A new analysis by a University of Chicago astronomer has come to terms with the Standard Model’s persistent “Hubble tension.”
Our universe is expanding, but our main way of measuring how fast that expansion is happening has yielded different answers. Over the past decade, astrophysicists have gradually been split into two camps: one that believes the difference is significant, and one that thinks it may be due to measurement errors.
If it turns out that errors cause mismatches, this will confirm our basic model of how the universe works. Another possibility introduces a thread that, when pulled, indicates that some new, missing fundamental physics is needed to reattach it together. For several years, each new evidence from telescopes has been swinging the argument back and forth, giving rise to the so-called “Hubble tension”.
Wendy Friedman, a famous astronomer and John and Marion Sullivan Professor of Astronomy and Astrophysics at the University of Chicago, made some original measurements of the expansion rate of the universe that resulted in a higher value for the Hubble constant. But in a new review paper accepted Astrophysical Journal مجلة, Friedman gives an overview of the most recent observations. Her conclusion: Recent observations are starting to fill the gap.
This means that there may be no conflict after all, and our standard model of the universe does not need much modification.
The rate at which the universe expands is called the Hubble constant, called UChicago alum Edwin Hubble, SB 1910, PhD 1917, which is credited with discovering the expansion of the universe in 1929. Scientists want to determine this rate precisely, because the Hubble constant is related to the age of the universe and how it evolved over time time.
A major wrinkle has emerged in the past decade when results for the two main measurement methods began to differ. But scientists still debate the significance of the mismatch.
One way to measure the Hubble constant is to look at the very faint light left over from the Big Bang, called the cosmic microwave background. This has been done in space and on Earth using facilities such as the Antarctic Telescope led by UChicago. Scientists can input these observations into their “standard model” of the early universe and run it in time to predict what the Hubble constant should be like today; They get an answer of 67.4 kilometers per second per megaparsec.
The other way is to look at the stars and galaxies in the nearby universe, and measure their distances and how fast they are moving away from us. Friedman was a leading expert in this method for several decades; In 2001, her team made one of the most remarkable measurements using the Hubble Space Telescope to image stars called Cepheids. The value they found was 72. Friedman continued to measure the Cepheids in the ensuing years, reviewing more telescope data each time. However, in 2019, she and her colleagues published an answer based on an entirely different method using stars called red giants. The idea was to verify the Cepheids in an independent way.
Red giants are very large, luminous stars that always reach the same peak brightness before quickly fading out. If scientists can accurately measure the actual or intrinsic peak brightness of the red giants, they can then measure the distances to their host galaxies, an essential but tricky part of the equation. The main question is how accurate these measurements are.
The first version of this calculation in 2019 used a single very close galaxy to calibrate the luminosity of red giant stars. Over the past two years, Friedman and her collaborators have run the numbers for several different galaxies and star groups. “There are now four independent ways to calibrate the luminosity of a red giant, and they agree 1% of each other,” Friedman said. “This suggests to us that this is a really good way to measure distance.”
“I really wanted to look carefully at both the Cepheids and the red giants. I know very well their strengths and weaknesses,” Friedman said. “I came to the conclusion that we don’t need new fundamental physics to explain the differences in local and distant rates of expansion. The data for the new red giant appears to be consistent.”
University of Chicago graduate student Taylor Hoyt, who has been making measurements of red giant stars in anchor galaxies, added, “We continue to measure and test red giant sub-stars in various ways, and they continue to exceed our expectations.”
The value of the Hubble constant obtained by the Friedman team from the red giants is 69.8 km/s/million segments – about the same as the value derived from the cosmic microwave background experiment. “No new physics is required,” Friedman said.
Calculations using Cepheid stars still give higher numbers, but according to Friedman’s analysis, the difference may not be alarming. “Cepheid stars have always been a little bit noisier and a bit more complicated to fully understand; they are young stars in active star-forming regions in galaxies, and that means there is potential for things like dust or pollution from other stars to throw off your measurements,” she explained.
In her opinion, the conflict could be resolved with better data.
Next year, when the James Webb Space Telescope is expected to launch, scientists will start collecting those new observations. Friedman and his collaborators have already had time on the telescope for a major program to make more measurements of both Cepheid giant stars and red giant stars. “Webb will give us higher sensitivity and accuracy, and the data will really get better, very soon,” she said.
But in the meantime, she wanted to take a closer look at the existing data, and what she found was that a lot of it actually agreed.
“That’s the way science goes,” Friedman said. “You kick the tires to see if something deflates, and so far, no punctures.”
Some scholars who have been in favor of intrinsic incompatibility may be disappointed. But for Friedman, either answer is exciting.
“There is still room for new physics, but even if there is no room for new physics, it will show that our Standard Model is fundamentally correct, which is also a profound conclusion to be made,” she said. “Here’s the interesting thing about science: We don’t know the answers up front. We learn as we go forward. It’s a really exciting time to be in this field.”
Reference: “Measurements of the Hubble Constant: Tensions in Perspective” by Wendy Friedman, June 30, 2021, Astrophysical Journal مجلة.