Showing papers on "Big Rip published in 2014"

Sequestering the Standard Model Vacuum Energy

Abstract: We propose a very simple reformulation of General Relativity, which completely sequesters from gravity all of the vacuum energy from a matter sector, including all loop corrections and renders all contributions from phase transitions automatically small. The idea is to make the dimensional parameters in the matter sector functionals of the 4-volume element of the universe. For them to be nonzero, the universe should be finite in spacetime. If this matter is the Standard Model of particle physics, our mechanism prevents any of its vacuum energy, classical or quantum, from sourcing the curvature of the universe. The mechanism is consistent with the large hierarchy between the Planck scale, electroweak scale and curvature scale, and early universe cosmology, including inflation. Consequences of our proposal are that the vacuum curvature of an old and large universe is not zero, but very small, that wDE ≃ −1 is a transient, and that the universe will collapse in the future.

Universe in a black hole with spin and torsion

Abstract: The conservation law for the angular momentum in curved spacetime requires that the antisymmetric part of the affine connection (the torsion tensor) is a variable in the principle of least action. The coupling between spin and torsion generates gravitational repulsion in fermionic matter at extremely high densities and avoids the formation of singularities in black holes. We show that every black hole in the presence of torsion forms a nonsingular, closed, nearly flat, homogeneous, and isotropic universe on the other side of its event horizon. Quantum particle production in such a universe can generate a period of exponential expansion which creates an enormous amount of matter in that universe. Accordingly, our Universe may have originated from the interior of a black hole existing in another universe.

No compelling cosmological models come out of magnetic universes which are based on nonlinear electrodynamics

Abstract: Here we investigate the cosmic dynamics of Friedmann-Robertson-Walker universes – flat spatial sections – which are driven by nonlinear electrodynamics (NLED) Lagrangians. We pay special attention to the check of the sign of the square sound speed since, whenever the latter quantity is negative, the corresponding cosmological model is classically unstable against small perturbations of the background energy density. Besides, based on causality arguments, one has to require that the mentioned small perturbations of the background should propagate at most at the local speed of light. We also look for the occurrence of curvature singularities. Our results indicate that several cosmological models which are based in known NLED Lagrangians, either are plagued by curvature singularities of the sudden and/or big rip type, or are violently unstable against small perturbations of the cosmological background – due to negative sign of the square sound speed – or both. In addition, causality issues associated with superluminal propagation of the background perturbations may also arise.

Black hole universe with a cosmological constant

Abstract: Time evolution of a black hole lattice universe with a positive cosmological constant $\mathrm{\ensuremath{\Lambda}}$ is simulated. The vacuum Einstein equations are numerically solved in a cubic box with a black hole in the center. Periodic boundary conditions on all pairs of opposite faces are imposed. Configurations of marginally trapped surfaces are analyzed. We describe the time evolution of not only black hole horizons, but also cosmological horizons. Defining the effective scale factor by using the area of a surface of the cubic box, we compare it with that in the spatially flat dust dominated Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) universe with the same value of $\mathrm{\ensuremath{\Lambda}}$. It is found that the behavior of the effective scale factor is well approximated by that in the FLRW universe. Our result suggests that local inhomogeneities do not significantly affect the global expansion law of the Universe irrespective of the value of $\mathrm{\ensuremath{\Lambda}}$.

Holographic dark energy model with linearly varying deceleration parameter and generalised Chaplygin gas dark energy model in Bianchi type-I universe

Abstract: The paper deals with a spatially homogeneous and anisotropic Bianchi type-I universe filled with two minimally interacting fluids; matter and holographic dark energy components. The nature of the holographic dark energy for Bianchi type-I space time is discussed. An exact solution to Einstein’s field equations in Bianchi type-I line element is obtained using the assumption of linearly varying deceleration parameter. Under the suitable condition, it is observed that the anisotropy parameter of the universe approaches to zero for large cosmic time and the coincidence parameter increases with increasing time. We established a correspondence between the holographic dark energy models with the generalised Chaplygin gas dark energy model. We also reconstructed the potential and dynamics of the scalar field which describes the Chaplygin cosmology. Solution of the field equations shows that a big rip type future singularity will occur for this model. It has been observed that the solutions are compatible with the results of recent observations.

