The cosmos is a grand expanse that keeps stretching, making the task of measuring its size and expansion rate akin to finding a reliable yardstick in an infinite stretch of fabric.
Amidst this challenge, the Hubble constant (H0) emerges as a crucial tool, offering insights into the universe’s expansion speed, age, and dimensions.
Yet, a lingering debate over H0’s exact value underscores a gap in our understanding of the universe’s fundamental physics. Resolving this debate hinges on refining the accuracy of cosmic distance measurements, particularly those based on stellar observations.
A groundbreaking study published in The Astrophysical Journal Letters brings us closer to this goal.
Spearheaded by EPFL’s Professor Richard I. Anderson, Nolan Koblischke (formerly an EPFL undergraduate research intern, now at the University of Toronto), and Laurent Eyer of the University of Geneva, the research brings a fresh perspective on cosmic distance measurements through the study of red giant stars.
Red giants, which are stars in a later stage of evolution, become larger, cooler, and adopt a reddish hue as they burn hydrogen in their outer layers.
These stars trace a distinct path on astronomical charts, known as the “red giant branch,” culminating in a point known as the tip of the red giant branch (TRGB). The TRGB marks a pivotal moment where helium ignition in these stars reverses their brightness trend.
The TRGB serves as a “standard candle” in astronomy, a celestial marker whose intrinsic brightness is known, allowing astronomers to calculate distances to distant galaxies by comparing the TRGB’s known and observed brightness.
This method, akin to gauging the distance of a light bulb based on its luminosity, has been refined by the researchers’ study, which utilized data from the Optical Gravitational Lensing Experiment (OGLE) and the ESA’s Gaia mission to examine red giants in the Large Magellanic Cloud (LMC).
The LMC, a companion galaxy to the Milky Way, provides an invaluable context for understanding stellar physics.
A significant revelation of this study is the discovery that all stars at the TRGB exhibit periodic brightness variations, akin to the Earth’s seismic waves causing oscillations.
These acoustic oscillations within the stars, previously known but undervalued for distance measurements, have now been recognized as a key to differentiating stars by age.
This differentiation allows for more nuanced and accurate cosmic distance measurements, as younger red giants near the TRGB are slightly dimmer than their older counterparts.
The study correlates these oscillations to the stars’ ages, with older stars oscillating at lower frequencies, much like a baritone’s deeper voice compared to a tenor’s.
This nuanced understanding of red giant stars promises to enhance the accuracy of cosmic distance measurements crucial for cosmology and for refining the Hubble constant.
By distinguishing the ages of red giants at the TRGB, astronomers can improve measurements of the local universe and further scrutinize the Hubble constant tension.
The implications of this study are profound, offering a path toward resolving long-standing debates in astrophysics and potentially unveiling new insights into the universe’s underlying physical processes.
The research findings can be found in The Astrophysical Journal Letters.
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