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Register a new userThe impact of peculiar velocities on supernova cosmology
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We study correlated fluctuations of Type~Ia supernova observables due to peculiar velocities of both the observer and the supernova host galaxies, and their impact on cosmological parameter estimation. We demonstrate using the CosmicFlows-3 dataset that at low redshifts the corrections for peculiar velocities in the JLA catalogue have been systematically underestimated. By querying a horizon-size N-body simulation we find that compared to a randomly placed observer, an observer in an environment like our local Universe will see 2-8 times stronger correlations between supernovae in the JLA catalogue. Hence the covariances usually employed assuming a typical observer are unphysical and underestimate the effects of coherent motion of the supernova host galaxies. Contrary to previous studies which asserted that this should have negligible effect on cosmological parameter estimation, we find that when peculiar velocities are treated consistently the JLA data favours significantly smaller values of the dark energy density than in the standard $Lambda$CDM model. A joint fit to simultaneously determine the cosmological parameters and the bulk flow indicates that the latter is around 250 km/s even beyond 200$h^{-1}$ Mpc. The local bulk flow is thus an essential nuisance parameter which must be included in cosmological model fitting when analysing supernova data.
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We present an analysis of peculiar velocities and their effect on supernova cosmology. In particular, we study (a) the corrections due to our own motion, (b) the effects of correlations in peculiar velocities induced by large-scale structure, and (c) uncertainties arising from a possible local under- or over-density. For all of these effects we present a case study of their impact on the cosmology derived by the Sloan Digital Sky Survey-II Supernova Survey (SDSS-II SN Survey). Correcting supernova redshifts for the CMB dipole slightly over-corrects nearby supernovae that share some of our local motion. We show that while neglecting the CMB dipole would cause a shift in the derived equation of state of Delta w ~ 0.04 (at fixed matter density) the additional local-motion correction is currently negligible (Delta w<0.01). We use a covariance-matrix approach to statistically account for correlated peculiar velocities. This down-weights nearby supernovae and effectively acts as a graduated version of the usual sharp low-redshift cut. Neglecting coherent velocities in the current sample causes a systematic shift of ~2% in the preferred value of w and will therefore have to be considered carefully when future surveys aim for percent-level accuracy. Finally, we perform n-body simulations to estimate the likely magnitude of any local density fluctuation (monopole) and estimate the impact as a function of the low-redshift cutoff. We see that for this aspect the low-z cutoff of z=0.02 is well-justified theoretically, but that living in a putative local density fluctuation leaves an indelible imprint on the magnitude-redshift relation.
We quantify the effect of supernova Type Ia peculiar velocities on the derivation of cosmological parameters. The published distant and local Ia SNe used for the Supernova Legacy Survey first-year cosmology report form the sample for this study. While previous work has assumed that the local SNe are at rest in the CMB frame (the No Flow assumption), we test this assumption by applying peculiar velocity corrections to the local SNe using three different flow models. The models are based on the IRAS PSCz galaxy redshift survey, have varying beta = Omega_m^0.6/b, and reproduce the Local Group motion in the CMB frame. These datasets are then fit for w, Omega_m, and Omega_Lambda using flatness or LambdaCDM and a BAO prior. The chi^2 statistic is used to examine the effect of the velocity corrections on the quality of the fits. The most favored model is the beta=0.5 model, which produces a fit significantly better than the No Flow assumption, consistent with previous peculiar velocity studies. By comparing the No Flow assumption with the favored models we derive the largest potential systematic error in w caused by ignoring peculiar velocities to be Delta w = +0.04. For Omega_Lambda, the potential error is Delta Omega_Lambda = -0.04 and for Omega_m, the potential error is Delta Omega_m < +0.01. The favored flow model (beta=0.5) produces the following cosmological parameters: w = -1.08 (+0.09,-0.08), Omega_m = 0.27 (+0.02,-0.02) assuming a flat cosmology, and Omega_Lambda = 0.80 (+0.08,-0.07) and Omega_m = 0.27 (+0.02,-0.02) for a w = -1 (LambdaCDM) cosmology.
