The 'barytone' of red giants refines cosmic distance measurements

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Large Magellanic Cloud. Credit: CTIO/NOIRLab/NSF/AURA/SMASH/D. Nightever (Montana State University) Image Processing: Travis Rector (University of Alaska Anchorage), Mahdi Zamani & David De Martin.

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Large Magellanic Cloud. Credit: CTIO/NOIRLab/NSF/AURA/SMASH/D. Nightever (Montana State University) Image Processing: Travis Rector (University of Alaska Anchorage), Mahdi Zamani & David De Martin.

New observations of red giant stars provide key insights into cosmic distance measurements and more precise measurements of the universe's expansion.

In an ever-expanding universe, measuring cosmic distances is like trying to find a reliable ruler in a vast, ever-stretching fabric. A tool used by astrophysicists is the Hubble constant, (H0), which sets how fast the universe is expanding and the age and observable size of the universe.

However, there is disagreement on the value of H0, due to conflicting measurements obtained from different celestial objects. The debate means that our understanding of the fundamental physics of the universe is incomplete. The stakes are high, and the key to finding a resolution is to significantly improve the accuracy of distance measurements based on stars.

Now, A study Published in Astrophysical Journal Letters EPFL Professor Richard I. Anderson, former EPFL Undergraduate Summer Research Intern Nolan Koblischke (now University of Toronto), and Laurent Iyer (University of Geneva) are refining cosmic distance measurements using acoustic signals from red giants. „We found that the acoustic oscillations of red giant stars tell us how best to measure cosmic distances using the 'end of the red giant branch' method,” says Anderson.

Measuring cosmic distances with red giants

Let's explain some terms. „Red giants” are aging stars. They adopt a red color as they eject hydrogen at their cores and use up external hydrogen, which makes them larger and cooler.

In astronomical maps, this evolution leads to the „red giant branch”, a deviation due to the star's increased brightness. The tip of the red giant branch (DRGB) is a critical point where these stars ignite helium and change their luminosity evolution.

Marked by the less bright stars above it on the map, the DRGB serves as a „standard candle” for cosmic distance measurements: by comparing its known brightness with the observed brightness of distant galaxies, astronomers can calculate the distance, much like estimating a light bulb. distance by its luminosity.

Singing in the dark

The researchers analyzed data from the Optical Gravitational Lensing Experiment (OGLE) and the ESA Gaia mission, which investigates red giants in the nearby subgalactic Large Magellanic Cloud (LMC). Physics of stars.

In a surprising twist, the scientists found that all the stars in the TRGB actually vary in brightness from time to time; Sound waves travel through the stars like earthquakes on Earth, causing them to oscillate. Although these oscillations were previously known, their importance for distance measurements was missed. But now, they've allowed researchers to differentiate stars by age, providing a more precise approach to measuring distances across the universe.

Anderson explains, „Younger red giant stars near the DRGB are slightly less luminous than their older cousins, and the sonic oscillations we observe as brightness fluctuations allow us to understand what kind of star we're dealing with: older stars oscillate. A low frequency—a baritone with a deeper voice than a tenor Like singing.”

This distinction is important for ensuring the most accurate distance measurements needed to obtain a better map of the cosmological and local universe, since every galaxy contains red giant stars.

The study also identifies several improvements to the DRGP distance method that are essential to understanding recent discussions of the Hubble constant tension. „We can now distinguish the age of the red giants that make up the DRGB, and based on that we can further improve the Hubble standard measurement,” says Anderson.

„Such improvements will further test the Hubble constant tension and lead to new insights into the fundamental physical processes that determine how the universe evolves.”

More information:
Richard I. Anderson et al., Small-Aptitude Red Giants, interpret the nature of the tip of the red giant branch as a constant candle, Astrophysical Journal Letters (2024) DOI: 10.3847/2041-8213/ad284d

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