Acceleration of the universe: a reconstruction of the effective equation of state

The present work is based upon a parametric reconstruction of the effective or total equation of state in a model for the universe with accelerated expansion. The constraints on the model parameters are obtained by maximum likelihood analysis using the supernova distance modulus data, observational Hubble data, baryon acoustic oscillation data and cosmic microwave background shift parameter data. For statistical comparison, the same analysis has also been carried out for the wCDM dark energy model. Different model selection criteria (Akaike information criterion (AIC)) and (Bayesian Information Criterion (BIC)) give the clear indication that the reconstructed model is well consistent with the wCDM model. Then both the models (w_{eff}(z) model and wCDM model) have also been presented through (q_0 ,j_0 ) parameter space. Tighter constraint on the present values of dark energy equation of state parameter (w_{DE}(z = 0)) and cosmological jerk (j_0) have been achieved for the reconstructed model.Comment: 11 pages, 8 figure

A parametric reconstruction of the cosmological jerk from diverse observational data sets

A parametric reconstruction of the jerk parameter, the third order derivative of the scale factor expressed in a dimensionless way, has been discussed. Observational constraints on the model parameters have been obtained by Maximum Likelihood Analysis of the models using Supernova Distance Modulus data (SNe), Observational Hubble Data (OHD), Baryon Acoustic Oscillation (BAO) data and CMB shift parameter data (CMBShift). The present value of the jerk parameter has been kept open to start with, but the plots of various cosmological parameter like deceleration parameter $q(z)$, jerk parameter $j(z)$, dark energy equation of state parameter $w_{DE}(z)$ indicate that the reconstructed models are very close to a $\Lambda$CDM model with a slight inclination towards a non-phantom behaviour of the evolution.Comment: 12 pages, 44 figures; Accepted for publication in Phys. Rev.

A Reconstruction of Quintessence Dark Energy

With a parametric form of the equation of state parameter of dark energy, a quintessence potential has been reconstructed. The potential is found to be a generalization of a double exponential potential. The constraints on the parameters are obtained by maximum likelihood analysis using observational Hubble data, type Ia supernova data, baryon acoustic oscillation data and the CMB shift parameter data. Th emodel shows preference towards the phantom behaviour of dark energy

Acceleration of the Universe in f(R) Gravity Models

A general formalism for the investigation of the late time dynamics of the universe for any analytic f(R) gravity model, along with a cold dark matter, has been discussed in the present work. The formalism is then elucidated with two examples. The values of the parameters of the models are chosen in such a way that they are consistent with the basic observational requirement.Comment: 6 pages; 5 figures. To appear in Astrophysics and Space Scienc

Reconstructing late-time cosmology with kinematical models

The present work is based on the reconstruction of late-time cosmological dynamics using purely kinematical models. The models are constructed from different parameterizations of the deceleration parameter. The models are confronted with cosmological observations which are completely independent of any fiducial assumption about the background cosmological model. Different kinematical parameters, namely the present Hubble parameter, present value of the deceleration parameter and the redshift of transition from decelerated to accelerated phase of expansion are constrained by Markov Chanin Monte Charlo (MCMC) analysis using the observation data sets. The evolution of different cosmological quantities are studied for the present models. The evolution of linear matter density contrast has been studied for the present kinematical models and the result is compared with the standard cosmological constant dark energy scenario. The thermodynamic nature of the universe has also been emphasized in the present context.Comment: 12 pages, 9 figure

Spherical collapse in DGP braneworld cosmology

The DGP (Dvali, Gabadaze and Porrati) braneworld cosmology gives an alternative to dark energy. In DGP cosmology the alleged cosmic acceleration is generated by the modification of gravity theory. Nonlinear evolution of matter density contrast in DGP braneworld cosmology is studied in the present work. The semi-analytic approach of spherical collapse of matter overdensity is adopted in the present context to study the nonliner evolution. Further, the number count of galaxy clusters along the redshift is studied for the DGP cosmology using Press-Schechter and Sheth-Tormen mass function formalisms. It is observed that for same values of cosmological parameters, the DGP model enhances the number of galaxy cluster compared to the standard $\Lambda$CDM scenario.Comment: 14 pages, 6 figures. arXiv admin note: text overlap with arXiv:2008.0379

Spherical collapse in a dark energy model with reconstructed effective equation of state