Loop quantum cosmology and the fate of cosmological singularities

Abstract: Singularities in general relativity such as the big bang and big crunch, and exotic singularities such as the big rip are the boundaries of the classical space- times. These events are marked by a divergence in the curvature invariants and the breakdown of the geodesic evolution. Recent progress on implementing techniques of loop quantum gravity to cosmological models reveals that such singularities may be generically resolved because of the quantum gravitational e ects. Due to the quantum geometry, which replaces the classical di erential geometry at the Planck scale, the big bang is replaced by a big bounce without any assumptions on the matter content or any fine tuning. In this manuscript, we discuss some of the main features of this approach and the results on the generic resolution of singularities for the isotropic as well as anisotropic models. Using e ective spacetime description of the quantum the- ory, we show the way quantum gravitational e ects lead to the universal bounds on the energy density, the Hubble rate and the anisotropic shear. We discuss the geodesic completeness in the e ective spacetime and the resolution of all of the strong singular- ities. It turns out that despite the bounds on energy density and the Hubble rate, there can be divergences in the curvature invariants. However such events are geodesically extendible, with tidal forces not strong enough to cause inevitable destruction of the in-falling objects.

Wormholes minimally violating the null energy condition

Abstract: We consider novel wormhole solutions supported by a matter content that minimally violates the null energy condition. More specifically, we consider an equation of state in which the sum of the energy density and radial pressure is proportional to a constant with a value smaller than that of the inverse area characterising the system, i.e., the area of the wormhole mouth. This approach is motivated by a recently proposed cosmological event, denoted the little sibling of the big rip, where the Hubble rate and the scale factor blow up but the cosmic derivative of the Hubble rate does not [1]. By using the cut-and-paste approach, we match interior spherically symmetric wormhole solutions to an exterior Schwarzschild geometry, and analyse the stability of the thin-shell to linearized spherically symmetric perturbations around static solutions, by choosing suitable properties for the exotic material residing on the junction interface radius. Furthermore, we also consider an inhomogeneous generalization of the equation of state considered above and analyse the respective stability regions. In particular, we obtain a specific wormhole solution with an asymptotic behaviour corresponding to a global monopole.

Dark Matter and Dark Energy in the Universe

Abstract: Cosmological and astronomical observations predict that the present Universe is passing through an accelerating phase of expansion. The Universe emerged out of an exponential phase in the very early Universe. The scalar field of the standard model of particle physics when used in cosmology admits such a phase of expansion known as inflation. The most favourable condition for inflation with scalar field to admit an Inflationary scenario is that the potential energy must dominate over the kinetic energy which one obtains with a flat potential. Thereafter the Universe enters into a matter dominated phase when the field oscillates at the minimum of the potential. But it is not possible to accommodate the present accelerating phase in the Einstein’s gravity. It is known from observational analysis that about 73 % matter is responsible for the late phase expansion and 23 % matter called Dark Matter is responsible for a stable galaxy. We discuss here the relevant fields and theories that are useful for describing the late Universe. DOI: http://dx.doi.org/10.3126/bibechana.v11i0.10374 BIBECHANA 11(1) (2014) 8-16

Primordial Magnetic Fields from the Post-Inflationary Universe

Abstract: We explore cosmological magnetogenesis in the post-inflationary universe, when the inflaton oscillates around its potential minimum and the universe is effectively dominated by cold matter. During this epoch prior to reheating, large-scale magnetic fields can be significantly produced by the cosmological background. By considering magnetogenesis both during and after inflation, we demonstrate that magnetic fields stronger than 10^{-15} G can be generated on Mpc scales without having strong couplings in the theory, or producing too large electric fields that would dominate the universe.

Exploring scalar field dynamics with Gaussian processes

Abstract: The origin of the accelerated expansion of the Universe remains an unsolved mystery in Cosmology. In this work we consider a spatially flat Friedmann-Robertson-Walker (FRW) Universe with non-relativistic matter and a single scalar field contributing to the energy density of the Universe. Properties of this scalar field, like potential, kinetic energy, equation of state etc. are reconstructed from Supernovae and BAO data using Gaussian processes. We also reconstruct energy conditions and kinematic variables of expansion, such as the jerk and the slow roll parameter. We find that the reconstructed scalar field variables and the kinematic quantities are consistent with a flat ΛCDM Universe. Further, we find that the null energy condition is satisfied for the redshift range of the Supernovae data considered in the paper, but the strong energy condition is violated.

