Friedmann-like cosmological models without singularity

The Einstein-Cartan theory of gravitation (general relativity with spin) provides a specific spin-spin contact interaction of matter, in addition to the usual long-range gravity. This new interaction enables us to prevent singularities in Cosmological models. It is shown how this mechanism works in the case when the standard Einstein-Cartan equations are valid at a microphysical level, and some spin-spin terms remain from the averaging procedure for randomly distributed spins. In contrast with the case of aligned spin distributions, it is possible to take over the isotropic and spatially homogeneous (i.e., Friedmannian) models into the Einstein-Cartan theory. These models can be made free from singularity, thanks to the self-interaction of spinning fluid.     

Entanglement inside the cosmological apparent horizon

Possible connections between quantum entanglement and cosmological eras are considered. In particular, assuming that two epochs are each other entangling, by measuring the entanglement degree, it is possible to recover dynamical properties of the universe. In particular, the effects of dark energy could be due to the entanglement between states, since a negative pressure arises at late times. In this process, we choose as ruler to quantify the “entanglement weight”, the so-called negativity of entanglement. It follows that a natural anti-gravitational effect occurs when the cosmological eras are entangled. Thus, dark energy could be seen as a straightforward consequence of entanglement. Specifically, our results can be compared with observational data. In doing so, it is possible to show that a pressureless term is recovered at a certain epoch dominating over dark energy and ruling the structure formation.     

Creation of gravitons near the cosmological singularity

Equations are obtained for the local rate of creation of gravitons in a Friedmann universe, filled with a relativistic perfect gas consisting of particles with nonzero rest mass. It is shown that as the singularity is approached, when 3P, the rate of creation increases as the energy density in the universe and can for this reason become arbitrarily large.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 95–98, April, 1981.     

Is the cosmological singularity thermodynamically possible?

The four broad approaches that have been suggested heretofore to eliminate the initial singularity from cosmology are briefly reviewed. None is satisfactory, basically because one does not know enough about the microphysics involved in the process. Thermodynamics has often been used in such dilemmas, and it is proposed to answer the question of whether there was a Friedmann-like singularity in the universe by exploiting the bound on specific entropy that has been established for finite system. It is made applicable to the universe by considering only a causally connected spacelike region within the particle horizon of a given observer. It is found that the specific entropy of radiation in such a region can exceed the bound if the observer is too early in the universe. Faith in the bound leads to the conclusion that the Friedmann models cannot be extrapolated back to nearer than a few Planck-Wheeler times from the singularity. The Friedmann initial singularity thus appears to be thermodynamically unacceptable.     

Generalised cosmological friedmann equations without gravitational singularity

Within the framework of the gauge approach to gravitation, including terms in the lagrangian quadratic in the curvature and torsion tensors, a generalised Friedman equation for a homogeneous isotropic cosmology is obtained. This equation avoids the gravitational singularity with infinite mass density.     

Event horizon and scalar potential

The introduction of a scalar potential with a more general schema than general Relativity eliminates the event horizon. Among possible solutions, the Schwarzschild one represents a singular case. A study of the geodesic properties of the matching with an approximated interior Solution are given. A new definition of the gravitational mass and function is deduced.     

Creation of fermion pairs near the cosmological singularity

The energy density ? and the pressure P of fermion pairs near the homogeneous isotropic singularity are obtained. In general case the contributions of the real pairs and the virtual ones to the ? and the P are approximately equal. The contribution of the real pairs is leading when the law of the expansion is a(t) = a₁t^q, q ? 1 ? 1. In this case all the matter in the Friedman's cosmological model can be created by a quantum explosion.     

A scalar polynomial singularity without an event horizon

It is shown that the solution of the field equations for a static spherically symmetric scalar field has a scalar polynomial singularity and no event horizon. The solution does not develop from nonsingular data on any Cauchy surface. The possible existence of a universal scalar field, the conformal diagram and geodesies of the solution, and the energy and momentum of the field present are discussed.     

A primordial cosmological scenario and the horizon problem

The coupling of the Einstein-Hilbert Lagrangian with a conformal field in an arbitrary even dimension leads, after reduction to four dimensions, to the ordinary gravity coupled non-trivially to two scalar fields. The vacuum solutions display an initial singularity, but with an infinite proper distance between any two points of the space-time. There is an initial contraction phase followed by an expansion phase. This is a kind of anti-Big Bang scenario. The coupling to a radiative fluid preserves this feature; however, the temperature of the radiative fluid is initially zero, increasing afterwards: its maximum value is related to the moment where the horizon crosses the radius of the Universe. The entropy in the three-dimensional space is inversely proportional to the volume of the internal space, and it can be very high today. We perform also a perturbative study of this model during the contraction phase. There is no explosive growth of the scalar perturbations.     

Studying the cosmological apparent horizon with quasistatic coordinates

This article aims at a natural generalization of the static coordinates to the (n?+?1)-dimensional Friedmann–Lemaître–Robertson–Walker (FLRW) Universe. After demonstrating a no-go theorem, we put forward the quasistatic coordinates for the FLRW Universe. Then, the quasistatic coordinates are utilized to study the unified first law and the scalar-type perturbations on the cosmological apparent horizon.     

The singularity of homogeneous cosmological models with electromagnetic fields

The regime of approaching the singularity in the presence of electromagnetic fields has been found. The rotation angles of the Kasner axes are calculated.     

Stochastic behavior of multidimensional cosmological models near a singularity

NITsPV. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 107–111, November, 1994.     

Positive cosmological constant,non-local gravity and horizon entropy

We discuss a class of (local and non-local) theories of gravity that share same properties: (i) they admit the Einstein spacetime with arbitrary cosmological constant as a solution; (ii) the on-shell action of such a theory vanishes and (iii) any (cosmological or black hole) horizon in the Einstein spacetime with a positive cosmological constant does not have a non-trivial entropy. The main focus is made on a recently proposed non-local model. This model has two phases: with a positive cosmological constant Λ>0$\Lambda >0$ and with zero Λ. The effective gravitational coupling differs essentially in these two phases. Generalizing the previous result of Barvinsky we show that the non-local theory in question is free of ghosts on the background of any Einstein spacetime and that it propagates a standard spin-2 particle. Contrary to the phase with a positive Λ, where the entropy vanishes for any type of horizon, in an Einstein spacetime with zero cosmological constant the horizons have the ordinary entropy proportional to the area. We conclude that, somewhat surprisingly, the presence of any, even extremely tiny, positive cosmological constant should be important for the proper resolution of the entropy problem and, possibly, the information puzzle.     

Quantum effects near the singularity in a general cosmological scenario

Joshi and Joshi obtained a differential equation for quantum uncertainty under some very general conditions and inferred the divergence of the uncertainty on the basis of an approximate solution. The present note exactly solves the equation and confirms the divergence.