Models of the universe

In this paper a theory of models of the universe is proposed. We refer to such models ascosmological models, where a cosmological model is defined as an Einstein-inextendible Einstein spacetime. A cosmological model isabsolute if it is a Lorentz-inextendible Einstein spacetime,predictive if it is globally hyperbolic, andnon-predictive if it is nonglobally-hyperbolic. We discuss several features of these models in the study of cosmology. As an example, any compact Einstein spacetime is always a non-predictive absolute cosmological model, whereas a noncompact complete Einstein spacetime is an absolute cosmological model which may be either predictive or non-predictive. We discuss the important role played by maximal Einstein spacetimes. In particular, we examine the possible proper Lorentz-extensions of such spacetimes, and show that a spatially compact maximal Einstein spacetime is exclusively either a predictive cosmological model or a proper sub-spacetime of a non-predictive cosmological model. Provided that the Strong Cosmic Censorship conjecture is true, a generic spatially compact maximal Einstein spacetime must be a predictive cosmological model. It isconjectured that the Strong Cosmic Censorship conjecture isnot true, and converting a vice to a virtue it is argued that the failure of the Strong Cosmic Censorship conjecture would point to what may be general relativity's greatest prediction of all, namely,that general relativity predicts that general relativity cannot predict the entire history of the universe.     

Limiting energy density and a regular accelerating universe in Riemann-Cartan spacetime

Isotropic cosmology built in the Riemann-Cartan spacetime by using sufficiently general expression of gravitational Lagrangian is investigated. It is shown that cosmological equations obtained by certain restrictions on indefinite parameters of gravitational Lagrangian lead to limiting energy density at the beginning of cosmological expansion and all cosmological models filled with usual gravitating matter satisfying standard energy conditions are regular with respect to energy density, spacetime metrics with its time derivative and torsion functions. At asymptotics cosmological solutions of spatially flat models coincide with that of standard ΛCDM-model for accelerating Universe.     

Monopole production in the very early universe in a first-order phase transition

The implications for magnetic monopole production of a first-order phase transition in the evolution of the early universe are discussed. Because of cosmological limits on the monopole density, strong constraints on the properties of the phase transition are required. The universe is likely to supercool, with the phase transition terminating either because the metastable state becomes unstable or because the universe is filled by “bubbles” of the broken symmetry state. Both scenarios are complicated by the exponential expansion of the supercooled universe, owing to the large cosmological term existing before the phase transition. To achieve the required dilution of the monopole/entropy ratio in either case would require a fortuitous arrangement of the parameters of the grand unified theory. We also comment on the implications of a first-order phase transition for the generation of a baryon excess. In the appendix, we elaborate a conjecture concerning the theory of nucleation in finite-temperature quantum field theory.     

Antimatter in the universe

The models leading to a high abundance of antimatter in the universe are discussed. Special attention is payed to the model of antimatter creation in the form of compact stellar-like objects. Such objects can contribute significantly to the cosmological dark matter. Observational signatures of antimatter in the Galaxy are discussed.     

Perfect fluid quantum anisotropic universe: merits and challenges

The present paper deals with quantization of perfect fluid anisotropic cosmological models. Bianchi type V and IX models are discussed following Schutz’s method of expressing fluid velocities in terms of six potentials. The wave functions are found for several examples of equations of state. In one case a complete wave packet could be formed analytically. The initial singularity of a zero proper volume can be avoided in this case, but it is plagued by the usual problem of non-unitarity of anisotropic quantum cosmological models. It is seen that a particular operator ordering alleviates this problem.     

Modified gravity in contemporary universe

Astronomical data in favor of cosmological acceleration and possible explanations of accelerated expansion of the universe are discussed. Main attention is paid to gravity modifications at small curvature which could induce accelerated cosmological expansion. It is shown that gravitating systems with mass density rising with time evolve to a singular state with infinite curvature scalar. The universe evolution during the radiation-dominated epoch is studied in the R ²-extended gravity theory. Particle production rate by the oscillating curvature and the back reaction of particle production on the evolution of R are calculated in one-loop approximation. Possible implications of the model for cosmological creation of non-thermal dark matter are discussed.     

Hierarchical and relativistic models of the universe

The Treder and Barbour-Bertotti models of relativistic Lagrangians using only relative coordinates and velocities are compared in light of Leibnitz-Mach ideas. The plausible relationship between cosmological and vacuum asymmetries is noted, and the possibility of a modernized hierarchical model is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 86–90, July, 1978.     

Structural instability of Friedmann–Robertson–Walker cosmological models

Cosmological singularity and asymptotic behavior of scale factor of generalized cosmological models are analyzed in respect of their structural stability. It is shown, that cosmological singularity is structurally unstable for the majority of models with barotropic perfect fluid with strong energy condition. Inclusion of Λ-term extends the set of structurally stable cosmological models.     

CP and CPT symmetry violations,entropy and the expanding universe

The conjecture linking together statistical and cosmological time arrows implies interpreting the contracting phase in an oscillating cosmological model as a time-inverted expansion. The Fitch-Cronin effect now requires a redefinition of which is matter and which is antimatter in this time-inverted picture. However, ifCPT is also broken, then theT-violating effects yield a different set of laws altogether for such reactions.Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, U.S. Air Force, under AFOSR grant number EOOAR-68-0010, through the European Office of Aerospace Research.     

