Showing papers on "Deceleration parameter published in 2018"

The Cardassian expansion revisited: constraints from updated Hubble parameter measurements and type Ia supernova data

Abstract: Motivated by an updated compilation of observational Hubble data (OHD) which consist of 51 points in the redshift range 0.07

Localization accuracy of compact binary coalescences detected by the third-generation gravitational-wave detectors and implication for cosmology

Abstract: We use the Fisher information matrix to investigate the angular resolution and luminosity distance uncertainty for coalescing binary neutron stars (BNSs) and neutron star-black hole binaries (NSBHs) detected by the third-generation (3G) gravitational-wave (GW) detectors. Our study focuses on an individual 3G detector and a network of up to four 3G detectors at different locations including the United States, Europe, China, and Australia for the proposed Einstein Telescope (ET) and Cosmic Explorer (CE) detectors. In particular, we examine the effect of the Earth's rotation, as GW signals from BNS and low-mass NSBH systems could be hours long for 3G detectors. In this case, an individual detector can be effectively treated as a detector network with long baselines formed by the trajectory of the detector as it rotates with the Earth. Therefore, a single detector or two-detector networks could also be used to localize the GW sources effectively. We find that a time-dependent antenna beam-pattern function can help better localize BNS and NSBH sources, especially edge-on ones. The medium angular resolution for one ET-D detector is around $150\text{ }\text{ }{\mathrm{deg}}^{2}$ for BNSs at a redshift of $z=0.1$, which improves rapidly with a decreasing low-frequency cutoff ${f}_{\mathrm{low}}$ in sensitivity. The medium angular resolution for a network of two CE detectors in the United States and Europe, respectively, is around $20\text{ }\text{ }{\mathrm{deg}}^{2}$ at $z=0.2$ for the simulated BNS and NSBH samples. While for a network of two ET-D detectors, the similar angular resolution can be achieved at a much higher redshift of $z=0.5$. The angular resolution of a network of three detectors is mainly determined by the baselines between detectors regardless of the CE or ET detector type. The medium angular resolution of BNS for a network of three detectors of the ET-D or CE type in the United States, Europe, and Australia is around $10\text{ }\text{ }{\mathrm{deg}}^{2}$ at $z=2$. We discuss the implications of our results for multimessenger astronomy and, in particular, for using GW sources as independent tools to constrain the Hubble constant ${H}_{0}$, the deceleration parameter ${q}_{0}$, and the equation-of-state (EoS) of dark energy. We find that, in general, if 10 BNSs or NSBHs at $z=0.1$ with known redshifts are detected by 3G networks consisting of two ET-like detectors, ${H}_{0}$ can be measured with an accuracy of 0.9%. If 1000 face-on BNSs at $zl2$ are detected with known redshifts, we are able to achieve $\mathrm{\ensuremath{\Delta}}{q}_{0}=0.002$ for the deceleration parameter, or $\mathrm{\ensuremath{\Delta}}{w}_{0}=0.03$ and $\mathrm{\ensuremath{\Delta}}{w}_{a}=0.2$ for EoS of dark energy, respectively.

Evidence for anisotropy of cosmic acceleration

Abstract: Observations reveal a `bulk flow' in the local Universe which is faster and extends to much larger scales than is expected around a typical observer in the standard $\Lambda$CDM cosmology. This is expected to result in a scale-dependent dipolar modulation of the acceleration of the expansion rate inferred from observations of objects within the bulk flow. From a maximum-likelihood analysis of the Joint Lightcurve Analysis (JLA) catalogue of Type Ia supernovae we find that the deceleration parameter, in addition to a small monopole, indeed has a much bigger dipole component aligned with the CMB dipole which falls exponentially with redshift $z$: $q_0 = q_\mathrm{m} + \vec{q}_\mathrm{d}.\hat{n}\exp(-z/S)$. The best fit to data yields $q_\mathrm{d} = -8.03$ and $S = 0.0262~(\Rightarrow d \sim 100~\mathrm{Mpc})$, rejecting isotropy ($q_\mathrm{d} = 0$) with $3.9\sigma$ statistical significance, while $q_\mathrm{m} = -0.157$ and consistent with no acceleration ($q_\mathrm{m} = 0$) at $1.4\sigma$. Thus the cosmic acceleration deduced from supernovae may be an artefact of our being non-Copernican observers, rather than evidence for a dominant component of `dark energy' in the Universe.

