The accelerated expansion of the universe has now been confirmed by several independent observations including those of high redshift type Ia supernovae, and the cosmic microwave background combined with the large scale structure of the Universe. In this work we introduce families of analytical solutions for the scale factor different from the current literature. The knowledge of the scale factor behavior might shed some light on these questions since the entire evolution of a homogeneous isotropic Universe is contained in the scale factor. We use different parameters for these solutions and with these parameters we stablish a connection with the equation of state for different dark energy scenarios.
We study the entropic considerations on the Universe system and the Universe-Black hole system, filled by cosmological constant or exotic quintessence-like and phantom-like fields having negative pressure, using their relevant entropic bounds. It turns out that for both systems these considerations single out the cosmological constant, among the negative pressure candidate fields, as the viable cosmological field.
The tests of the constancy of the fundamental constants are tests of the local position invariance and thus of the equivalence principle. We summarize the various constraints that have been obtained and then describe the connection between varying constants and extensions of general relativity. To finish, we discuss the link with cosmology, and more particularly with the acceleration of the Universe. We take the opportunity to summarize various possibilities to test general relativity (but also the Copernican principle) on cosmological scales.
Modified Newtonian dynamics (MOND) can be obtained by modifying the entropic formulation of gravity, this is achieved by considering the quantum statistical nature of the degrees of freedom on the holographic screen. Through this frame work, we find some constraints on a cosmological extension for MOND, with no additional auxiliary fields. The connections between MOND to conformal gravity and Rindler force gravity are examined. These two alternative gravity theories are subsequently considered as possible cosmological extensions of MOND.
The objective of this paper is not simply to present an historical overview of Einstein's cosmological considerations, but to discuss the central role they played in shaping the paradigm of relativistic cosmology. This, we'll show, was a result of both his actions and, perhaps more importantly, his inactions. Accordingly, discussion won't simply be restricted to Einstein's considerations, as we'll analyse relevant contributions to the relativistic expansion paradigm during the approximately twenty years following Slipher's first redshift measurements in 1912. Our aim is to shed some light on w…
We construct explicit solutions for scalar, vector and tensor perturbations in a less known setting, a flat universe filled by an isotropic elastic solid with pressure and shear modulus proportional to energy density. The solutions generalize the well known formulas for cosmological perturbations in a universe filled by ideal fluid.
In this work, a cosmological model inspired by string/M- theory with fermionic field is taken into consideration. Here it is investigated whether the introduction of a non-Dirac fermionic field--characterized by an \textit{interaction term}--affects the cosmological evolution. The self-interaction potential is considered as a combination of the scalar and pseudo-scalar invariants. It is observed that the fermionic field under consideration behaves like an inflation field for the early Universe and later on, as a dark energy field. The late time acceleration becomes more prominent by the additi…
We propose a new approach for multiverse analysis based on computational complexity, which leads to a new family of "computational" measure factors. By defining a cosmology as a space-time containing a vacuum with specified properties (for example small cosmological constant) together with rules for how time evolution will produce the vacuum, we can associate global time in a multiverse with clock time on a supercomputer which simulates it. We argue for a principle of "limited computational complexity" governing early universe dynamics as simulated by this supercomputer, which translates to a …
It is shown that the antiscalar approach to dark energy, whereby the energy-momentum tensor of the scalar field has the sign opposite to that of the rest of the matter, follows from the considerations of thermodynamic stability, as well as from the static limit of the Einstein-Maxwell equations. The same limit also demonstrates that the resulting background antiscalar field proves to be of quasi-electrostatic origin.
In a recent propposal, the cosmological constant has been considered as as a new thermodynamical variable and its change is related to the radiation produced by black holes. Using this consideration and by modelling the primordial black holes as Schwarzschild-de Sitter holes,we have constrained the total mass of primordial black holes evaporated by now, giving an estimate of the order of 1.624x10^24 solar masses.
