Showing papers on "Big Rip published in 2021"

A $w$ phantom transition at $z_t<0.1$ as a resolution of the Hubble tension

Abstract: A rapid transition of the dark energy equation of state parameter $w$ at a transition redshift $z_t z_t$ to $w z_t$. Such a Late $w$ Phantom Transition ($LwPT$) avoids the discontinuity of $H(z)$ suggested in previous studies and thus does not require a step in the Pantheon Hubble diagram which is strongly constrained. We demonstrate that such an ultra low $z$ abrupt feature of $w(z)$ provides a better fit to cosmological data compared to smooth late time deformations of $H(z)$ that also address the Hubble tension. The strongly present day phantom dark energy behavior implied by this class of models hints towards a rapid approach of a Big Rip singularity which for $z_t=0.02$ will rip the universe in less than 3.5 billion years. Early hints of such effect may be observable in the dynamics of the nearest and largest bound systems ({\it e.g.} Virgo structures). The $LwPT$ can be generically induced by a phantom scalar field frozen by Hubble friction mimicking the cosmological constant and currently entering its ghost instability phase as Hubble friction decreases below the field dynamical scale.

Negative cosmological constant in the dark sector

Abstract: We consider the possibility that the dark sector of our Universe contains a negative cosmological constant dubbed λ. For such models to be viable, the dark sector should contain an additional component responsible for the late-time accelerated expansion rate (X). We explore the departure of the expansion history of these models from the concordanceΛ cold dark matter (ΛCDM) model. For a large class of our models, the accelerated expansion is transient with a nontrivial dependence on the model parameters. All models with wX>−1 will eventually contract and we derive an analytical expression for the scale factor a(t) in the neighborhood of its maximal value. We find also the scale factor for models ending in a Big Rip in the regime where dustlike matter density is negligible compared to λ. We address further the viability of such models, in particular when a high H0 is taken into account. While we find no decisive evidence for a nonzero λ, the best models are obtained with a phantom behavior on redshifts z≳1 with a higher evidence for nonzero λ. An observed value for h substantially higher than 0.70 would be a decisive test of their viability.

Rip cosmological models in extended symmetric teleparallel gravity.

Abstract: In this paper we have investigated some rip cosmological models in an extended symmetric teleparallel gravity theory. We consider the form $f(Q,T)=aQ^m+bT$ in the Einstein-Hilbert action and expressed the field equations and the dynamical parameters in terms of the non-metricity $Q$. Three rip models such as Little Rip, Big Rip and Pseudo Rip are presented. The energy conditions and the cosmographic parameters are derived and analysed for all these models.

Cosmological dynamics and bifurcation analysis of the general non-minimally coupled scalar field models

Abstract: Non-minimally coupled scalar field models are well-known for providing interesting cosmological features. These include a late time dark energy behavior, a phantom dark energy evolution without singularity, an early time inflationary universe, scaling solutions, convergence to the standard $\Lambda$CDM, etc. While the usual stability analysis helps us determine the evolution of a model geometrically, bifurcation theory allows us to precisely locate the parameters' values describing the global dynamics without a fine-tuning of initial conditions. Using the center manifold theory and bifurcation analysis, we show that the general model undergoes a transcritical bifurcation, which predicts us to tune our models to have certain desired dynamics. We obtained a class of models and a range of parameters capable of describing a cosmic evolution from an early radiation era towards a late time dark energy era over a wide range of initial conditions. There is also a possible scenario of crossing the phantom divide line. We also find a class of models where the late time attractor mechanism is indistinguishable from that of a structurally stable general relativity based model; thus, we can elude the big rip singularity generically. Therefore, bifurcation theory allows us to select models that are viable with cosmological observations.

Cosmological aspects of f(R, T) gravity in a simple model with a parametrization of q

Abstract: In this paper, we have considered a quadratic variation of the deceleration parameter (q) as a function of cosmic time (t) which describes a smooth transition from the decelerating phase of the Universe to an accelerating one and also show some distinctive feature from the standard model. The logical move of this article is against the behavior of the future Universe, i.e., whether the Universe expands forever or ends with a Big Rip, and we observe that the outcome of the considered parametrization comes in favor of Big Rip future of the Universe. The whole setup of the parametrization and solution is taken in f(R, T) theory of gravity for a spatially flat Friedmann–Lemaitre–Robertson–Walker (FLRW) geometry. Furthermore, we have considered the functional form of f(R, T) function as $$ f(R)+f(T)$$ , where a quadratic correction of the geometric term R is adopted as the function f(R), and a linear matter term f(T). We have investigated some features of the model by examining the behavior of physical parameters. Our primary goal here is to discuss the physical dynamics of the model in f(R, T) gravity. We have found that the EoS parameter also has the same singularity as that of the Hubble parameter, i.e., at the initial phase and at the Big Rip. The EoS parameter is explored in some detail for our choice of f(R, T) function considered here. Different cases for f(R, T) functional form for different values of the coupling parameters are discussed, and the evolution of the physical parameters is shown graphically.

