Dylan L. Jow, Douglas Scott

We investigate recent claims for a detection of "Hawking points" (positions on the sky with unusually large temperature gradients between rings) in the cosmic microwave background (CMB) temperature maps at the 99.98% confidence level. We find that, after marginalization over the size of the rings, an excess is detected in Planck satellite maps at only an 87% confidence level (i.e., little more than 1$\sigma$). Therefore, we conclude that there is no statistically significant evidence for the presence of Hawking points in the CMB.

Andrew Repp, István Szapudi

A reliable model of galaxy bias is necessary for interpreting data from future dense galaxy surveys. Conventional bias models are inaccurate, in that they can yield unphysical results ($\delta_g < -1$) for voids that might contain half of the available cosmological information. For this reason, we present a physically-motivated bias model based on an analogy with the Ising model. With only two free parameters, the model produces sensible results for both high- and low-density regions. We also test the model using a catalog of Millennium Simulation galaxies in cubical survey pixels with side lengths from $2h^{-1}$--$31h^{-1}$Mpc, at redshifts from 0 to 2. We find the Ising model markedly superior to linear and quadratic bias models on scales smaller than $10h^{-1}$Mpc, while those conventional models fare better on scales larger than $30h^{-1}$Mpc. While the largest scale where the Ising model is applicable might vary for a specific galaxy catalog, it should be superior on any scale with a non-negligible fraction of cells devoid of galaxies.

Constantina Nicolaou, Ofer Lahav, Pablo Lemos, William Hartley, Jonathan Braden

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 the 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.

Suvodip Mukherjee, Guilhem Lavaux, François R. Bouchet, Jens Jasche, Benjamin D. Wandelt, Samaya M. Nissanke, Florent Leclercq, Kenta Hotokezaka

Gravitational wave (GW) sources are an excellent probe of the luminosity distance and offer a novel measure of the Hubble constant, $H_0$. This estimation of $H_0$ from standard sirens requires an accurate estimation of the cosmological redshift of the host galaxy of the GW source, after correcting for its peculiar velocity. Absence of an accurate peculiar velocity correction affects both the precision and accuracy of the measurement of $H_0$, particularly for nearby sources. We propose a framework to incorporate such a peculiar velocity correction for GW sources. The implementation of our method to the event GW170817 combined with the Very Large Baseline Interferometry (VLBI) observation leads to a revised value of $H_0= 69.3^{+ 4.5}_{-4.0}$ km s$^{-1}$Mpc$^{-1}$; more importantly, it demonstrates that our method will allow to achieve in practice unbiased, accurate measurements of $H_0$ at the precision promised by standard siren forecasts.