Bharat Ratra’s research while affiliated with Kansas State University and other places

Gamma-ray bursts (GRBs) are promising cosmological probes for exploring the Universe at intermediate redshifts (z). We analyze 151 Fermi-observed long GRBs (datasets A123 and A28) to simultaneously constrain the Amati correlation and cosmological parameters within six spatially flat and nonflat dark energy models. We find that these datasets are standardizable via a single Amati correlation, suggesting their potential for cosmological analyses. However, constraints on the current value of the nonrelativistic matter density parameter from A123 and the combined A123 + A28 data exhibit $>2\sigma$ tension with those derived from a joint analysis of better-established Hubble parameter [H(z)] and baryon acoustic oscillation (BAO) data for most considered cosmological models. This tension indicates that these GRB data are unsuitable for jointly constraining cosmological parameters with better-established H(z) + BAO and similar data. Although the A28 data constraints are consistent with the H(z) + BAO data constraints, its limited sample size (28 GRBs) and high intrinsic scatter ($\sim0.7$) diminishes its statistical power compared to existing datasets.

We study spatially-flat dynamical dark energy parametrizations, w(z)CDM, with redshift-dependent dark energy equation of state parameter w(z) expressed using three different quadratic and other polynomial forms (as functions of $1-a$, where a is the scale factor), without and with a varying cosmic microwave background (CMB) lensing consistency parameter $A_L$. We use Planck CMB anisotropy data (P18 and lensing) and a large, mutually-consistent non-CMB data compilation that includes Pantheon+ type Ia supernova, baryon acoustic oscillation (BAO), Hubble parameter (H(z)), and growth factor ($f\sigma_8$) measurements, but not recent DESI BAO data. The six w(z)CDM ($+A_L$) parametrizations show higher consistency between the CMB and non-CMB data constraints compared to the XCDM ($+A_L$) and $w_0 w_a$CDM ($+A_L$) cases. Constraints from the most-restrictive P18+lensing+non-CMB data compilation on the six w(z)CDM ($+A_L$) parametrizations indicate that dark energy dynamics is favored over a cosmological constant by $\gtrsim 2\sigma$ when $A_L = 1$, but only by $\gtrsim 1\sigma$ when $A_L$ is allowed to vary (and $A_L>1$ at $\sim2\sigma$ significance). Non-CMB data dominate the P18+lensing+non-CMB compilation at low z and favor quintessence-like dark energy. At high z P18+lensing data dominate, favoring phantom-like dark energy with significance from $1.5\sigma$ to $2.9 \sigma$ when $A_L = 1$, and from $1.1\sigma$ to $1.8\sigma$ when $A_L$ varies. These results suggest that the observed excess weak lensing smoothing of some of the Planck CMB anistropy multipoles is partially responsible for the $A_L = 1$ cases $\gtrsim 2\sigma$ evidence for dark energy dynamics over a cosmological constant.

We test the standardizability of a homogeneous sample of 41 lower-redshift ($0.00415\leq z \leq 0.474$) active galactic nuclei (AGNs) reverberation-mapped (RM) using the broad H$\alpha$ and H$\beta$ emission lines. We find that these sources can be standardized using four radius$-$luminosity ($R-L$) relations incorporating H$\alpha$ and H$\beta$ time delays and monochromatic and broad H$\alpha$ luminosities. Although the $R-L$ relation parameters are well constrained and independent of the six cosmological models considered, the resulting cosmological constraints are weak. The measured $R-L$ relations exhibit slightly steeper slopes than predicted by a simple photoionization model and steeper than those from previous higher-redshift H$\beta$ analyses based on larger datasets. These differences likely reflect the absence of high-accreting sources in our smaller, lower-redshift sample, which primarily comprises lower-accreting AGNs. The inferred cosmological parameters are consistent within 2$\sigma$ (or better) with those from better-established cosmological probes. This contrasts with our earlier findings using a larger, heterogeneous sample of 118 H$\beta$ AGNs, which yielded cosmological constraints differing by $\gtrsim 2\sigma$ from better-established cosmological probes. Our analysis demonstrates that sample homogeneity$-$specifically, the use of a consistent time-lag determination method$-$is crucial for developing RM AGNs as a cosmological probe.

