arXiv subjects

Ruchika

Publications and source records attributed to Ruchika.

23 records · Page 2Linked to original sources

arXiv2019

Cosmology With Low-Redshift Observations: No Signal For New Physics

We analyse various low-redshift cosmological data from Type-Ia Supernova, Baryon Acoustic Oscillations, Time-Delay measurements using Strong-Lensing, $H(z)$ measurements using Cosmic Chronometers and growth measurements from large scale structure observations for $Λ$CDM and some different dark energy models. By calculating the Bayesian Evidence for different dark energy models, we find out that the $Λ$CDM still gives the best fit to the data with $H_{0}=70.3^{+1.36}_{-1.35}$ Km/s/Mpc (at $1σ$). This value is in $2σ$ or less tension with various low and high redshift measurements for $H_{0}$ including SH0ES, Planck-2018 and the recent results from H0LiCOW-XIII. The derived constraint on $S_{8}=σ_{8}\sqrt{Ω_{m0}/{0.3}}$ from our analysis is $S_{8} = 0.76^{+0.03}_{-0.03}$, fully consistent with direct measurement of $S_{8}$ by KiDS+VIKING-450+DES1 survey. We hence conclude that the $Λ$CDM model with parameter constraints obtained in this work is consistent with different early and late Universe observations within $2σ$. We therefore, do not find any compelling reason to go beyond concordance $Λ$CDM model.

astro-ph.CO↗

arXiv2019

Model independent constraints on dark energy evolution from low-redshift observations

Knowing the late time evolution of the Universe and finding out the causes for this evolution are the important challenges of modern cosmology. In this work, we adopt a model-independent cosmographic approach and approximate the Hubble parameter considering the Pade approximation which works better than the standard Taylor series approximation for $z>1$. With this, we constrain the late time evolution of the Universe considering low-redshift observations coming from SNIa, BAO, $H(z)$, $H_{0}$ , strong-lensing time-delay as well as the Megamaser observations for angular diameter distances. We confirm the tensions with $Λ$CDM model for low-redshifts observations. The present value of the equation of state for the dark energy has to be phantom-like and for other redshifts, it has to be either phantom or should have a phantom crossing. For lower values of $Ω_{m0}$, multiple phantom crossings are expected. This poses serious challenges for single, non-interacting scalar field models for dark energy. We derive constraints on the {\it statefinders} $(r,s)$ and these constraints show that a single dark energy model cannot fit data for the whole redshift range $0\leq z\leq 2$: in other words, we need multiple dark energy behaviors for different redshift ranges. Moreover, the constraint on sound speed for the total fluid of the Universe, and for the dark energy fluid (assuming them being barotropic), rules out the possibility of a barotropic fluid model for unified dark sector and barotropic fluid model for dark energy, as fluctuations in these fluids are unstable as $c_{s}^2 < 0$ due to constraints from low-redshift observations.

astro-ph.CO↗

arXiv2017

Bayesian Evidences for Dark Energy models in light of current obsevational data

We do a comprehensive study of the Bayesian evidences for a large number of dark energy models using a combination of latest cosmological data from SNIa, CMB, BAO, Strong lensing time delay, Growth measurements, measurements of Hubble parameter at different redshifts and measurements of angular diameter distance by Megamaser Cosmology Project . We consider a variety of scalar field models with different potentials as well as different parametrisations for the dark energy equation of state. Among 21 models that we consider in our study, we do not find strong evidences in favour of any evolving dark energy model compared to $Λ$CDM. For the evolving dark energy models, we show that purely non-phantom models have much better evidences compared to those models that allow both phantom and non-phantom behaviours. Canonical scalar field with exponential and tachyon field with square potential have highest evidences among all the models considered in this work. We also show that a combination of low redshift measurements decisively favours an accelerating $Λ$CDM model compared to a non-accelerating power law model.

astro-ph.CO↗

arXiv2017

The Price of Shifting the Hubble Constant

An anisotropic measurement of the baryon acoustic oscillation (BAO) feature fixes the product of the Hubble constant and the acoustic scale $H_0 r_d$. Therefore, regardless of the dark energy dynamics, to accommodate a higher value of $H_0$ one needs a lower $r_d$ and so necessarily a modification of early time cosmology. One must either reduce the age of the Universe at the drag epoch or else the speed of sound in the primordial plasma. The first can be achieved, for example, with dark radiation or very early dark energy, automatically preserving the angular size of the acoustic scale in the Cosmic Microwave Background (CMB) with no modifications to post-recombination dark energy. However it is known that the simplest such modifications fall afoul of CMB constraints at higher multipoles. As an example, we combine anisotropic BAO with geometric measurements from strong lensing time delays from H0LiCOW and megamasers from the Megamaser Cosmology Project to measure $r_d$, with and without the local distance ladder measurement of $H_0$. We find that the best fit value of $r_d$ is indeed quite insensitive to the dark energy model, and is also hardly affected by the inclusion of the local distance ladder data.

astro-ph.CO↗

arXiv2017

Is it time to go beyond $Λ$CDM universe?

Concordance $Λ$CDM universe is the simplest model that is consistent with a large variety of cosmological observations till date. But few recent observations indicate inconsistencies in $Λ$CDM model. In this paper, we consider the combination of recent SnIa+Bao+Cmb+Growth+$H(z)$+$H_{0}$ measurements to revisit the constraints on the dark energy evolution using the widely studied CPL parametrisation for the dark energy equation of state. Although the reconstructed behaviour for the dark energy equation of state confirms the inconsistency of $Λ$CDM at $95\%$ confidence level, the reconstructed $Om$ diagnostic which is a {\it null test} for $Λ$CDM, still allows the concordance $Λ$CDM behaviour with a lower range of $Ω_{m0}$ than that obtained by Planck-2015. {\it This confirms that $Λ$CDM is still the best choice for the dark energy model}. We also measure the parameter $S = σ_{8}\sqrt{Ω_{m0}/0.3} = 0.728 \pm 0.023$ which is consistent with its recent measurement by KiDS survey. The confidence contour in the $Ω_{m0}-σ_{8}$ parameter plane is also fully consistent with KiDS survey measurement.

astro-ph.CO↗