Hubble tension is the difference in results that scientists get when they try to measure the rate at which our Universe expands. Find out how a new study may have split the difference and loosened the tension between two camps of researchers.
The Hubble tension refers to the discrepancies scientists have found between two ways of measuring how fast our Universe is expanding. That rate of speed is called the Hubble Constant, and it has a long history of being tricky to pin down.
The discrepancy matters because cosmologists need an accurate expansion rate to estimate the age of the Universe. They know the approximate size of the Universe, and they know it’s expanding.
By reversing that expansion, they can work their way back to the origin of the Universe at the Big Bang event. Science has a rough idea of the velocity of the Universe’s expansion. Still, it hasn’t managed to determine its exact rate.
Science Hasn’t Pinned Down Precise Expansion Rate
Part of the Hubble tension problem is that scientists approach the speed measurement in two ways. One way is to measure the distance to a particular class of stars called Cepheids. The other is to measure the fluctuations in the trace radiation left over from the Big Bang that astronomers call the Cosmic Microwave Background (CMB).
When astronomers use the Cepheid technique, they consistently get a result of roughly 72 kilometres per second per megaparsec (km/s/Mpc). However, when they use the CMB, the answer comes in at around 67.4 km/s/Mpc.
That doesn’t sound like a huge difference, and it isn’t. Everyone assumed that with more accurate research, the numbers would get closer together.
Numbers Have Only Become More Stubborn
Frustratingly, they haven’t. Instead, the numbers have only become more stubborn as astrophysicists take what they believe are more and more accurate measurements.
The discrepancy has divided the astrophysics community into two camps. Some scientists still think that the difference is simply due to measurement error. In contrast, others think the difference points to something more profound.
If the latter group is correct, we might need a whole new standard model of the Universe. On the other hand, we need to find better observation techniques if the variation is because of measurement error. This is the crux of the Hubble tension.
New Study Loosens Hubble Tension
A new study from the University of Chicago seems to ease the Hubble tension between the two camps. Professor Wendy Freedman made some of the Cepheid technique measurements that arrived at the 72 km/s/Mpc figure.
Professor Freedman has just published a new review paper in the Astrophysical Journal. In it, she demonstrates that the gap between the two techniques is narrowing to the point where there may be no genuine tension at all, and the Standard Model has been right all along.
Starting in 2019, Professor Freedman and her team began observing another class of stars called red giants to cross-check the results scientists have arrived at with the Cepheids.
Observing Another Class of Stars Called Red Giants
Professor Freedman explains, “I really wanted to look carefully at both the Cepheids and red giants. I know their strengths and weaknesses well.” So, for the last two years, she and her team have been observing various galaxies and star populations.
As their name implies, red giants are huge, and they’re also extremely bright. So when astronomers can pinpoint their intrinsic peak brightness, they provide excellent measuring sticks for determining how far away distant galaxies are.
Measuring their intrinsic peak brightness has led Professor Freedman to believe that she may have resolved the Hubble tension. “I have come to the conclusion that we do not require fundamental new physics to explain the differences in the local and distant expansion rates. The new red giant data show that they are consistent.”
Splits the Difference Between the Cepheid and CMB Methods
The team’s findings put the Hubble Constant at 69.8 km/s/Mpc, which splits the difference between the Cepheid and CMB methods. So, why do we get a much closer figure using the red giant measurements?
“The Cepheid stars have always been a little noisier and a little more complicated to fully understand,” Professor Freedman explained. “They are young stars in the active star-forming regions of galaxies, and that means there’s potential for things like dust or contamination from other stars to throw off your measurements.”
University of Chicago graduate student Taylor Hoyt is a member of the research team. As he puts it, “We keep measuring and testing the red giant branch stars in different ways, and they keep exceeding our expectations.” As many suspected, the Hubble tension seems to come down to measurement error when using Cepheid stars as yardsticks for estimating the distance between galaxies.
James Webb Telescope Will Provide New Opportunities
NASA plans to launch the replacement for the Hubble Space Telescope later this year. It will be called the James Webb Space Telescope. It will provide new opportunities to collect better data related to the Hubble Constant.
The next step for Professor Freedman and her team will be to use the time they’ve been awarded on the telescope for a new program to measure more Cepheid and red giant stars. “The Webb will give us higher sensitivity and resolution,” Freedman explained, “and the data will get better really, really soon.”
“Data Will Get Better Really, Really Soon”
As we piece together the new story of how our Universe began, scientists need a definite answer to when the Big Bang took place. To determine that point in time, we need to know precisely the rate at which our Universe expands.
Professor Freedman’s research seems to lead us toward that knowledge while eliminating the division caused by the Hubble tension. By following the truth wherever it leads, she and her team are helping us find the answers we need.
“That’s the interesting thing about science: We don’t know the answers in advance. We’re learning as we go. It is a really exciting time to be in the field.”
We always have more to learn if we dare to know.
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