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For nearly three decades now, it\u2019s been clear that the expansion of the Universe is speeding up. Some unknown quantity, dramatically dubbed \u2018dark energy\u2019, is pushing the Universe apart. But the rate at which the Universe\u2019s expansion is increasing \u2013 called the Hubble Constant \u2013 hasn\u2019t yet been nailed down to a single number.<\/p>\n
Not for lack of trying.<\/p>\n
<\/p>\n In fact, there are multiple ways of measuring it. The problem is that these methods don\u2019t agree with each other. They each give different numbers, which is a confounding \u2013 and exciting \u2013 puzzle. It means there may be new physics to uncover, if we look carefully.<\/p>\n This mystery is known as the Hubble tension, and it\u2019s 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.<\/p>\n In a new paper this week, three Swiss scientists describe a method for significantly improving one measurement technique.<\/p>\n 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 \u2018Tip-of-the-Red-Giant-Branch\u2019, or TRGB.<\/p>\n When stars in the TRGB ignite helium, they achieve a known, reliably measured level of brightness: they become \u2018standard candles\u2019, making distance measurements between them more accurate.<\/p>\n But that brightness isn\u2019t perfectly constant: there are oscillations \u2013 sound waves rippling through the layers of the star. Scientists knew about these acoustic oscillations from previous studies of stellar evolution, but they hadn\u2019t yet been accounted for in attempts at resolving the Hubble tension.<\/p>\n That\u2019s what this new paper sets out to do.<\/p>\n \u201cYounger red giant stars near the TRGB are a little less bright than their older cousins,\u201d says lead author Richard Anderson. \u201cThe acoustic oscillations that we observe as brightness fluctuations allow us to understand which type of star we\u2019re dealing with: the older stars oscillate at lower frequency \u2013 just like a baritone sings with a deeper voice than a tenor!\u201d<\/p>\n \u201cNow 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,\u201d says Anderson.<\/p>\n That\u2019s good news, securing new confidence in our understanding of how the Universe expands. However, by itself, it isn\u2019t 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.<\/p>\n 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.<\/p>\n Learn more:<\/p>\n \u201cThe baritone of Red Giants refines cosmic distance measurements.\u201d EPFL.<\/p>\n Richard Anderson, Nolan Koblischke, and Laurent Eyer, \u201cSmall-amplitude Red Giants Elucidate the Nature of the Tip of the Red Giant Branch as a Standard Candle.\u201d ApJL, March 7, 2024.<\/p>\nLike this:<\/h3>\n