scholarly journals COSMIC ACCELERATION AND CONCORDANCE FROM CAUSAL BACKREACTION WITH RECURSIVE NONLINEARITIES

2013 ◽  
Vol 22 (13) ◽  
pp. 1330026 ◽  
Author(s):  
BRETT BOCHNER
Keyword(s):  
Dark Energy ◽  
Large Scale ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Clustering Model ◽  
Hierarchical Nature ◽  
The Universe ◽  
Far Future ◽  
Apparent Acceleration

We review the causal backreaction paradigm, in which the need for Dark Energy is eliminated via the generation of an apparent cosmic acceleration from the causal flow of inhomogeneity information coming in from distant structure-forming regions. The formalism detailed here incorporates the effects of "recursive nonlinearities": the process by which already-established metric perturbations will subsequently act to slow-down all future flows of inhomogeneity information. Despite such effects, we find viable cosmological models in which causal backreaction successfully serves as a replacement for Dark Energy, via the adoption of relatively large values for the dimensionless "strength" of the clustering evolution functions being modeled. These large values are justified by the hierarchical nature of clustering and virialization in the universe, which occurs on multiple cosmic length scales simultaneously; moreover, the clustering model amplitudes needed to match the apparent acceleration can be moderated via the incorporation of a model parameter representing the late-time slow-down of clustering due to astrophysical feedback processes. In summary, an alternative cosmic concordance can be achieved for a matter-only universe in which the apparent acceleration observed is generated entirely by causal backreaction effects. Lastly, considering the long-term fate of the universe, while the possibility of an "eternal" acceleration due to causal backreaction seems unlikely, this conclusion does not take into account the large-scale breakdown of cosmological isotropy in the far future, or the eventual ubiquity of gravitationally-nonlinear perturbations.

scholarly journals A look into the cosmological consequences of a dark energy model with higher derivatives of H in the framework of Chameleon Brans–Dicke cosmology

2019 ◽  
Vol 28 (11) ◽  
pp. 1950149 ◽  
Author(s):  
Antonio Pasqua ◽  
Surajit Chattopadhyay ◽  
Aroonkumar Beesham
Keyword(s):  
Dark Energy ◽  
Dark Energy Model ◽  
Late Time ◽  
Statefinder Parameters ◽  
Eos Parameter ◽  
Higher Derivatives ◽  
De Formula ◽  
Derivatives Of ◽  
The Universe ◽  
Far Future

In this paper, we study some relevant cosmological features of a Dark Energy (DE) model with Granda–Oliveros cut-off, which is just a specific case of Nojiri–Odintsov holographic DE [S. Nojiri and S. D. Odintsov, Gen. Relativ. Gravit. 38 (2006) 1285] unifying phantom inflation with late-time acceleration, in the framework of Chameleon Brans–Dicke (BD) cosmology. Choosing a particular ansatz for some of the quantities involved, we derive the expressions of some important cosmological quantities, like the Equation of State (EoS) parameter of DE [Formula: see text], the effective EoS parameter [Formula: see text], the pressure of DE [Formula: see text] and the deceleration parameter [Formula: see text]. Moreover, we study the behavior of statefinder parameters [Formula: see text] and [Formula: see text], of the cosmographic parameters [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] and of the squared speed of the sound [Formula: see text] for both case corresponding to noninteracting and interacting Dark sectors. We also plot the quantities we have derived and we calculate their values for [Formula: see text] (i.e. for the beginning of the universe history), for [Formula: see text] (i.e. for far future) and for the present time, indicated with [Formula: see text]. The EoS parameters have been tested against various observational values available in the literature.

scholarly journals Accelerating cosmology in modified gravity: From convenient F(R) or string-inspired theory to bimetric F(R) gravity

2014 ◽  
Vol 11 (02) ◽  
pp. 1460006 ◽  
Author(s):  
Shin'ichi Nojiri ◽  
Sergei D. Odintsov
Keyword(s):  
Dark Energy ◽  
Modified Gravity ◽  
Early Time ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Reconstruction Method ◽  
Free Theory ◽  
Acceleration Of The Universe ◽  
The Universe ◽  
Unified Description

We consider modified gravity which may describe the early-time inflation and/or late-time cosmic acceleration of the universe. In particular, we discuss the properties of F(R), F(G), string-inspired and scalar-Einstein–Gauss–Bonnet gravities, including their FRW equations and fluid or scalar-tensor description. Simplest accelerating cosmologies are investigated and possibility of unified description of the inflation with dark energy is described. The cosmological reconstruction program which permits to get the requested universe evolution from modified gravity is developed. As some extension, massive F(R) bigravity which is ghost-free theory is presented. Its scalar-tensor form turns out to be the easiest formulation. The cosmological reconstruction method for such bigravity is presented. The unified description of inflation with dark energy in F(R) bigravity turns out to be possible.