By using gamma-ray burst (GRB) data to simultaneously constrain Amati correlation parameters and cosmological parameters in six spatially flat and nonflat dark energy cosmological models, we show that an updated 220 GRB version of the Jia et al. [1] GRB data compilation are standardizable through the Amati correlation and so can be used for cosmological analyses. However, the resulting GRB data constraints on the current value of the nonrelativistic matter density parameter, Ω m 0, are in > 2σ tension with those from a joint analysis of better-established Hubble parameter [H(z)] and baryon acoustic oscillation (BAO) data for most of the cosmological models we consider, indicating that these GRB data cannot be jointly used with better-established H(z) + BAO data to constrain cosmological parameters.

We study the performance of the spatially-flat dynamical dark energy (DE) $w_0w_a$CDM parameterization, with redshift-dependent DE fluid equation of state parameter $w(z) = w_0 + w_a z/(1+z)$, with and without a varying CMB lensing consistency parameter $A_L$, against Planck cosmic microwave background (CMB) data (P18 and lensing) and a combination of non-CMB data composed of baryonic acoustic oscillation (BAO) measurements that do not include DESI BAO data, Pantheon+ type Ia supernovae (SNIa) observations, Hubble parameter [H(z)] measurements, and growth factor ($f\sigma_8$) data points. From our most restrictive data set, P18+lensing+non-CMB, for the $w_0w_a$CDM+$A_L$ parameterization, we obtain $w_0=-0.879\pm 0.060$, $w_a=-0.39^{+0.26}_{-0.22}$, the asymptotic limit $w(z\to\infty) = w_0+w_a=-1.27^{+0.20}_{-0.17}$, and $A_L=1.078^{+0.036}_{-0.040}$ (all $1\sigma$ errors). This joint analysis of CMB and non-CMB data favors DE dynamics over a cosmological constant at $\sim 1\sigma$ and $A_L>1$ at $\sim 2\sigma$, i.e. more smoothing of the Planck CMB anisotropy data than is predicted by the best-fit model. For the $w_0w_a$CDM parameterization with $A_L=1$ the evidence in favor of DE dynamics is larger, $\sim 2\sigma$, suggesting that at least part of the evidence for DE dynamics comes from the excess smoothing of the Planck CMB anisotropy data. For the $w_0w_a$CDM parameterization with $A_L=1$, there is a difference of $2.8\sigma$ between P18 and non-CMB cosmological parameter constraints and $2.7\sigma$ between P18+lensing and non-CMB constraints. When $A_L$ is allowed to vary these tensions reduced to $1.9\sigma$ and $2.1\sigma$ respectively. Our P18+lensing+non-CMB data compilation positively favors the $w_0w_a$CDM parameterization without and with a varying $A_L$ parameter over the flat $\Lambda$CDM model, and $w_0w_a$CDM+$A_L$ is also positively favored over $w_0w_a$CDM.

