One of the greatest challenges that scientists have had to face until now is establishing a precise scale that can allow them to measure distances in the Universe. One method that has had successful results is the one that combines supernovae with Cepheid variables, which also allows to accurately establish the Hubble constant that measures the expansion of the Universe.

Nevertheless, despite its success, the Cepheid-supernovae method has had a stumbling block: how the relation between Cepheids’ luminosities and its chemical composition affect the measurements, that is, its metallicity. Additionally, there is also the problem that the value of the Hubble constant, obtained by this method, seems to have a significant difference with the value determined by currently operating Plank Satellite’s study, which measures the cosmic microwave background’s fluctuations, the most distant trace of the Big Bang explosion we know.

For this reason, according to Wolfgang Gieren –MAS’ associate researcher and astronomer of Universidad de Concepción’s Department of Astronomy and the Center for Excellence in Astrophysics and Associated Technologies CATA– it is necessary to establish a maximum precision in the Cepheids method, considering that this method depends on metallicity. With that, it can be possible to elucidate if the Hubble constant is accurate.

Although previous researches have not been clear to specify this point yet, a recent study of MAS researcher, along with his team: astronomers Piotr Wielgorski, Grzegorz Pietrzynski and Dariusz Graczyk, all from Universidad de Concepción’s Department of Astronomy, has changed this scenario. The study proves in a very clear and precise way that this dependence between Cepheids’ luminosities and its metallicity is close to zero, that is, it is not important neither for the measurement of distances in the Universe nor for the determination of the Hubble constant. Therefore, when measuring the distance of a distant galaxy with the Cepheid method, this will always generate accurate results, independent of Cepheids’ metallicity in that distant galaxy, which is usually unknown.

“Our determination of the metallicity effect is quite precise (3%,) 10 times more precise that other published studies, and it represents a major step towards the goal of measuring the Hubble constant through the Cepheids-supernovae method with an accuracy high enough to understand if the discrepancy with the value obtained by the Plank mission is real or not. If it is real, this could have consequences for our current cosmological models,” he explains.

The Valuable Magellanic Clouds

The study, which was published in the prestigious Astrophysical Journal, used the abundant Cepheid populations in the two Magellanic Clouds since their relation between their pulsation period and their “apparent magnitude” is well known. “Metallicities of Cepheids located in the two Magellanic Clouds are accurately known and are different enough to test the effect that this difference in metallicity has on the relation between pulsation and luminosity periods of Cepheids, which is the instrument used to measure their distances,” Gieren explains.

Cepheids used in this study were classified by OGLE Project in Chile that observed them in visible light, and by Infrared Survey Facility (IRSF,) a Japanese project carried out in South Africa that observed the same Cepheids in infrared light. MAS’ researcher’s team is closely collaborating with researchers from both projects.

“Comparing distances of the two Magellanic Clouds derived from their respective Cepheids in different photometric bands with those derived with very high accuracy from eclipsing binary systems in both Clouds that we previously studied in 2013 and 2014, we have been able to establish that luminosities, or Cepheid’s absolute magnitudes, have a dependence of metallicities consistent with zero, particularly in the near-infrared photometry range in which measurements of distances to galaxies with Cepheids provide the best precision. This scientific result takes us closer to our great goal: measuring the Hubble constant with an accuracy of 1%, a dream that astronomers have had for almost a century, since Edwin Hubble discovered that Universe is expanding.”

Perhaps, a conclusion that also establishes a new challenge to scientists, since it implies that it might be necessary to check some aspects of the cosmological models used by the Plank Mission’s team on their Hubble constant’s calculations.

Image: period-luminosity relations of Cepheids in both Magellanic Clouds, in different optical and near-infrared photometric bands.