arXiv subjects

Bharat Ratra

Publications and source records attributed to Bharat Ratra.

At least 55 records · Page 3Linked to original sources

arXiv2020

Cosmological constraints from HII starburst galaxy apparent magnitude and other cosmological measurements

We use HII starburst galaxy apparent magnitude measurements to constrain cosmological parameters in six cosmological models. A joint analysis of HII galaxy, quasar angular size, baryon acoustic oscillations peak length scale, and Hubble parameter measurements result in relatively model-independent and restrictive estimates of the current values of the non-relativistic matter density parameter $Ω_{\rm m_0}$ and the Hubble constant $H_0$. These estimates favor a 2.0$σ$ to 3.4$σ$ (depending on cosmological model) lower $H_0$ than what is measured from the local expansion rate. The combined data are consistent with dark energy being a cosmological constant and with flat spatial hypersurfaces, but do not strongly rule out mild dark energy dynamics or slightly non-flat spatial geometries.

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arXiv2020

Using quasar X-ray and UV flux measurements to constrain cosmological model parameters

Risaliti and Lusso have compiled X-ray and UV flux measurements of 1598 quasars (QSOs) in the redshift range $0.036 \leq z \leq 5.1003$, part of which, $z \sim 2.4 - 5.1$, is largely cosmologically unprobed. In this paper we use these QSO measurements, alone and in conjunction with baryon acoustic oscillation (BAO) and Hubble parameter [$H(z)$] measurements, to constrain cosmological parameters in six different cosmological models, each with two different Hubble constant priors. In most of these models, given the larger uncertainties, the QSO cosmological parameter constraints are mostly consistent with those from the $H(z)$ + BAO data. A somewhat significant exception is the non-relativistic matter density parameter $Ω_{m0}$ where the QSO data favors $Ω_{m0} \sim 0.5 - 0.6$ in most models. Consequently in joint analyses of QSO data with $H(z)$ + BAO data the one-dimensional $Ω_{m0}$ distributions shift slightly toward larger values. A joint analysis of the QSO + $H(z)$ + BAO data is consistent with the current standard model, spatially-flat $Λ$CDM, but mildly favors closed spatial hypersurfaces and dynamical dark energy. Since the higher $Ω_{m0}$ values favored by the QSO data appear to be associated with the $z \sim 2 - 5$ part of these data, and conflict somewhat with strong indications for $Ω_{m0} \sim 0.3$ from most $z < 2.5$ data as well as from the cosmic microwave background anisotropy data at $z \sim 1100$, in most models, the larger QSO data $Ω_{m0}$ is possibly more indicative of an issue with the $z \sim 2 - 5$ QSO data than of an inadequacy of the standard flat $Λ$CDM model.

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arXiv2020

CMB distance priors revisited: effects of dark energy dynamics, spatial curvature, primordial power spectrum, and neutrino parameters

As a physical and sufficient compression of the full CMB data, the CMB distance priors, or shift parameters, have been widely used and provide a convenient way to include CMB data when obtaining cosmological constraints. In this paper, we revisit this data vector and examine its stability under different cosmological models. We find that the CMB distance priors are an accurate substitute for the full CMB data when probing dark energy dynamics. This is true when the primordial power spectrum model is directly generalized from the power spectrum of the model used in the derivation of the distance priors from the CMB data. We discover a difference when a non-flat model with the untilted primordial inflation power spectrum is used to measure the distance priors. This power spectrum is a radical change from the more conventional tilted primordial power spectrum and violates fundamental assumptions for the reliability of the CMB shift parameters. We also investigate the performance of CMB distance priors when the sum of neutrino masses $\sum m_ν$ and the effective number of relativistic species $N_{\text{eff}}$ are allowed to vary. Our findings are consistent with earlier results: the neutrino parameters can change the measurement of the sound horizon from CMB data, and thus the CMB distance priors. We find that when the neutrino model is allowed to vary, the cold dark matter density $ω_{c}$ and $N_{\text{eff}}$ need to be included in the set of parameters that summarize CMB data, in order to reproduce the constraints from the full CMB data. We present an updated and expanded set of CMB distance priors which can reproduce constraints from the full CMB data within $1σ$, and are applicable to models with massive neutrinos, as well as non-standard cosmologies.

