Showing papers on "Big Rip published in 2020"

Lagrangian formulation of Omoriis law and analogy with the cosmic Big Rip

Abstract: A recent model predicting Omori's law giving the number of aftershocks per unit time following an earthquake involves a differential equation analogous to the Friedmann equation of cosmology. The before-shock phase is analogous to an accelerating universe approaching a Big Rip, the main shock to the Big Rip singularity, and the aftershock to a contracting universe. The analogy provides some physical intuition and Lagrangian and Hamiltonian formulations for Omori's law and its generalizations.

Lagrangian formulation of Omori’s law and analogy with the cosmic Big Rip

Abstract: A recent model predicting Omori’s law giving the number of aftershocks per unit time following an earthquake involves a differential equation analogous to the Friedmann equation of cosmology. The beforeshock phase is analogous to an accelerating universe approaching a Big Rip, the main shock to the Big Rip singularity, and the aftershock to a contracting universe. The analogy provides some physical intuition and Lagrangian and Hamiltonian formulations for Omori’s law and its generalizations.

$$f(\mathcal {R},\varphi ,\chi )$$f(R,φ,χ) cosmology with Noether symmetry

Abstract: This paper is devoted to explore modified $$f(\mathcal {R})$$ theories of gravity using Noether symmetry approach. For this purpose, Friedmann–Robertson–Walker spacetime is chosen to investigate the cosmic evolution. The study is mainly divided into two parts: Firstly Noether symmetries of metric $$f(\mathcal {R})$$ gravity are revisited and some new class of solutions with the help of conserved quantities are reported. It is shown that different scenarios of cosmic evolution can be discussed using Noether symmetries and one of the case indicates the chances for the existence of Big Rip singularity. Secondly, $$f(\mathcal {R})$$ theory coupled with scalar field has been discussed in detail. The Noether equations of modified gravity are reported with three subcases for flat Friedmann–Robertson–Walker universe. It is concluded that conserved quantities are quite helpful to find some important exact solutions in the cosmological contexts. Moreover, the scalar field involved in the modified gravity plays a vital role in the cosmic evolution and an accelerated expansion phase can be observed for some suitable choices of $$f(\mathcal {R},\varphi ,\chi )$$ gravity models.

Tilted universe with big rip singularity in Lyra geometry

Abstract: We have examined tilted cosmological models by using conformally flat space-time with wet dark fluid in Lyra geometry In order to solve the field equations we have considered a power law In this

No evidence of beyond-soft-corrections to the Born-Infeld paradigm in the cosmic history

Abstract: The Dieterici fluid model can be applied for describing the growth of the dark energy amount and the expenses of dark matter leading to a transition between decelerating and accelerating phases of the Universe. This may challenge the Generalized Chaplygin Gas paradigm. However, in this paper we will show that the cosmic chronometers data do not favor the Dieterici proposal because of the tension between the estimates of the cosmological parameters when different redshift ranges of the cosmic history of the universe are considered, and because the model would be affected by a Big Rip singularity. Our analysis relies on a Bayesian Machine Learning technique which allows us to place tight constraints on our forecast.

Cosmic expansion with matter creation and bulk viscosity

Abstract: We explore the cosmological implications at effective level of matter creation effects in a dissipative fluid for a Friedmann-Lemaitre-Robertson-Walker geometry; we also perform a statistical analysis for this kind of model. By considering an inhomogeneous ansatz for the particle production rate, which we obtain for a created matter of dark matter type, we can have a quintessence scenario or a future singularity known as a little rip; this is in dependence of the value of a constant parameter, $\ensuremath{\eta}$, which characterizes the matter production effects. The dimensionless age of this kind of universe is computed, showing that this number is greater than the standard cosmology value; this is typical of universes with the presence of dark energy. The inclusion of baryonic matter is studied. We implement the construction of the particle production rate for a dissipative fluid by considering two approaches for the expression of the bulk viscous pressure: we find that, in the Eckart model, we have a big rip singularity leading to a catastrophic matter production, and in the truncated version of the Israel-Stewart model this rate remains bounded, which leads to a quintessence scenario. For a nonadiabatic dissipative fluid, we obtain a positive temperature, and the cosmic expansion obeys the second law of thermodynamics.

$f(\mathcal{R},\varphi,\chi)$ Cosmology with Noether Symmetry.

