Measuring Hubble's Constant to 1% using Pulsating Stars

Abstract: Hubble's constant, H0, quantifies the expansion rate of the Universe today and is of fundamental importance for cosmology. For example, H0 is related to the age of the Universe, its observable size, and its critical density, among others. Yet, recent observations have established a 5 sigma discrepancy between H0 measured in the local Universe and the value predicted by flat Lambda CDM cosmology based on observations of the early Universe. To clarify this looming cosmological crisis, the H1PStars project seeks to measure H0 to an accuracy of 1% using stellar standard candles that calibrate the absolute magnitudes of type-Ia supernovae. Starting with a brief overview of the current Hubble tension, I will discuss recent improvements in the calibration of and upcoming opportunities for the nearest rungs of the distance ladder. In turn, I will discuss ongoing work to improve the absolute calibration of Cepheid luminosities, to mitigate biases affecting Cepheid distances, and to obtain new insights into Cepheid stars from a stellar variability perspective that will help to improve our astrophysical basis for using Cepheids as highly accurate distance tracers. I will close by discussing these improvements in the context of the 1% measurement of Hubble’s constant required to understand the ongoing cosmological crisis.