Showing papers on "Big Rip published in 2015"

Cosmology with a stiff matter era

Abstract: We consider the possibility that the Universe is made of a dark fluid described by a quadratic equation of state $P=K{\ensuremath{\rho}}^{2}$, where $\ensuremath{\rho}$ is the rest-mass density and $K$ is a constant. The energy density $\ensuremath{\epsilon}=\ensuremath{\rho}{c}^{2}+K{\ensuremath{\rho}}^{2}$ is the sum of two terms: a rest-mass term $\ensuremath{\rho}{c}^{2}$ that mimics ``dark matter'' ($P=0$) and an internal energy term $u=K{\ensuremath{\rho}}^{2}=P$ that mimics a ``stiff fluid'' ($P=\ensuremath{\epsilon}$) in which the speed of sound is equal to the speed of light. In the early universe, the internal energy dominates and the dark fluid behaves as a stiff fluid ($P\ensuremath{\sim}\ensuremath{\epsilon}$, $\ensuremath{\epsilon}\ensuremath{\propto}{a}^{\ensuremath{-}6}$). In the late universe, the rest-mass energy dominates and the dark fluid behaves as pressureless dark matter ($P\ensuremath{\simeq}0$, $\ensuremath{\epsilon}\ensuremath{\propto}{a}^{\ensuremath{-}3}$). We provide a simple analytical solution of the Friedmann equations for a universe undergoing a stiff matter era, a dark matter era, and a dark energy era due to the cosmological constant. This analytical solution generalizes the Einstein--de Sitter solution describing the dark matter era, and the $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model describing the dark matter era and the dark energy era. Historically, the possibility of a primordial stiff matter era first appeared in the cosmological model of Zel'dovich where the primordial universe is assumed to be made of a cold gas of baryons. A primordial stiff matter era also occurs in recent cosmological models where dark matter is made of relativistic self-gravitating Bose-Einstein condensates (BECs). When the internal energy of the dark fluid mimicking stiff matter is positive, the primordial universe is singular like in the standard big bang theory. It expands from an initial state with a vanishing scale factor and an infinite density. We consider the possibility that the internal energy of the dark fluid is negative (while, of course, its total energy density is positive), so that it mimics anti-stiff matter. This happens, for example, when the BECs have an attractive self-interaction with a negative scattering length. In that case, the primordial universe is nonsingular and bouncing like in loop quantum cosmology. At $t=0$, the scale factor is finite and the energy density is equal to zero. The universe first has a phantom behavior where the energy density increases with the scale factor, then a normal behavior where the energy density decreases with the scale factor. For the sake of generality, we consider a cosmological constant of arbitrary sign. When the cosmological constant is positive, the Universe asymptotically reaches a de Sitter regime where the scale factor increases exponentially rapidly with time. This can account for the accelerating expansion of the Universe that we observe at present. When the cosmological constant is negative (anti--de Sitter), the evolution of the Universe is cyclic. Therefore, depending on the sign of the internal energy of the dark fluid and on the sign of the cosmological constant, we obtain analytical solutions of the Friedmann equations describing singular and nonsingular expanding, bouncing, or cyclic universes.

Universe acceleration and nonlinear electrodynamics

Abstract: A new model of nonlinear electrodynamics with a dimensional parameter $\beta$ coupled to gravity is considered. We show that an accelerated expansion of the universe takes place if the nonlinear electromagnetic field is the source of the gravitational field. A pure magnetic universe is investigated and the magnetic field drives the universe to accelerate. In this model, after the big bang, the universe undergoes inflation, and the accelerated expansion and then decelerates approaching Minkowski spacetime asymptotically. We demonstrate the causality of the model and a classical stability at the deceleration phase.

