Red giant stars create new way to measure how quickly the universe is expanding
The universe is expanding, but the rate of that expansion is proving to be elusive. A new measurement of the universe’s expansion rate falls between two previously established rates, leading to more questions than answers.
About a century ago, Carnegie Institute of Science astronomer Edwin Hubble made the discovery that the universe had continued to grow after the Big Bang. But the rate of that expansion, deemed the Hubble constant, is difficult to determine. Objects in the universe can act like mile markers, so understanding their distance allows for the measurement of the speed at which they’re moving away from us. The ratio of dividing that distance by velocity can determine the Hubble constant.
Different teams of scientists have determined three different ways to arrive at that rate, and all three rates disagree.
A study published Tuesday in The Astrophysical Journal reveals the latest method.
“The Hubble constant is the cosmological parameter that sets the absolute scale, size and age of the universe; it is one of the most direct ways we have of quantifying how the universe evolves,” said Wendy Freedman, a study co-author and professor at the University of Chicago’s department of astronomy and astrophysics. “The discrepancy that we saw before has not gone away, but this new evidence suggests that the jury is still out on whether there is an immediate and compelling reason to believe that there is something fundamentally flawed in our current model of the universe.”
Stars called Cepheids have been used to take measurements because they pulse regularly. That pulsation rate, as well as their brightness, can determine their distance from Earth.
"From afar two bells may well appear to be the same, listening to their tones can reveal that one is actually much larger and more distant, and the other is smaller and closer," said Barry Madore, study co-author and astronomer at Carnegie. "Likewise, comparing how bright distant Cepheids appear to be against the brightness of nearby Cepheids enables us to determine how far away each of the stars' host galaxies are from Earth."
The Cepheids method established the expansion rate as 74.0 kilometers per second per megaparsec, a unit of vast distance in space (1 megaparsec = 3.3 million light years).
The second method uses cosmic background radiation, which is the afterglow of the Big Bang and the oldest visible light astronomers can observe. These ripples can be mapped and modeled to predict the expansion rate, which placed it at 67.4 kilometers per second per megaparsec.
That discrepancy alone could require new physics.
But the latest study is relying on a different kind of stars called red giants. It's a late stage in a star's life cycle where a helium flash occurs and temperatures rise millions of degrees until the entire star's structure is rearranged. The brightness of these stars in different galaxies can be measured, so the researchers used the Hubble Space Telescope to look for them in different galaxies.
"Think of it as scanning a crowd to identify the tallest person—that's like the brightest red giant experiencing a helium flash," said Christopher Burns, study co-author and research associate at the Carnegie Institute of Science. "If you lived in a world where you knew that the tallest person in any room would be that exact same height—as we assume that the brightest red giant's peak brightness is the same—you could use that information to tell you how far away the tallest person is from you in any given crowd."
The Hubble constant calculated by measuring these stars resulted in 69.8, which is right between the previous two rates.
Instead of being an expected tiebreaker, the new rate raises more questions about the current understanding of the universe.
"We're like that old song, 'Stuck in the Middle with You,'" Madore said. "Is there a crisis in cosmology? We'd hoped to be a tiebreaker, but for now the answer is: not so fast. The question of whether the standard model of the universe is complete or not remains to be answered."