How Universe Evolved? New Methodology Throws Fresh Light

Every day, scientific methods continue to advance, which deepens our understanding of the universe. The best cosmic yardsticks have just been calibrated with unparalleled accuracy by experts at the Ecole polytechnique federale de Lausanne (EPFL), revealing new light on the phenomenon known as the Hubble tension.

A study carried out by the Stellar Standard Candles and Distances research group, led by Richard Anderson at EPFL's Institute of Physics, adds a new piece to the Hubble tension. Their research, published in Astronomy & Astrophysics, achieved the most accurate calibration of Cepheid stars-a type of variable star whose luminosity fluctuates over a defined period - for distance measurements to date based on data collected by the European Space Agency's (ESA's) Gaia mission. This new calibration further amplifies the Hubble tension.

The new EPFL study is so important because it strengthens the first rung of the distance ladder by improving the calibration of Cepheids as distance tracers. Indeed, the new calibration allows us to measure astronomical distances to within +- 0.9%, and this lends strong support to the late Universe measurement. Additionally, the results obtained at EPFL, in collaboration with the SH0ES team, helped to refine the H0 measurement, resulting in improved precision and an increased significance of the Hubble tension.

"Our study confirms the 73 km/s/Mpc expansion rate, but more importantly, it also provides the most precise, reliable calibrations of Cepheids as tools to measure distances to date," says Anderson. "We developed a method that searched for Cepheids belonging to star clusters made up of several hundreds of stars by testing whether stars were moving together through the Milky Way. Thanks to this trick, we could take advantage of the best knowledge of Gaia's parallax measurements while benefiting from the gain in precision provided by the many cluster member stars. This has allowed us to push the accuracy of Gaia parallaxes to their limit and provides the firmest basis on which the distance ladder can be rested."

Why does a difference of just a few km/s/Mpc matter, given the vast scale of the universe? "This discrepancy has a huge significance," says Anderson. "Suppose you wanted to build a tunnel by digging into two opposite sides of a mountain. If you've understood the type of rock correctly and if your calculations are correct, then the two holes you're digging will meet in the centre. But if they don't, that means you've made a mistake -- either your calculations are wrong or you're wrong about the type of rock. That's what's going on with the Hubble constant. The more confirmation we get that our calculations are accurate, the more we can conclude that the discrepancy means our understanding of the Universe is mistaken and that the Universe isn't quite as we thought."

The discrepancy has many other implications. It calls into question the very fundamentals, like the exact nature of dark energy, the time-space continuum, and gravity. "It means we have to rethink the basic concepts that form the foundation of our overall understanding of physics," says Anderson.

His research group's study makes an important contribution in other areas, too. "Because our measurements are so precise, they give us insight into the geometry of the Milky Way," says Mauricio Cruz Reyes, a PhD student in Anderson's research group and lead author of the study. "The highly accurate calibration we developed will let us better determine the Milky Way's size and shape as a flat-disk galaxy and its distance from other galaxies, for example. Our work also confirmed the reliability of the Gaia data by comparing them with those taken from other telescopes."

(With inputs from ANI)