LRS Bianchi type-I cosmological model in f ( R , T ) theory of gravity with Λ ( T )

Abstract: The locally rotationally symmetric (LRS) Bianchi type-I cosmological models have been investigated in f(R,T) theory of gravity, where R is the Ricci scalar and T is the trace of the energy momentum tensor, for some choices of the functional f(R,T)=f 1(R)+f 2(T). The exact solutions of the field equations are obtained for the linearly varying deceleration parameter q(t) proposed by Akarsu and Dereli (2012). Keeping an eye on the accelerating nature of the universe in the present epoch, the dynamics and physical behaviour of the models have been discussed. It is interesting to note that in one of the model, the universe ends with a big rip. By taking different functional forms for f 2(T) we have investigated whether or not the Big Rip can be avoided. We found that, the Big Rip situation can not be avoided and may be inherent in the linearly varying deceleration parameter. We have also applied the State-finder diagnostics to get the geometrical dynamics of the universe at different phases.

The little sibling of the big rip singularity

Abstract: In this paper, we present a new cosmological event, which we named the little sibling of the big rip. This event is much smoother than the big rip singularity. When the little sibling of the big rip is reached, the Hubble rate and the scale factor blow up, but the cosmic derivative of the Hubble rate does not. This abrupt event takes place at an infinite cosmic time where the scalar curvature explodes. We show that a doomsday a la little sibling of the big rip is compatible with an accelerating universe, indeed at present it would mimic perfectly a ΛCDM scenario. It turns out that, even though the event seems to be harmless as it takes place in the infinite future, the bound structures in the universe would be unavoidably destroyed on a finite cosmic time from now. The model can be motivated by considering that the weak energy condition should not be strongly violated in our universe, and it could give us some hints about the status of recently formulated nonlinear energy conditions.

Eddington–Born–Infeld cosmology: a cosmographic approach, a tale of doomsdays and the fate of bound structures

Abstract: The Eddington-inspired-Born–Infeld scenario (EiBI) can prevent the big bang singularity for a matter content whose equation of state is constant and positive. In a recent paper [Bouhmadi-Lopez et al. (Eur. Phys. J. C 74:2802, 2014)] we showed that, on the contrary, it is impossible to smooth a big rip in the EiBI setup. In fact the situations are still different for other singularities. In this paper we show that a big freeze singularity in GR can in some cases be smoothed to a sudden or a type IV singularity under the EiBI scenario. Similarly, a sudden or a type IV singularity in GR can be replaced in some regions of the parameter space by a type IV singularity or a loitering behaviour, respectively, in the EiBI framework. Furthermore, we find that the auxiliary metric related to the physical connection usually has a smoother behaviour than that based on the physical metric. In addition, we show that bound structures close to a big rip or a little rip will be destroyed before the advent of the singularity and will remain bound close to a sudden, big freeze or type IV singularity. We then constrain the model following a cosmographic approach, which is well known to be model independent, for a given Friedmann–Lemaitre–Robertson–Walker geometry. It turns out that among the various past or present singularities, the cosmographic analysis can pick up the physical region that determines the occurrence of a type IV singularity or a loitering effect in the past. Moreover, to determine which of the future singularities or doomsdays is more probable, observational constraints on the higher-order cosmographic parameters are required.

Sequestration of vacuum energy and the end of the universe.

Abstract: Recently, we proposed a mechanism for sequestering the standard model vacuum energy that predicts that the Universe will collapse. Here we present a simple mechanism for bringing about this collapse, employing a scalar field whose potential is linear and becomes negative, providing the negative energy density required to end the expansion. The slope of the potential is chosen to allow for the expansion to last until the current Hubble time, about 10 10 years, to accommodate our Universe. Crucially, this choice is technically natural due to a shift symmetry. Moreover, vacuum energy sequestering selects radiatively stable initial conditions for the collapse, which guarantee that immediately before the turnaround the Universe is dominated by the linear potential which drives an epoch of accelerated expansion for at least an e fold. Thus, a single, technically natural choice for the slope ensures that the collapse is imminent and is preceded by the current stage of cosmic acceleration, giving a new answer to the “why now?” problem.

Singular cosmological evolution using canonical and ghost scalar fields

Abstract: We demonstrate that finite time singularities of Type IV can be consistently incorporated in the Universe's cosmological evolution, either appearing in the inflationary era, or in the late-time regime. While using only one scalar field instabilities can in principle occur at the time of the phantom-divide crossing, when two fields are involved we are able to avoid such instabilities. Additionally, the two-field scalar-tensor theories prove to be able to offer a plethora of possible viable cosmological scenarios, at which various types of cosmological singularities can be realized. Amongst others, it is possible to describe inflation with the appearance of a Type IV singularity, and phantom late-time acceleration which ends in a Big Rip. Finally, for completeness, we also present the Type IV realization in the context of suitably reconstructed F(R) gravity.