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arXiv2020

Quasar X-ray and UV flux, baryon acoustic oscillation, and Hubble parameter measurement constraints on cosmological model parameters

We use the Risaliti & Lusso (2015) compilation of 808 X-ray and UV flux measurements of quasars (QSOs) in the redshift range $0.061 \leq z \leq 6.28$, alone and in conjuction with baryon acoustic oscillation (BAO) and Hubble parameter [$H(z)$] measurements, to constrain cosmological parameters in six cosmological models. The QSO data constraints are significantly weaker than, but consistent with, those from the $H(z)$ + BAO data. A joint analysis of the QSO + $H(z)$ + BAO data is consistent with the current standard model, spatially-flat $Λ$CDM, but mildly favors closed spatial hypersurfaces and dynamical dark energy.

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arXiv2020

Using SPTpol, Planck 2015, and non-CMB data to constrain tilted spatially-flat and untilted non-flat $Λ$CDM, XCDM, and $ϕ$CDM dark energy inflation cosmologies

We use six tilted spatially-flat and untilted non-flat dark energy cosmological models in analyses of South Pole Telescope polarization (SPTpol) cosmic microwave background (CMB) data, alone and in combination with Planck 2015 CMB data and non-CMB data, namely, the Pantheon Type Ia supernovae apparent magnitudes, a collection of baryon acoustic oscillation data points, Hubble parameter measurements, and growth rates. Although the cosmological models that best-fit the Planck CMB and non-CMB data do not provide good fits to the SPTpol data, with the $χ^2$'s exceeding the expected value, given the uncertainties, in each model the cosmological parameter constraints from the SPTpol data and from the Planck CMB and non-CMB data are largely mutually consistent. When the smaller angular scale SPTpol data are used jointly with either the Planck data alone or with the Planck CMB and the non-CMB data to constrain untilted non-flat models, spatially-closed models remain favored over their corresponding flat limits. When used in conjunction with Planck data, non-CMB data (baryon acoustic oscillation measurements in particular, from six experiments) have significantly more constraining power than the SPTpol data.

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arXiv2019

Planck 2015 constraints on spatially-flat dynamical dark energy models

We determine constraints on spatially-flat tilted dynamical dark energy XCDM and $ϕ$CDM inflation models by analyzing Planck 2015 cosmic microwave background (CMB) anisotropy data and baryon acoustic oscillation (BAO) distance measurements. XCDM is a simple and widely used but physically inconsistent parameterization of dynamical dark energy, while the $ϕ$CDM model is a physically consistent one in which a scalar field $ϕ$ with an inverse power-law potential energy density powers the currently accelerating cosmological expansion. Both these models have one additional parameter compared to standard $Λ$CDM and both better fit the TT + lowP + lensing + BAO data than does the standard tilted flat-$Λ$CDM model, with $Δχ^2 = -1.26\ (-1.60)$ for the XCDM ($ϕ$CDM) model relative to the $Λ$CDM model. While this is a 1.1$σ$ (1.3$σ$) improvement over standard $Λ$CDM and so not significant, dynamical dark energy models cannot be ruled out. In addition, both dynamical dark energy models reduce the tension between the Planck 2015 CMB anisotropy and the weak lensing $σ_8$ constraints.

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arXiv2019

Measuring the Hubble constant and spatial curvature from supernova apparent magnitude, baryon acoustic oscillation, and Hubble parameter data

Cosmic microwave background (CMB) anisotropy (spatial inhomogeneity) data provide the tightest constraints on the Hubble constant, matter density, spatial curvature, and dark energy dynamics. Other data, sensitive to the evolution of only the spatially homogeneous part of the cosmological model, such as Type Ia supernova apparent magnitude, baryon acoustic oscillation distance, and Hubble parameter measurements, can be used in conjunction with the CMB data to more tightly constrain parameters. Recent joint analyses of CMB and such non-CMB data indicate that slightly closed spatial hypersurfaces are favored in nonflat untilted inflation models and that dark energy dynamics cannot be ruled out, and favor a smaller Hubble constant. We show that the constraints that follow from these non-CMB data alone are consistent with those that follow from the CMB data alone and so also consistent with, but weaker than, those that follow from the joint analyses of the CMB and non-CMB data.