Spontaneous creation of the universe from nothing

Abstract: An interesting idea is that the universe could be spontaneously created from nothingbut no rigorousproof has been given. In this paperwe present such a proof based on the analytic solutions of the Wheeler-DeWitt equation (WDWE). Explicit solutions of the WDWE for the special operator ordering factorp= -2 (or 4) show thatonce a small true vacuum bubble is created by quantum fluctuations of themetastable false vacuumit can expand exponentially no matter whether the bubble is closed,flat,or open.The exponential expansion will end when the bubble becomes large and thus the early universe appears.With the de Broglie-Bohm quantum trajectory theorywe show explicitly that it is the quantum potentialthat plays the role of the cosmological constant and provides the power for the exponential expansion of thetrue vacuum bubble. So it is clear that the birth of the early universe completely depends on the quantum nature of the theory.

Stability analysis and future singularity of the $m^2 R \Box^{-2} R$ model of non-local gravity

Abstract: We analyse the classical stability of the model proposed by Maggiore and Mancarella, where gravity is modified by a term $\sim m^2 R \Box^{-2} R$ to produce the late-time acceleration of the expansion of the universe. Our study takes into account all excitations of the metric that can potentially drive an instability. There are some subtleties in identifying these modes, as a non-local field theory contains dynamical fields which yet do not correspond to degrees of freedom. Since some of them are ghost-like, we clarify the impact of such modes on the stability of the solutions of interest that are the flat space-time and cosmological solutions. We then find that flat space-time is unstable under scalar perturbations, but the instability manifests itself only at cosmological scales, i.e. out of the region of validity of this solution. It is therefore the stability of the FLRW solution which is relevant there, in which case the scalar perturbations are known to be well-behaved by numerical studies. By finding the analytic solution for the late-time behaviour of the scale factor, which leads to a big rip singularity, we argue that the linear perturbations are bounded in the future because of the domination of Hubble friction. In particular, this effect damps the scalar ghost perturbations which were responsible for destabilizing Minkowski space-time. Thus, the model remains phenomenologically viable.

Thermodynamics in Little Rip cosmology in the framework of a type of f(R, T) gravity

Abstract: The cosmological reconstruction of the Little Rip model in f (R, T) gravity is investigated, where R is the curvature scalar and T the trace of the energy momentum tensor. The model perfectly reproduces the present stage of the universe, characterized by the $ \Lambda$ CDM model, without singularity at future finite-time (without the Big Rip). The input parameters are determined according to Supernovae cosmology data and perfectly fit with the WMAP around the Little Rip. Moreover, the thermodynamics is considered in this Little Rip cosmology and it is illustrated that the second law of thermodynamics is always satisfied around the Little Rip universe for the temperature inside the horizon being the same as that of the apparent horizon. Moreover, we show the existence of a stable fixed point in the Little Rip universe which confirms that this is actually a late-time attractor in the phantom-dominated universe. The linear perturbation analysis is performed around the critical points, showing that the Little Rip model obtained is stable.

Cosmological singularities in Born-Infeld determinantal gravity

Abstract: The Born-Infeld determinantal gravity has been recently proposed as a way to smooth the Big Bang singularity. This theory is formulated on the Weitzenbock space-time and the teleparallel representation is used instead of the standard Riemannian representation. We find that although this theory is shown to be singularity-free for certain region of the parameter space in which the divergence of the Hubble rate at the high energy regime is substituted by a de-Sitter stage or a bounce in a Friedmann-Lemaitre-Robertson-Walker universe, cosmological singularities such as Big Rip, Big Bang, Big Freeze, and Sudden singularities can emerge in other regions of the configuration space of the theory. We also show that all these singular events exist even though the Universe is filled with a perfect fluid with a constant equation of state.

Each instant of time a new Universe

Abstract: We present an alternative view of quantum evolution in which each moment of time is viewed as a new “universe” and time evolution is given by correlations between them. [Editors note: for a video of the talks given by Prof. Aharonov at the Aharonov-80 conference in 2012 at Chapman University, see quantum.chapman.edu/talk-3 and quantum.chapman.edu/talk-30 and by Prof. Popescu, see quantum.chapman.edu/talk-31.]