An improved model-independent assessment of the late-time cosmic expansion

Abstract: In the current work, we have implemented an extension of the standard Gaussian Process formalism, namely the Multi-Task Gaussian Process with the ability to perform a joint learning of several cosmological data simultaneously. We have utilised the "low-redshift" expansion rate data from Supernovae Type-Ia (SN), Baryon Acoustic Oscillations (BAO) and Cosmic Chronometers (CC) data in a joint analysis. We have tested several possible models of covariance functions and find very consistent estimates for cosmologically relevant parameters. In the current formalism, we also find provisions for heuristic arguments which allow us to select the best-suited kernel for the reconstruction of expansion rate data. We also utilised our method to account for systematics in CC data and find an estimate of $H_0 = 68.52^{+0.94 + 2.51 (sys)}_{-0.94} $ $\textrm{km/s Mpc}^{-1}$ and a corresponding $r_d = 145.61^{+2.82}_{ - 2.82 - 4.3 (sys)} $ Mpc as our primary result. Subsequently, we find constraints on the present deceleration parameter $q_0 = -0.52 \pm 0.06$ and the transition redshift $z_T = 0.64^{+0.12}_{-0.09}$. All the estimated cosmological parameters are found to be in good agreement with the standard $\Lambda$CDM scenario. Including the local model-independent $H_0$ estimate to the analysis we find $H_0 = 71.40^{ + 0.30 + 1.65 (sys)}_{- 0.30 } $ $\textrm{km/s Mpc}^{-1}$ and the corresponding $r_d = 141.29^{ + 1.31 }_{-1.31-2.63 (sys)}$ Mpc. Also, the constraints on $r_d H_0$ remain consistent throughout our analysis and also with the model-dependent CMB estimate. Using the $\mathcal{O}m(z)$ diagnostic, we find that the concordance model is very consistent within the redshift range $z \lesssim 2$ and mildly discrepant for $z \gtrsim 2$.

An improved model-independent assessment of the late-time cosmic expansion

Abstract: In the current work, we have implemented an extension of the standard Gaussian Process formalism, namely the Multi-Task Gaussian Process with the ability to perform a joint learning of several cosmological data simultaneously. We have utilised the "low-redshift" expansion rate data from Supernovae Type-Ia (SN), Baryon Acoustic Oscillations (BAO) and Cosmic Chronometers (CC) data in a joint analysis. We have tested several possible models of covariance functions and find very consistent estimates for cosmologically relevant parameters. In the current formalism, we also find provisions for heuristic arguments which allow us to select the best-suited kernel for the reconstruction of expansion rate data. We also utilised our method to account for systematics in CC data and find an estimate of $H_0 = 68.52^{+0.94 + 2.51 (sys)}_{-0.94} $ $\textrm{km/s Mpc}^{-1}$ and a corresponding $r_d = 145.61^{+2.82}_{ - 2.82 - 4.3 (sys)} $ Mpc as our primary result. Subsequently, we find constraints on the present deceleration parameter $q_0 = -0.52 \pm 0.06$ and the transition redshift $z_T = 0.64^{+0.12}_{-0.09}$. All the estimated cosmological parameters are found to be in good agreement with the standard $\Lambda$CDM scenario. Including the local model-independent $H_0$ estimate to the analysis we find $H_0 = 71.40^{ + 0.30 + 1.65 (sys)}_{- 0.30 } $ $\textrm{km/s Mpc}^{-1}$ and the corresponding $r_d = 141.29^{ + 1.31 }_{-1.31-2.63 (sys)}$ Mpc. Also, the constraints on $r_d H_0$ remain consistent throughout our analysis and also with the model-dependent CMB estimate. Using the $\mathcal{O}m(z)$ diagnostic, we find that the concordance model is very consistent within the redshift range $z \lesssim 2$ and mildly discrepant for $z \gtrsim 2$.