Big Rip and Big Crunch Cosmological Models in a Gravitational Field with Torsion

Abstract: The gravitational field with torsion is being constructed by using the parametrized absolute parallelism geometry. A generalized law of variation of Hubble’s parameter in evolutionary cosmological models is used. The cosmological models under the influence of the gravitational field with torsion are obtained and discussed. A new model of the Universe is presented using a special class of Riemann–Cartan geometry. This model is oscillating with expansion and contraction at different stages. It behaves normally as the conventional Big Bang model in the first half-age until it reaches the moment of a Big Rip, then reverses its behavior as a result of a changes in the pressure and torsion until it reaches a Big Crunch at the end of the second half-age. We suppose that the Big Rip singularity is replaced by a regular maximum of the scale factor at the Big Rip due to a possible physical mechanism of quantum nature. The positivity condition for the energy density of matter leads to exclusion of open and closed universes.

Dark energy and cosmological horizon thermal effects

Abstract: We investigate various dark energy models by taking into account the thermal effects induced from Hawking radiation on the apparent horizon of the Universe, for example, near a finite-time future singularity. If the dark energy density increases as the Universe expands, the Universe's evolution reaches a type II singularity (or sudden future singularity). The second derivative of the scale factor diverges, but the first derivative remains finite. Quasi--de Sitter evolution can change to sudden future singularity in the case of with an effective cosmological constant larger than the maximum possible value of the energy density of the Universe. Another interesting feature of the cosmological solution is the possibility of a transition between deceleration and acceleration for quintessence dark energy with a simple equation of state. Finally, we investigate which fluid component can remedy big rip singularities and other crushing-type singularities.

Cosmological Models with Big Rip and Pseudo Rip Scenarios in Extended Theory of Gravity

Abstract: In this paper, we have presented the big rip and pseudo rip cosmological models in an extended theory of gravity. The matter field is considered to be that of perfect fluid. The geometrical parameters are adjusted in such a manner that it matches the prescriptions given by cosmological observations, to be specific to the range of Hubble tension $(H_{0})$. The models favor phantom behavior. The violation of strong energy conditions are shown in both the models, as it has become essential in an extended gravity. The representative values of the coupling parameter are significant on the evolution of the universe.

Little rip in classical and quantum f ( R ) cosmology

Abstract: The little rip is a cosmological abrupt event predicted by some phantom dark energy models that could describe the future evolution of our Universe. This event can be interpreted as a big rip singularity delayed indefinitely, although in those models bounded structures will be destroyed in a finite cosmic time in the future. In this work, we analyze the little rip cosmology from a classical and quantum point of view within the scheme of alternative metric $f(R)$ theories of gravity. The quantum analysis is performed in the framework of $f(R)$ quantum geometrodynamics by means of the modified Wheeler-DeWitt equation. In this context, we show that the DeWitt criterion can be satisfied. Similar to what happens in general relativity, this result points toward the avoidance of the little rip in $f(R)$ quantum cosmology.

Classical and Quantum f(R) Cosmology: The Big Rip, the Little Rip and the Little Sibling of the Big Rip

Abstract: The big rip, the little rip and the little sibling of the big rip are cosmological doomsdays predicted by some phantom dark-energy models that could describe the future evolution of our universe. When the universe evolves towards either of these future cosmic events, all bounded structures and, ultimately, space–time itself are ripped apart. Nevertheless, it is commonly believed that quantum gravity effects may smooth or even avoid these classically predicted singularities. In this review, we discuss the classical and quantum occurrence of these riplike events in the scheme of metric f(R) theories of gravity. The quantum analysis is performed in the framework of f(R) quantum geometrodynamics. In this context, we analyze the fulfilment of the DeWitt criterion for the avoidance of these singular fates. This review contains as well new unpublished work (the analysis of the equation of state for the phantom fluid and a new quantum treatment of the big rip and the little sibling of the big rip events).

