The Cepheid distance ladder: from the local Gaia calibration to distant galaxies.

by Louise Breuval (Observatoire de Paris, LESIA)

Charpak (LPNHE)




Cepheids are peculiar stars: their luminosity vary in time with a constant period ranging from a few days up to about sixty days. They are particularly useful tools for measuring astronomical distances, thanks to the empirical relation between their pulsation period and intrinsic luminosity, the period-luminosity (PL) relation. Indeed, by observing the apparent luminosity of such stars and by comparing it to the brightness predicted by the PL relation, it is then possible to derive their distance. 
However, this law between Cepheids period and brightness is still not known with a sufficient accuracy. Its imprecision currently represents the main source of uncertainty in the determination of the Hubble constant, that describes the expansion rate of the Universe. Measuring the Hubble constant is paramount since its value is at the center of a major controversy, often called crisis: its recent empirical estimate by Riess et al. (2019), 74.0 +/- 1.4 km/s/Mpc, differs by about 4 sigma from the value estimated by Planck Collaboration (2018) based on a Lambda-CDM model and the Planck CMB data, 67.4 +/- 0.5 km/s/Mpc. This discrepancy could possibly provide evidence for a breach in the standard cosmological model that currently describes our Universe.
Calibrating the Cepheid PL relation is therefore of critical importance and requires the prior measurement of a set of precise distances. However, Cepheids distances are difficult to access since they are often affected by systematics and saturation. We present an original and alternative method that aims to calibrate the PL relation, not based on Cepheid distances as commonly done, but on the distances of Cepheids resolved companions and on average distances of star clusters hosting Cepheids. From an initial Hubble constant of 76.2 +/- 2.4 km/s/Mpc, we derive a revised value of 72.8 +/- 1.9 km/s/Mpc.