Imagine measuring the ever-expanding universe with a ruler that constantly stretches. That's the challenge cosmologists face when determining the Hubble constant (H0), a value reflecting the universe's expansion rate and ultimately its age and size. Conflicting H0 measurements derived from various objects highlight the need for better cosmic distance measurement tools.
Enter red giant stars, the heroes of a new study published in The Astrophysical Journal Letters. Led by EPFL professor Richard I. Anderson, the research team including Nolan Koblischke (University of Toronto) and Laurent Eyer (University of Geneva) unveils a method to refine cosmic distances using the unique acoustics of these aging stars.
Red giants, known for their reddish hue, are crucial distance markers. As they exhaust core hydrogen and burn outer hydrogen, they swell and cool, forming a distinct “red giant branch” on astronomical diagrams. The tip of this branch (TRGB), where red giants ignite helium, serves as a “standard candle” for astronomers. By comparing a TRGB star's known intrinsic brightness to its observed brightness in distant galaxies, astronomers can gauge distances.
But here's the twist: the study reveals all TRGB stars aren't created equal. They pulsate with sound waves, like faint stellar earthquakes. While known, this detail was overlooked for distance measurements. The researchers, however, saw an opportunity.
“These acoustic oscillations act like fingerprints, allowing us to distinguish young TRGB stars from older ones,” explains Anderson. Younger stars are slightly dimmer, and their sound waves vibrate faster, akin to a tenor's voice. Older stars, like baritones, resonate at lower frequencies.
This distinction is crucial. With the ability to differentiate red giants by age, astronomers can obtain more nuanced distance measurements across vast cosmic distances. Since red giants reside in most galaxies, this method has the potential to create a more accurate map of the local universe.
“By incorporating the age factor, we can significantly improve H0 measurements derived from the TRGB method,” says Anderson. This refined data will further challenge the “Hubble constant tension,” a discrepancy between H0 values obtained from local and early universe observations. Resolving this tension could lead to groundbreaking discoveries about the fundamental forces shaping the universe's evolution.
Source: Ecole Polytechnique Federale de Lausanne