In this work we investigate the systematic uncertainties that arise from the calculation of the peculiar velocity when estimating the Hubble constant ($H_0$) from gravitational wave standard sirens. We study the GW170817 event and the estimation of the peculiar velocity of its host galaxy, NGC 4993, when using Gaussian smoothing over nearby galaxies. NGC 4993 being a relatively nearby galaxy, at $sim 40 {rm Mpc}$ away, is subject to a significant effect of peculiar velocities. We demonstrate a direct dependence of the estimated peculiar velocity value on the choice of smoothing scale. We show that when not accounting for this systematic, a bias of $sim 200 {rm km s ^{-1}}$ in the peculiar velocity incurs a bias of $sim 4 {rm km s ^{-1} Mpc^{-1}}$ on the Hubble constant. We formulate a Bayesian model that accounts for the dependence of the peculiar velocity on the smoothing scale and by marginalising over this parameter we remove the need for a choice of smoothing scale. The proposed model yields $H_0 = 68.6 ^{+14.0}_{-8.5}~{rm km s^{-1} Mpc^{-1}}$. We demonstrate that under this model a more robust unbiased estimate of the Hubble constant from nearby GW sources is obtained.
Using Planck data combined with the Meta Catalogue of X-ray detected Clusters of galaxies (MCXC), we address the study of peculiar motions by searching for evidence of the kinetic Sunyaev-Zeldovich effect (kSZ). By implementing various filters designed to extract the kSZ generated at the positions of the clusters, we obtain consistent constraints on the radial peculiar velocity average, root mean square (rms), and local bulk flow amplitude at different depths. For the whole cluster sample of average redshift 0.18, the measured average radial peculiar velocity with respect to the cosmic microwave background (CMB) radiation at that redshift, i.e., the kSZ monopole, amounts to $72 pm 60$ km s$^{-1}$. This constitutes less than 1% of the relative Hubble velocity of the cluster sample with respect to our local CMB frame. While the linear $Lambda$CDM prediction for the typical cluster radial velocity rms at $z=0.15$ is close to 230km s$^{-1}$, the upper limit imposed by Planck data on the cluster subsample corresponds to 800 km s$^{-1}$ at 95% confidence level, i.e., about three times higher. Planck data also set strong constraints on the local bulk flow in volumes centred on the Local Group. There is no detection of bulk flow as measured in any comoving sphere extending to the maximum redshift covered by the cluster sample. A blind search for bulk flows in this sample has an upper limit of 254 km s$^{-1}$ (95% confidence level) dominated by CMB confusion and instrumental noise, indicating that the Universe is largely homogeneous on Gpc scales. In this context, in conjunction with supernova observations, Planck is able to rule out a large class of inhomogeneous void models as alternatives to dark energy or modified gravity. The Planck constraints on peculiar velocities and bulk flows are thus consistent with the $Lambda$CDM scenario.
Accurate simulations of the 21-cm signal from the Epoch of Reionization require the generation of maps at specific frequencies given the values of astrophysical and cosmological parameters. The peculiar velocities of the hydrogen atoms producing the 21-cm radiation result in a shift in the observed frequency of the 21-cm radiation and changes the amplitude of the signal itself. This is not an effect we can remove but instead needs to be accurately modelled to ensure we infer the correct physical parameters from an observation. We describe the full calculation of the distortion of the 21-cm signal, and propose a new code that integrates the 21-cm intensity along the line of sight for each individual light cone pixel to fully capture the intensity contributions from multiple redshifts. This algorithm naturally deals with the typical divergences found in standard approaches, allowing for large optical depths and 21-cm absorption events at high redshifts. We find the new method results in up to a 60% decrease in power on the largest scales on the sky, and an increase of over 80% on the smallest scales on the sky. We find that the new implementation of the light cone results in a longer tail of bright temperatures in the brightness temperature distribution, as a result of the successful circumventing of a previous cap that had to be implemented to avoid a divergence in the brightness temperature. We conclude that this full treatment of the evolution of the light cone pixel can be an important effect.
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