Cosmological phase transitions are thought to have taken place at the early Universe imprinting their properties on the observable Universe. There is strong evidence that, through the dynamics of a scalar field that lead a second order phase transition, inflation shaped the Universe accounting for the most conspicuous features of the observed Universe. It is argued that inflation has also striking implications for the vacuum energy. Considerations for subsequent second order phase transitions are also discussed.
The solution to the current extending Universe problem, and the description of all stages of evolution compels scientists to consider various cosmological models. Scalar - tensor models are rather simple and also allow us to clearly define the separate stages of evolution. Furthermore, other cosmological models are reduced. Our work takes into consideration the non-minimally interacted scalar field and the spinor field. The spinor field has been considered to establish a better understanding of the stages of evolution in our Universe.
The present work deals with scalar field cosmology in the framework of a quantum gravity modified Einstein-Hilbert Lagrangian with variable $G$ and $Λ$. Using Renormalization group, variable $G$ behaves as a minimally coupled filed (not the scalar-tensor theory) and variable $Λ$ can be interpreted as a potential function. The point Lagrangian for this model in the background of homogeneous and isotropic flat FLRW space-time model experiences point-like Noether symmetry and equivalent potential function $Λ(G)$ is determined. Using a point transformation in the $3D$ augmented space is found that…
We confront dark energy models which are currently similar to $Λ$CDM theory with observational data which include the SNe data, matter density perturbations and baryon acoustic oscillations data. DE cosmology under consideration may evolve to Big Rip, type II or type III future singularity, or to Little Rip or Pseudo-Rip universe. It is shown that matter perturbations data define more precisely the possible deviation from $Λ$CDM model than consideration of SNe data only. The combined data analysis proves that DE models under consideration are as consistent as $Λ$CDM model. We demonstrate that …
In the present work, we have searched the existence of $Λ$CDM-type cosmological model in anisotropic Heckmann-Schucking space-time. The matter source that is responsible for the present acceleration of the universe consist of cosmic fluid with $p = ωρ$, where $ω$ is the equation of state parameter. The Einstein's field equations have been solved explicitly under some specific choice of parameters that isotropizes the model under consideration. It has been found that the derived model is in good agreement with recent SN Ia observations. Some physical aspects of the model has been discussed in d…
The dynamics of metric perturbations is explored in the gravity theory with anomaly-induced quantum corrections. Our first purpose is to derive the equation for gravitational waves in this theory on the general homogeneous and isotropic background, and then verify the stability of such background with respect to metric perturbations. The problem under consideration has several interesting applications. Our first purpose is to explore the stability of the classical cosmological solutions in the theory with quantum effects taken into account. There is an interesting literature about stability of…
We proved that strictly stationary Einstein-Maxwell-axion-dilaton spacetime with negative cosmological constant could not support a nontrivial configuration of complex scalar fields. We considered the general case of the arbitrary number of U(1) gauge fields in the theory under consideration.
We study the Vaidya black hole surrounded by the exotic quintessence-like, phantom-like and cosmological constant-like fields by means of entropic considerations. Explicitly, we show that for this thermodynamical system, the requirement for the identification of D-bound and Bekenstein entropy bound can be considered as a thermodynamical criterion by which one can rule out the quintessence-like and phantom-like fields, and prefer the cosmological constant as a viþable cosmological field.
With due consideration of reasonable cosmological assumptions within the limit of the present cosmological scenario, we have analysed a spherically symmetric metric in 5D setting within the framework of Lyra manifold. The model universe is predicted to be a DE model, dominated by vacuum energy. The model represents an oscillating model, each cycle evolving with a big bang and ending at a big crunch, undergoing a series of bounces. The universe is isotropic and undergoes super-exponential expansion. The value of Hubble's parameter is measured to be $H=67.0691$ which is very close to $H_0=67.36\…
We investigate the dynamical features of a large family of running vacuum cosmologies for which $Λ$ evolves as a polynomial in the Hubble parameter. Specifically, using the critical point analysis we study the existence and the stability of singular solutions which describe de-Sitter, radiation and matter dominated eras. We find several classes of $Λ(H)$ cosmologies for which new analytical solutions are given in terms of Laurent expansions. Finally, we show that the Milne universe and the $R_{h}=ct$ model can be seen as perturbations around a specific $Λ(H)$ model, but this model is unstable.