Singular cosmological evolution using canonical and phantom scalar fields

Abstract: We demonstrate that finite time singularities of Type IV can be consistently incorporated in the Universe's cosmological evolution, either appearing in the inflationary era, or in the late-time regime. While using only one scalar field instabilities can in principle occur at the time of the phantom-divide crossing, when two fields are involved we are able to avoid such instabilities. Additionally, the two-field scalar-tensor theories prove to be able to offer a plethora of possible viable cosmological scenarios, at which various types of cosmological singularities can be realized. Amongst others, it is possible to describe inflation with the appearance of a Type IV singularity, and phantom late-time acceleration which ends in a Big Rip. Finally, for completeness, we also present the Type IV realization in the context of suitably reconstructed $F(R)$ gravity.

An exposition on Friedmann cosmology with negative energy densities

Abstract: How would negative energy density affect a classic Friedmann cosmology? Although never measured and possibly unphysical, certain realizations of quantum field theories leaves the door open for such a possibility. In this paper we analyze the evolution of a universe comprising varying amounts of negative energy forms. Negative energy components have negative normalized energy densities, Ω 1/3. Assuming that such energy forms generate pressure like perfect fluids, the attractive or repulsive nature of negative energy components are reviewed. The Friedmann equation is satisfied only when negative energy forms are coupled to a greater magnitude of positive energy forms or positive curvature. We show that the solutions exhibit cyclic evolution with bounces and turnovers.The future and fate of such universes in terms of curvature, temperature, acceleration, and energy density are reviewed. The end states are dubbed ``big crunch," `` big void," or ``big rip" and further qualified as ``warped",``curved", or ``flat",``hot" versus ``cold", ``accelerating" versus ``decelerating" versus ``coasting". A universe that ends by contracting to zero energy density is termed ``big poof." Which contracting universes ``bounce" in expansion and which expanding universes ``turnover" into contraction are also reviewed.

Viscosity-Induced Crossing of the Phantom Barrier

Abstract: We show explicitly, by using astrophysical data plus reasonable assumptions for the bulk viscosity in the cosmic fluid, how the magnitude of this viscosity may be high enough to drive the fluid from its position in the quintessence region at present time t = 0 across the barrier w = −1 into the phantom region in the late universe. The phantom barrier is accordingly not a sharp mathematical divide, but rather a fuzzy concept. We also calculate the limiting forms of various thermodynamical quantities, including the rate of entropy production, for a dark energy fluid near the future Big Rip singularity.

Filaments from the galaxy distribution and from the velocity field in the local universe

Abstract: The cosmic web that characterizes the large-scale structure of the Universe can be quantified by a variety of methods. For example, large redshift surveys can be used in combination with point process algorithms to extract long curvilinear filaments in the galaxy distribution. Alternatively, given a full 3D reconstruction of the velocity field, kinematic techniques can be used to decompose the web into voids, sheets, filaments and knots. In this Letter, we look at how two such algorithms – the Bisous model and the velocity shear web – compare with each other in the local Universe (within 100 Mpc), finding good agreement. This is both remarkable and comforting, given that the two methods are radically different in ideology and applied to completely independent and different data sets. Unsurprisingly, the methods are in better agreement when applied to unbiased and complete data sets, like cosmological simulations, than when applied to observational samples. We conclude that more observational data is needed to improve on these methods, but that both methods are most likely properly tracing the underlying distribution of matter in the Universe.

