansion of the universe is slowing down toward its ultimate reversal. It is argued that none of these notions is incontrovertible and that some of them involve serious inconsistencies. An alternative approach is proposed which assumes that the universe is expanding uniformly and that it constitutes a fundamental reference frame for light propagation as implied by the Robertson-Walker metric. This approach leads to a model of the universe which is possibly infinite but without a specific observational horizon, and which satisfies the requirements of relativity. It is shown that the proposed model is theoretically consistent and that recent astronomical evidence supports its assumptions and predictions; it therefore presents a serious challenge to commonly held views about the universe.     

The Solution of Tachyon Inflation in Curved Universe

In this paper, we have considered the curved universe which is filled by tachyonic field. We have found the exact solutions for the field, pressure, density, and scale factor and some cosmological parameters. In such universe, we have investigated the role of tachyonic field in different stages of k for the evolution of the universe. Finally we draw the graphs for the scale factor, Hubble’s parameter, energy density, pressure, acceleration parameter, equation of state and potential for the different values of k. Also we obtained the exact form of field which shows that the tachyonic field has the kink form.     

Intermediate inflation in Gauss–Bonnet brane world

In this article we study an intermediate inflationary universe models using the Gauss–Bonnet brane. General conditions required for these models to be realizable are derived and discussed. We use recent astronomical observations to constraint the parameters appearing in the model.     

Phantom cosmology with non-minimally coupled real scalar field

We find that the expansion of the universe is accelerating by analyzing the recent observation data of type Ia supernova (SN-Ia). It indicates that the equation of state of the dark energy might be smaller than -1, which leads to the introduction of phantom models featured by its negative kinetic energy to account for the regime of equation of state parameter w < -1. In this paper the possibility of using a non-minimally coupled real scalar field as phantom to realize the equation of state parameter w < -1 is discussed. The main equations which govern the evolution of the universe are obtained. Then we rewrite them with the observable quantities.     

The Dynamics of Chung-Freese Braneworld

It is shown that the ansatz of Chung-Freese models a new braneworld, and while superluminal, this braneworld does not suffer from the pathologies of other FTL methods. It is also shown that our universe is a trapped surface inside a universe with large extra dimensions, and in order to preserve compatibility with the Standard Model (SM), we present two trapping mechanisms that keeps SM fields inside our brane. For the Chung-Freese braneworld we develop a similar formalism to Morris domain walls, and we present our universe as a low level state in a potential well with the bulk in the highest levels of potential. Finally, we show that the Israel condition can also act as a trapping mechanism for SM fields inside the brane. Our model uses the idea that fermions are allowed to escape to the bulk inspired by the work of Havoudiasl-Hewett-Rizzo [2].     

Cosmological Models with Variable <Emphasis Type="Italic">G</Emphasis> in <Emphasis Type="Italic">C</Emphasis>-Field Cosmology

Cosmological models with variable G in C-field cosmology for barotropic fluid distribution in FRW space-time are investigated. To get the deterministic model of the universe, we have assumed that G=R ⁿ where R is the scale factor and n the constant. To obtain the results in terms of cosmic time t, we have assumed n=−1. We find that for n=−1, Creation field (C) and spatial volume increase with time, G and ρ (matter density) decreases with time, the model represent accelerating universe. Thus inflationary scenario exists in the model. The model is also free from horizon. The results so obtained match with the astronomical observations.     

暗能量:观测证据和理论研究

自1998年发现宇宙的加速膨胀以来,大量的天文观测显示宇宙中存在压强为负的暗能量成分.暗能量已经成为目前宇宙学和理论物理的最重要的研究课题之一.文章介绍了暗能量存在的天文观测证据和理论研究现状与展望.     

暗能量：观测证据和理论研究

自1998年发现宇宙的加速膨胀以来,大量的天文观测显示宇宙中存在压强为负的暗能量成分.暗能量已经成为目前宇宙学和理论物理的最重要的研究课题之一.文章介绍了暗能量存在的天文观测证据和理论研究现状与展望.     

Study of Thermodynamic Quantities in Generalized Gravity Theories

In this work, we have studied the thermodynamic quantities like temperature of the universe, heat capacity and squared speed of sound in generalized gravity theories like Brans-Dicke, Hořava-Lifshitz and f(R) gravities. We have considered the universe filled with dark matter and dark energy. Also we have considered the equation of state parameters for open, closed and flat models. We have observed that in all cases the equation of state behaves like quintessence. The temperature and heat capacity of the universe are found to decrease with the expansion of the universe in all cases. In Brans-Dicke and f(R) gravity theories the squared speed of sound is found to exhibit increasing behavior for open, closed and flat models and in Hořava-Lifshitz gravity theory it is found to exhibit decreasing behavior for open and closed models with the evolution of the universe. However, for flat universe, the squared speed of sound remains constant in Hořava-Lifshitz gravity.     