Consideration of vacuum polarization in quantum electrodynamics may affect the momentum dispersion relation for photons for a non-trivial background, due to appearance of curvature dependent terms in the effective action. We investigate the effect of a positive cosmological constant $Λ$ on this at one loop order for stationary $Λ$-vacuum spacetimes. To the best of our knowledge, so far it has been shown that $Λ$ affects the propagation in a time dependent black hole spacetime. Here we consider the static de Sitter cosmic string and the Kerr-de Sitter spacetime to show that there can be some no…
A difficult task to deal with is the analytical treatment of models composed by three real scalar fields, once their equations of motion are in general coupled and hard to be integrated. In order to overcome this problem we introduce a methodology to construct three-field models based on the so-called "extension method". The fundamental idea of the procedure is to combine three one-field systems in a non-trivial way, to construct an effective three scalar field model. An interesting scenario where the method can be implemented is within inflationary models, where the Einstein-Hilbert Lagrangia…
We explore the structure of the spin foam-like vertex expansion in loop quantum cosmology and discuss properties of the corresponding amplitudes, with the aim of elucidating some of the expansion's useful properties and features. We find that the expansion is best suited for consideration of conceptual questions and for investigating short-time, highly quantum behavior. In order to study dynamics at cosmological scales, the expansion must be carried to very high order, limiting its direct utility as a calculational tool for such questions. Conversely, it is unclear that the expansion can be tr…
Recent studies suggest that the cosmological constant affects the gravitational bending of photons although the orbital equation for light in Schwarzschild-de Sitter space time is free from cosmological constant. Here we argued that the very notion of cosmological constant independency of photon orbit in the Schwarzschild-de Sitter space time is not proper. Consequently the cosmological constant has some clear contribution to the deflection angle of light rays. We stressed the importance of the study of photon trajectories from the reference objects in bending calculation, particularly for asy…
We analytically study the expansion of a Bose-Einstein condensate in a ring-shaped trap with an increasing central radius. The evolution of the ground state is described using a scaling transform. Additionally, the dynamics of excited azimuthal modes over the varying ground state is analyzed through a generalization of the Bogoliubov-de Gennes approach. Our results explain some of the features observed in recent experiments focused on testing the applicability of the system as a parallel of cosmological inflationary models. The radial dynamics, which corresponds to the inflaton field of the co…
Many probability measures in the multiverse depend exponentially on some observable parameters, giving rise to potential problems such as youngness bias, Q-catastrophe etc. In this paper we explore a possibility that the exponential runaway dependence should be viewed not as a problem, but as a feature that may help us to fix all parameters in the landscape, including the value of the cosmological constant, without using anthropic considerations.
We investigate the main features of the flat Friedmann-Lemaıtre-Robertson-Walker cosmological models in the f(T) teleparallel gravity. In particular, a general approach to find out exact cosmological solutions in f (T) gravity is discussed. Instead of taking into account phenomenological models, we consider as a selection criterion, the existence of Noether symmetries in the cosmological f(T) point-like Lagrangian. We find that only power-law models admit extra Noether symmetries. A complete analysis of such cosmological models is developed.
We consider black holes which form from an initially spherically symmetric super-Hubble perturbation of a cosmological background filled by a perfect fluid $p=w ρ$ with $w\in (0,1]$. Previous work has shown that when $w = 1/3$ (radiation), there is a critical threshold for black hole formation ($δ_c$), which, to a very good approximation, only depends upon the curvature of the compaction function around its peak value. We find that this generalizes to all $w\gtrsim 1/3$; for smaller $w$s the knowledge of the full shape of the compaction function is necessary. We provide analytic approximations…
SNIA and CMB datasets have shown both of evolving Newton's "constant" and a signature of the coupling of scalar field to matter. These observations motivate the consideration of the scalar-matter coupling in Jordan frame in the framework of scalar-tensor gravity. So far, majority of the works on the coupling of scalar matter has performed in Einstein frame in the framework of minimally coupled scalar fields. In this work, we generalize the original scalar-tensor theories of gravity introducing a direct coupling of scalar to matter in the Jordan frame. The combined consideration of both evolvin…
We introduce consideration of a new factor, synchronisation of spacetime Mixmaster oscillations, that may play a simplifying role in understanding the nature of the general inhomogeneous cosmological solution to Einstein's equations. We conjecture that, on approach to a singularity, the interaction of spacetime Mixmaster oscillations in different regions of an inhomogeneous universe can produce a synchronisation of these oscillations through a coupling to their mean field in the way first demonstrated by the Kuramoto coupled oscillator model.
