Research Projects

Classical pulsating stars such as Cepheid variables are primary standard candles in astronomy for the extragalactic distance determinations. These stellar distance indicators have known intrinsic luminosities that form the first step of the cosmic distance ladder used to determine the Hubble constant - the present expansion rate of the Universe. The Hubble constant is a key cosmological parameter, which provides stringent constraints on the nature of dark energy – a mysterious force known to be accelerating the cosmic expansion. Currently, the Hubble constant values based on a variety of standard candles observed with the Gaia/Hubble/James Webb space and several ground-based telescopes, are in an intriguing discord with its measurement from the Planck space mission based on cosmic microwave background observations in the early universe. This discrepancy is known as the Hubble tension and points to new physics in the cosmological model. Given the stakes, this Hubble tension must be evaluated against all sources of measurement errors in the rate of cosmic expansion based on both the local (late) and early universe probes. One of the main sources of uncertainties in the local determinations of the Hubble constant is the absolute luminosity scales of stellar standard candles such as classical Cepheids, RR Lyrae, Mira, and tip of the red giant branch. These luminosity calibrations have not yet reached percent-level accuracy despite unprecedented geometric distances of standard candles in publically available Gaia space mission data. The main caveats in achieving self-consistent, accurate and precise luminosity scales are the uncertainties associated with Gaia parallax offsets, lack of complementary multi-band photometry and high-resolution spectroscopy, and highly debated effects of composition and age on the luminosities, among other uncertainties. Our objective is to strategically address these shortcomings in the luminosity scales of stellar standard candles in view of the nominal five-year Gaia mission data release (expected in mid 2026). For this purpose, we plan to use multi-wavelength datasets from time-domain surveys and our observational programs on the state-of-the-art 4-10m class telescopes, open-source 1D stellar evolution and pulsation models, and innovative data science and machine-learning tools leveraging expertise in a multidisciplinary context. With the final Gaia data release, we will provide first-ever percent-level precise luminosity calibrations of stellar distance indicators. A cross-investigation of percent-level precise luminosity scales of a variety of stellar standard candles with high accuracy and fidelity will allow us to firmly rule out (or confirm) measurement uncertainties as the origin of the Hubble tension. Our precise luminosity calibrations will also be the primary resource for the astronomical distance determination in the era of space and ground-based large observational facilities upcoming in the next few years.