Cosmological evolution with quadratic gravity and nonideal fluids

Abstract: Some cosmological models based on the gravitational theory $$f(R)=R+\zeta R^2$$ , and on fluids obeying to the equations of state of Redlich–Kwong, Berthelot, and Dieterici are proposed for describing smooth transitions between different cosmic epochs. A dynamical system analysis reveals that these models contain fixed points which correspond to an inflationary, a radiation dominated and a late-time accelerating epoch, and a nonsingular bouncing solution, the latter being an asymptotic fixed point of the compactified phase space. The infinity of the compactified phase space is interpreted as a region in which the non-ideal behaviors of the previously mentioned cosmic fluids are suppressed. Physical constraints on the adopted dimensionless variables are derived by demanding the theory to be free from ghost and tachyonic instabilities, and a novel cosmological interpretation of such variables is proposed through a cosmographic analysis. The different effects of the equation of state parameters on the number of equilibrium solutions and on their stability nature are clarified. Some generic properties of these models, which are not sensitive to the particular fluid considered, are identified, while differences are critically examined by showing that the Redlich–Kwong scenario admits a second radiation-dominated epoch and a Big Rip Singularity.

Cosmological models generated by utilizing a varying polynomial deceleration parameter

Abstract: The current paper is concerned with extracting a novel law for a varying polynomial deceleration parameter. This law covers each of Berman’s laws, a linearly varying deceleration parameter, a periodic Universe with a varying deceleration parameter of the second degree, and many other models. Additionally, this novel law tells us about the behavior of the Universe. Furthermore, it gives appropriate results with data (SNIa data and CMB shift), particularly of the late time of the Universe. In accordance with this law, one can expect several patterns in the evolution of the Universe like Big Rip, Big Crunch, periodic Universe, and dark energy.

Three Impossible Theories

Abstract: I will begin by conjecturing a cosmological generalization of black hole complementarity (also known as the central dogma). I will then discuss three theories and argue that they are inconsistent with second law of thermodynamics if the cosmological version of the dogma is correct. The three theories are: the big rip; cyclic cosmology; and the Farhi-Guth-Guven mechanism for creating inflating universes behind black hole horizons.

Non-local gravity in bouncing cosmology scenarios

Abstract: In this work, we analyzed the improved Deser-Woodard non-local gravity over the background of five different bouncing cosmologies, whose premise is to avoid the initial singular state of the universe. We developed the numerical solution for the non-local distortion function, which encompass the modifications to the Einstein-Hilbert action, using the reconstruction procedure and we have found that three classes of bouncing have a viable cosmological solution. Afterwards, we discussed the physical aspects and outcomes of the evolution of the distortion function throughout the bouncing point for three physical classes specifically: the symmetric bounce, the super bounce and the Big Rip universe bounce.

Cosmological constraints of interacting phantom dark energy models

Abstract: In this paper, we consider three phantom dark energy models, in the context of interaction between the dark components namely cold dark matter (CDM) and dark energy (DE). The first model, known as $w_{\textrm{d}}$CDM can induce a big rip singularity (BR) while the two remaining induce future abrupt events known as the Little Rip (LR) and Little Sibling of the Big Rip (LSBR). These phantom DE models can be distinguished by their equation of state. We invoke a new phenomenon such as the interaction between CDM and DE given that it could solve or alleviate some of the problems encountered in standard cosmology. We aim to find out the effect of such an interaction on the cosmological parameters of the studied models, as well as, the persistence or the disappearance of the singularity and the abrupt events induced by the models under study. We choose an interaction term proportional to DE density, i.e. $Q=\lambda H \rho_{\textrm{d}}$, since the case where $Q\propto \rho_{\textrm{m}}$ could lead to a large scale instability at early time. We also do not claim at all that $Q=\lambda H \rho_{\textrm{d}}$ is the ideal choice since it suffers from a negative CDM density in the future. By the use of a Markov Chain Monte Carlo (MCMC) approach, and by assuming a flat FLRW Universe, we constrain the cosmological parameters of each of the three phantom DE models studied. Furthermore, by the aid of the corrected Akaike Information Criterion ($\text{AIC}_{c}$) tool, we compare our phantom DE models. Finally, a perturbative analysis of phantom DE models under consideration is performed based on the best fit background parameters.

Phantom-like dark energy from quantum gravity

Abstract: We analyse the emergent cosmological dynamics corresponding to the mean field hydrodynamics of quantum gravity condensates, in the tensorial group field theory formalism. We focus in particular on the cosmological effects of fundamental interactions, and on the contributions from different quantum geometric modes. The general consequence of such interactions is to produce an accelerated expansion of the universe, which can happen both at early times, after the quantum bounce predicted by the model, and at late times. Our main result is that, while this fails to give a compelling inflationary scenario in the early universe, it produces naturally a phantom-like dark energy dynamics at late times, compatible with cosmological observations. By recasting the emergent cosmological dynamics in terms of an effective equation of state, we show that it can generically cross the phantom divide, purely out of quantum gravity effects without the need of any additional phantom matter. Furthermore, we show that the dynamics avoids any Big Rip singularity, approaching instead a de Sitter universe asymptotically.

