Pune: A pioneering study led by Professor Anupam Bhardwaj from the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, has used 40 oxygen-rich Mira variable stars across 18 galactic clusters to make a breakthrough in measuring the universe’s rate of expansion. Leveraging the European Space Agency’s Gaia mission for precise cluster distances, the team established a highly accurate period-luminosity relation, achieving an independent calibration of the cosmic distance ladder separate from traditional Cepheid variable methods.
Accurate Calibration for the Hubble Constant
By monitoring the luminosities and pulsation periods of these Mira variables and applying absolute calibration using Gaia’s data, the team succeeded in determining the Hubble constant with 3.7% precision. This new anchor bypasses Cepheids, using Miras to independently calibrate supernovae host galaxies, and provides the most precise cosmic expansion rate achieved by such stars to date. The study, published in the Astrophysical Journal, included contributions from Nobel Laureate Adam Riess and European Southern Observatory astronomer Dr. Marina Rejkuba.
Prof. Bhardwaj highlights, “Mira variables allowed us to establish a three-anchor baseline of the distance ladder, showing that metallicity affects their luminosity much less than Cepheids, making them a promising alternative for measuring cosmic expansion.” Adam Riess adds, “The agreement between Cepheid and Mira-based Hubble constants suggests the persistent Hubble tension is likely not due to measurement errors, and may point towards new physics.”
Mira Variable Stars: The “Wonderful” Standard Candles
Mira variables, giant stars that pulsate between brightness phases over periods from 100 to 1,000 days, are prototypical “standard candles” — objects with known true brightness enabling precise cosmic distance measurements. Their regular variability and relatively cool temperatures (about 3,000 Kelvin) provide astronomers a robust tool for calibrating the extragalactic distance ladder. The new work demonstrates that Mira variables’ relation between brightness and pulsation provides accurate means for estimating galactic distances and mapping the universe’s expansion.
Impact and Future Directions
While uncertainties in the Mira-based Hubble constant remain due to limited supernovae host galaxies with known Miras, upcoming surveys like the Rubin Observatory are expected to discover more Mira stars, further refining these measurements. This promises to improve our understanding of the universe’s size and age and may help resolve the long-standing Hubble tension: the discrepancy between expansion rates measured using nearby stars and those inferred from the early universe’s cosmic microwave background.
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Source: PIB
