The universe is expanding, but how fast exactly? The answer seems to depend on whether you are estimating the cosmic expansion rate or H, called the Hubble constant.₀Based on the Big Bang echo (cosmic microwave background or CMB), are you measuring H? ₀ directly based on modern stars and galaxies. Known as the Hubble voltage, this problem has baffled astrophysicists and cosmologists around the world.

The work by the Stellar Standard Candles and Distances research group, led by Richard Anderson of the EPFL Institute for Physics, adds a new piece to the puzzle. published work, Astronomy and Astrophysics. mission. This new calibration further increases the Hubble voltage.

Hubble constant (H ₀ ) is named after the astrophysicist who, together with Georges Lemaître, discovered this phenomenon in the late 1920s. This is measured in kilometers per second per megaparsec (km/s/Mpc), where 1 Mpc equals about 3.26 million light years.

The best direct measurement of H ₀ uses a “cosmic distance ladder” determined by the absolute brightness calibration of Cepheids, the first step of which has now been recalibrated by the EPFL study. In turn, Cepheids set the next rung of the ladder; here, supernovas – powerful explosions of stars at the end of their lives – follow the expansion of space.

This distance scale as measured by the supernova, H₀For the Dark Energy Equations of State (SH0ES) group led by 2011 Nobel laureate in physics Adam Riess, H’s ₀ 73.0±1.0 km/h/Mpc..

First radiation after the Big Bang

H ₀ It can also be determined by interpreting CMB, the ubiquitous microwave radiation left over from the Big Bang more than 13 billion years ago. However, this method of measuring the “early universe” should assume the most detailed physical understanding of how the universe evolved, making it model dependent. ESA’s Planck satellite provided the most complete data on the CMB and, according to this method, H₀ It is 67.4 ± 0.5 km/h/Mpc.

Hubble voltage falls within this discrepancy of 5.6 km/s/Mpc depending on whether the CMB method (early universe) or the distance ladder method (late universe) is used. The accuracy of measurements made by either method means that there is a problem with understanding the fundamental laws of physics that govern the universe. Of course, this important question underscores how important it is for astrophysicists’ methods to be reliable.

The new EPFL study is crucial because it strengthens the first rung of the distance ladder and improves the calibration of Cepheids as distance indicators. Indeed, the new calibration allows us to measure astronomical distances with ±0.9% accuracy, providing strong support for late measurements of the universe. In addition, the results obtained in collaboration with the SH0ES team at EPFL helped improve the H measurement.₀this led to an increase in accuracy and an increase in the value of the Hubble voltage.

“Our work confirms the expansion rate of 73 km/s/Mpc, but more importantly provides the most accurate and reliable calibrations of Cepheids as distance measuring tools to date,” says Anderson.

“We have developed a method that searches for Cepheids belonging to star clusters of several hundred stars, checking whether stars are moving together in the Milky Way. With this trick, we were able to take advantage of the best knowledge of Gaia’s parallax measurements while taking advantage of the increased precision provided by the cluster’s many stellar members. allowed us to push the accuracy of Gaia’s parallaxes to their limits and provided the strongest foundation on which to build the ladder of distance.”

Rethinking basic concepts

Why does a difference of only a few km/s/Mpc matter given the vastness of the universe? “This discrepancy is huge,” Anderson says.

“Let’s say you want to build a tunnel by digging both sides of a mountain. If you determine the rock type correctly and your calculations are correct, the two holes you dug will meet in the center. But if not, then you made a mistake – either your calculations are wrong or you are wrong about the breed type.

“This is what happens with the Hubble constant. The more we confirm that our calculations are correct, the more we can conclude that the inconsistency means that our understanding of the universe is wrong, that the universe is not exactly what we thought it was.”

Disagreement has many other consequences. This questions fundamentals such as the precise nature of dark energy, the space-time continuum, and gravity. “This means we need to rethink the fundamental concepts that underpin our overall understanding of physics,” Anderson says.

The research group’s research also makes important contributions in other fields. “Because our measurements are so precise, they give us insight into the geometry of the Milky Way,” says Mauricio Cruz Reyes. Student in Anderson’s research group and lead author of the study. “The extremely precise calibration we developed will allow us to better determine, for example, the size and shape of the Milky Way as a flat disk galaxy, and its distance to other galaxies. Our study also validated the reliability of the Gaia data by comparing it with data received from other telescopes”.