Harnessing Dark Energy in Five-dimensional Brans-Dicke Universe

Abstract: In trying to explain the present accelerated expansion of the universe in the light of a five-dimensional Brans-Dicke theory, it is found that the fifth dimension itself here acts as a source of dark energy. It may be taken as a curvature-induced form of dark energy, in one case of which it behaves similar to that form of dark energy arising out of the cosmological constant which is the most commonly accepted form of dark energy. It is also found that this new type of dark energy is free from big rip singularity, and may be taken as a viable form of dark energy which can explain some of physical mysteries of the universe.

New futures for cosmological models.

Abstract: The discovery of accelerated expansion of the universe opened the possibility of new scenarios for the doom of our spacetime, besides aeternal expansion and a final contraction. In this paper we review the chances which may await our universe. In particular, there are new possible singular fates (sudden singularities, big rip...), but there also other evolutions which cannot be considered as singular. In addition to this, some of the singular fates are not strong enough in the sense that the spacetime can be extended beyond the singularity. For deriving our results we make use of generalised power and asymptotic expansions of the scale factor of the universe.

Torsion and shear effect on a Big Rip model in a gravitational field

Abstract: In this paper, we investigate the effects of torsion and shears within the framework of the gravitational field with torsion in early and late cosmology. General relativity and torsion field equations are constructed using absolute parallel geometry. The Big Rip model of the Universe has been presented using a special class of Riemann–Cartan geometry and the law of variation of Hubble’s parameter. The model does not depend on the curvature constant. The positive condition of the energy density of the matter is satisfied in this model. This cosmological model shows that the torsion and shear effect is strong at the beginning of the Big Bang and at the end of the universe. Through the examination of precise cases of the parameters and initial conditions, we can show that for suitable ranges of the parameters, the dynamic torsion scalar model can exhibit features similar to those of the currently observed accelerating universe. The relationship between the torsion and shear scalars is investigated, and their impact on the accelerating universe is addressed apart from the idea of dark energy.

Cosmological dynamics and bifurcation analysis of the general non-minimal coupled scalar field models

Abstract: Non-minimal coupled scalar field models are well-known for providing interesting cosmological features. These include a late-time dark energy behavior, a phantom dark energy evolution without singularity, an early-time inflationary Universe, scaling solutions, convergence to the standard $$\Lambda $$ CDM, etc. While the usual stability analysis helps us determine the evolution of a model geometrically, bifurcation theory allows us to precisely locate the parameters’ values describing the global dynamics without a fine-tuning of initial conditions. Using the center manifold theory and bifurcation analysis, we show that the general model undergoes a transcritical bifurcation, predicting us to tune our models to have certain desired dynamics. We obtained a class of models and a range of parameters capable of describing a cosmic evolution from an early radiation era towards a late time dark energy era over a wide range of initial conditions. There is also a possible scenario of crossing the phantom divide line. We also find a class of models where the late time attractor mechanism is indistinguishable from a structurally stable general relativity-based model; thus, we can elude the big rip singularity generically. Therefore, bifurcation theory allows us to select models that are viable with cosmological observations.

Cosmological models with Big rip and Pseudo rip Scenarios in extended theory of gravity

Abstract: In this paper, we have presented the big rip and pseudo rip cosmological models in an extended theory of gravity. The matter field is considered to be that of perfect fluid. The geometrical parameters are adjusted in such a manner that it matches the prescriptions given by cosmological observations, to be specific to the range of Hubble tension $(H_{0})$. The models favor phantom behavior. The violation of strong energy conditions are shown in both the models, as it has become essential in an extended gravity. The representative values of the coupling parameter are significant on the evolution of the universe.

Cosmological evolution with quadratic gravity and nonideal fluids

Abstract: Some cosmological models based on the gravitational theory $f(R) = R+\zeta R^2$, and on fluids obeying to the equations of state of Redlich-Kwong, Berthelot, and Dieterici are proposed for describing smooth transitions between different cosmic epochs. A dynamical system analysis reveals that these models contain fixed points which correspond to an inflationary, a radiation dominated and a late-time accelerating epoch, and a nonsingular bouncing solution, the latter being an asymptotic fixed point of the compactified phase space. The infinity of the compactified phase space is interpreted as a region in which the non-ideal behaviors of the previously mentioned cosmic fluids are suppressed. Physical constraints on the adopted dimensionless variables are derived by demanding the theory to be free from ghost and tachyonic instabilities, and a novel cosmological interpretation of such variables is proposed through a cosmographic analysis. The different effects of the equation of state parameters on the number of equilibrium solutions and on their stability nature are clarified. Some generic properties of these models, which are not sensitive to the particular fluid considered, are identified, while differences are critically examined by showing that the Redlich-Kwong scenario admits a second radiation-dominated epoch and a Big Rip Singularity.

