Deep sounds from red giant stars can reveal their distance from us, offering a fresh method to map out the universe. This discovery is especially significant at a time when the accuracy of cosmic distances is under debate, challenging our understanding of the universe, which some experts view as a crisis in cosmology.
The consensus among astronomers is that the universe is not only expanding but doing so at an accelerated pace. Yet, the two primary methods for measuring this expansion yield conflicting results, creating what is known as the Hubble tension. Initially, the margin of error in these methods allowed for some agreement. But with technological advancements enhancing our precision, this overlap has vanished.
This discrepancy suggests an error in at least one of the measurement techniques or the interpretations derived from them. Since our perception of the universe’s expansion significantly influences other astronomical understandings, resolving this discrepancy has become a critical goal. A new method could potentially resolve this conflict.
A group of researchers has proposed that a subset of red giant stars could serve as an alternative metric for gauging the universe’s expansion rate. Although their research has been limited to nearby examples, the potential for broader application exists.
As stars exhaust their hydrogen fuel and begin to cool, they expand into red giants. At a certain point in their evolution, when they start to fuse helium, they reach a stage known as the “tip of the red giant branch” (TRGB). These stars, especially red giants, exhibit brightness variations due to internal sound waves.
The researchers focused on TRGB stars within the Magellanic Clouds, categorizing them based on their vibration periods. They discovered that stars with slower vibrations have consistent characteristics, making them valuable for astronomical measurements, whereas stars with quicker vibrations tend to be younger and metal-rich.
Richard Anderson, from the École Polytechnique Fédérale de Lausanne, explained that younger red giants near the TRGB are slightly dimmer than their older counterparts. The observed brightness fluctuations, caused by acoustic oscillations, help identify the star’s age. Older stars oscillate at lower frequencies, akin to how a baritone’s voice is deeper than a tenor’s.
Understanding a star’s position on the TRGB allows for a more accurate calculation of its true brightness. This, combined with the observed light, enables the measurement of distances to TRGB stars in distant galaxies. While TRGB stars have long been considered for distance measurement, uncertainties regarding their true luminosity have made this method less reliable than using Cepheid variables. However, timing the oscillations could improve accuracy.
Measuring these stars’ movements and the galaxies they inhabit through their redshift is relatively straightforward. By correlating distance with movement speed, we can better understand the universe’s expansion.
Anderson highlights that the acoustic oscillations of red giant stars provide an optimal approach for measuring cosmic distances using the TRGB method. This technique is akin to another existing method that relies on the consistent peak brightness of Type Ia supernovae. Although TRGB stars are not as luminous as supernovae, making them unsuitable for measuring vast distances, their abundance and the ease with which they can be observed make them useful for calibrating measurements in closer galaxies where supernovae have been detected.
If the resolution to the Hubble tension lies in reevaluating our observations or interpretations of Type Ia supernovae, TRGB stars could offer a path to discovery.
This research is detailed in an open-access article published in the Astrophysical Journal Letters.