Allan Sandage and the Cosmic Expansion

TL;DR

Allan Sandage's work provided strong evidence for the universe's expansion, explained local velocity effects on cosmic microwave background observations, and refined the Hubble constant using multiple calibration methods.

Contribution

The paper details Sandage's comprehensive analysis of cosmic expansion, local velocity corrections, and the precise calibration of the Hubble constant using Cepheids, SNe Ia, and TRGBs.

Findings

Linear expansion holds down to local scales (~300 km/s).

Local velocities explain the CMB dipole anisotropy.

Hubble constant determined as 62.3 km/s/Mpc with specified uncertainties.

Abstract

This is an account of Allan Sandage's work on (1) The character of the expansion field. For many years he has been the strongest defender of an expanding Universe. He later explained the CMB dipole by a local velocity of 220 +/- 50 km/s toward the Virgo cluster and by a bulk motion of the Local supercluster (extending out to ~3500 km/s) of 450-500 km/s toward an apex at l=275, b=12. Allowing for these streaming velocities he found linear expansion to hold down to local scales (~300 km/s). (2) The calibration of the Hubble constant. Probing different methods he finally adopted - from Cepheid-calibrated SNe Ia and from independent RR Lyr-calibrated TRGBs - H_0 = 62.3 +/- 1.3 +/- 5.0 km/s/Mpc.