Gravitational energy in the expanding universe

The role of gravitational energy in the evolution of the universe is examined. In co-moving coordinates, calculation of the Landau-Lifshitz pseudotensor for FRW models reveals that: (i) the total energy of a spatially closed universe irrespective of the equation of state of the cosmic fluid is zero at all times, (ii) the total energy enclosed within any finite volume of the spatially flat universe is zero at all times, (iii) during inflation the vacuum energy driving the accelerated expansion and ultimately responsible for the creation of matter (radiation) in the universe, is drawn from the energy of the gravitational field. In a similar fashion, certain cosmological models which abandon adiabaticity by allowing for particle creation, use the gravitational energy directly as an energy source.     

On the rotation of the universe

We consider different interpretations of the rotation of the metagalaxy and cosmological models of the universe with rotation. The Muradyan formula for the angular momentum of the metagalaxy is obtained, starting from the hierarchic concept of reality. It is established that the angular velocities of rotation of matter in the Gedel and Ozsvath-Schücking models of the universe have the same order 10^–11 rad/year. Possible local effects of a rotating universe are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 22–25, January, 1985.     

Induction of strong gravitation and the heirarchical structure of the universe

A nontraditional approach to strong gravitation is proposed from the position of induced gravitation. Induction of strong gravitation constants and a strong gravitation “cosmological term” due to quantum effects in curved space is considered. The relationship between induced gravitation and the concept of a heirarchical structure of the universe is studied.     

Anisotropic Bianchi-I universe with phantom field and cosmological constant

We study an anisotropic Bianchi-I universe in the presence of a phantom field and a cosmological constant. Cosmological solutions are obtained when the kinetic energy of the phantom field is of the order of anisotropy and dominates over the potential energy of the field. The anisotropy of the universe decreases and the universe transits to an isotropic flat FRW universe accommodating the present acceleration. A class of new cosmological solutions is obtained for an anisotropic universe in case an initial anisotropy exists which is bigger than the value determined by the parameter of the kinetic part of the field. Later, an autonomous system of equations for an axially symmetric Bianchi-I universe with phantom field in an exponential potential is studied. We discuss the stability of the cosmological solutions.      

Conformal anomalies and massless particle generation in a Fridman universe

The general structure of conformai anomalies of the energy-momentum tensor is established. The energy density of massless particles generated during the cosmological expansion is calculated in the lowest-order perturbation theory. Estimates are made that indicate that the energy density of generated massless particles in the early universe can significantly exceed the energy density of the generated massive particles.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 54–58, August, 1984.We are grateful to E. S. Fradkin, V. P. Frolov, I. V. Tyutin, and B. L. Voronov for useful discussions.     

An inhomogeneous model universe behaving homogeneously

We present a new model universe based on the junction of FRW to flat Lemaitre–Tolman–Bondi (LTB) solutions of Einstein equations along our past light cone, bringing structures within the FRW models. The model is assumed globally to be homogeneous, i.e. the cosmological principle is valid. Local inhomogeneities within the past light cone are modeled as a flat LTB, whereas those outside the light cone are assumed to be smoothed out and represented by a FRW model. The model is singularity free, always FRW far from the observer along the past light cone, gives way to a different luminosity distance relation as for the CDM/FRW models, a negative deceleration parameter near the observer, and correct linear and non-linear density contrast. As a whole, the model behaves like a FRW model on the past light cone with a special behavior of the scale factor, Hubble and deceleration parameter, mimicking dark energy. Paper in honor of Bahram Mashhoon’s 60th birthday.     

Models of the universe containing matter and gravitational radiation

The manner in which a field of gravitational radiation modifies the cosmological background space-time of the model universe containing it is considered. The cosmological equations for the models are solved and a number of examples of the resulting universes are presented in diagrammatic and tabular form.     

A new look at the accelerating universe

We show that the cosmological model having zero cosmological constant, but containing the vacuum energy of a simple quantized free scalar field of low mass (VCDM model), agrees with the cosmic microwave background radiation (CMBR) and supernova (SNe-Ia) data at least as well as the classical cosmological model with a small nonzero cosmological constant. We also show that in the VCDM model the ratio of vacuum pressure to vacuum energy density is less than -1. We display the VCDM results for a set of parameters that give a very good fit to the CMBR power spectrum, and show that the same parameters also give a good fit to the SNe-Ia data.     

A non-singular homogeneous universe with torsion

The Raychaudhuri equation and other relations for a spinning fluid in the framework of the Einstein-Cartan theory are obtained. It is shown that non-singular cosmological models of the Bianchi types I÷VIII are possible. As an example, a class of solutions for the I and the V type is presented.     

Exploring the expansion history of the universe

Exploring the recent expansion history of the universe promises insights into the cosmological model, the nature of dark energy, and potentially clues to high energy physics theories and gravitation. We examine the extent to which precision distance-redshift observations can map out the history, including the acceleration-deceleration transition, and the components and equations of state of the energy density. We consider the ability to distinguish between various dynamical scalar field models for the dark energy, as well as higher dimension and alternate gravity theories. Finally, we present a new, advantageous parametrization for the study of dark energy.