Warm-intermediate inflationary universe model in braneworld cosmologies

Warm-intermediate inflationary universe models in the context of braneworld cosmologies are studied. This study is done in the weak and strong dissipative regimes. We find that the scalar potentials and dissipation coefficients in terms of the scalar field evolve as type-power-law and powers of logarithms, respectively. General conditions required for these models to be realizable are derived and discussed. We also study the scalar and tensor perturbations for each regime. We use recent astronomical observations to constrain the parameters appearing in the braneworld models.     

Models of neutrino masses and baryogenesis

Majorana masses of the neutrino implies lepton number violation and is intimately related to the lepton asymmetry of the universe, which gets related to the baryon asymmetry of the universe in the presence of the sphalerons during the electroweak phase transition. Assuming that the baryon asymmetry of the universe is generated before the electroweak phase transition, it is possible to discriminate different classes of models of neutrino masses. While see-saw mechanism and the triplet Higgs mechanism are preferred, the Zee-type radiative models and the R-parity breaking models requires additional inputs to generate baryon asymmetry of the universe during the electroweak phase transition.     

The energy of the Universe

References to energy of the universe have focussed upon the matter contribution, whereas the conservation laws must include a gravitational contribution as well. The conservation laws as applied to FRW cosmologies suggest a zero total energy irrespective of the spatial curvature when the value of the cosmological constant is taken to be zero. This result provides a useful constraint on models of the early universe and lends support to currently studied theories of the universe arising as a quantum fluctuation of the vacuum.     

But Still, It Moves: Tides, Stellar Parallax, and Galileo’s Commitment to the Copernican Theory

Galileo found the Copernican heliocentric theory of the universe so persuasive owing to its mathematical elegance that he embraced it even when his theory of the tides stood in opposition to it. Further support for Galileo’s deep commitment to the Copernican heliocentric theory is found in his recently discovered unpublished observations of the double star Mizar in 1617, which exhibited no annual stellar parallax and hence indicated that the Earth does not move, in contradiction to the Copernican heliocentric theory. Further, Galileo did not mention this contradiction in his Dialogue Concerning the Two Chief World Systems of 1632. I conclude that he was so deeply committed to the Copernican heliocentric theory that he was unswayed even when observations undermined it, and I suggest that if he had published his observations on the double star Mizar, general acceptance of the Copernican heliocentric theory by astronomers would have been delayed even more than it was. Christopher M. Graney teaches physics and astronomy at Jefferson Community College in Louisville, Kentucky, and runs the college’s observatory at Otter Creek Park in Louisville.     

A note on infinities in eternal inflation

In some well-known scenarios of open-universe eternal inflation, developed by Vilenkin and co-workers, a large number of universes nucleate and thermalize within the eternally inflating mega-universe. According to the proposal, each universe nucleates at a point, and therefore the boundary of the nucleated universe is a space-like surface nearly coincident with the future light cone emanating from the point of nucleation, all points of which have the same proper-time. This leads the authors to conclude that at the proper-time t  =  t _nuc at which any such nucleation occurs, an infinite open universe comes into existence. We point out that this is due entirely to the supposition of the nucleation occurring at a single point, which in light of quantum cosmology seems difficult to support. Even an infinitesimal space-like length at the moment of nucleation gives a rather different result—the boundary of the nucleating universe evolves in proper time and becomes infinite only in an infinite time. The alleged infinity is never attained at any finite time.     

The universe dynamics from topological considerations

We explore the possibility that the dynamics of the universe can be reproduced choosing appropriately the initial global topology of the Universe. In this work we start with two concentric spherical three-dimensional branes S ³, with radius a ₁ < a ₂ immersed in a five-dimensional space-time. The novel feature of this model is that in the interior brane there exist only spin-zero fundamental fields (scalar fields), while in the exterior one there exist only spin-one fundamental interactions. As usual, the bulk of the universe is dominated by gravitational interactions. In this model, like in the Ekpyrotic one, the Big Bang is consequence of the collision of the branes and causes the existence of the particles predicted by the standard model in the exterior brane (our universe). The scalar fields on the interior brane interact with the spin-one fields on the exterior one only through gravitation, they induce the effect of Scalar Field Dark Matter with an ultra-light mass on the exterior one. We discuss two different regimes where the energy density and the brane tension are compared, with the aim to obtain the observed dynamics of the universe after the collision of the branes.