Observational constraints on the jerk parameter with the data of the Hubble parameter

Abstract: We study the accelerated expansion phase of the universe by using the kinematic approach. In particular, the deceleration parameter q is parametrized in a model-independent way. Considering a generalized parametrization for q, we first obtain the jerk parameter j (a dimensionless third time derivative of the scale factor) and then confront it with cosmic observations. We use the latest observational dataset of the Hubble parameter H(z) consisting of 41 data points in the redshift range of $$0.07 \le z \le 2.36$$ , larger than the redshift range that covered by the Type Ia supernova. We also acquire the current values of the deceleration parameter $$q_0$$ , jerk parameter $$j_0$$ and transition redshift $$z_t$$ (at which the expansion of the universe switches from being decelerated to accelerated) with $$1\sigma $$ errors ( $$68.3\%$$ confidence level). As a result, it is demonstrate that the universe is indeed undergoing an accelerated expansion phase following the decelerated one. This is consistent with the present observations. Moreover, we find the departure for the present model from the standard $$\Lambda $$ CDM model according to the evolution of j. Furthermore, the evolution of the normalized Hubble parameter is shown for the present model and it is compared with the dataset of H(z).

Tsallis holographic dark energy in the brane cosmology

Abstract: We study some cosmological features of Tsallis holographic dark energy (THDE) in Cyclic, DGP and RS II braneworlds. In our setup, a flat FRW universe is considered filled by a pressureless source and THDE with the Hubble radius as the IR cutoff, while there is no interaction between them. Our result shows that although suitable behavior can be obtained for the system parameters such as the deceleration parameter, the models are not always stable during the cosmic evolution at the classical level.

A periodic varying deceleration parameter in f(R, T) gravity

Abstract: The phenomenon of accelerated expansion of the present universe and a cosmic transit aspect is explored in the framework of a modified gravity theory known as f(R, T) gravity (where R is the Ricci scalar and T is the trace of the energy–momentum tensor of the matter content). The cosmic transit phenomenon signifies a signature flipping behavior of the deceleration parameter. We employ a periodic varying deceleration parameter and obtained the exact solution of field equations. The dynamical features of the model including the oscillatory behavior of the EOS parameter are studied. We have also explored the obvious violation of energy–momentum conservation in f(R, T) gravity. The periodic behavior of energy conditions for the model are also discussed with a wide range of the free parameters.

Cosmology in modified f(R,T)-gravity theory in a variant Λ(T) scenario-revisited

Abstract: Three new cosmological models of the present Universe are obtained with f(R,T) modified theory of gravity proposed by Harko et al. [Phys. Rev. D 84 (2011) 024020, arXiv:1104.2669 [gr-qc]] in a general class of Bianchi space-time. In this paper, we have generalized the modified f(R,T) field equations with Λ(T)-gravity, where R and T denote the curvature scalar and the trace of the stress–energy–momentum tensor, respectively. To find the deterministic solutions we have considered the linearly varying deceleration parameter q proposed by Akarsu and Dereli [Cosmological models with linearly varying deceleration parameter, Int. J. Theor. Phys. 51 (2011) 612]. We have made the analyses of the variation of pressure, energy density and cosmological term with cosmic time. It is observed that our derived models are unstable in early time whereas they are stable at late and future time (i.e. at present epoch). The physical and geometric properties of all three models are studied in detail.

Constraints on a generalized deceleration parameter from cosmic chronometers

Abstract: In this paper, we have proposed a generalized parametrization for the deceleration parameter q in order to study the evolutionary history of the universe. We have shown that the proposed model can ...

FRW type Kaluza–Klein modified holographic Ricci dark energy models in Brans–Dicke theory of gravitation

Abstract: In this paper, we investigate non-Ricci, non-compact Friedmann–Robertson–Walker type Kaluza–Klein cosmology in the presence of pressureless matter and modified holographic Ricci dark energy in the frame work of Brans and Dicke (Phys Rev 124:965, 1961) scalar–tensor theory of gravitation. We solve the field equations of this theory using a hybrid expansion law for the five dimensional scale factor. We have also used a power law and a form of logarithmic function of the scale factor for the Brans–Dicke scalar field. Consequently, we obtain two interesting cosmological models of the Kaluza–Klein universe. We have evaluated the cosmological parameters, namely, the equation of state parameter, the deceleration parameter, and the density parameters. To check the stability of our models we use the squared speed of sound. Some well-known cosmological ( $$\omega _{de}$$ – $$\omega ^{\prime }_{de}$$ and statefinder) planes are constructed for our models. We have also analyzed the physical behavior of these parameters through graphical representation. It is observed that the FRW type Kaluza–Klein dark energy models presented are compatible with the present day cosmological observations.

