Emergent universe scenario via Quintom matter

The emergent universe scenario provides a possible alternative to bouncing cosmology to avoid the Big Bang singularity problem. In this Letter we study the realization of the emergent universe scenario by making use of Quintom matter with an equation of state across the cosmological constant boundary. We will show explicitly the analytic and numerical solutions of emergent universe in two Quintom models, which are a phenomenological fluid and a nonconventional spinor field, respectively.     

Emergent universe from scale invariant two measures theory

The dilaton-gravity sector of a linear in the scalar curvature, scale invariant Two Measures Field Theory (TMT), is explored in detail in the context of closed FRW cosmology and shown to allow stable emerging universe solutions. The model possesses scale invariance which is spontaneously broken due to the intrinsic features of the TMT dynamics. We study the transition from the emerging phase to inflation, and then to a zero cosmological constant phase. We also study the spectrum of density perturbations and the constraints that impose on the parameters of the theory.     

Gauging universe expansion via scalar fields

In this study, we investigate the expansion of the FRLW universe in the open, closed, and flat geometries. The universe is dominated by a scalar field (spatially homogeneous) as a source of dark energy. We consider the three different classes of scalar fields – quintessence, tachyonic, and phantom field – for our analysis. A mathematical analysis is carried out by considering these three scalar fields with exponential and power-law potentials. Both potentials give exponential expansion in the open, closed, and flat FRLW universes. It is found that quintessence, tachyonic, and phantom scalar fields are indistinguishable under the slow roll approximation.     

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.     

Holographic dark energy and the universe expansion acceleration

By incorporating the holographic principle in a time-depending Λ-term cosmology, new physical bounds on the arbitrary parameters of the model can be obtained. Considering then the dark energy as a purely geometric entity, for which no equation of state has to be introduced, it is shown that the resulting range of allowed values for the parameters may explain both the coincidence problem and the universe accelerated expansion, without resorting to any kind of additional structures.     

Spherically Symmetric Solutions of Light Galileon

We have been studied the model of light Galileon with translational shift symmetry ? → ? + c. The matter Lagrangian is presented in the form \(\mathcal {L}_{\phi }= -\eta (\partial \phi )^{2}+\beta G^{\mu \nu }\partial _{\mu }\phi \partial _{\nu }\phi \). We have been addressed two issues: the first is that, we have been proven that, this type of Galileons belong to the modified matter-curvature models of gravity in type of \(f(R,R^{\mu \nu }T_{\mu \nu }^{m})\). Secondly, we have been investigated exact solution for spherically symmetric geometries in this model. We have been found an exact solution with singularity at r = 0 in null coordinates. We have been proven that the solution has also a non-divergence current vector norm. This solution can be considered as an special solution which has been investigated in literature before, in which the Galileon’s field is non-static (time dependence). Our scalar-shift symmetrized Galileon has the simple form of ? = t, which it is remembered by us dilaton field.     

The arrow of time and the expansion of the universe

It is pointed out that a model used to test any suggestion regarding the arrow of time in a cosmological contect, must have sufficient complexity. The clain of Zeh that the arrow of time defined by a universal wavefunction does not permit a reversal of cosmic expansion to be observed, is refuted on these grounds.     

The expansion of the universe and the cosmological constant problem

The discovery that the expansion of the universe is accelerating in time is a major discovery which still awaits adequate explanation. It is generally agreed that this implies a cosmic repulsion as a result of the existence of a cosmological constant ∧>0. However, estimates of ∧, based on calculations of the zero-point fluctuations of quantum fields are too large by over a hundred orders of magnitude. This result is obtained by summing the zero-point energies up to a large cutoff energy Ω, based on the Planck scale. Since there is no compelling reason for this choice, we argue that since all known quantum electrodynamic (QED) effects involves interaction with matter, a preferred choice should be based on causality and other considerations, leading to a much lower value for ∧.     

Logarithmically slow expansion of hot bubbles in gases

We predict a logarithmically slow expansion of hot bubbles in gases in the process of cooling. A model problem is first solved, when the temperature has compact support. Then the temperature profile decaying exponentially at large distances is considered. The periphery of the bubble is shown to remain essentially static ("glassy") in the process of cooling until it is taken over by a logarithmically slowly expanding "core." An analytical solution to the problem is obtained by matched asymptotic expansion. This problem gives an example of how logarithmic corrections enter dynamic scaling.     

Accelerated expansion of the universe filled up with the scalar gravitons

The concept of the scalar graviton as the source of the dark matter and dark energy of gravitational origin is applied to study the evolution of the isotropic homogeneous Universe. A realistic self-consistent solution to the modified pure gravity equations which correctly describes the accelerated expansion of the spatially flat Universe is found and investigated. It is argued that the scenario with the scalar gravitons filling up the Universe may emulate the LCDM model, thus reducing the true dark matter in the given context to an artifact. The text was submitted by the authors in English.     

A neutrino-dominated universe

Relic neutrinos produced during the early evolution of the universe will be abundant today (n _v n ) and, if they have a small mass (3 <m _v < 10 eV), may supply the dominant contribution to the total mass density. We review the data on the mass on various scales galaxies, binaries, small groups, large clusters) and conclude that ordinary matter (nucleons) is capable of accounting for the inferred mass on all scales except that of clusters of galaxies. Were the mass in clusters mainly in nucleons, too much helium and too little deuterium would have been produced during primordial nucleosynthesis. Relic neutrinos withm _v > 3 eV are heavy enough to collapse into clusters of galaxies; form _v < 10 eV they are too light to collapse along with binaries and small groups. Such neutrinos would supply the dominant contribution to the mass in the universe.This essay received the first award from the Gravity Research Foundation for the year 1980-Ed.     

A gravitino-rich universe

Journal of High Energy Physics - Recently it has been shown that the two-sphere partition function of a gauged linear sigma model of a Calabi-Yau manifold yields the exact quantum Kähler...     

A point-particle model universe

The observable cosmos is modeled as a set of point-particles, representing the galaxies, which perturb a dust-filled, Robertson-Walker space-time. The analysis proceeds only to first order in=8G/c ² and employs a metric suggested by McVittie [General Relativity and Cosmology (Chapman and Hall, London, 1965)], whose original work this paper seeks to develop. Necessary and sufficient conditions are found for the metric to give rise to an energy tensor of a chosen form appropriate to the modeling. In particular, a second-order equation is found which governs a certain time-independent potential. A class of solutions to this equation is established, and the associated singularities of the mass density are shown to be of a Dirac type.