How fast is the universe expanding?
Over the past 20 years, scientists have measured the “Hubble constant” (the rate at which the universe is expanding) in two different ways, leading them to come up with different answers as they wonder if there’s something missing from their models of how the universe works.
But new measurements from the James Webb Space Telescope suggest that the collision, also known as the “Hubble tension,” may not exist after all.
After analyzing Webb’s data, University of Chicago cosmologist Wendy Friedman and her colleagues measured the distances to 10 nearby galaxies and a new value for the Hubble constant. Their measurement (70 kilometers per second per megaparsec) overlaps with other leading methods for determining the Hubble constant.
“This is a very interesting story,” said Friedman, a distinguished astronomer and the John and Marion Sullivan University Professor of Astronomy and Astrophysics at the University of Chicago. “Based on these new JWST data and three independent methods, we find no strong evidence for Hubble tension. Instead, the standard cosmological model explaining the evolution of the universe appears to be valid.”
“Confirming the reality of the tension in the Hubble constant would have profound implications for fundamental physics and modern cosmology.”
When scientists take these measurements, they test them in a variety of ways to make sure they are as accurate as possible.
The Cosmic Microwave Background (CMB) is one of the main approaches to determining the light left over from the Big Bang’s aftermath. The current best estimate of the Hubble constant from this method is very accurate: 67.4 kilometers per second per megaparsec.
Another approach is to use the brightness of stars to measure the expansion of galaxies in our nearby universe: Measuring the distance and speed at which galaxies are moving away from us tells us how fast the universe is expanding.
This method had previously yielded a higher value for the Hubble constant, about 74 kilometers per second per megaparsec.
This is a big difference, and scientists say the standard model of the evolution of the universe may be missing something important. This apparent discrepancy is known as the “Hubble tension.”
Scientists used Webb’s data to measure 10 nearby galaxies, laying the groundwork for measuring the expansion rate of the universe.
They used three different methods to verify their results: The first method uses a type of star called a Cepheid variable, whose brightness changes predictably over time.
The second method, called the “tip of the red giant branch,” measures the limit on the luminosity of low-mass stars. The third method uses carbon stars, which have stable colors and luminosities in the near-infrared. The new analysis combines all three methods, using them within the same galaxy for the first time.
In both cases, the values were close to the Cosmic Microwave Background result of 67.4 kilometers per second per megaparsec, within the margin of error.
“To have such good matches from three stars that are very different types from ours is a strong indication that we’re on the right track.” Friedman said.
“Future observations by JWST will be crucial in confirming or disproving the Hubble tension and assessing its impact on cosmology.” “This is a very real threat,” said Barry Madore, a study co-author at the Carnegie Institution for Science and visiting professor at the University of Chicago.
Journal References:
- Wendy L. Friedman, Barry F. Madore, Ying-Sung Zhang, Taylor J. Hoyt, Abigail J. Lee, and Kayla A. Owens. Chicago-Carnegie Hubble Program (CCHP) Status Report: Three Independent Astrophysical Determinations of the Hubble Constant Using the James Webb Space Telescope. The Astrophysical Journal. DOI: 10.48550/arXiv.2408.06153
