The present expansion rate of the Universe, also called the Hubble constant (H0), is a key cosmological parameter that parametrizes the age of the Universe. Currently, the Hubble constant values based on a variety of standard candles observed with the Gaia/Hubble/James Webb space telescopes and ground-based telescopes, are in an intriguing discord with the measurement from the Planck space mission. This Hubble tension is most dominant at the ~5 sigma level between the H0 values inferred with flat CDM theory and Cosmic Microwave Background observations from the Planck satellite in comparison with the H0 values obtained from the cosmic distance ladder with Classical Cepheids (CC) as primary and Type Ia Supernovae as secondary distance indicators. This discrepancy points to new physics in the cosmological model and is one of the most hotly debated topics in the current astrophysical literature (e.g. Verde et al. 2019, Riess et al. 2022, 2024, Abdalla et al. 2022 and references therein). Confirming the H0 tension is expected to have significant consequences for both fundamental physics and modern cosmology. While theoretical efforts to test possible limitations of the lambda CDM scenario are in progress, there are also indications of residual and/or unknown systematic uncertainties in the local distance scale (see e.g. Freedman 2021, De Somma et al. 2022, Freedman et al. 2024).

One of the crucial steps towards resolving or confirming this Hubble tension is to provide Cepheid independent calibrations of the cosmic distance ladder using stellar populations of different ages and compositions in different stellar environments. In this context older standard candles, RR Lyrae stars, Type II Cepheids, AGB stars (Mira variables and J-AGB carbon stars), and the tip of the red giant branch (TRGB) play a vital role by providing alternate routes and independent calibrations for the first rung of the cosmic distance ladder.

In the proposed symposium, we aim to discuss open problems and perspectives in the investigation of these stellar standard candles in view of the upcoming Gaia data release 4, expected in late 2026. We plan to invite experts in observational and theoretical areas, to trigger new collaborations and ideas, and contribute towards evaluating this Hubble tension.

Programme

Invited speakers