A new measurement could change our understanding of the universe
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OK! RS Puppis, a type of variable star known as a Cepheid variable. Credit: Hubble Legacy Archive, NASA, ESA
The universe is expanding, but how fast? The answer seems to depend on whether you estimate the cosmic expansion rate—referred to as the Hubble constant, or H0—based on the Big Bang echo (the cosmic microwave background, or CMB) or measure H0 directly based on present-day stars and galaxies. This problem, known as the Hubble tension, has puzzled astrophysicists and cosmologists around the world.
A study conducted by the Stellar Standard Candles and Distance research group, led by Richard Anderson at EPFL’s Institute of Physics, adds a new piece to the puzzle. Their research, published in Astronomy & Astrophysics, has achieved the most accurate calibration of Cepheid stars—a type of variable star whose brightness fluctuates over a period—for distance measurements yet based on data from collected by the European Space Agency’s (ESA) Gaia. mission. This new calibration further amplifies the Hubble voltage.
The Hubble constant (H0) is named after the astrophysicist who – together with Georges Lemaître – discovered the phenomenon in the late 1920s. It is measured in kilometers per second per megaparsec (km/s/Mpc), where 1 Mpc is about 3.26 million light years.
The best direct measurement of H0 uses a “cosmic distance scale”, the first scale of which is determined by the absolute luminosity calibration of Cepheids, now recalibrated by the EPFL study. Cepheids, on the other hand, calibrate the next rung of the scale, where supernovae—powerful explosions of stars at the end of their lives—track the expansion of space itself.
This distance scale, measured from the supernova, H0, for the Dark Energy Equation of State (SH0ES) team led by Adam Riess, winner of the 2011 Nobel Prize in Physics, places H0 at 73.0 ± 1.0 km /s/Mpc.
The first radiation after the Big Bang
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 the “early universe” must assume more detailed physical understanding of how the universe evolves, making it model-dependent. ESA’s Planck satellite has provided the most complete data for 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 scale method (late universe) is used. The implication, provided the measurements made in both methods are correct, is that there is something wrong with the understanding of the fundamental physical laws that govern the universe. Of course, this major issue underscores how essential it is that astrophysicists’ methods are reliable. Position on the sky, position in proper motion space, and color-magnitude diagram for different groups of Cepheids. Background stars are shown in gray and cluster membership probability is color coded. Light colors indicate high probability. Cepheids are shown as labeled using large red filled circles. Cepheids detected as cluster members by HDBSCAN also display a superimposed symbol to illustrate the probability of membership. Credit: Astronomy and Astrophysics (2023). DOI: 10.1051/0004-6361/202244775
The new EPFL study is so important because it strengthens the first scale of the distance scale 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 measurement of the universe. Furthermore, 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 accurate and reliable calibration of Cepheids as a means of measuring distances to date,” says Anderson.
“We developed a method that looked for Cepheids belonging to star clusters consisting of several hundred stars by testing whether the stars are moving together through the Milky Way. Thanks to this trick, we could take advantage of the best knowledge of the measurements of Gaia’s parallaxes while taking advantage of the gain in accuracy provided by multiple cluster member stars. This has allowed us to push the accuracy of Gaia parallaxes to their limit and provides the strongest basis on which the distance scale can rest.”
Rethinking basic concepts
Why does a difference of only a few km/s/Mpc matter, given the vast 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 got the type of rock right 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’ve made a mistake – either your calculations are wrong or you’re wrong about the type of stone.
“This is what is happening with the Hubble constant. The more confirmation we get that our calculations are correct, the more we can conclude that the discrepancy means that our understanding of the universe is wrong, that the universe is not as we thought.”
The discrepancy has many other implications. It questions the very basics, such as the exact nature of dark energy, the space-time continuum, and gravity. “This means we have to rethink the basic concepts that form the foundation of our general understanding of physics,” says Anderson.
The study of his research group 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, a Ph.D. student in Anderson’s research group and lead author of the study. “The very precise calibration we 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 comparing them with those obtained by other telescopes”.
More information: Mauricio Cruz Reyes et al, A 0.9% calibration of the Galactic Cepheid luminosity scale based on Gaia DR3 data of open clusters and Cepheids, Astronomy and Astrophysics (2023). DOI: 10.1051/0004-6361/202244775
Journal information: Astronomy & Astrophysics