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The new measurement could change our understanding of the universe

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RS Puppis is a type of variable star known as a Cepheid variable. Credit: Hubble Legacy Archives, NASA, European Space Agency

The universe is expanding, but exactly at what rate? The answer depends on whether you estimate the rate of cosmic expansion, called the Hubble constant or H.– based on the echo of the Big Bang (cosmic microwave background, or CMB) as measured by H It is directly related to today’s stars and constellations. This problem, known as Hubble stress, has baffled astrophysicists and cosmologists around the world.

A study by the Stellar Standard Candles and Distances research group led by Richard Anderson of the Institute of Physics at EPFL adds a new piece to the puzzle. Their studies have been published Astronomy and Astrophysics, performed the most accurate calibration of Cepheid stars – a type of variable star whose brightness varies over a period of time – for distance measurements to date based on data collected by the European Space Agency’s (ESA) Gaia mission. This new calibration further increases the Hubble tension.

Hubble’s constant (H) is named after the astrophysicist who discovered the phenomenon in the late 1920s by Georges Lemaître, and is measured in kilometers per second per megasec (km/v/Mpc), where 1 Mpc is about 3.26 million light-years away.

The best direct measurement of H It uses a “cosmic distance scale”, the first of which is determined by absolute calibration of the brightness of Cepheids, which has now been recalibrated by the EPFL study. In contrast, Cepheids calibrates the next rung of the ladder, tracking supernovae — the powerful explosions of stars at the end of their lives — that expand space.

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This distance scale, measured by the supernova, is H.The Dark Energy Equation of State (SH0ES) group led by 2011 Nobel Laureate in Physics Adam Rice, H. 73.0 ± 1.0 km/sec/mpc.

The first radiation after the Big Bang

M It can also be determined by interpreting CMB radiation, the ubiquitous microwave radiation left over from the Big Bang 13 billion years ago. However, this method of measuring the “early universe” requires a very detailed physical understanding of how the universe formed, making it model dependent. The European Space Agency (ESA) Planck satellite provided the most detailed data on the CMB, according to this method, provided by H. 67.4 ± 0.5 km/sec/mpc.

The Hubble tension represents this discrepancy of 5.6 km/s/million particles, depending on whether the CMB (early universe) or distance ladder method (late universe) is used. If the measurements made by both methods are correct, the implication is that there is something wrong with our understanding of the basic physical laws that govern the universe. Of course, this key issue underscores how essential the reliability of astrophysicists’ methods is.

Position on the sky, position in the appropriate motion space, and color scale chart for different groups of Cepheids. Background stars are shown in gray and the probability of group membership is color-coded. Lighter colors indicate higher probability. Cepheids are shown as indicated using large solid red circles. Cepheids detected as members of a cluster by HDPSCAN have a superscript to indicate membership probability. debt: Astronomy and Astrophysics (2023) DOI: 10.1051/0004-6361/202244775

The new EPFL study is very important because it improves the first order of distance measurement by improving the calibration of Cepheids as a distance observer. Indeed, the new calibration allows astronomical distances to be measured to within ±0.9%, providing strong support for later measurements of the universe. In addition, the results obtained at EPFL, together with the SH0ES group, made it possible to develop H measurement, resulting in greater precision and greater emphasis on the Hubble effort.

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“Our study confirms an expansion rate of 73 km/s/mpc, but more importantly, it provides the most accurate and reliable calibrations of kyphids as distance measuring instruments to date,” explains Anderson.

“We developed a method to search for Cepheids in clusters of several hundred stars by testing whether the stars move together in the Milky Way. Thanks to this trick, we were able to take advantage of Gaia’s excellent cognitive parallax measurements. This allowed us to push the resolution of observations of Gaia to their limits and provides a strong foundation on which the distance scale can rest. .

Review the basic concepts

Given the vastness of the universe, why is a difference of a few km/s/m3 significant? “This gap is of enormous importance,” Anderson says.

“Suppose you want to dig a tunnel through two opposite sides of a mountain. If you get the rock type right and your calculations are correct, the two holes you drill will meet in the center. This isn’t the case, which means you’ve made a mistake – either your calculations are wrong or you’re wrong about the rock type.

This is what happens with the Hubble constant. And the more we confirm the accuracy of our calculations, the more we conclude that the discrepancy means that our understanding of the universe is wrong and that the universe is not what we thought it was. ”

Spacing also has many other effects. It challenges fundamentals such as the exact nature of dark energy, temporal continuity and gravity. “This means that we need to rethink the fundamental concepts that form the basis of our general understanding of physics,” Anderson says.

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His research group’s research makes important contributions to other fields as well. “Our measurements are so precise that they give us insight into the geometry of the Milky Way,” says Mauricio Cruz Reis, a PhD student in the Andersen research group and lead author of the study. “The highly accurate 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. Our work has confirmed the reliability of the data from Gaia by comparing it with data from other telescopes.

More information:
Mauricio Cruz Reyes et al, 0.9% calibration of the Galactic Cepheid Luminometer based on Gaia DR3 data from open and cepheid collections. Astronomy and Astrophysics (2023) DOI: 10.1051/0004-6361/202244775

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Stuart Wagner
Stuart Wagner
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