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arXiv2019

Using the tilted flat-$Λ$CDM and the untilted non-flat $Λ$CDM inflation models to measure cosmological parameters from a compilation of observational data

We use the physically-consistent tilted spatially-flat and untilted non-flat $Λ$CDM inflation models to constrain cosmological parameter values with the Planck 2015 cosmic microwave background (CMB) anisotropy data and recent Type Ia supernovae measurements, baryonic acoustic oscillations (BAO) data, growth rate observations, and Hubble parameter measurements. The most dramatic consequence of including the four non-CMB data sets is the significant strengthening of the evidence for non-flatness in the non-flat $Λ$CDM model, from 1.8$σ$ for the CMB data alone to 5.1$σ$ for the full data combination. The BAO data is the most powerful of the non-CMB data sets in more tightly constraining model parameter values and in favoring a spatially-closed Universe in which spatial curvature contributes about a percent to the current cosmological energy budget. The untilted non-flat $Λ$CDM model better fits the large-angle CMB temperature anisotropy angular spectrum and is more consistent with the Dark Energy Survey constraints on the current value of the rms amplitude of mass fluctuations ($σ_8$) as a function of the current value of the nonrelativistic matter density parameter ($Ω_m$) but does not provide as good a fit to the smaller-angle CMB temperature anisotropy data as does the tilted flat-$Λ$CDM model. Some measured cosmological parameter values differ significantly between the two models, including the reionization optical depth and the baryonic matter density parameter, both of whose 2$σ$ ranges (in the two models) are disjoint or almost so.

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arXiv2019

Baryon acoustic oscillation, Hubble parameter, and angular size measurement constraints on the Hubble constant, dark energy dynamics, and spatial curvature

In this paper we use all available baryon acoustic oscillation, Hubble parameter, and quasar angular size data to constrain six dark energy cosmological models, both spatially flat and non-flat. Depending on the model and data combination considered, these data mildly favor closed spatial hypersurfaces (by as much as $1.7σ$) and dark energy dynamics (up to a little over $2σ$) over a cosmological constant $Λ$. The data also favor, at $1.8σ$ to $3.4σ$, depending on the model and data combination, a lower Hubble constant than what is measured from the local expansion rate.

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arXiv2019

First study of reionization in tilted flat and untilted non-flat dynamical dark energy inflation models

We examine the effects of dark energy dynamics and spatial curvature on cosmic reionization by studying reionization in tilted spatially-flat and untilted non-flat XCDM and $ϕ$CDM dynamical dark energy inflation models that best fit the Planck 2015 cosmic microwave background (CMB) anisotropy and a large compilation of non-CMB data. We carry out a detailed statistical study, based on a principal component analysis and a Markov chain Monte Carlo analysis of a compilation of lower-redshift reionization data, to estimate the uncertainties in the cosmological model reionization histories. We find that, irrespective of the nature of dark energy, there are significant differences between the reionization histories of the spatially-flat and non-flat models. Although both the flat and non-flat models can accurately match the low-redshift ($z\lesssim6$) reionization observations, there is a clear discrepancy between high-redshift ($z>7$) Lyman-$α$ emitter data and the predictions from non-flat models. This is solely due to the fact that the non-flat models have a significantly larger electron scattering optical depth, $τ_{\rm el}$, compared to the flat models, which requires an extended and much earlier reionization scenario supported by more high-redshift ionizing sources in the non-flat models. Non-flat models also require strong redshift evolution in the photon escape fraction, that can become unrealistically high ($\gtrsim1$) at some redshifts. However, $τ_{\rm el}$ is about 0.9-$σ$ lower in the tilted flat $Λ$CDM model when the new Planck 2018 data are used and this reduction will partially alleviate the tension between the non-flat model predictions and the data.

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arXiv2019

Cosmological magnetic braking and the formation of high-redshift, super-massive black holes

We study the effect of magnetic braking due to a primordial magnetic field in the context of the formation of massive ($\gtrsim 10^{4} M_\odot$) direct collapse black holes (DCBHs) at high redshifts. Under the assumption of axial symmetry, we analytically compute the effect of magnetic braking on the angular momentum of gas collapsing into the potential well of massive dark matter haloes ($\simeq 10^{7-9} M_\odot$) which are spun up by gravitational tidal torques. We find that a primordial magnetic field of strength $B_0\simeq 0.1$~nG (comoving) can remove the initial angular momentum gained by the in-falling gas due to tidal torques, thus significantly lowering the angular momentum barrier to the formation of DCBHs. These magnetic field strengths are consistent with the bounds on primordial fields from astrophysical and cosmological measurements and they are large enough to seed observed galactic magnetic fields.

