The distance scale of the universe

Abstract

We critically review the methods currently being used to determine extragalactic distances. Within the Milky Way, direct parallaxes and traditional main-sequence fitting for both young open clusters and old globular clusters tie us directly to high-luminosity, variable stars used in extragalactic studies: Cepheids and RR Lyrae stars. These, in turn, inform a calibration of the tip of the red giant branch as well as red clump stars. Apart from the Milky Way, we focus on the Large Magellanic Cloud as an important stopping point at which a large variety of possible distance indicators have been evaluated and differentially tested against each other. Beyond the Local Group many stellar distance indicators fall below current detection and/or resolution limits, and they must be replaced with methods employing higher luminosity (and often much more rare) objects. These methods include the properties of nuclear masers, surface brightness fluctuations, the Tully–Fisher relation, and finally various types of supernovae. Ultimately a calibration of the expansion rate of the universe can provide distances using observed recessional velocities scaled by the Hubble constant.