scholarly journals Cosmic acceleration sourced by modification of gravity without extra degrees of freedom

2019 ◽  
Vol 16 (08) ◽  
pp. 1950128
Author(s):  
Abhineet Agarwal ◽  
R. Myrzakulov ◽  
S. K. J. Pacif ◽  
M. Sami ◽  
Anzhong Wang
Keyword(s):  
Degrees Of Freedom ◽  
Large Scale ◽  
Cosmic Acceleration ◽  
Jordan Frame ◽  
Model Parameters ◽  
Late Time ◽  
Baryonic Matter ◽  
Narrow Region ◽  
Cluster Data ◽  
Age Constraints

In this paper, we investigate a scenario in which late-time cosmic acceleration might arise due to coupling between dark matter and baryonic matter without resorting to dark energy or large-scale modification of gravity associated with extra degrees of freedom. The scenario can give rise to late-time acceleration in Jordan frame and no acceleration in Einstein frame — generic modification of gravity caused by disformal coupling. Using a simple parametrization of the coupling function, in maximally disformal case, we constrain the model parameters by using the age constraints due to globular cluster data. We also obtain observational constraints on the parameters using [Formula: see text] datasets. In this case, we distinguish between phantom and non-phantom acceleration and show that the model can give rise to phantom behavior in a narrow region of parameter space.

scholarly journals Cosmic acceleration from coupling of baryonic and dark matter components: Analysis and diagnostics

2019 ◽  
Vol 28 (06) ◽  
pp. 1950083 ◽  
Author(s):  
Abhineet Agarwal ◽  
R. Myrzakulov ◽  
S. K. J. Pacif ◽  
M. Shahalam
Keyword(s):  
Dark Matter ◽  
Degrees Of Freedom ◽  
Cosmic Acceleration ◽  
Jordan Frame ◽  
Model Parameters ◽  
Late Time ◽  
Baryonic Matter ◽  
Acoustic Oscillations ◽  
Type Ia ◽  
Components Analysis

In this paper, we examine a scenario in which late-time cosmic acceleration might arise due to the coupling between baryonic matter and dark matter without the presence of extra degrees of freedom. In this case, one can obtain late-time acceleration in Jordan frame and not in Einstein frame. We consider two different forms of parametrization of the coupling function, and put constraints on the model parameters by using an integrated dataset of Hubble parameter, Type Ia supernova and baryon acoustic oscillations. The models under consideration are consistent with the observations. In addition, we perform the statefinder and [Formula: see text] diagnostics, and show that the models exhibit a distinctive behavior due to the phantom characteristic in future which is a generic feature of the underlying scenario.

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.

scholarly journals An accelerating cosmological model from a parametrization of Hubble parameter

2019 ◽  
Vol 35 (05) ◽  
pp. 2050011 ◽  
Author(s):  
S. K. J. Pacif ◽  
Md Salahuddin Khan ◽  
L. K. Paikroy ◽  
Shalini Singh
Keyword(s):  
Dark Energy ◽  
Cosmological Model ◽  
Hubble Parameter ◽  
Cosmological Parameters ◽  
Cosmic Time ◽  
Solution Procedure ◽  
Cosmic Acceleration ◽  
Model Parameters ◽  
Late Time ◽  
Field Equations

In view of late-time cosmic acceleration, a dark energy cosmological model is revisited wherein Einstein’s cosmological constant is considered as a candidate of dark energy. Exact solution of Einstein field equations (EFEs) is derived in a homogeneous isotropic background in classical general relativity. The solution procedure is adopted in a model-independent way (or the cosmological parametrization). A simple parametrization of the Hubble parameter (H) as a function of cosmic time t is considered which yields an exponential type of evolution of the scale factor (a) and also shows a negative value of deceleration parameter at the present time with a signature flip from early deceleration to late acceleration. Cosmological dynamics of the model obtained have been discussed illustratively for different phases of the evolution of the universe. The evolution of different cosmological parameters is shown graphically for flat and closed cases of Friedmann–Lemaitre–Robertson–Walker (FLRW) spacetime for the presented model (open case is incompatible to the present scenario). We have also constrained our model parameters with the updated (36 points) observational Hubble dataset.

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 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.

scholarly journals Reconstruction of cosmic history from a simple parametrization of H

2017 ◽  
Vol 14 (07) ◽  
pp. 1750111 ◽  
Author(s):  
Shibesh Kumar Jas Pacif ◽  
Ratbay Myrzakulov ◽  
Shynaray Myrzakul
Keyword(s):  
Hubble Parameter ◽  
Functional Form ◽  
Cosmic Acceleration ◽  
Model Parameters ◽  
Late Time ◽  
Text Data ◽  
Simple Modification ◽  
Big Rip ◽  
Cosmic History

In this paper, we propose a simple parametrization of the Hubble parameter (HP) [Formula: see text] in order to explain the late-time cosmic acceleration. We show that our proposal covers many models obtained in different schemes of parametrization under one umbrella. We demonstrate that a simple modification in the functional form of HP can give rise to interesting cosmological phenomena such as big rip singularity, bounce and others. We have also constrained the model parameters using the latest 28 points of [Formula: see text] data for three cases which admit transition from deceleration to acceleration.

scholarly journals Observational constraints on the generalized α attractor model

2018 ◽  
Vol 27 (05) ◽  
pp. 1850058 ◽  
Author(s):  
M. Shahalam ◽  
Ratbay Myrzakulov ◽  
Shynaray Myrzakul ◽  
Anzhong Wang
Keyword(s):  
Cosmic Acceleration ◽  
Asymptotic Region ◽  
Model Parameters ◽  
Observational Constraints ◽  
Late Time ◽  
Freezing And Thawing ◽  
Energy Models ◽  
Slow Roll ◽  
Attractor Model ◽  
Original Potential

We study the generalized [Formula: see text] attractor model in the context of the late time cosmic acceleration. The model interpolates between the scaling freezing and thawing dark energy models. In the slow roll region, the original potential is modified whereas the modification ceases in the asymptotic region and the effective potential behaves as the quadratic one. In our setting, the field rolls slowly around the present epoch and mimics the scaling behavior in the future. We obtain observational constraints on the model parameters by using an integrated database (SN+Hubble+BAO+CMB).

scholarly journals Conformal (In)Equality

2018 ◽  
Vol 168 ◽  
pp. 08001
Author(s):  
Young-Hwan Hyun ◽  
Yoonbai Kim ◽  
Seokcheon Lee
Keyword(s):  
Dark Energy ◽  
Modified Gravity ◽  
Large Scale ◽  
Growth Index ◽  
Gravity Theory ◽  
Jordan Frame ◽  
Model Parameters ◽  
Modified Gravity Theories ◽  
Gravity Theories ◽  
The Universe

The current accelerating expansion of the Universe is explained either by dark energy or by modified gravity theories. Both of them can explain exactly the same background evolution of the Universe, however this degeneracy may be broken when the observation of large scale structure formation is taken into account. Two observables are parameterized by the so-called dark energy equation of state, ω and the growth index parameter, γ. From these observed parameters, one may reconstruct the model parameters of the so-called scalar-tensor gravity theory, one of the modified gravity theories. Especially, the scalar-tensor gravity theory is described both in Jordan frame and in Einstein frame. If cosmological observations are interpreted in one frame, then all of the observables should also be interpreted in that frame. This explicitly shows conformal inequality of cosmological observables.

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