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The universe is expanding, but at what speed exactly? The answer seems to depend on whether you estimate the rate of cosmic expansion – referred to as the Hubble constant, or H—based on the echo of the Big Bang (the cosmic microwave background, or CMB) or H It is directly dependent on the stars and galaxies of today. This problem, known as the Hubble tension, has baffled astrophysicists and cosmologists around the world.
A study by the Stellar Standard Candles and Distances group led by Richard Anderson at the EPFL Institute of Physics adds a new piece to the puzzle. Their research has been published in Astronomy and astrophysics, has achieved the most accurate calibration of Cepheid stars — a type of variable star whose luminosity fluctuates over a specific period — for distance measurements to date based on data collected by the European Space Agency (ESA’s) Gaia mission. This new calibration further amplifies the Hubble tension.
Hubble constant (H) is named after the astrophysicist who discovered this phenomenon with Georges Lemaitre in the late 1920s. It is measured in kilometers per second per megasec (km/s/Mpc), where 1 Mpc equals about 3.26 million light-years.
The best direct measure of H. It uses a “cosmological distance scale”, the first rung of which is set by the absolute calibration of the luminosity of the Cepheid, now recalibrated by the EPFL study. Cepheids, in contrast, calibrate the next rung of the ladder, tracking supernovae—the powerful explosions of stars at the end of their lives—the expansion of space itself.
This distance scale, measured in supernovae, H.for the Dark Energy Equation of State (SH0ES) team led by Adam Riess, 2011 Nobel Prize winner in Physics, puts H at 73.0 ± 1.0 km/sec/mpc.
The first radiation after the Big Bang
h It can also be determined by interpreting CMB radiation – the omnipresent microwave radiation left over from the Big Bang more than 13 billion years ago. However, the “early universe” measurement method must assume the most detailed physical understanding of how the universe evolved, making it model dependent. The European Space Agency (ESA) Planck satellite has provided the most complete data on the CMB and, according to this method, H 67.4 ± 0.5 km/sec/mpc.
The Hubble tension indicates this discrepancy of 5.6 km/sec/million blocks, depending on whether the CMB (early universe) or distance ladder method (late universe) is used. The implication, provided that the measurements made by both methods are correct, is that there is something wrong with the understanding of the basic physical laws that govern the universe. Of course, this key issue underscores how important it is for astrophysicists’ methods to be reliable.
The new EPFL study is very important because it strengthens the first rung of the distance ladder by improving the calibration of Cepheids as distance trackers. In fact, the new calibration allows us to measure astronomical distances within ± 0.9%, and this provides strong support for the late entropy measurement. In addition, the results obtained at EPFL helped, in collaboration with the SH0ES team, to improve H. measurement, resulting in improved accuracy and increased significance of the Hubble tension.
“Our study confirms the rate of expansion of 73 km/s/Mpc, but more importantly, it also provides the most accurate and reliable calibrations of kyphids as distance measuring tools to date,” Anderson says.
“We developed a method that searched for Cepheids belonging to star clusters of several hundred stars by testing whether stars move together across the Milky Way. Thanks to this trick, we can take advantage of the best knowledge of Gaia’s measurements of parallax while taking advantage of the increase in resolution offered by many of the cluster’s member stars. This has allowed us to push the resolution of Gaia’s views to their limits and provides the strongest foundation on which the distance ladder can rest.”
Rethink basic concepts
Why is a difference of a few kilometers/second/Mpc important, given the vast scale of the universe? “This discrepancy is of great importance,” Anderson says.
“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, the two holes you drill will meet in the center. But if they don’t, then you’ve made a mistake – either your calculations are wrong or you’re wrong about the type of rock.
This is what happens with the Hubble constant. The more confirmation we get of the accuracy of our calculations, the more we conclude that the discrepancy means that our understanding of the universe is wrong, that the universe is not quite what we thought it was.”
Contradiction has many other effects. It calls into question the fundamentals, such as the exact nature of dark energy, temporal continuity, and gravity. “This means that we have to rethink the fundamental concepts that form the basis of our general understanding of physics,” Anderson says.
His group’s study makes an important contribution to other fields as well. “Because our measurements are so precise, they give us insight into the geometry of the Milky Way,” says Mauricio Cruz Reyes, PhD. Student in Anderson’s research group and lead author of the study. “The high-resolution calibration we have developed will allow us to better determine the size and shape of the Milky Way 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 it with that from other telescopes.”
Mauricio Cruz Reyes et al, 0.9% calibration of the Cepheid galactic glossometer based on Gaia DR3 data for open clusters and Cepheids, Astronomy and astrophysics (2023). DOI: 10.1051/0004-6361/202244775
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