The quantum realm of the "Little Sibling" of the Big Rip singularity

Abstract: We analyse the quantum behaviour of the ``Little Sibling'' of the Big Rip singularity (LSBR) [1]. The quantisation is carried within the geometrodynamical approach given by the Wheeler-DeWitt (WDW) equation. The classical model is based on a Friedmann-Lemaitre-Robertson-Walker Universe filled by a perfect fluid that can be mapped to a scalar field with phantom character. We analyse the WDW equation in two setups. In the first step, we consider the scale factor as the single degree of freedom, which from a classical perspective parametrises both the geometry and the matter content given by the perfect fluid. We then solve the WDW equation within a WKB approximation, for two factor ordering choices. On the second approach, we consider the WDW equation with two degrees of freedom: the scale factor and a scalar field. We solve the WDW equation, with the Laplace-Beltrami factor-ordering, using a Born-Oppenheimer approximation. In both approaches, we impose the DeWitt (DW) condition as a potential criterion for singularity avoidance. We conclude that in all the cases analysed the DW condition can be verified, which might be an indication that the LSBR can be avoided or smoothed in the quantum approach.

On the stability of Einstein static universe in doubly general relativity scenario

Abstract: By presenting a relation between the average energy of the ensemble of probe photons and the energy density of the universe, in the context of gravity’s rainbow or the doubly general relativity scenario, we introduce a rainbow FRW universe model. By analyzing the fixed points in the flat FRW model modified by two well-known rainbow functions, we find that the finite time singularity avoidance (i.e. Big Bang) may still remain as a problem. Then we follow the “emergent universe” scenario in which there is no beginning of time and consequently there is no Big-Bang singularity. Moreover, we study the impact of high energy quantum gravity modifications related to the gravity’s rainbow on the stability conditions of an “Einstein static universe” (ESU). We find that independent of the particular rainbow function, the positive energy condition dictates a positive spatial curvature for the universe. In fact, without raising a nonphysical energy condition in the quantum gravity regimes, we can observe agreement between gravity’s rainbow scenario and the basic assumption of the modern version of the “emergent universe”. We show that in the absence and presence of an energy-dependent cosmological constant $$\Lambda (\epsilon )$$ , a stable Einstein static solution is available versus the homogeneous and linear scalar perturbations under the variety of the obtained conditions. Also, we explore the stability of ESU against the vector and tensor perturbations.

Future dynamics in \(f(R)\) theories

Abstract: The $$f(R)$$ gravity theories provide an alternative way to explain the current cosmic acceleration without invoking a dark energy matter component used in the cosmological modeling in the framework of general relativity. However, the freedom in the choice of the functional forms of $$f(R)$$ gives rise to the problem of the degeneracy among these gravity theories on theoretical and (or) observational grounds. In this paper we examine the question as to whether the future dynamics can be used to break the degeneracy between $$f(R)$$ gravity theories by investigating the dynamics of spatially homogeneous and isotropic dust flat models in two $$f(R)$$ gravity theories, namely the well-known $$f(R) = R + \alpha R^{n}$$ gravity and another by Aviles et al., whose motivation comes from the cosmographic approach to $$f(R)$$ gravity. We perform a detailed numerical study of the dynamics of these theories taking into account the recent constraints on the cosmological parameters made by the Planck Collaboration. We demonstrate that besides being useful for discriminating between these two $$f(R)$$ gravity theories, the future dynamics technique can also be used to determine the finite-time behavior as well as the fate of the Universe in the framework of these $$f(R)$$ gravity theories. There also emerges from our analysis the result that one still can have a dust flat FLRW solution with a big rip, if gravity is governed by $$f(R) = R + \alpha R^n $$ . We also show that FLRW dust solutions with $$f''<0$$ do not necessarily lead to singularities.

Impacts of different SNLS3 light-curve fitters on cosmological consequences of interacting dark energy models

