Researchers Leverage Mira Stars to Refine Cosmic Expansion Rate

A recent study led by Professor Anupam Bhardwaj from the Inter-University Centre for Astronomy and Astrophysics (IUCAA) has made significant strides in determining the rate of cosmic expansion. This groundbreaking research utilized 40 oxygen-rich Mira variable stars, located in 18 stellar clusters within our galaxy, to create a more precise method for calibrating the universe’s expansion rate. The findings were published in the prestigious Astrophysical Journal.

The research team monitored these Mira stars over an extended period, establishing their mean luminosities and pulsation periods. The European Space Agency’s Gaia mission provided critical geometric distances to these star clusters, situated between 13,000 and 55,000 light-years from Earth. This collaboration enabled the team to achieve an absolute calibration of the stellar luminosities of the Mira variables, enhancing measurement precision in cosmic studies.

New Insights on Cosmic Distance Calibration

The study established a new “absolute” period-luminosity relationship for these Mira stars, which serves as an independent calibration of supernovae used in the cosmic distance ladder, without relying on traditional Cepheid variables. This innovative approach allowed the team to calculate the Hubble constant with an impressive precision of 3.7 percent.

“We used Miras in our galaxy as anchors for the first time to determine the most precise cosmic expansion rate based on these cool stars,” said Professor Bhardwaj. He emphasized that the Mira variables provided a three-anchor baseline calibration of the extragalactic distance ladder, complemented by additional Mira variables from two external galaxies. This research highlights that metal abundance affects Mira luminosity three times less than Cepheid variables, positioning Miras as a promising alternative for determining the Hubble constant.

Co-author Adam Riess, a Nobel Laureate affiliated with the Space Telescope Science Institute and Johns Hopkins University, noted the implications of the study for ongoing discussions regarding the Hubble tension. “The consistency between Cepheid and Mira-anchored Hubble constant values further suggests the Hubble tension is unlikely due to measurement errors and points to a more fundamental cause, including the possibility of new physics,” he stated.

Implications for Cosmology

Dr. Marina Rejkuba, another co-author and staff astronomer at the European Southern Observatory, highlighted the study’s significance in merging stellar astrophysics with cosmology. “This study ensures our understanding of the potential of Mira variable stars as a new, well-calibrated anchor for Hubble constant determination,” she remarked.

While this calibration aligns the precision of Miras with that of Cepheids, the overall uncertainty in the Mira-based Hubble constant measurement is still affected by the limited number of galaxies with known Miras, currently only two supernova host galaxies. Nevertheless, advancements such as those anticipated from the Rubin Observatory are expected to unveil a greater number of Miras in supernova host galaxies, enhancing the ability to accurately map the universe’s age and size.

Mira, also known as Omicron Ceti, is a star noted for its remarkable brightness fluctuations over time. First measured by astronomers in the 17th century, Mira became the prototype for an entire class of stars known as Mira variables. These stars undergo regular cycles of expansion and contraction, typically affecting their brightness over intervals ranging from 100 to 1,000 days.

As stars in the late stages of their life, Mira variables possess surface temperatures around 3,000 Kelvin, roughly half that of the Sun. Their unique properties establish a strong correlation between their brightness and the duration of their pulsation cycles, allowing astronomers to use them as “standard candles.” This concept is crucial for calculating distances in the universe, contributing to what astronomers refer to as the extragalactic distance ladder.

The current scientific challenge known as the “Hubble tension” arises from discrepancies in the measured values of the Hubble constant. Observations using nearby stars, such as Cepheid variables and Type Ia supernovae, yield a higher value than calculations based on early universe observations, including cosmic microwave background data. This ongoing debate suggests that the universe may be expanding at a faster rate than predicted by established cosmological models, prompting scientists to explore potential unknown physics or necessary updates to existing models.

Research like this recent study involving Mira and other variable stars continues to play a pivotal role in deciphering the complexities of the cosmos, potentially reshaping our understanding of the universe’s expansion and its fundamental properties.