Statefinder diagnostic and constraints on the Palatini f(R) gravity theories

Abstract: We focus on a series of f(R) gravity theories in Palatini formalism to investigate the probabilities of producing late-time acceleration for the flat Friedmann-Robertson-Walker (FRW) universe. We apply a statefinder diagnostic to these cosmological models for chosen series of parameters to see if they can be distinguished from one another. The diagnostic involves the statefinder pair {r, s}, where r is derived from the scale factor a and its higher derivatives with respect to the cosmic time t, and s is expressed by r and the deceleration parameter q. In conclusion, we find that although two types of f(R) theories: (i) f(R) = R + αRm – βR −n and (ii) f(R) = R + α ln R – β can lead to late-time acceleration, their evolutionary trajectories in the r – s and r – q planes reveal different evolutionary properties, which certainly justify the merits of the statefinder diagnostic. Additionally, we utilize the observational Hubble parameter data (OHD) to constrain these models of f(R) gravity. As a result, except for m = n = 1/2 in case (i), α = 0 in case (i) and case (ii) allow the ΛCDM model to exist in the 1σ confidence region. After applying the statefinder diagnostic to the best-fit models, we find that all the best-fit models are capable of going through the deceleration/acceleration transition stage with a late-time acceleration epoch, and all these models turn to the de Sitter point ({r, s} = {1, 0}) in the future. Also, the evolutionary differences between these models are distinct, especially in the r – s plane, which makes the statefinder diagnostic more reliable in discriminating cosmological models.

Plane symmetric modified holographic Ricci dark energy model in Saez-Ballester theory of gravitation

Abstract: The present work is devoted to investigation of non-static plane symmetric universe filled with matter and anisotropic modified holographic Ricci dark energy components within the framework of a scalar-tensor theory formulated by Saez and Ballester (1986). To get the determinate solution of the model we have used scalar expansion proportional to the shear scalar, linearly varying deceleration parameter and energy density of modified holographic Ricci dark energy. We study the equation of state parameter ( ω Λ ) , deceleration parameter (q), squared speed of sound ( v s 2 ) and ω Λ - ω Λ ′ and r - s planes for our dark energy model. The equation of state parameter provides a quintom-like behavior of the universe. The r - s plane corresponds to Λ CDM limit, Chaplygin and phantom regions, whereas ω Λ - ω Λ ′ trajectories correspond to both thawing and freezing regions.

Falsifying cosmological models based on a non-linear electrodynamics

Abstract: Recently, the nonlinear electrodynamics (NED) has been gaining attention to generate primordial magnetic fields in the Universe and also to resolve singularity problems. Moreover, recent works have shown the crucial role of the NED on the inflation. This paper provides a new approach based on a new model of NED as a source of gravitation to remove the cosmic singularity at the big bang and explain the cosmic acceleration during the inflation era on the background of stochastic magnetic field. Also, we found a realization of a cyclic Universe, free of initial singularity, due to the proposed NED energy density. In addition, we explore whether a NED field without or with matter can be the origin of the late-time acceleration. For this we obtain explicit equations for H(z) and perform a MCMC analysis to constrain the NED parameters by using 31 observational Hubble data (OHD) obtained from cosmic chronometers covering the redshift range $$0< z < 1.97$$ ; and with the joint-light-analysis (JLA) SNIa compilation consisting in 740 data points in the range $$0.01

