A new measurement that can change the understanding of the universe

Illustration of a Cepheid

Illustration of a Cepheid -EPFL


Data from ESA’s Gaia mission have yielded the most precise calibration of Cepheids for cosmic distance measurements. It is a type of variable star whose luminosity fluctuates in a defined period.

The universe is expanding, but how fast exactly? The answer seems to depend on whether you estimate the rate of cosmic expansion, known as the Hubble constant or H0, based on the echo of the Big Bang (the cosmic microwave background, or CMB), or if you measure H0 directly based on the stars and current galaxies. This problem, known as the Hubble stress, has puzzled astrophysicists and cosmologists around the world.

The new research, 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 puzzle, as the new calibration of the Cepheids further amplifies the Hubble strain.

The Hubble constant (H0) is named after the astrophysicist who, along with Georges Lemaitre, discovered the phenomenon in the late 1920s. It 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 H0 uses a “cosmic distance ladder”, whose first step is established by the absolute calibration of the brightness of the Cepheids, now recalibrated by the EPFL study. In turn, the Cepheids gauge the next rung of the ladder, where supernovae, powerful explosions of stars at the end of their lives, chart the expansion of space itself.

This distance scale, as measured by supernovae, H0, for the Dark Energy Equation of State (SH0ES) team led by Adam Riess, winner of the 2011 Nobel Prize in Physics, puts H0 at 73.0 +/- 1 .0 km/s/Mpc.

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

The Hubble tension refers to 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 implication, as long as the measurements made in both methods are correct, is that there is something wrong in understanding the basic physical laws that govern the universe. Naturally, this main theme underscores how essential it is that astrophysicists’ methods be reliable.

The new EPFL study -published in Astronomy & Astrophysics- It is important because, according to its authors, it strengthens the first rung of the distance scale by improving the calibration of the Cepheids as distance tracers. In fact, the new calibration allows us to measure astronomical distances with an accuracy of +/- 0.9%, and this provides strong support for the measurement of the late universe. In addition, the results obtained at EPFL, in collaboration with the SH0ES team, helped to refine the measurement of H0, which resulted in higher precision and greater significance of the Hubble strain.

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

“We developed a method that searched for Cepheids belonging to star clusters made up of several hundred stars by testing whether the stars move together through the Milky Way. Thanks to this trick, we were able to take advantage of the best knowledge of Gaia’s parallax measurements while benefiting of the gain in precision provided by the many member stars of the cluster. This has allowed us to push the precision of Gaia’s parallaxes to its limit and provides the firmest foundation on which the distance scale can rest.”

Why is a difference of a few km/s/Mpc important, given the large scale of the universe? “This discrepancy is of great importance,” says Anderson.

“Suppose you wanted to build a tunnel by digging into two opposite sides of a mountain. If you have understood the type of rock correctly and if your calculations are correct, then the two holes you are digging will meet in the center. But if they don’t, that means you made a mistake, or your calculations are wrong, or you are wrong about the type of rock.

“That’s what’s happening 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 wrong, that the universe is not exactly as we thought.”

The discrepancy has many other implications. It casts doubt on fundamentals, such as the exact nature of dark energy, the space-time continuum, and gravity. “It means that we have to rethink the basic concepts that form the basis of our general understanding of physics,” says Anderson.

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Written by Editor TLN

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