Red Giants Offer a New Way to Measure Distance in the Universe

For nearly three decades now, it’s been clear that the expansion of the Universe is speeding up. Some unknown quantity, dramatically dubbed ‘dark energy’, is pushing the Universe apart. But the rate at which the Universe’s expansion is increasing – called the Hubble Constant – hasn’t yet been nailed down to a single number.

Not for lack of trying.

In fact, there are multiple ways of measuring it. The problem is that these methods don’t agree with each other. They each give different numbers, which is a confounding – and exciting – puzzle. It means there may be new physics to uncover, if we look carefully.

This mystery is known as the Hubble tension, and it’s only becoming more intractable as measurement techniques become more precise. So astronomers are on the hunt for new and better ways to measure the expansion of the Universe.

In a new paper this week, three Swiss scientists describe a method for significantly improving one measurement technique.

The method uses a specific subset of red giant stars: old stars that have burned away most of the hydrogen in their cores. As they age, red giants get larger, less dense, and dimmer. But at a certain point in their evolution, they switch from burning hydrogen to burning helium, a change that causes a dramatic uptick in brightness. Stars in this phase of their life are considered to have reached the ‘Tip-of-the-Red-Giant-Branch’, or TRGB.

When stars in the TRGB ignite helium, they achieve a known, reliably measured level of brightness: they become ‘standard candles’, making distance measurements between them more accurate.

But that brightness isn’t perfectly constant: there are oscillations – sound waves rippling through the layers of the star. Scientists knew about these acoustic oscillations from previous studies of stellar evolution, but they hadn’t yet been accounted for in attempts at resolving the Hubble tension.

That’s what this new paper sets out to do.

“Younger red giant stars near the TRGB are a little less bright than their older cousins,” says lead author Richard Anderson. “The acoustic oscillations that we observe as brightness fluctuations allow us to understand which type of star we’re dealing with: the older stars oscillate at lower frequency – just like a baritone sings with a deeper voice than a tenor!”

“Now that we can distinguish the ages of the red giants that make up the TRGB, we will be able to further improve the Hubble constant measurement based thereon,” says Anderson.

That’s good news, securing new confidence in our understanding of how the Universe expands. However, by itself, it isn’t likely to resolve the Hubble tension. The widest gap amongst different Hubble constant measurements is between recent Universe observations: type 1A supernovae, cepheid variables, kilonovae, and red giants; and early Universe observations: especially the cosmic microwave background.

That tension remains. Still, the more confident we can be about the accuracy of our measurements, the more sure we can be that there is something new about how the Universe works waiting to be discovered. Accounting for the TRGB oscillations is a concrete step in that direction.

Learn more:

The baritone of Red Giants refines cosmic distance measurements.” EPFL.

Richard Anderson, Nolan Koblischke, and Laurent Eyer, “Small-amplitude Red Giants Elucidate the Nature of the Tip of the Red Giant Branch as a Standard Candle.” ApJL, March 7, 2024.