Constraining Bianchi Type I Universe With Type Ia Supernova and H(z) Data

Abstract: We use recent 36 observational Hubble data (OHD) in the redshift range $0.07\leq z\leq 2.36$, latest \textgravedbl joint light curves\textacutedbl (JLA) sample, comprised of 740 type Ia supernovae (SNIa) in the redshift range $0.01\leq z \leq 1.30$, and their joint combination datasets to constrain anisotropic Bianchi type I (BI) dark energy (DE) model. To estimate model parameters, we apply Hamiltonian Monte Carlo technique. We also compute the covariance matrix for BI dark energy model by considering different datasets to compare the correlation between model parameters. To check the acceptability of our fittings, all results are compared with those obtained from 9 year WMAP as well as Planck (2015) collaboration. Our estimations show that at 68\% confidence level the dark energy equation of state (EOS) parameter for OHD or JLA datasets alone varies between quintessence and phantom regions whereas for OHD+JLA dataset this parameter only varies in phantom region. It is also found that the current cosmic anisotropy is of order $\sim10^{-3}$ which imply that the OHD and JLA datasets do not put tight constraint on this parameter. Therefore, to constraint anisotropy parameter, it is necessary to use high redshif dataset namely cosmic microwave background (CMB). Moreover, from the calculation of $p$-value associated with $\chi^{2}$ statistic we observed that non of the $\omega \mbox{BI}$ and flat $\omega\mbox{CDM}$ models rule out by OHD or JLA datasets. The deceleration parameter is obtained as $q=-0.46^{+0.89 +0.36}_{-0.41 -0.37}$, $q=-0.619^{+0.12 +0.20}_{-0.095 -0.24}$, and $q=-0.52^{+0.080 +0.014}_{-0.046 -0.15}$ for OHD, SNIa, and OHD+SNIa data respectively.

Anisotropic model with decaying cosmological term

Abstract: A spatially homogeneous and anisotropic locally rotationally symmetric Bianchi type-I spacetime with cosmological term $\varLambda $ in $f(R,T) $ theory has been studied. The exact solution of the field equations is obtained under a variation law of the Hubble parameter $(H) $ which yields a time dependent deceleration parameter (Banerjee and Das in Gen. Relativ. Gravit. 37:10, 2005). The model presents a cosmological scenario which describes early deceleration and late time acceleration. The physical parameters of the model have been analysed.

Gauging the cosmic acceleration with recent type Ia supernovae data sets

Abstract: We revisit a model-independent estimator for cosmic acceleration based on type Ia supernovae distance measurements. This approach does not rely on any specific theory for gravity, energy content, nor parametrization for the scale factor or deceleration parameter and is based on falsifying the null hypothesis that the Universe never expanded in an accelerated way. By generating mock catalogs of known cosmologies, we test the robustness of this estimator, establishing its limits of applicability. We detail the pros and cons of such an approach. For example, we find that there are specific counterexamples in which the estimator wrongly provides evidence against acceleration in accelerating cosmologies. The dependence of the estimator on the ${H}_{0}$ value is also discussed. Finally, we update the evidence for acceleration using the recent UNION2.1 and Joint Light-Curve Analysis samples. Contrary to recent claims, available data strongly favor an accelerated expansion of the Universe in complete agreement with the standard $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model.

Quantifying the evidence for the current speed-up of the Universe with low and intermediate-redshift data. A more model-independent approach

Abstract: We reconstruct in this paper the deceleration and jerk parameters as functions of the cosmological redshift from data on cosmic chronometers (CCH), baryon acoustic oscillations (BAOs), and the Pantheon+MCT compilation of supernovae of Type Ia (SnIa). The reconstruction is carried out with the Weighted Function Regression method, previously introduced by Gomez-Valent & Amendola (2018). It improves the usual cosmographic approach by automatically implementing Occam's razor criterion. This makes our procedure to be more free of model and parametrization dependencies than many other analyses in the literature. The reconstructed functions are fully compatible with the predictions for the concordance model. In addition, we also discuss the confidence level at which we can claim that the Universe (assumed to be flat, homogeneous and isotropic) is currently accelerating. According to Jeffreys' scale and jargon, we find moderate evidence in favor of such speed-up using the data on SnIa+CCH, and very strong one when we also use data on BAOs. The measured current value of the deceleration parameter in the latter case reads $q_0\sim -0.60\pm 0.10$, and for the deceleration-acceleration transition redshift we find $z_t\sim 0.8\pm 0.10$. The former is $\sim 6\sigma$ away from $0$. This is in stark contrast, for instance, with the $\sim 17\sigma$ that are found in the context of the flat $\Lambda$CDM even without including the BAOs data. This indicates that cosmography and Occam's razor criterion play a crucial role in this discussion, and that estimating the evidence for positive acceleration only in the framework of a particular cosmological model or parametrization is clearly insufficient.

FLRW cosmological models with quark and strange quark matters in f(R,T) gravity.