Abstract: This paper is devoted to explore modified $f(\mathcal{R})$ theories of gravity using Noether symmetry approach. For this purpose, Friedmann-Robertson-Walker spacetime is chosen to investigate the cosmic evolution. The study is mainly divided into two parts: Firstly Noether symmetries of metric $f(\mathcal{R})$ gravity are revisited and some new class of solutions with the help of conserved quantities are reported. It is shown that different scenarios of cosmic evolution can be discussed using Noether symmetries and one of the case indicates the chances for the existence of Big Rip singularity. Secondly, $f(\mathcal{R})$ theory coupled with scalar field has been discussed in detail. The Noether equations of modified gravity are reported with three subcases for flat Friedmann-Robertson-Walker universe. It is concluded that conserved quantities are quite helpful to find some important exact solutions in the cosmological contexts. Moreover, the scalar field involved in the modified gravity plays a vital role in the cosmic evolution and an accelerated expansion phase can be observed for some suitable choices of $f(\mathcal{R},\varphi,\chi)$ gravity models.

$f(R)$ Gravity Phase Space in the Presence of Thermal Effects

Abstract: In this paper, we shall consider $f(R)$ gravity and its cosmological implications, when an extra matter term generated by thermal effects is added by hand in the Lagrangian. We formulate the equations of motion of the theory as a dynamical system, that can be treated as an autonomous one only for specific solutions for the Hubble rate, which are of cosmological interest. Particularly, we focus our analysis on subspaces of the total phase space, corresponding to (quasi-)de Sitter accelerating expansion, matter-dominated and radiation-dominated solutions. In all the aforementioned cases, the dynamical system is an autonomous dynamical system. With regard to the thermal term effects, these are expected to significantly affect the evolution near a Big Rip singularity, and we also consider this case in terms of the corresponding dynamical system, in which case the system is non-autonomous, and we attempt to extract analytical and numerical solutions that can assess the specific cases. This course is taken twice: the first for the vacuum theory and the second when two perfect fluids (dust and radiation) are included as matter sources in the field equations. In both cases, we reach similar conclusions. The results of this theory do not differ significantly from the results of the pure $f(R)$ in the de Sitter and quasi-de Sitter phases, as the same fixed points are attained, so for sure the late-time era de Sitter is not affected. However, in the matter-dominated and radiation-dominated phases, the fixed points attained are affected by the presence of the thermal term, so surely the thermal effects would destroy the matter and radiation domination eras.

Future soft singularities, Born-Infeld-like fields, and particles

Abstract: We consider different scenarios of the evolution of the Universe, where the singularities or some nonanalyticities in the geometry of the spacetime are present, trying to answer the following question: is it possible to conserve some kind of notion of particle corresponding to a chosen quantum field present in the universe when the latter approaches the singularity? We study scalar fields with different types of Lagrangians, writing down the second-order differential equations for the linear perturbations of these fields in the vicinity of a singularity. If both independent solutions are regular, we construct the vacuum state for quantum particles as a Gaussian function of the corresponding variable. If at least one of two independent solutions has a singular asymptotic behavior, then we cannot define the creation and the annihilation operators and construct the vacuum. This means that the very notion of particle loses sense. We show that at the approaching to the big rip singularity, particles corresponding to the phantom scalar field driving the evolution of the universe must vanish, while particles of other fields still can be defined. In the case of the model of the universe described by the tachyon field with a special trigonometric potential, where the big brake singularity occurs, we see that the (pseudo) tachyon particles do not pass through this singularity. Adding to this model some quantity of dust, we slightly change the characteristics of this singularity and tachyon particles survive. Finally, we consider a model with the scalar field with the cusped potential, where the phantom divide line crossing occurs. Here the particles are well defined in the vicinity of this crossing point.

Conformal cyclic evolution of phantom energy dominated universe

Abstract: From the Wheeler Dewitt solutions, the scale factor of the initial universe is discussed. In this study scale factors from Wheeler Dewitt solutions, loop quantum gravity, and phantom energy dominated stages are compared. Certain modifications have been attempted in scale factor and quantum potentials driven by canonical quantum gravity approaches. Their results are discussed in this work. Despite increment of phantom energy density avoidance of big rip is reported. Scale factors predicted from various models is discussed in this work. Relationship between scale factors and smooth continuation of aeon is discussed by the application of conformal cyclic cosmology. Quantum potentials for various models are correlated and a correction parameter is included on the cosmological constant. Phantom energy dominated, final stage non-singular evolution of the universe is reported. Eternal increment of phantom energy density without interacting with dark matter is reported for the consequence of evolution of the future universe. Also, the non-interacting solutions of phantom energy and dark matter are explained. As the evolution continues even after the final singularity is approached, the validity of conformal cyclic cosmology is predicted. Non zero values for the scale factor for the set of eigenvalues are reported with a graph

An interacting holographic dark energy model within an induced gravity brane

Abstract: In this paper, we present a model for the late-time evolution of the universe where a dark energy-dark matter interaction is invoked. Dark energy is modeled through an holographic Ricci dark energy...