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arXiv2019

Observational constraints on the tilted flat-XCDM and the untilted nonflat XCDM dynamical dark energy inflation parameterizations

We constrain tilted spatially-flat and untilted nonflat XCDM dynamical dark energy inflation parameterizations using Planck 2015 cosmic microwave background (CMB) anisotropy data and recent baryonic acoustic oscillations distance measurements, Type Ia supernovae data, Hubble parameter observations, and growth rate measurements. Inclusion of the four non-CMB data sets results in a significant strengthening of the evidence for nonflatness in the nonflat XCDM model from 1.1$σ$ for the CMB data alone to 3.4$σ$ for the full data combination. In this untilted nonflat XCDM case the data favor a spatially-closed model in which spatial curvature contributes a little less than a percent of the current cosmological energy budget; they also mildly favor dynamical dark energy over a cosmological constant at 1.2$σ$. These data are also better fit by the flat-XCDM parameterization than by the standard $Λ$CDM model, but only at 0.3$σ$ significance. Current data is unable to rule out dark energy dynamics. The nonflat XCDM parameterization is compatible with the Dark Energy Survey limits on the present value of the rms mass fluctuations amplitude ($σ_8$) as a function of the present value of the nonrelativistic matter density parameter ($Ω_m$), however it does not provide as good a fit to the higher multipole CMB temperature anisotropy data as does the standard tilted flat-$Λ$CDM model. A number of measured cosmological parameter values differ significantly when determined using the tilted flat-XCDM and the nonflat XCDM parameterizations, including the baryonic matter density parameter and the reionization optical depth.

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arXiv2019

Gaussian Processes, Median Statistics, Milky Way Rotation Curves

We use the Iocco et al. (2015) compilation of 2,780 circular velocity measurements to analyze the Milky Way rotation curve. We find that the error bars for individual measurements are non-gaussian, and hence instead derive median statistics binned central circular velocity values and error bars from these data. We use these median statistics central values and error bars to fit the data to simple, few parameter, rotation curve functions. These simple functions are unable to adequately capture the significant small scale spatial structure in these data and so provide poor fits. We introduce and use the Gaussian Processes (GP) method to capture this small scale structure and use it to derive Milky Way rotation curves from the binned median statistics circular velocity data. The GP method rotation curves have significant small-scale spatial structure superimposed on a broad rise to galactocentric radius $R\approx7$ kpc and a decline at larger $R$. We use the GP method median statistics rotation curve to measure the Oort $A$ and $B$ constants and other characteristic rotation curve quantities. We study correlations in the residual circular velocities (relative to the GP method rotation curve). Along with other evidence for azimuthal asymmetry of the Milky Way circular rotation velocity field, we find that larger residual circular velocities seem to favor parts of spiral arms.

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arXiv2018

Planck 2015 constraints on the non-flat XCDM inflation model

We study Planck 2015 cosmic microwave background (CMB) anisotropy data using the energy density inhomogeneity power spectrum generated by quantum fluctuations during an early epoch of inflation in the non-flat XCDM model. Here dark energy is parameterized using a fluid with a negative equation of state parameter but with the speed of fluid acoustic inhomogeneities set to the speed of light. We use this simple parameterization of dynamical dark energy, that is relatively straightforward to use in a computation, in a first attempt to gain some insight into how dark energy dynamics and non-zero spatial curvature jointly affect the CMB anisotropy data constraints. Unlike earlier analyses of non-flat models, we use a physically consistent power spectrum for energy density inhomogeneities. We find that the Planck 2015 data in conjunction with baryon acoustic oscillation measurements are reasonably well fit by a closed XCDM model in which spatial curvature contributes a percent of the current cosmological energy density budget. In this model, the measured Hubble constant and non-relativistic matter density parameter are in good agreement with values determined using most other data. Depending on parameter values, the closed XCDM model has reduced power, relative to the tilted, spatially-flat $Λ$CDM case, and appears to partially alleviate the low multipole CMB temperature anisotropy deficit and can help partially reconcile the CMB anisotropy and weak lensing $σ_8$ constraints, at the expense of somewhat worsening the fit to higher multipole CMB temperature anisotropy data. However, the closed XCDM inflation model does not seem to improve the agreement much, if at all, compared to the closed $Λ$CDM inflation case, even though it has one more free parameter. Our results are interesting but tentative; a more thorough analysis is needed to properly gauge their significance.