Abstract: In this paper, we have studied the magnetized quark matter (QM) and strange quark matter (SQM) distributions in the presence of $ f(R,T)$ gravity in the background of Friedmann--Lema\^itre--Robertson--Walker (FLRW) metric. To get exact solutions of modified field equations we have used $f(R,T) = R + 2 f(T)$ model given by Harko et al. with two different parametrization of geometrical parameters \textit{i.e.} the parametrization of the deceleration parameter $ q $, and the scale factor $ a $ in hybrid expansion form. Also, we have obtained Einstein Static Universe (ESU) solutions for QM and SQM distributions in $f(R,T)$ gravity and General Relativity (GR). All models in $f(R,T)$ gravity and GR for FRW and ESU Universes with QM also SQM distributions, we get zero magnetic field. These results agree with the solutions of Akta{\c{s} and Ayg\"un in $f(R,T)$ gravity. However, we have also discussed the physical consequences of our obtained models.

An interacting new holographic dark energy in the framework of fractal cosmology

Abstract: In this paper, we study an interacting holographic dark energy model in the framework of fractal cosmology. The features of fractal cosmology could pass ultraviolet divergencies and also make a better understanding of the universe in different dimensions. We discuss a fractal FRW universe filled with the dark energy and cold dark matter interacting with each other. It is observed that the Hubble parameter embraces the recent observational range while the deceleration parameter demonstrates an accelerating universe and a behavior similar to $\Lambda \mbox{CDM}$ . Plotting the equation of state shows that it lies in phantom region for interaction mode. We use $\mathit{Om}$ -diagnostic tool and it shows a phantom behavior of dark energy which is a condition of avoiding the formation of black holes. Finally we execute the StateFinder diagnostic pair and all the trajectories for interacting and non-interacting state of the model meet the fixed point $\Lambda \mbox{CDM}$ at the start of the evolution. A behavior similar to Chaplygin gas also can be observed in statefinder plane. We find that new holographic dark energy model (NHDE) in fractal cosmology expressed the consistent behavior with recent observational data and can be considered as a model to avoid the formation of black holes in comparison with the main model of NHDE in the simple FRW universe. It has also been observed that for the interaction term varying with matter density, the model generates asymptotic de-Sitter solution. However, if the interaction term varies with energy density, then the model shows Big-Rip singularity. Using our modified CAMB code, we observed that the interacting model suppresses the CMB spectrum at low multipoles $l<50$ and enhances the acoustic peaks. Based on the observational data sets used in this paper and using Metropolis-Hastings method of MCMC numerical calculation, it seems that the best value with $1\sigma $ and $2\sigma $ confidence interval are $\Omega _{m0}=0.278^{+0.008~+0.010} _{-0.007~-0.009}$ , $H_{0}=69.9^{+0.95~+1.57}_{-0.95~-1.57}$ , $r_{c}=0.08^{+0.02~+0.027}_{-0.002~-0.0027}$ , $\beta =0.496^{+0.005~+0.009} _{-0.005~-0.009}$ , $c= 0.691^{+0.024~+0.039}_{-0.025~-0.037}$ and $b^{2}=0.035$ according to which we find that the proposed model in the presence of interaction is compatible with the recent observational data.

A stable flat universe with variable cosmological constant in $f(R,T)$ gravity

Abstract: In this paper, a general FRW cosmological model has been constructed in f(R,T) gravity reconstruction with variable cosmological constant. A number of solutions to the field equations has been generated by utilizing a form for the Hubble parameter that leads to Berman’s law of constant deceleration parameter q = m − 1. The possible decelerating and accelerating solutions have been investigated. For (q > 0) we get a stable flat decelerating radiation-dominated universe at q = 1. For (q < 0) we get a stable accelerating solution describing a flat universe with positive energy density and negative cosmological constant. Nonconventional mechanisms that are expected to address the late-time acceleration with negative cosmological constant have been discussed.