Grand rip and grand bang/crunch cosmological singularities

Abstract: The present accelerated expansion of the Universe has enriched the list of possible scenarios for its fate, singular or not. In this paper a unifying framework for analyzing such behaviors is proposed, based on generalized power and asymptotic expansions of the barotropic index $w$, or equivalently of the deceleration parameter $q$, in terms of the time coordinate. Besides well-known singular and nonsingular future behaviors, other types of strong singularities appear around the phantom divide in flat models, with features similar to those of big rip or big bang/crunch, which we have dubbed ``grand rip'' and ``grand bang/crunch,'' respectively, since energy density and pressure diverge faster than ${t}^{\ensuremath{-}2}$ in coordinate time. In addition to this, the scale factor does not admit convergent generalized power series around these singularities with a finite number of terms with negative powers.

Emergent universe in spatially flat cosmological model

Abstract: The scenario of an emergent universe provides a promising resolution to the big bang singularity in universes with positive or negative spatial curvature. It however remains unclear whether the scenario can be successfully implemented in a spatially flat universe which seems to be favored by present cosmological observations. In this paper, we study the stability of Einstein static state solutions in a spatially flat Shtanov-Sahni braneworld scenario. With a negative dark radiation term included and assuming a scalar field as the only matter energy component, we find that the universe can stay at an Einstein static state past eternally and then evolve to an inflation phase naturally as the scalar field climbs up its potential slowly. In addition, we also propose a concrete potential of the scalar field that realizes this scenario.

Stability analysis and future singularity of the m2 R □-2 R model of non-local gravity

Abstract: We analyse the classical stability of the model proposed by Maggiore and Mancarella, where gravity is modified by a term ~ m2 R -2 R to produce the late-time acceleration of the expansion of the universe. Our study takes into account all excitations of the metric that can potentially drive an instability. There are some subtleties in identifying these modes, as a non-local field theory contains dynamical fields which yet do not correspond to degrees of freedom. Since some of them are ghost-like, we clarify the impact of such modes on the stability of the solutions of interest that are the flat space-time and cosmological solutions. We then find that flat space-time is unstable under scalar perturbations, but the instability manifests itself only at cosmological scales, i.e. out of the region of validity of this solution. It is therefore the stability of the FLRW solution which is relevant there, in which case the scalar perturbations are known to be well-behaved by numerical studies. By finding the analytic solution for the late-time behaviour of the scale factor, which leads to a big rip singularity, we argue that the linear perturbations are bounded in the future because of the domination of Hubble friction. In particular, this effect damps the scalar ghost perturbations which were responsible for destabilizing Minkowski space-time. Thus, the model remains phenomenologically viable.

Relativistic viscous universe models

Abstract: The research on relativistic universe models with viscous fl uids is reviewed. Viscosity may have been of significance during the early inflatio nary era, and may also be of importance for the late time evolution of the Universe. Bulk viscosity and shear viscosity cause exponential decay of anisotropy, while nonlinear viscosity causes powerlaw decay of anisotropy. We consider also the influence from t urbulence, in connection with future singularities of the universe (Big Rip and Littl e Rip). Finally, we review some recent developments of causal cosmology theories.

Cosmology in ghost-free bigravity theory with twin matter fluids: The origin of dark matter

Abstract: We study dynamics of Friedmann-Lemaitre-Robertson-Walker (FLRW) spacetime based on the ghost-free bigravity theory. Assuming the coupling parameters guaranteeing the existence of de Sitter space as well as Minkowski spacetime, we find two stable attractors for spacetime with "twin" dust matter fields: One is de Sitter accelerating universe and the other is matter dominated universe. Although a considerable number of initial data leads to de Sitter universe, we also find matter dominated universe or spacetime with a future singularity for some initial data. The cosmic no-hair conjecture does not exactly hold, but the accelerating expansion can be found naturally. The $\Lambda$-CDM model is obtained as an attractor. We also show that the dark matter component in the Friedmann equation, which originates from another twin matter, can be about 5 times larger than the baryonic matter, by choosing the appropriate coupling constants.

Holographic dark energy with linearly varying deceleration parameter and escaping big rip singularity of the Bianchi type-V universe

Abstract: The present work deals with the accretion of two minimally interacting fluids: dark matter and a hypothetical isotropic fluid as the holographic dark energy components onto black hole and wormhole in a spatially homogeneous and anisotropic Bianchi type-V universe. To obtain an exact solution of the Einstein’s field equations, we use the assumption of linearly varying deceleration parameter. Solution describes effectively the actual acceleration and indicates a big rip type future singularity of the universe. We have studied the evolution of the mass of black hole and the wormhole embedded in this anisotropic universe in order to reproduce a stable universe protected against future-time singularity. It is observed that the accretion of these dark components leads to a gradual decrease and increase of black hole and wormhole mass respectively. Finally, we have found that contrary to our previous case (Sarkar in Astrophys. Space. Sci. 341:651, 2014a), the big rip singularity of the universe with a divergent Hubble parameter of this dark energy model may be avoided by a big trip.