MODIFIED GRAVITY THEORY DUE TO POSITIVE AS WELL AS NEGATIVE CURVATURE INDUCED ACCELERATION

2012 ◽  
Vol 21 (02) ◽  
pp. 1250012
Author(s):  
PRIYADARSHI MAJUMDAR
Keyword(s):  
Dark Energy ◽  
Modified Gravity ◽  
Negative Curvature ◽  
Gravity Theory ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Negative Power ◽  
Modified Gravity Theory ◽  
Phase Q ◽  
The Universe

We formulate a modified gravity theory that eliminates the need for dark energy and is stable for a Lagrangian containing R, R2 as well as 1/R terms (i.e. nonlinear contributions of the Ricci curvature with a non-analytic model of f(R) at R = 0) without considering any matter-dominated era. The terms with positive powers (1, 2) of the curvature support the inflationary epoch while the terms with negative power (-1) serves as effective dark energy, supporting current cosmic acceleration. We present a few analytical solutions of evolution equation for the deceleration parameter q as a function of Hubble parameter H and time t; specially in one solution, the universe evolves continuously from q = 1 (a radiation-dominated epoch) to q = -1/2 (dark-energy-dominated late-time accelerating phase) when the universe is sufficiently old. The solution is supported by numerical results. The transition from the decelerated (q > 0) to the accelerated phase (q < 0) of expansion takes place smoothly without having to resort to a study of asymptotic behavior.

scholarly journals Dark energy in Horndeski theories after GW170817: A review

2019 ◽  
Vol 28 (05) ◽  
pp. 1942005 ◽  
Author(s):  
Ryotaro Kase ◽  
Shinji Tsujikawa
Keyword(s):  
Dark Energy ◽  
Large Scale ◽  
Equations Of Motion ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Dark Energy Density ◽  
Static Approximation ◽  
Binary Neutron Star ◽  
Energy Models ◽  
Gravitational Theories

The gravitational wave (GW) event GW170817 from a binary neutron star merger together with the electromagnetic counterpart showed that the speed of GWs [Formula: see text] is very close to that of light for the redshift [Formula: see text]. This places tight constraints on dark energy models constructed in the framework of modified gravitational theories. We review models of the late-time cosmic acceleration in scalar–tensor theories with second-order equations of motion (dubbed Horndeski theories) by paying particular attention to the evolution of dark energy equation of state and observables relevant to the cosmic growth history. We provide a gauge-ready formulation of scalar perturbations in full Horndeski theories and estimate observables associated with the evolution of large-scale structures, cosmic microwave background and weak lensing by employing a so-called quasi-static approximation for the modes deep inside the sound horizon. In light of the recent observational bound of [Formula: see text], we also classify surviving dark energy models into four classes depending on different structure-formation patterns and discuss how they can be observationally distinguished from each other. In particular, the nonminimally coupled theories in which the scalar field [Formula: see text] has a coupling with the Ricci scalar [Formula: see text] of the form [Formula: see text], including [Formula: see text] gravity, can be tightly constrained not only from the cosmic expansion and growth histories but also from the variation of screened gravitational couplings. The cross-correlation of integrated Sachs–Wolfe signal with galaxy distributions can be a key observable for placing bounds on the relative ratio of cubic Galileon density to total dark energy density. The dawn of GW astronomy will open up a new window to constrain nonminimally coupled theories further by the modified luminosity distance of tensor perturbations.

ANALYTICAL CONSIDERATIONS ABOUT THE COSMOLOGICAL CONSTANT AND DARK ENERGY

2009 ◽  
Vol 24 (28n29) ◽  
pp. 5427-5444 ◽  
Author(s):  
EVERTON M. C. ABREU ◽  
LEONARDO P. G. DE ASSIS ◽  
CARLOS M. L. DOS REIS
Keyword(s):  
Dark Energy ◽  
Cosmological Constant ◽  
Large Scale ◽  
Weak Interactions ◽  
High Redshift ◽  
Fundamental Constant ◽  
Scale Factor ◽  
Cosmic Acceleration ◽  
Accelerated Expansion ◽  
The Universe

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. Another way of presenting this kinematic property of the universe is to postulate the existence of a new and exotic entity, with negative pressure, the dark energy (DE). In spite of observationally well established, no single theoretical model provides an entirely compelling framework within which cosmic acceleration or DE can be understood. At present all existing observational data are in agreement with the simplest possibility that the cosmological constant be a candidate for DE. This case is internally self-consistent and noncontradictory. The extreme smallness of the cosmological constant expressed in either Planck, or even atomic units means only that its origin is not related to strong, electromagnetic, and weak interactions. Although in this case DE reduces to only a single fundamental constant we still have no derivation from any underlying quantum field theory for its small value. From the principles of quantum cosmologies, for example, it is possible to obtain the reason for an inverse-square law for the cosmological constant with no conflict with observations. Despite the fact that this general expression is well known, 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 mentioned above 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 establish a connection with the equation of state for different DE scenarios.