We estimate the amount of vorticity generated at second order in cosmological perturbation theory from the coupling between first order energy density and non-adiabatic pressure, or entropy, perturbations. Assuming power law input spectra for the source terms, and working in a radiation background, we calculate the wave number dependence of the vorticity power spectrum and its amplitude. We show that the vorticity generated by this mechanism is non-negligible on small scales, and hence should be taken into consideration in current and future CMB experiments.
This article deals with the study of Bianchi type-I universe in the context of f(R,T) gravity. Einstein's field equations in f(R,T) gravity has been solved in presence of cosmological constant ? and quadratic equation of state. Here we have discussed two classes of f(R,T) gravity i.e. f(R,T) = R + 2f(T) and f(R,T) = f_1(R) + f_2(T). A set of models has been taken into consideration based on the plausible relation. Also we have studied the some physical and kinematical properties of the models.
In the context of extended Teleparallel gravity theories with a 3+1 dimensions Gauss-Bonnet analog term, we address the possibility of these theories reproducing several well-known cosmological bouncing scenarios in a four-dimensional Friedmann-Lemaître-Robertson-Walker geometry. We shall study which types of gravitational Lagrangians are capable of reconstructing bouncing solutions provided by analytical expressions for symmetric, oscillatory, superbounce, the matter bounce and singular bounce. Some of the Lagrangians discovered are both analytical at the origin having Minkowski and Schwarzsc…
Unimodular gravity is an appealing approach to address the cosmological constant problem. In this scenario, the vacuum energy density of quantum fields does not gravitate and the cosmological constant appears merely as an integration constant. Recently, it has been shown that energy diffusion that may arise in quantum gravity and in theories with spontaneous collapse is compatible with this framework by virtue of its restricted diffeomorphism invariance. New studies suggest that this phenomenon could lead to higher-order equations in the context of homogeneous and isotropic Universe, affecting…
Whereas current cosmological observations suggest that the universe is dominated by a positive cosmological constant ($Λ> 0$), the AdS/CFT correspondence tells us that the case $Λ<0$ is still worthy of consideration. In this paper we study the McVittie solution with $Λ<0$. Following a related study, the solution is understood here by way of a systematic construction of conformal diagrams based on detailed numerical integrations of the null geodesic equations. As in the pure Robertson - Walker case, we find that $Λ<0$ ensures collapse to a Big Crunch, a feature which completely dominates the gl…
We find the membrane equations which describe the leading order in $1/D$ dynamics of black holes in the $D\rightarrow\infty$ limit for the most general four-derivative theory of gravity in the presence of a cosmological constant. We work up to linear order in the parameter determining the strength of the four-derivative corrections to the gravity action and hence there are no ghost modes in the theory. We find that the effective membrane equations we obtain are the covariant version of the membrane equations in absence of the cosmological constant. We also find the world-volume stress tensor f…
An additional variation of the Einstein-Hilbert action with respect to the Planck mass provides a constraint on the average Ricci scalar that prevents vacuum energy from gravitating. Consideration of the evolution of the inhomogeneous matter distribution in the Universe with evaluation of the averaging constraint on disconnected matter cells that ultimately form isolated gravitationally bound structures yields a backreaction effect that self-consistently produces the cosmological constant of the background. A uniform prior on our location in the formation of these isolated structures implies a…
