Presentation #402.06 in the session “How Gaia Reveals the Galaxy’s Secrets: Results on the Galactic Scale, Part 2”.
The modern extra-galactic distance ladder, as a path to the Hubble constant, has been simplified to three rungs: (1) the calibration of a stellar standard candle in the near-field using distances derived from geometric methods, (2) the calibration of Supernova Type Ia (SNe Ia) in the local volume, and (3) the use of SNe Ia in the Hubble Flow to determine the Hubble Constant. Prior to Gaia, the first-rung was established using highly-precise, but extremely-expensive, parallax measurements using the HST-FGS and its observational expense limited the application to a handful of key stars (RR Lyrae, Cepheids, Type II Cepheids) to calibrate their period-luminosity relationships (PL). Gaia is, and will continue to be, transformational in the absolute calibration of standard candles by vastly increasing the number of stars used in calibrations. Moreover, by vastly expanding the volume probed by trigonometric parallaxes, Gaia also opens up the use of statistical standard candles (Red Clump, Tip of the Red Giant Branch) and calibrates PL relations for new classes of variable stars (Miras), as well as permitting full characterization of systematic terms (metallicity). In this talk, I will contextualize these advancements and forecast what to expect as Gaia approaches end-of-mission performance in terms of Galactic and Extra-Galactic distances, as well as highlighting the next systematic terms for improvment.