“There may be no conflict after all” in the Expanded Universe debate

Our universe is expanding, but our primary means of measuring the speed at which this expansion is occurring has yielded different answers. Over the past decade, astrophysicists have gradually split into two camps: one who thinks the difference is significant and one who thinks it may be due to measurement errors.

If it turns out that the errors caused the incompatibility, that would confirm our basic model of how a working con file works. Another possibility introduces a thread which, when pulled, indicates that a new missing fundamental physics is needed to reattach it together. For several years, each new evidence from telescopes has shifted the argument, giving rise to the so-called “Hubble Tension”.

Wendy Friedman, famous astronomer and John and Marion Sullivan professor of astronomy and astrophysics at the University of Chicago, made original measurements of the rate of expansion of the universe which resulted in a higher value for the constant from Hubble. But in a new journal article accepted Astrophysics Journal ةلةFriedman provides an overview of the most recent sightings. His conclusion: recent observations are starting to fill the void.

This means there might not be a conflict after all, and our standard model of the universe doesn’t need a lot of tweaking.

The rate at which the universe expands is called the Hubble constant, called UChicago alum Edwin Hubble, SB 1910, Ph.D. 1917, who is credited with discovering the expansion of the universe in 1929. Scientists want to determine this rate with precision, because the Hubble constant is related to the age of the universe and its evolution over time.

A major wrinkle has appeared over the past decade when the results of the two main measurement methods have started to differ. But scientists are still debating the importance 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 cosmic diffuse background. This has been done in space and on Earth using facilities such as the Antarctic Telescope led by UChicago. Scientists can enter these observations into their “standard model” of the early universe and run it in time to predict what the Hubble constant should look 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 near 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 most remarkable measurements using the Hubble Space Telescope to image stars called Cepheids. The value they found was 72. Friedman continued to measure 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 independently verify the Cepheids.

Red giants are very large bright stars that always reach the same peak in brightness before quickly disappearing. If scientists can accurately measure the actual or intrinsic maximum luminosity of red giants, then they can measure distances to their host galaxies – an essential but delicate 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 nearby galaxy to calibrate the brightness of the red giant stars. Over the past two years, Friedman and colleagues have calculated the numbers for several different galaxies and star groups. “There are now four independent ways of calibrating the brightness of a red giant, and they match within 1% of each other,” Friedman said. “It shows us that this is a very good way to measure distance.”

“I really wanted to take a close look at both the Cepheids and the Red Giant,” Friedman said. I know their strengths and weaknesses very well. “I came to the conclusion that we don’t need fundamental new physics to explain the differences in local and distant expansion rates. The data from the new red giant shows that they are consistent. “

“We continue to measure and test the red giant branch stars in a variety of ways, and they continue to exceed our expectations,” added Taylor Hoyt, a graduate student at the University of Chicago, who has performed giant star measurements. red in galaxies anchors.

The value of the Hubble constant obtained by Friedman’s team on the red giants is 69.8 km / s / mcm – roughly the same as the value derived from the cosmic diffuse background experiment. “There’s 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. “The Cepheid stars have always been a little louder and a little more complicated to fully understand; These are young stars in active star-forming regions in galaxies, which means there is a potential for things like dust or pollution from other stars to get rid of your measurements, ”a- she explained.

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

Next year, when the James Webb Space Telescope is due to launch, scientists will start collecting these new observations. Friedman and his colleagues 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 precision, and the data will really 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 much of it matched.

“This is how science works,” Friedman said. “You kick the tires to see if something deflates, and so far there hasn’t been a puncture.”

Some researchers who have been in favor of inherent 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 would show that our Standard Model is basically correct, which is also a profound conclusion to be drawn.” , she said. “Here’s the interesting thing about science: we don’t know the answers ahead of time. We learn by progressing. It really is an exciting time to be on the pitch.

More information:
“Hubble Constant Measurements: Tensions in Perspective”. Wendy Friedman et al. Journal of Astrophysics مجلة2021.