The present work deals with the study of spherical collapse of matter density contrast in a latetime cosmological model with a reconstructed effective equation of state. The linear and nonlinear evolution of matter density contrast are studied. The variation of the critical density at collapse of the spherical overdense region along redshift are also investigate. Further the number count of collapsed object or the dark matter halos, equivalent to the number count of galaxy clusters, are also studied. Two different halo mass function formulations, namely the Press-Schechter mass function and the Sheth-Tormen mass function, are adopetd to study the cluster number count along the redshift. Similar analysis is also carried out for wCDM dark energy model to have a direct comparison with the reconstructed effective equation of state model. These two models are highly degenerate at the background and linear level of matter perturbation. But the nonlinear evolution of matter overdensity breaks the degeneracy. The reconstructed effective equation of state model shows a substantial suppression in the cluster number count compared to wCDM for redshift z > 0.5, and for z < 0.5, the number count is slightly higher than that of wCDM.Comment: 12 pages, 5 figure

Clustering of dark matter in interacting tachyon dark energy with Λ\LambdaCDM background

One of the non-canonical descriptions of scalar field dark energy is the tachyon. The present work is devoted to study the dynamics of dark matter overdensity in a conformally coupled tachyon field dark energy model. The model is tuned to mimic the $\Lambda$CDM cosmology at background level. The semi-analytic spherical collapse model of dark matter overdensity is adopted to study the nonlinear evolution. The effects of non-minimal coupling in the energy budget on the clustering of dark matter is investigated. It is observed that the growth rate of matter overdensity is higher in presence of the non-minimal coupling. The critical density at collapse is suppressed in case of interaction. Further the number counts of dark matter halos or galaxy clusters along redshift are studied using the Press-Schechter and Sheth- Tormen halo mass functions. Suppression in the number of dark matter halos is observed when the interaction is allowed. A comparison of cluster number count in the present model and $\Lambda$CDM is carried out. The present model allowing the interaction produces much lower number of galaxy clusters compared to $\Lambda$CDM, but without interaction the cluster number count is slightly higher than $\Lambda$CDM cluster count.Comment: 15 pages, 6 figure

Astronomical bounds on a cosmological model allowing a general interaction in the dark sector

Non-gravitational interaction between two barotropic dark fluids, namely the pressureless dust and the dark energy in a spatially flat Friedmann-Lema\^{i}tre-Robertson-Walker model has been discussed. It is shown that for the interactions which are linear in terms the energy densities of the dark components and their first order derivatives, the net energy density is governed by a second order differential equation with constant coefficients. Taking a generalized interaction, which includes a number of already known interactions as special cases, the dynamics of the universe is described for three types of the dark energy equation of state, namely that of interacting quintessence, interacting vacuum energy density and interacting phantom. The models have been constrained using the standard cosmological probes, Supernovae type Ia data from joint light curve analysis and the observational Hubble parameter data. Two geometric tests, the cosmographic studies and the $Om$ diagnostic have been invoked so as to ascertain the behaviour of the present model vis-a-vis the $\Lambda$-cold dark matter model. We further discussed the interacting scenarios taking into account the thermodynamic considerations.Comment: 20 pages; 4 tables; 14 figures; Published version in MNRA

Assessment of the cosmic distance duality relation using Gaussian Process

Two types of distance measurement are important in cosmological observations, the angular diameter distance $d_A$ and the luminosity distance $d_L$. In the present work, we carried out an assessment of the theoretical relation between these two distance measurements, namely the cosmic distance duality relation, from type Ia supernovae (SN-Ia) data, the Cosmic Chronometer (CC) Hubble parameter data, and baryon acoustic oscillation (BAO) data using Gaussian Process. The luminosity distance curve and the angular diameter distance curve are extracted from the SN-Ia data and the combination of BAO and CC data respectively using the Gaussian Process. The distance duality relation is checked by a non-parametric reconstruction using the reconstructed $H$, $d_L$, and the volume-averaged distance $D_v$. We compare the results obtained for different choices of the covariance function employed in the Gaussian Process. It is observed that the theoretical distance duality relation is in well agreement with the present analysis in 2$\sigma$ for the overlapping redshift domain $0 \leq z \leq 2$ of the reconstruction.Comment: 10 pages, 8 sets of figures, accepted for publication in MNRA