Towards a 1% measurement of the Hubble constant: accounting for time dilation in variable-star light curves

European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
e-mail: randerso@eso.org

Received: 27 August 2019
Accepted: 23 September 2019

Abstract

Assessing the significance and implications of the recently established Hubble tension requires the comprehensive identification, quantification, and mitigation of uncertainties and/or biases affecting H₀ measurements. Here, we investigate the previously overlooked distance scale bias resulting from the interplay between redshift and Leavitt laws in an expanding Universe: Redshift-Leavitt bias (RLB). Redshift dilates oscillation periods of pulsating stars residing in supernova-host galaxies relative to periods of identical stars residing in nearby (anchor) galaxies. Multiplying dilated log P with Leavitt Law slopes leads to underestimated absolute magnitudes, overestimated distance moduli, and a systematic error on H₀. Emulating the SH0ES distance ladder, we estimate an associated H₀ bias of (0.27 ± 0.01)% and obtain a corrected H₀ = 73.70 ± 1.40 km s⁻¹ Mpc⁻¹. RLB becomes increasingly relevant as distance ladder calibrations pursue greater numbers of ever more distant galaxies hosting both Cepheids (or Miras) and type-Ia supernovae. The measured periods of oscillating stars can readily be corrected for heliocentric redshift (e.g. of their host galaxies) in order to ensure H₀ measurements free of RLB.