COSMIC ACCELERATION AND MODIFIED GRAVITY

2007 ◽  
Vol 16 (12a) ◽  
pp. 2065-2074 ◽  
Author(s):  
MARK TRODDEN
Keyword(s):  
General Relativity ◽  
Dark Energy ◽  
Modified Gravity ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Late Time Acceleration ◽  
Acceleration Of The Universe ◽  
The Universe

I briefly discuss some attempts to construct a consistent modification to general relativity (GR) that might explain the observed late-time acceleration of the Universe and provide an alternative to dark energy. I describe the issues facing extensions to GR, illustrate these with a specific example, and discuss the resulting observational and theoretical obstacles.

scholarly journals A gravitational puzzle

2011 ◽  
Vol 369 (1957) ◽  
pp. 4998-5002
Author(s):  
Robert R. Caldwell
Keyword(s):  
Gravitational Lensing ◽  
Large Scale ◽  
Current Data ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Microwave Background ◽  
Scale Free ◽  
Cosmological Observations ◽  
Stress Energy ◽  
The Relationship

The challenge to understand the physical origin of the cosmic acceleration is framed as a problem of gravitation. Specifically, does the relationship between stress–energy and space–time curvature differ on large scales from the predictions of general relativity. In this article, we describe efforts to model and test a generalized relationship between the matter and the metric using cosmological observations. Late-time tracers of large-scale structure, including the cosmic microwave background, weak gravitational lensing, and clustering are shown to provide good tests of the proposed solution. Current data are very close to proving a critical test, leaving only a small window in parameter space in the case that the generalized relationship is scale free above galactic scales.

Phase space analysis of the Umami Chaplygin model

2021 ◽  
pp. 2150052
Author(s):  
Qihong Huang ◽  
Ruanjing Zhang ◽  
Jun Chen ◽  
He Huang ◽  
Feiquan Tu
Keyword(s):  
Phase Space ◽  
Cosmic Acceleration ◽  
Accelerated Expansion ◽  
Late Time ◽  
Phase Space Analysis ◽  
Space Analysis ◽  
History Of ◽  
Evolutionary Trajectories ◽  
The Universe ◽  
Universe Evolution

In this paper, we analyze the universe evolution and phase space behavior of the Umami Chaplygin model, where the Umami Chaplygin fluid replaces both a dark energy and a dark and baryonic matter. We find the Umami Chaplygin model can be stable against perturbations under some conditions and can be used to explain the late-time cosmic acceleration. The results of phase space analysis show that there exists a late-time accelerated expansion attractor with [Formula: see text], which indicates the Umami Chaplygin fluid can behave as a cosmological constant. Moreover, the Umami Chaplygin model can describe the expansion history of the universe. The evolutionary trajectories of the statefinder diagnostic pairs and the finite time future singularities are also discussed.

scholarly journals Observational constraints of a new unified dark fluid and the H0 tension

2019 ◽  
Vol 490 (2) ◽  
pp. 2071-2085 ◽  
Author(s):  
Weiqiang Yang ◽  
Supriya Pan ◽  
Andronikos Paliathanasis ◽  
Subir Ghosh ◽  
Yabo Wu
Keyword(s):  
Dark Matter ◽  
Dark Energy ◽  
Large Scale ◽  
Cold Dark Matter ◽  
Early Time ◽  
Observational Constraints ◽  
Minimal Extension ◽  
Energy Evolution ◽  
The Universe ◽  
Different Sources

ABSTRACT Unified cosmological models have received a lot of attention in astrophysics community for explaining both the dark matter and dark energy evolution. The Chaplygin cosmologies, a well-known name in this group have been investigated matched with observations from different sources. Obviously, Chaplygin cosmologies have to obey restrictions in order to be consistent with the observational data. As a consequence, alternative unified models, differing from Chaplygin model, are of special interest. In the present work, we consider a specific example of such a unified cosmological model, that is quantified by only a single parameter μ, that can be considered as a minimal extension of the Λ-cold dark matter cosmology. We investigate its observational boundaries together with an analysis of the universe at large scale. Our study shows that at early time the model behaves like a dust, and as time evolves, it mimics a dark energy fluid depicting a clear transition from the early decelerating phase to the late cosmic accelerating phase. Finally, the model approaches the cosmological constant boundary in an asymptotic manner. We remark that for the present unified model, the estimations of H0 are slightly higher than its local estimation and thus alleviating the H0 tension.

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