Gagandeep S. Anand, R. Brent Tully, Luca Rizzi, Adam G. Riess, Wenlong Yuan

The tip of the red giant branch has been used to measure distances to 500 nearby galaxies with the Hubble Space Telescope (HST) which are available in the Color-Magnitude Diagrams and Tip of the Red Giant Branch (CMDs/TRGB) catalog on the Extragalactic Distance Database (EDD). Our established methods are employed to perform an independent reduction of the targets presented by the Carnegie-Chicago Hubble Program (CCHP) in the series of papers culminating in Freedman (2021). Our distinct methodology involves modeling the observed luminosity function of red giant branch and asymptotic giant branch stars, which differs from the edge-detection algorithms employed by the CCHP. We find excellent agreement between distances for 11 hosts with new imaging, all at D < 20 Mpc. However, we are unable to measure the TRGB for 4 of the 5 hosts that use archival data designed to measure distances with Cepheids, all at D > 23 Mpc. With two new HST observations taken in the halo of the megamaser host NGC 4258, the first with the same ACS F606W and F814W filters and the post-servicing electronics used for SN Ia hosts, we then calibrate our TRGB distance scale to the geometric megamaser distance. Using our TRGB distances, we find a value of the Hubble Constant of $H_{0}$ = 71.5 $\pm$ 1.8 km/s/Mpc when using either the Pantheon or Carnegie Supernova Project (CSP) samples of supernovae. In the future, the James Webb Space Telescope will extend measurements of the TRGB to additional hosts of SN Ia and surface brightness fluctuation measurements for separate paths to $H_{0}$.

Sayali Atul Bhatkar

We consider a generic scattering process that takes place in a region of size R inside the static patch of the de Sitter spacetime such that R is smaller than the curvature length scale of the background. The effect of curvature can thus be studied perturbatively. We obtain the asymptotic electromagnetic field generated by the scattering process including the leading order correction due to the presence of de Sitter background and discuss its universal aspects. We finally caculate the resultant first order corrections to the flat spacetime velocity memory effect.

Farnik Nikakhtar, Ravi K. Sheth, Idit Zehavi

The baryon acoustic oscillation feature can be used as a standard cosmological ruler. In practice, for sub-percent level accuracy on the distance scale, it must be standardized. The physical reason why is understood, so we use this to develop an algorithm which improves the estimated scale. The algorithm exploits the fact that, over the range of scales where the initial correlation function is well-fit by a polynomial, the leading order effects which distort the length of the ruler can be accounted for analytically. Tests of the method in numerical simulations show that it provides simple and fast reconstruction of the full shape of the BAO feature, as well as sub-percent determination of the linear point in the correlation function of biased tracers with minimal assumptions about the underlying cosmological model or the nature of the observed tracers. Our results also suggest that, for least squares estimators of the correlation function, half-integer generalized Laguerre functions are a particularly useful choice.