Conformal cyclic evolution of phantom energy dominated universe

Abstract: From the Wheeler-Dewitt solutions, the scale factor of the initial universe is discussed. In this study, scale factors from Wheeler-Dewitt solutions, loop quantum gravity, and phantom energy dominated stages are compared. Certain modifications have been attempted in scale factor and quantum potentials driven by canonical quantum gravity approaches. Their results are discussed in this work. Despite an increment of phantom energy density, avoidance of Big Rip is reported. Scale factors predicted from various models are discussed in this work. The relationship between scale factors and the smooth continuation of Aeon is discussed by the application of conformal cyclic cosmology. Quantum potentials for various models are correlated and a correction parameter is included in the cosmological constant. Phantom energy dominated, final stage non-singular evolution of the universe is found. Eternal increment of phantom energy density without interacting with dark matter is reported for the consequence of the evolution of the future universe. Also, the non-interacting solutions of phantom energy and dark matter are explained. As the evolution continues even after the final singularity is approached, the validity of conformal cyclic cosmology is predicted. Non zero values for the scale factor for the set of eigenvalues are presented. Results are compared with supersymmetric classical cosmology. The non-interacting solutions are compared with SiBI solutions

Constructions of f(R,G,) gravity from some expansions of the Universe

Abstract: Here we propose the extended modified gravity theory named f(R,G,𝒯) gravity where R is the Ricci scalar, G is the Gauss–Bonnet invariant, and 𝒯 is the trace of the stress-energy tensor. We derive the gravitational field equations in f(R,G,𝒯) gravity by taking the least action principle. Next we construct the f(R,G,𝒯) in terms of R, G and 𝒯 in de Sitter as well as power-law expansion. We also construct f(R,G,𝒯) if the expansion follows the finite-time future singularity (big rip singularity). We investigate the energy conditions in this modified theory of gravity and examine the validity of all energy conditions.

A varying Dark Energy effective speed of sound parameter in the phantom Universe

Abstract: We analyse the phenomenological effects of a varying Dark Energy (DE) effective speed of sound parameter, $c^{2}_{\textrm{sd}}$, on the cosmological perturbations of three phantom DE models. Each of these models induce a particular abrupt future event known as Big Rip (BR), Little Rip (LR), and Little Sibling of the Big Rip (LSBR). In this class of abrupt events, all the bound structures in the Universe would be ripped apart at a finite cosmic time. We compute the evolution of the perturbations, $f\sigma_{8}$ growth rate and forecast the current matter power spectrum. We vary the $c^{2}_{\textrm{sd}}$ parameter in the interval $[0,1]$ and compute the relative deviation with respect $c^{2}_{\textrm{sd}}=1$. In addition, we analyse the effect of gravitational potential sign flip that occurs at very large scale factors as compared with the current one.

The change of the Universe epochs within the scope of Bianchi type-V model

Abstract: We investigate the Bianchi type-V cosmological model with dark matter and anisotropic dark energy. It is assumed that the two sources minimally interact, so their energy-momentum tensors are conserved separately. The proportionality condition that relates the shear scalar $$(\sigma )$$ with the expansion scalar $$(\theta )$$ and the assumption of a diagonal energy–momentum tensor lead to some restriction on the metric functions. A special law is introduced for skewness parameter that describes the deviation of pressure from isotropy. This law can lead to models: the little rip, the big rip and the hybrid expansion. The hybrid expansion law provides a description of the transition of the Universe from deceleration to acceleration. The Universe behavior is discussed depending on the numerical parameters of the models.

Big Rip: heating by Hawking radiation and a possible connection to conformal cyclic cosmology

Abstract: In the Big Rip cosmological scenario, a FRW universe containing dark energy with $w < -1$ in its equation of state (phantom energy) expands in such way that, in a finite time span, the scale factor diverges to infinity and the size of the cosmic horizon goes to zero. Here we revisit this scenario in light of the fact that Hawking radiation is expected to be generated at the apparent horizon of a FRW universe, and show that the energy density and temperature of that radiation both diverge at the Big Rip. We then use this fact to propose a new variant of Penrose's conformal cyclic cosmology model in which the future spacetime metric becomes conformally invariant at the Big Rip instead of in the remote future of a de Sitter universe; this removes the need for mass decay from the model and makes it consistent with current physical laws.