Red giants with deep voices could solve the confusion in cosmology

The transmission of deep tones within red giant stars can tell us their distances, creating a new way to measure the universe. That could be useful to astronomers under any circumstances – but even more so when measurements of cosmic distances have called into question our models of the universe, which some see as a crisis in cosmology.

Astronomers agree that the universe is expanding, and that the rate of expansion is increasing. However, the two main ways of measuring growth produce conflicting results, known as the Hubble tension. At one time, the error bars in each method were so wide that there was some overlap. But now that new instruments have improved our precision, that has disappeared.

At least one of the measurements (or the conclusions we draw from them) must be wrong – but we don’t know which one, or why. Because so many other things about the way we see the universe are derived from that rate of expansion, solving this issue has become a priority. Perhaps another method could act as a tiebreaker.

That’s the hope of a team of researchers who have published evidence showing that a subcategory of red giant stars could provide an alternative measure of the universe’s expansion rate. Their studies were still too close to home to be used in that way, but the potential is there.

As stars near the end of their lives and run out of hydrogen to fuse, they cool and swell, becoming red giants. Eventually they begin to fuse helium instead, and around the time of the transition they are known as ‘top of the red giant branch’ (TRGB) stars.

Many stars – especially red giants – experience variations in brightness due to giant sound waves bouncing back and forth within them. The researchers divided TRGB stars in the Magellanic Clouds into populations based on the length of time they take to vibrate. They found that the group with slower vibrations has the consistent characteristics that make them valuable to astronomers, while those with higher pitches are younger and likely richer in metals.

“Younger red giant stars near the TRGB are slightly less bright than their older cousins,” study author Richard Anderson of the École Polytechnique Fédérale de Lausanne said in a statement. “The acoustic oscillations that we observe as brightness fluctuations allow us to understand what type of star we are dealing with: the older stars oscillate at a lower frequency – just like a baritone sings with a deeper voice than a tenor!”

Knowing a star’s location on the TRGB allows us to calculate its true brightness more accurately. By combining this with the amount of light we see, we can measure the distance to TRGB stars in more distant galaxies. The potential of using TRGB stars to measure distances has been known for some time, but previous attempts were plagued by uncertainty about the actual brightness and were therefore considered less reliable than using Cepheid variables. Timing the oscillations could solve this.

Measuring the motion of these stars and the galaxies in which they occur, using their redshift is relatively simple. Matching distance and speed of movement provides a way to measure the growth of the universe.

“We found that the acoustic oscillations of red giant stars tell us how to best measure cosmic distances using the ‘tip of the red giant branch’ method,” Anderson said.

Using objects of known intrinsic brightness is similar to one of the existing measures using Type Ia supernovae. These are cherished by astronomers because their brightness is consistent at their peak, giving us a good idea of ​​how bright they really are. By matching this with our measured brightness and redshift, we can compare distance and expansion, as with the TRGB stars, creating one side of the Hubble strain. The conflicting results come from measurements of the cosmic background radiation.

TRGB stars are not as bright as supernovae, so we cannot use this method over equally large distances. On the other hand, they are more frequent and can be measured at our leisure, rather than occurring only briefly. Consequently, we can use TRGB stars to calibrate our measurements to closer galaxies where supernovae have been observed.

If, as some suspect, the solution to the Hubble tension is that there is something wrong with our Type Ia supernova observations or the way we interpret them, this could be the way to find out.

The research has been published open access in the journal Astrophysical Journal Letters

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