Abstract: We explore the cosmological consequences of interacting dark energy (IDE) models using the SNLS3 supernova samples. In particular, we focus on the impacts of different SNLS3 light-curve fitters (LCF) (corresponding to "SALT2", "SiFTO", and "Combined" sample). Firstly, making use of the three SNLS3 data sets, as well as the Planck distance priors data and the galaxy clustering data, we constrain the parameter spaces of three IDE models. Then, we study the cosmic evolutions of Hubble parameter $H(z)$, deceleration diagram $q(z)$, statefinder hierarchy $S^{(1)}_3(z)$ and $S^{(1)}_4(z)$, and check whether or not these dark energy diagnosis can distinguish the differences among the results of different SNLS3 LCF. At last, we perform high redshift cosmic age test using three old high redshift objects (OHRO), and explore the fate of the Universe. We find that, the impacts of different SNLS3 LCF are rather small, and can not be distinguished by using $H(z)$, $q(z)$, $S^{(1)}_3(z)$, $S^{(1)}_4(z)$, and the age data of OHRO. In addition, we infer, from the current observations, how far we are from a cosmic doomsday in the worst case, and find that the "Combined" sample always gives the largest 2$\sigma$ lower limit of the time interval between "big rip" and today, while the results given by the "SALT2" and the "SiFTO" sample are close to each other. These conclusions are insensitive to a specific form of dark sector interaction. Our method can be used to distinguish the differences among various cosmological observations.

Cosmology with nonminimal kinetic coupling and a Higgs-like potential

Abstract: We consider cosmological dynamics in the theory of gravity with the scalar field possessing the nonminimal kinetic coupling to curvature given as κGμν,μ,ν, and the Higgs-like potential . Using the dynamical system method, we analyze stationary points, their stability, and all possible asymptotical regimes of the model under consideration. We show that the Higgs field with the kinetic coupling provides an existence of accelerated regimes of the Universe evolution. There are three possible cosmological scenarios with acceleration: (i) The late-time de Sitter epoch when the Hubble parameter tends to the constant value, as t → ∞, while the scalar field tends to zero, 0(t)→ , so that the Higgs potential reaches its local maximum . (ii) The Big Rip when H(t)~(t*−t)−1 → ∞ and (t)~(t*−t)−2 → ∞ as t → t*. (iii) The Little Rip when H(t)~ t1/2 → ∞ and (t)~ t1/4 → ∞ as t → ∞. Also, we derive modified slow-roll conditions for the Higgs field and demonstrate that they lead to the Little Rip scenario.

Stability of cylindrical thin shell wormhole during evolution of universe from inflation to late time acceleration

Abstract: In this paper, we consider the stability of cylindrical wormholes during evolution of universe from inflation to late time acceleration epochs. We show that there are two types of cylindrical wormholes. The first type is produced at the corresponding point where k black F-strings are transited to BIon configuration. This wormhole transfers energy from extra dimensions into our universe, causes inflation, loses it’s energy and vanishes. The second type of cylindrical wormhole is created by a tachyonic potential and causes a new phase of acceleration. We show that wormhole parameters grow faster than the scale factor in this era, overtake it at ripping time and lead to the destruction of universe at big rip singularity.

The Bohmian Approach to the Problems of Cosmological Quantum Fluctuations

Abstract: There are two kinds of quantum fluctuations relevant to cosmology that we focus on in this article: those that form the seeds for structure formation in the early universe and those giving rise to Boltzmann brains in the late universe. First, structure formation requires slight inhomogeneities in the density of matter in the early universe, which then get amplified by the effect of gravity, leading to clumping of matter into stars and galaxies. According to inflation theory, quantum fluctuations form the seeds of these inhomogeneities. However, these quantum fluctuations are described by a quantum state which is homogeneous and isotropic, and this raises a problem, connected to the foundations of quantum theory, as the unitary evolution alone cannot break the symmetry of the quantum state. Second, Boltzmann brains are random agglomerates of particles that, by extreme coincidence, form functioning brains. Unlikely as these coincidences are, they seem to be predicted to occur in a quantum universe as vacuum fluctuations if the universe continues to exist for an infinite (or just very long) time, in fact to occur over and over, even forming the majority of all brains in the history of the universe. We provide a brief introduction to the Bohmian version of quantum theory and explain why in this version, Boltzmann brains, an undesirable kind of fluctuation, do not occur (or at least not often), while inhomogeneous seeds for structure formation, a desirable kind of fluctuation, do.