An Idea about Negative Cosmic Time in the Big Bang-Big Rip Cosmological Model

Abstract: In this article, we assume that the beginning of the universe was before the Big Bang. In the beginning, all matter in the universe was combined in an infinitesimal spherical shape. This sphere was compressed to an incomprehensible value for a period, and then exploded and expanded time and space. We are referring to the negative time before the Big Bang. The evolution of the universe before the Big Bang, passing through the moment of the explosion to the end of the universe at the Big Rip, has been studied. In this article, we try to answer the questions; did the universe exist before the Big Bang? What is the origin of the universe and how did it arise? What are the stages of the evolution of the universe until the moment of the Big Rip? What is the length of time for the stages of this development?

An anisotropic Kantowski-Sachs universe with radiation, dust and a phantom fluid

Abstract: In the present work, we study the dynamical evolution of an homogeneous and anisotropic KS cosmological model, considering general relativity as the gravitational theory, such that there are three different perfect fluids in the matter sector. They are radiation, dust and phantom fluid. Our main motivation is determining if the present model tends to an homogeneous and isotropic FRW model, during its evolution. Also, we want to establish how the parameters and initial conditions of the model, quantitatively, influence the isotropization of the present model. In order to simplify our task, we use the Misner parametrization of the KS metric. In terms of that parametrization the KS metric has two metric functions: the scale factor $a(t)$ and $\beta(t)$, which measures the spatial anisotropy of the model. We solve, numerically, the Einstein's equations of the model and find a solution where the universe starts to expand from a, small, initial size and continues to expand until it ends in a {\it Big Rip} singularity. We explicitly show that for the expansive solution, after same time, the universe becomes isotropic. Based on that result, we can speculate that the expansive solution may represent an initial, anisotropic, stage of our Universe, that later, due to the expansion, became isotropic.

Cosmological aspects of $f(R,T)$ gravity in a simple model with a parametrization of $q$

Abstract: In this paper, we have considered a quadratic variation of the deceleration parameter ($q$) as a function of cosmic time ($t$) which describes a smooth transition from the decelerating phase of the Universe to an accelerating one and also show some distinctive feature from the standard model. The logical move of this article is against the behavior of the future Universe, \textit{i.e.} whether the Universe expands forever or ends with a Big Rip, and we observe that the outcome of the considered parametrization comes in favor of Big Rip future of the Universe. The whole set up of the parametrization and solution is taken in $f(R,T)$ theory of gravity for a spatially flat Friedmann-Lema\^itre-Robertson-Walker (FLRW) geometry. Furthermore, we have considered the functional form of $f(R,T)$ function as $% f(R)+f(T)$, where a quadratic correction of the geometric term $R$ is adopted as the function $f(R)$, and a linear matter term $f(T)$. We have investigated some features of the model by examining the behavior of physical parameters. Our primary goal here is to discuss the physical dynamics of the model in $f(R,T)$ gravity. We have found, the EoS parameter also has the same singularity as that of the Hubble parameter \textit{i.e.} at the initial phase and at the Big Rip. The EoS parameter is explored in some detail for our choice of $f(R,T)$ function considered here. Different cases for $f(R,T)$ functional form for different values of the coupling parameters are discussed, and the evolution of the physical parameters is shown graphically.

Classical and quantum $f(R)$ cosmology: The big rip, the little rip and the little sibling of the big rip

Abstract: The big rip, the little rip and the little sibling of the big rip are cosmological doomsdays predicted by some phantom dark energy models that could describe the future evolution of our Universe. When the universe evolves towards either of these future cosmic events all bounded structures and, ultimately, space-time itself are ripped apart. Nevertheless, it is commonly belief that quantum gravity effects may smooth or even avoid these classically predicted singularities. In this review, we discuss the classical and quantum occurrence of these riplike events in the scheme of metric $f(R)$ theories of gravity. The quantum analysis is performed in the framework of $f(R)$ quantum geometrodynamics. In this context, we analyse the fulfilment of the DeWitt criterion for the avoidance of these singular fates.