Cosmological singularities in interacting dark energy models with an $\omega(q)$ parametrization

Abstract: Future singularities arising in a family of models for the expanding Universe, characterized by sharing a convenient parametrization of the energy budget in terms of the deceleration parameter, are classified. Finite-time future singularities are known to appear in many cosmological scenarios, in particular, in the presence of viscosity or non-gravitational interactions, the last being known to be able to suppress or just change in some cases the type of the cosmological singularity. Here, a family of models with a parametrization of the energy budget in terms of the deceleration parameter are studied in the light of Gaussian processes using reconstructed data from $40$-value $H(z)$ datasets. Eventually, the form of the possible non-gravitational interaction between dark energy and dark matter is constructed from these smoothed $H(z)$ data. Using phase space analysis, it is shown that a non-interacting model with dark energy $\omega_\mathrm{de} = \omega_{0} + \omega_{1}q$ ($q$ being the deceleration parameter) may evolve, after starting from a matter dominated unstable state, into a de Sitter Universe (the solution being in fact a stable node). Moreover, for a model with interaction term $Q = 3 H b \rho_\mathrm{dm}$ ($b$ is a parameter and $H$ the Hubble constant) three stable critical points are obtained, what may have important astrophysical implications. In addition, part of the paper is devoted to a general discussion of the finite-time future singularities obtained from direct numerical integration of the field equations, since they appear in many cosmological scenarios and could be useful for future extended studies of the models here introduced. Numerical solutions for the new models, produce finite-time future singularities of Type I or Type III, or an $\omega$-singularity, provided general relativity describes the background dynamics.

Viscous cosmology in new holographic dark energy model and the cosmic acceleration

Abstract: In this work, we study a flat Friedmann–Robertson–Walker universe filled with dark matter and viscous new holographic dark energy. We present four possible solutions of the model depending on the choice of the viscous term. We obtain the evolution of the cosmological quantities such as scale factor, deceleration parameter and transition redshift to observe the effect of viscosity in the evolution. We also emphasis upon the two independent geometrical diagnostics for our model, namely the statefinder and the Om diagnostics. In the first case we study new holographic dark energy model without viscous and obtain power-law expansion of the universe which gives constant deceleration parameter and statefinder parameters. In the limit of the parameter, the model approaches to $$\Lambda CDM$$ model. In new holographic dark energy model with viscous, the bulk viscous coefficient is assumed as $$\zeta =\zeta _{0}+\zeta _{1}H$$ , where $$\zeta _{0}$$ and $$\zeta _{1}$$ are constants, and H is the Hubble parameter. In this model, we obtain all possible solutions with viscous term and analyze the expansion history of the universe. We draw the evolution graphs of the scale factor and deceleration parameter. It is observed that the universe transits from deceleration to acceleration for small values of $$\zeta $$ in late time. However, it accelerates very fast from the beginning for large values of $$\zeta $$ . By illustrating the evolutionary trajectories in $$r-s$$ and $$r-q$$ planes, we find that our model behaves as an quintessence like for small values of viscous coefficient and a Chaplygin gas like for large values of bulk viscous coefficient at early stage. However, model has close resemblance to that of the $$\Lambda CDM$$ cosmology in late time. The $$ Om$$ has positive and negative curvatures for phantom and quintessence models, respectively depending on $$\zeta $$ . Our study shows that the bulk viscosity plays very important role in the expansion history of the universe.

A new parametrization of dark energy equation of state leading to double exponential potential

Abstract: We show that a canonical scalar field with a phenomenological form of energy density or equivalently an equation of state parameter can provide the required transition from decelerated ($q>0$) to accelerated expansion ($q<0$) phase of the universe. We have used the latest Type Ia Supernova (SNIa) and Hubble parameter datasets to constrain the model parameters. It has been found that for each of these dataset, the transition in deceleration parameter $q$ takes place at the recent past ($z<1$). The future evolution of $q$ is also discussed in the context of the model under consideration. Furthermore, using those datasets, we have reconstructed $\omega_{\phi}(z)$, the equation of state parameter for the scalar field. The results show that the reconstructed forms of $q(z)$ and $\omega_{\phi}(z)$ do not differ much from the standard $\Lambda$CDM value at the current epoch. Finally, the functional form of the relevant potential $V(\phi)$ is derived by a parametric reconstruction from the observational dataset. The corresponding $V(\phi)$ comes out to be a double exponential potential which has a number of cosmological implications. Additionally, we have also studied the effect of this particular scalar field dark energy sector on the evolution of matter over-densities and compared it with the $\Lambda$CDM model.