Entropy and the Typicality of Universes

Abstract: The universal validity of the second law of thermodynamics is widely attributed to a finely tuned initial condition of the universe. This creates a problem: why is the universe atypical? We suggest that the problem is an artefact created by inappropriate transfer of the traditional concept of entropy to the whole universe. Use of what we call the relational $N$-body problem as a model indicates the need to employ two distinct entropy-type concepts to describe the universe. One, which we call entaxy, is novel. It is scale-invariant and decreases as the observable universe evolves. The other is the algebraic sum of the dimensionful entropies of branch systems (isolated subsystems of the universe). This conventional additive entropy increases. In our model, the decrease of entaxy is fundamental and makes possible the emergence of branch systems and their increasing entropy. We have previously shown that all solutions of our model divide into two halves at a unique `Janus point' of maximum disorder. This constitutes a common past for two futures each with its own gravitational arrow of time. We now show that these arrows are expressed through the formation of branch systems within which conventional entropy increases. On either side of the Janus point, this increase is in the same direction in every branch system. We also show that it is only possible to specify unbiased solution-determining data at the Janus point. Special properties of these `mid-point data' make it possible to develop a rational theory of the typicality of universes whose governing law, as in our model, dictates the presence of a Janus point in every solution. If our self-gravitating universe is governed by such a law, then the second law of thermodynamics is a necessary direct consequence of it and does not need any special initial condition.

Quantization of Gravitational System and its Cosmological Consequences

Abstract: We have found that the hierarchial problems appearing in cosmology is a manifestation of the quantum nature of the universe. The universe is still described by the same formulae that once hold at Planck's time. The universe is found to be governed by the Machian equation, $G M=Rc^2$, where $M$ and $R$ are mass and radius of the universe. A Planck's constant for different cosmic scales is provided. The status of the universe at different stages is shown to be described in terms of the fundamental constants (eg. \textcolor{blue}{$c, \hbar, G, \Lambda, H$}) only. The concept of maximal (minimal) acceleration, power, temperature, etc., is introduced and justified. The electromagnetic interactions are shown to be active at a cosmic level. Their contribution would exclude the inclusion of dark energy in cosmology.

Gauss-Bonnet modified gravity models with bouncing behavior

Abstract: The following issue is addressed: how the addition of a Gauss-Bonnet term (generically coming from most fundamental theories, as string and M theories), to a viable model, can change the specific properties, and even the physical nature, of the corresponding cosmological solutions? Specifically, brand new original dark energy models are obtained in this way with quite interesting properties, which exhibit, in a unified fashion, the three distinguished possible cosmological phases corresponding to phantom matter, quintessence, and ordinary matter, respectively. A model, in which the equation of state parameter, $w$, is a function of time, is seen to lead either to a singularity of the Big Rip kind or to a bouncing solution which evolves into a de Sitter universe with $w=-1$. Moreover, new Gauss-Bonnet modified gravity models with bouncing behavior in the early stages of the universe evolution are obtained and tested for the validity and stability of the corresponding solutions. They allow for a remarkably natural, unified description of a bouncing behavior at early times and accelerated expansion at present.

Viscosity-Induced Crossing of the Phantom Barrier

Abstract: We show explicitly, by using astrophysical data plus reasonable assumptions for the bulk viscosity in the cosmic fluid, how the magnitude of this viscosity may be high enough to drive the fluid from its position in the quintessence region at present time $t=0$ across the barrier $w=-1$ into the phantom region in the late universe. The phantom barrier is accordingly not a sharp mathematical divide, but rather a fuzzy concept. We also calculate the limiting forms of various thermodynamical quantities, including the rate of entropy production, for a dark energy fluid near the future Big Rip singularity.

Cosmological Models Coupled with Dark Matter in a Dissipative Universe

Abstract: We consider the cosmological system with two interacting fluids: dark energy and dark matter, in a homogeneous and isotropic universe with dissipation. The modified gravitational equation for dark matter is solved. The analytic representations for the Little Rip, the Pseudo Rip, and the bounce cosmology models with dissipation are obtained in terms of the thermodynamic parameters in the equation of state. We analyze the corrections in the energy density for dark matter, in view of the dissipative processes and the coupling with dark energy.