Probing the dark side of the Universe with weak gravitational lensing effects

Abstract: Arising from gravitational deflections of light rays by large-scale structures in the Universe, weak-lensing effects have been recognized as one of the most important probes in cosmological studies. In this paper, we review the main progress in weak-lensing analyses, and discuss the challenges in future investigations aiming to understand the dark side of the Universe with unprecedented precisions.

Third Quantization and Quantum Universes

Abstract: We study the third quantization of the Friedmann-Robertson-Walker cosmology with N-minimal massless fields. The third quantized Hamiltonian for the Wheeler-DeWitt equation in the minisuperspace consists of infinite number of intrinsic time-dependent, decoupled oscillators. The Hamiltonian has a pair of invariant operators for each universe with conserved momenta of the fields that play a role of the annihilation and the creation operators and that construct various quantum states for the universe. The closed universe exhibits an interesting feature of transitions from stable states to tachyonic states depending on the conserved momenta of the fields. In the classical forbidden unstable regime, the quantum states have googolplex growing position and conjugate momentum dispersions, which defy any measurements of the position of the universe.

Effective dark energy models and dark energy models with bounce in frames of F(T) gravity

Abstract: Various cosmological models in frames of F(T) gravity are considered. The general scheme of constructing effective dark energy models with various evolution is presented. It is showed that these models in principle are compatible with ΛCDM model. The dynamics of universe governed by F(T) gravity can mimics ΛCDM evolution in past but declines from it in a future. We also construct some dark energy models with the “real” (non-effective) equation-of-state parameter w such that w≤−1. It is showed that in F(T) gravity the Universe filled phantom field not necessarily ends its existence in singularity. There are two possible mechanisms permitting the final singularity. Firstly due to the nonlinear dependence between energy density and H 2 (H is the Hubble parameter) the universe can expands not so fast as in the general relativity and in fact Little Rip regime take place instead Big Rip. We also considered the models with possible bounce in future. In these models the universe expansion can mimics the dynamics with future singularity but due to bounce in future universe begin contracts.

UV surface brightness of galaxies from the local Universe to z ~ 5

Abstract: The Tolman test for surface brightness dimming was originally proposed as a test for the expansion of the Universe. The test, which is independent of the details of the assumed cosmology,is based on comparisons of the surface brightness (SB) of identical objects at different cosmological distances. Claims have been made that the Tolman test provides compelling evidence against a static model for the Universe. In this paper we reconsider this subject by adopting a static Euclidean Universe with a linear Hubble relation at all z (which is not the standard Einstein- de Sitter model),resulting in a relation between flux and luminosity that is virtually indistinguishable from the one used for LCDM models. Based on the analysis of the UV surface brightness of luminous disk galaxies from HUDF and GALEX datasets, reaching from the local Universe to z ~ 5 we show that the surface brightness remains constant as expected in a SEU. A re-analysis of previously-published data used for the Tolman test at lower redshift, when treated within the same framework, confirms the results of the present analysis by extending our claim to elliptical galaxies. We conclude that available observations of galactic SB are consistent with a static Euclidean model of the Universe. We do not claim that the consistency of the adopted model with SB data is sufficient by itself to confirm what would be a radical transformation in our understanding of the cosmos. However, we believe this result is more than sufficient reason to examine further this combination of hypotheses.

Interacting viscous dark energy in Bianchi type-I Universe

Abstract: A solution to the coincidence and Big Rip problems on the bases of an anisotropic space-time is proposed. To do so, we study the interaction between viscous dark energy and dark matter in the scope of the Bianchi type-I Universe. We parameterize the viscosity and the interaction between the two fluids by constants ζ 0 and σ respectively. A detailed investigation on the cosmological implications of this parametrization has been made. We have also performed a geometrical diagnostic by using the statefinder pairs {s,r} and {q,r} in order to differentiate between different dark energy models. Moreover, we fit the coupling parameter σ as well as the Hubble’s parameter H 0 of our model by minimizing the χ 2 through the age differential method, involving a direct measurement of H.