New holographic dark energy model with constant bulk viscosity in modified $$ gravity theory

Abstract: The aim of this paper is to study new holographic dark energy (HDE) model in modified $f(R,T)$ gravity theory within the framework of a flat Friedmann-Robertson-Walker model with bulk viscous matter content. It is thought that the negative pressure caused by the bulk viscosity can play the role of dark energy component, and drive the accelerating expansion of the universe. This is the motive of this paper to observe such phenomena with bulk viscosity. In the specific model $f(R,T)=R+\lambda T$ , where $R$ is the Ricci scalar, $T$ the trace of the energy-momentum tensor and $\lambda $ is a constant, we find the solution for non-viscous and viscous new HDE models. We analyze new HDE model with constant bulk viscosity, $\zeta =\zeta _{0}= \text{const.}$ to explain the present accelerated expansion of the universe. We classify all possible scenarios (deceleration, acceleration and their transition) with possible positive and negative ranges of $\lambda $ over the constraint on $\zeta _{0}$ to analyze the evolution of the universe. We obtain the solutions of scale factor and deceleration parameter, and discuss the evolution of the universe. We observe the future finite-time singularities of type I and III at a finite time under certain constraints on $\lambda $ . We also investigate the statefinder and $\mathit{Om}$ diagnostics of the viscous new HDE model to discriminate with other existing dark energy models. In late time the viscous new HDE model approaches to $\varLambda \mathit{CDM}$ model. We also discuss the thermodynamics and entropy of the model and find that it satisfies the second law of thermodynamics.

Ricci dark energy model with bulk viscosity

Abstract: In order to explore the possibility of bulk viscosity as a possible candidate of dark energy to explain the accelerating universe, we investigate the dissipative processes in the holographic Ricci dark energy (HRDE) model within the framework of the standard Eckart theory of relativistic thermodynamics. We assume that the flat Friedmann-Robertson-Walker universe is filled with pressureless dark matter and viscous HRDE. We obtain the exact solutions of non-viscous and viscous HRDE models, respectively. We plot the evolution of the scale factor and deceleration parameter to observe the transition phase in the viscous case. We also discuss two geometrical diagnostics, namely the statefinder and Om to discriminate from other existing dark energy models. In the non-viscous HRDE model, the power-law form of expansion is obtained which gives the constant deceleration parameter and statefinder pair. It does not show phase transition. In the viscous HRDE model, we consider all possible forms of bulk viscous coefficient (with constant or general form of viscous terms) and discuss the cosmological evolution in detail. We obtain the exponential expansion of the scale factor which gives the time-dependent deceleration parameter and statefinder pair. The model shows the transition from the decelerated phase to the accelerated phase depending on the values of the viscous term. It starts to accelerate in the past for large values of the viscous term. The trajectory of the statefinder pair shows that for small values of the viscous term, the trajectory is curved and starts from quintessence in early time and approaches to $\Lambda$ CDM model in late time. It is also observed that for large values of the viscous term, it behaves like the Chaplygin gas in early time but approaches to $\Lambda$ CDM model in late time. We also plot the trajectory of the deceleration parameter, statefinder pair and Om with the model parameter to observe the evolutionary behavior of the universe. It also shows a similar behavior except for the fact that it only behaves as quintessence in the past but approaches to $\Lambda$ CDM or the steady state model in late time. The results of viscous HRDE models show that the recent acceleration is well explained with the viscous term.

Impact of Collisional Matter on the Late-Time Dynamics of f(R,T) Gravity

Abstract: We study the cosmic evolution of non-minimally coupled f ( R , T ) gravity in the presence of matter fluids consisting of collisional self-interacting dark matter and radiation. We study the cosmic evolution in the presence of collisional matter, and we compare the results with those corresponding to non-collisional matter and the Λ -cold-dark-matter ( Λ CDM) model. Particularly, for a flat Friedmann–Lema i ^ tre–Robertson–Walker Universe, we study two non-minimally coupled f ( R , T ) gravity models and we focus our study on the late-time dynamical evolution of the model. Our study is focused on the late-time behavior of the effective equation of the state parameter ω e f f and of the deceleration parameter q as functions of the redshift for a Universe containing collisional and non-collisional dark matter fluids, and we compare both models with the Λ CDM model. As we demonstrate, the resulting picture is well accommodated to the latest observational data on the basis of physical parameters.