We study the performance of six Λ cold dark matter (ΛCDM) models, with four of them allowing for nonflat spatial hypersurfaces (nonzero current value of the spatial curvature density parameter Ωk) and three of them allowing for a nonunity value of the lensing consistency parameter AL. We also study a set of six XCDM models where the nonevolving cosmological constant Λ dark energy density is replaced by a dynamical dark energy density X-fluid parametrized by a nonevolving equation of state parameter w. For the nonflat models we consider two different primordial power spectra, Planck P(q), used by the Planck collaboration, and new P(q), resulting from quantum fluctuations in a not-necessarily-very-slow-roll nonflat inflation model. These models are constrained by and tested against: Planck 2018 CMB temperature and polarization power spectra data (P18); Planck 2018 CMB lensing potential power spectrum data (lensing); and, an updated compilation of baryon acoustic oscillation, type Ia supernova, Hubble parameter [H(z)], and growth factor [fσ8] data points [collectively denoted by non-CMB (new) data], individually and jointly. P18 data favor Ωk<0 (closed spatial geometry) for the ΛCDM and XCDM models and w<−1 (phantomlike dynamical dark energy) for the XCDM models while non-CMB (new) data favor Ωk>0 (open geometry) in the case of the ΛCDM models and Ωk<0 (closed geometry) and w>−1 (quintessencelike dynamical dark energy) for the XCDM models. When P18 and non-CMB (new) data are jointly analyzed there is weak evidence in favor of open spatial geometry and moderate evidence in favor of quintessencelike dynamical dark energy. On the other hand, regardless of data considered, AL>1 is always favored, with different degrees of evidence, even for P18+lensing+non−CMB (new) data. According to Akaike and deviance information criterion results, AL-varying models are positively favored over the flat ΛCDM model for P18+lensing+non−CMB (new) data. The XCDM model cosmological parameter constraints obtained from P18 or P18+lensing data and from non-CMB (new) data are incompatible at >3σ, ruling out the three AL=1 XCDM models at >3σ. In the nine models not ruled out by >3σ incompatibilities between parameter values determined from different datasets, for the P18+lensing+non-CMB (new) dataset we find little deviation from flat geometry and moderate deviation from a cosmological constant. In all six nonflat models that are not ruled out at >3σ, open geometry is mildly favored (by at most 0.8σ), and in all three XCDM+AL models (that are not ruled out at >3σ) quintessencelike dynamical dark energy is moderately favored (by at most 1.6σ). In the AL=1 nonflat ΛCDM cases, we find for P18+lensing+non−CMB (new) data Ωk=0.0009±0.0017 [0.0008±0.0017] for the Planck [new] P(q) model, favoring open geometry at 0.53σ [0.47σ]. Given these results, the flat ΛCDM model remains the simplest (largely) observationally-consistent cosmological model. Our cosmological parameter constraints obtained for the flat ΛCDM model (and other models), when P18+lensing+non−CMB (new) data are considered, are the most restrictive results to date.

To determine whether or not H ii starburst galaxies (H iiG) are standardizable candles, we study the correlation between the Hβ luminosity (L) and the velocity dispersion (σ) of the ionized gas from H iiG measurements by simultaneously constraining the L−σ relation parameters and the cosmological model parameters. We investigate six flat and nonflat relativistic dark energy cosmological models. We find that low-redshift and high-redshift H iiG data subsets are standardizable but obey different L−σ relations. Current H iiG data are too sparse and too nonuniformly distributed in redshift to allow for a determination of why the samples follow different relations, but it could be caused by the high-redshift sample containing relatively fewer intrinsically dimmer sources (Malmquist bias) or it could be a consequence of H iiG evolution. Until this issue is better understood, H iiG data cosmological constraints must be treated with caution.

We systematically explore the influence of the prior of the peak absolute magnitude ( M ) of Type Ia supernovae (SNe Ia) on the measurement of the Hubble constant ( H 0 ) from SNe Ia observations. We consider five different data-motivated M priors, representing varying levels of dispersion, and assume the spatially flat ΛCDM cosmological model. Different M priors lead to relative changes in the mean values of H 0 from 2% to 7%. Loose priors on M yield H 0 estimates consistent with both the Planck 2018 result and the SH0ES result at the 68% confidence level. We also examine the potential impact of peculiar velocity subtraction on the value of H 0 and show that it is insignificant for the SNe Ia observations with redshift z > 0.01 used in our analyses. We also repeat the analysis in the cosmography model and find very similar results. This suggests that our results are robust and model independent.

de Grijs & Bono compiled 211 independent measurements of the distance to galaxy M87 in the Virgo cluster from 15 different tracers and reported 31.03 ± 0.14 mag as the arithmetic mean of a subset of this compilation as the best estimate of the distance. We compute three different central estimates—the arithmetic mean, weighted mean, and the median—and corresponding statistical uncertainty for the full data set as well as three sub-compilations. We find that for all three central estimates the error distributions show that the data sets are significantly non-Gaussian. As a result, we conclude that the median is the most reliable of the three central estimates, as median statistics do not assume Gaussianity. We use median statistics to determine the systematic error on the distance by analyzing the scatter in the 15 tracer subgroup distances. From the 211 distance measurements, we recommend a summary M87 distance modulus of 31.08 − 0.04 + 0.05 (statistical) − 0.06 + 0.04 (systematic) mag, or combining the two errors in quadrature 31.08 − 0.07 + 0.06 mag, rounded to 16.4 ± 0.5 Mpc, all at 68.27% significance.