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The ongoing 'Hubble tension' in the universe's expansion debate

The ongoing ‘Hubble tension’ in the universe’s expansion debate

A new analysis by a University of Chicago astronomer has come to terms with the Standard Model in the current “Hubble effort”.

Our universe is expanding, but our basic way of measuring the speed at which that expansion is happening has given rise to different answers. Over the past 10 years or so, astrophysicists have gradually divided into two camps: one that thinks the difference is important, and another that thinks it may be due to measurement errors.

If errors turn out to be the root cause of mismatches, this will confirm our basic model of how the universe works. Another possibility introduces a thread that, when pulled, indicates that new, missing fundamental physics is needed to connect it. For several years, each new piece of telescope evidence upended the controversy, giving rise to the so-called “Hubble effort”.

Wendy Friedman, famous astronomer and professor of astronomy and astrophysics at the University of Chicago John and Marion Sullivan 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 article that was approved by Astrophysical Journal مجلةFriedman provides an overview of the latest scenes. His 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 doesn’t need much change.

The rate at which the universe is expanding is called the Hubble constant, named after University of California graduate Edwin Hubble, SP 1910, Ph.D. 1917, who discovered the expansion of the universe in 1929. Scientists want to determine this speed precisely, because the Hubble constant is related to the age and evolution of the universe. by the time.

A significant wrinkle has emerged over the past decade when results for the two main measurement methods have begun 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 by the Big Bang, called the diffuse cosmic background. This has been done in space and on Earth using facilities such as the Antarctic Telescope led by UChicago. Scientists can incorporate these observations into their “standard model” of the early universe and move it forward in time to predict what the Hubble constant should be like today; They get a response 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, his team made one of the landmark measurements using the Hubble Space Telescope to image stars called Cepheids. The value they came up with was 72. Friedman continued to measure the Cepheids over the following years, examining 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 refer to the Cepheids in an independent way.

Red giants are very large, bright stars that always reach the same peak in brightness before quickly fading out. If scientists can accurately measure the actual or intrinsic maximum brightness of the red giants, they can measure 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 his colleagues have been calculating numbers for many different galaxies and star clusters. “There are now four independent ways to calibrate the luminosity of a red giant, and they match 1% of each other,” Friedman said. “This tells us that this is a very good way to measure distance.”

“I really wanted to take a closer look 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 between local and distant rates of expansion. The new data from the red giant appears to be consistent. “

Taylor Hoyt, a graduate student at the University of Chicago who has made measurements of red giant stars in anchored galaxies, added, “We continue to measure and test stars in the red giant branch in various ways, and they still exceed our expectations. “

The value of the Hubble constant obtained by the Friedman team from the red giants is 69.8 km/s/mbs — practically the same value as derived from the diffuse cosmic background experiment. “There is no need for new physics,” Friedman said.

Calculations using Cepheid stars always give higher numbers, but according to Friedman’s analysis, the difference may not be alarming. “Cryptons have always been a little louder and somewhat more complex to fully comprehend; These are young stars in active star-forming regions in galaxies, meaning that it is possible for things like dust or pollution from other stars to skew your measurements,” she explained.

In his opinion, the conflict could be resolved with better data.

Next year, when the James Webb Space Telescope is scheduled to launch, scientists will begin collecting these new observations. Friedman and his colleagues have already got time on the telescope for a major program to make more measurements of Cepheid stars and red giant stars. “Webb will give us higher sensitivity and accuracy, and the data will improve 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 matched up.

“That’s how science works,” Friedman said. “You kick the tires to see if anything deflates, and so far there are no punctures. “

Some scientists looking for intrinsic mismatches 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 physics, it will show that our Standard Model is fundamentally correct, which is also a profound conclusion to be reached,” she declared. “Here’s the interesting thing about science: We don’t know the answers in advance. We learn during our journey. It’s really a fun time to be on the field.

Reference: “Measurements of the Hubble Constant: Tensions in Perspective” by Wendy Friedman, June 30, 2021, Astrophysical Journal مجلة.

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