Young stellar distance indicators and the extragalactic distance scale

TL;DR

This paper reviews the use of young stellar distance indicators, especially Cepheids, in establishing the extragalactic distance scale and their role in measuring the Hubble constant, highlighting current challenges and future prospects.

Contribution

It provides an accessible overview of the current state of Cepheid-based distance measurements, calibration methods, and their implications for the Hubble tension.

Findings

Cepheids support a 1.4% measurement of Hubble's constant.

Gaia parallaxes are crucial for calibrating Cepheid distances.

Upcoming facilities like JWST will enhance distance estimates.

Abstract

The extragalactic distance scale is perhaps the most important application of stellar distance indicators. Among these, classical Cepheids are high-accuracy standard candles that support a $1.4\%$ measurement of Hubble's constant, $H_0$. The accuracy of Cepheid distances is thus directly relevant for understanding the implications of the Hubble tension, the $>5\sigma$ discord among direct, late-Universe $H_0$ measurements and $H_0$ values inferred from the early Universe observations assuming $\Lambda$CDM cosmology. This invited review aims to provide an accessible overview of the state of the art distance ladder that has established the Hubble tension, with a focus on Cepheids, their absolute calibration using trigonometric parallaxes from the ESA mission Gaia, and other Cepheid-related systematics. New observational facilities such as JWST and upcoming large surveys will provide exciting avenues to further improve distance estimates based on Cepheids.