Dynamic wormholes with particle creation mechanism

Abstract: The present work deals with a spherically symmetric space–time which is asymptotically (at spatial infinity) FRW space–time and represents wormhole configuration: The matter component is divided into two parts—(a) dissipative but homogeneous and isotropic fluid, and (b) an inhomogeneous and anisotropic barotropic fluid. Evolving wormhole solutions are obtained when isotropic fluid is phantom in nature and there is a big rip singularity at the end. Here the dissipative phenomena is due to the particle creation mechanism in non-equilibrium thermodynamics. Using the process to be adiabatic, the dissipative pressure is expressed linearly to the particle creation rate. For two choices of the particle creation rate as a function of the Hubble parameter, the equation of state parameter of the isotropic fluid is constrained to be in the phantom domain, except in one choice, it is possible to have wormhole configuration with normal isotropic fluid.

Role of higher-dimensional evolving wormholes in the formation of a big rip singularity

Abstract: We study the four-dimensional Universe on the M2-M5 BIon in the thermal background. The BIon is a configuration in a flat space of a D-brane and a parallel anti-D-brane connected by a wormhole. When the branes and antibranes are well separated and the brane’s spike is far from the antibrane’s spike, the wormhole cannot be formed. However, when two branes are close to each other, they can be connected by a wormhole. Under this condition, there exist many channels for flowing energy from extra dimensions into our Universe. This energy dominates all other forms of energy, such as the gravitational repulsion, and brings our brief epoch of the Universe to an end in the big rip singularity. We show that at this singularity the Universe is destroyed, and one black M2-brane is formed. Finally, we test our model against WMAP, Planck, and BICEP2 data, and we obtain the ripping time. According to experimental data, the N ≃ 50 case leads to n s ≃ 0.96 , where N and n s are the number e -folds and the spectral index, respectively. This standard case may be found in 0.01 R Tensor-scalar 0.3 , where R Tensor-scalar is the tensor-to-scalar ratio. At this point, the big rip singularity occurs in a finite time t rip = 31 ( Gyr ) for WMAP and Planck data and t rip = 28 ( Gyr ) for BICEP2 data. By comparing this time with the time of the big rip in the brane-antibrane, we find that the wormhole in the BIonic system accelerates the destruction of the Universe.

FRW viscous cosmology with inhomogeneous equation of state and future singularity

Abstract: A universe media is considered as a bulk viscosity described by inhomogeneous equation of state (EOS) of the form p = (γ − 1)ρ + Λ(t), where Λ(t) is a time-dependent parameter. A generalized dynamical equation for the scale factor of the universe is proposed to describe the cosmological evolution, in which we assume the bulk viscosity and time-dependent parameter Λ are linear combination of two terms of the form: ζ = ζ0 + ζ1H and Λ(t) = Λ0 + Λ1H, i.e.one is constant and other is proportional to Hubble parameter H = ȧ a. In this framework, we demonstrate that this model can be used to explain the dark energy dominated universe, and the inhomogeneous term of specific form introduced in EOS, may lead to three kinds of fates of cosmological evolution: no future singularity, big rip or TypeIII singularity as presented by [S. Nojiri and S. D. Odintsov, Phys. Rev. D 72, 023003 (2005)].

Cosmology in time asymmetric extensions of general relativity

Abstract: We investigate the cosmological behavior in a universe governed by time asymmetric extensions of general relativity, which is a novel modified gravity based on the addition of new, time-asymmetric, terms on the Hamiltonian framework, in a way that the algebra of constraints and local physics remain unchanged. Nevertheless, at cosmological scales these new terms can have significant effects that can alter the universe evolution, both at early and late times, and the freedom in the choice of the involved modification function makes the scenario able to produce a huge class of cosmological behaviors. For basic ansatzes of modification, we perform a detailed dynamical analysis, extracting the stable late-time solutions. Amongst others, we find that the universe can result in dark-energy dominated, accelerating solutions, even in the absence of an explicit cosmological constant, in which the dark energy can be quintessence-like, phantom-like, or behave as an effective cosmological constant. Moreover, it can result to matter-domination, or to a Big Rip, or experience the sequence from matter to dark energy domination. Additionally, in the case of closed curvature, the universe may experience a cosmological bounce or turnaround, or even cyclic behavior. Finally, these scenarios can easily satisfy the observational and phenomenological requirements. Hence, time asymmetric cosmology can be a good candidate for the description of the universe.