New University of Chicago study may resolve mystery about measuring expanding universe
CHICAGO (CBS) -- It was 95 years ago that astronomer and University of Chicago alumnus Edwin Hubble made the discoveries that proved the universe is expanding—but the question of just how fast the expansion is happening remains a matter of debate.
But a UChicago-led analysis of new measurements from the NASA's James Webb Space Telescope may be helping put that debate to bed.
The U of C noted that over the past 20 years, two methods have been used for measuring the rate at which the universe is expanding at a given time—known as the Hubble constant, or H0—each of which resulted in different answers.
One method involves studying the light left over from the Big Bang—or the cosmic microwave background—which results in a figure for H0 of 67.4 kilometers per second per megaparsec, UChicago noted. A megaparsec is 3,260,000 light years, while a light year is about 6 trillion miles.
The other method, UChicago explained, involves measuring the expansion of galaxies in closer proximity, using stars for which there is already information about the brightness. Much like the lights on a moving car, a star will look fainter when it is far away, UChicago explained. Measurements of the distance and speed at which these nearby galaxies are moving away also results in a figure for the rate of the expansion of the universe, or again, H0.
However, measurements with this method have in the past been in conflict with those using cosmic microwave radiation. They have instead returned a figure for the H0 of around 74 kilometers per second per parsec.
The gap has been such that some scientists have wondered if there was something missing from the model of the evolution of the universe, UChicago said. They thought maybe since the first method uses leftover radiation from the beginning of the universe, while the other examines information from the present, something big and unknown could have changed about the universe at some point, UChicago said.
This conflict between the two H0 figures is called the "Hubble tension." But the new study led by UChicago cosmologist Wendy Freedman found there may not be any mystery there after all.
The James Webb Space Telescope was launched on Christmas Day 2021 as a successor to the Hubble Telescope, which was launched in 1990 and is also still operating. The Webb Telescope has captured sharp images and collected new and revolutionary data that has allowed for new discoveries about the universe, UChicago noted.
Freedman specializes in the second of the two methods of measuring the H0. For the latest study, she and her colleagues used the Webb Telescope to take measurements of 10 nearby galaxies for that very purpose.
The team cross-checked their results using three methods focusing on specific kinds of stars, UChicago said. One involved focusing on a Cepheid variable star—which has a predictable variation in its brightness over time; the second was a method called the "tip of the red giant branch"—which is based on the fact that low-mass stars have a fixed upper brightness limit, and the third focused on carbon stars—which have consistent colors and brightnesses in the near-infrared light spectrum, UChicago explained.
This was the first time all three methods were used simultaneously with a focus on the same galaxies, UChicago said. It also returned H0 values within the margin of error of the other method of measuring the H0—the one using cosmic microwave background radiation—at 67.4 kilometers per second per megaparsec, UChicago explained.
The foundations for the H0 date back to Hubble's own research, which was published in 1929. As explained by the Euclid Consortium, Hubble made his discoveries by measuring redshifts—as when light from a distant object moves away, its wavelength shifts toward the red end of the spectrum.
Hubble found both that the galaxies he looked at were moving away, and that the father the galaxy was away from us, the faster it was receding, as explained by the Euclid Consortium. His original estimate for H0 was 500 km per second per megaparsec, but modern estimates have refined it to the current figures.
The latest paper that may end the "Hubble tension" conflict was submitted Monday to the Astrophysical Journal. Freeman authored the paper with Barry Madore of the Carnegie Institution for Science, UChicago research scientist Sung Jang, Taylor Hoyt of the Lawrence Berkeley National Laboratory, and UChicago grad students Kayla Owens and Abby Lee.