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arXiv2018

Observational constraints on the tilted spatially-flat and the untilted nonflat $ϕ$CDM dynamical dark energy inflation models

We constrain spatially-flat tilted and nonflat untilted scalar field ($ϕ$) dynamical dark energy inflation ($ϕ$CDM) models by using Planck 2015 cosmic microwave background (CMB) anisotropy measurements and recent baryonic acoustic oscillation distance observations, Type Ia supernovae apparent magnitude data, Hubble parameter measurements, and growth rate data. We assume an inverse power-law scalar field potential energy density $V(ϕ)=V_0 ϕ^{-α}$. We find that the combination of the CMB data with the four non-CMB data sets significantly improves parameter constraints and strengthens the evidence for nonflatness in the nonflat untilted $ϕ$CDM case from $1.8σ$ for the CMB measurements only to more than $3.1σ$ for the combined data. In the nonflat untilted $ϕ$CDM model current observations favor a spatially closed universe with spatial curvature contributing about two-thirds of a percent of the present cosmological energy budget. The flat tilted $ϕ$CDM model is a 0.4$σ$ better fit to the data than is the standard flat tilted $Λ$CDM model: current data allow for the possibility that dark energy is dynamical. The nonflat tilted $ϕ$CDM model is in better accord with the Dark Energy Survey bounds on the rms amplitude of mass fluctuations now ($σ_8$) as a function of the nonrelativistic matter density parameter now ($Ω_m$) but it does not provide as good a fit to the larger-multipole Planck 2015 CMB anisotropy data as does the standard flat tilted $Λ$CDM model. A few cosmological parameter value measurements differ significantly when determined using the tilted flat and the untilted nonflat $ϕ$CDM models, including the cold dark matter density parameter and the reionization optical depth.

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arXiv2018

Dark matter deficient galaxies in the Illustris flat-$Λ$CDM model structure formation simulation

Surveying dark matter deficient galaxies (those with dark matter mass to stellar mass ratio $M_{\rm dm}/M_{\rm star}<1$) in the Illustris simulation of structure formation in the flat-$Λ$CDM cosmogony, we find $M_{\rm star} \approx 2 \times 10^8\, M_\sun$ galaxies that have properties similar to those ascribed by \citet{vanDokkumetal2018a} to the ultra-diffuse galaxy NGC1052-DF2. The Illustris simulation also contains more luminous dark matter deficient galaxies. Illustris galaxy subhalo 476171 is a particularly interesting outlier, a massive and very compact galaxy with $M_{\rm star} \approx 9 \times 10^{10}\, M_\sun$ and $M_{\rm dm}/M_{\rm star} \approx 0.1$ and a half-stellar-mass radius of $\approx 2$ kpc. If the Illustris simulation and the $Λ$CDM model are accurate, there are a significant number of dark matter deficient galaxies, including massive luminous compact ones. It will be interesting to observationally discover these galaxies, and to also more clearly understand how they formed, as they are likely to provide new insight into and constraints on models of structure formation and the nature of dark matter.

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arXiv2018

Planck 2015 constraints on the non-flat $ϕ$CDM inflation model

We perform Markov chain Monte Carlo analyses to put constraints on the non-flat $ϕ$CDM inflation model using Planck 2015 cosmic microwave background (CMB) anisotropy data and baryon acoustic oscillation distance measurements. The $ϕ$CDM model is a consistent dynamical dark energy model in which the currently accelerating cosmological expansion is powered by a scalar field $ϕ$ slowly rolling down an inverse power-law potential energy density. We also use a physically consistent power spectrum for energy density inhomogeneities in this non-flat model. We find that, like the closed-$Λ$CDM and closed-XCDM models, the closed-$ϕ$CDM model provides a better fit to the lower multipole region of the CMB temperature anisotropy data compared to that provided by the tilted flat-$Λ$CDM model. Also, like the other closed models, this model reduces the tension between the Planck and the weak lensing $σ_8$ constraints. However, the higher multipole region of the CMB temperature anisotropy data are better fit by the tilted flat-$Λ$CDM model than by the closed models.

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arXiv2018

Median Statistics Analysis of Deuterium Abundance Measurements and Spatial Curvature Constraints

Zavarygin et al. (2018) compiled a list of 15 deuterium abundance measurements, discarded two because the remaining 13 measurements are then consistent with gaussianity, and found that the weighted mean baryon density (Omega_b h^2) determined from the 13 measurements is mildly discrepant (1.6sigma) with that determined from the Planck 2015 cosmic microwave background anisotropy data in a flat cosmogony. We find that a median statistic central estimate of Omega_b h^2 from all 15 deuterium abundance measurements is a more accurate estimate, is very consistent with Omega_b h^2 estimated from Planck 2015 data in a flat cosmogony, but is about 2sigma lower than that found in a closed cosmogonical model from the Planck 2015 data.

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