Although the universe is expanding rapidly, our main methods of measuring how fast it is happening have given us different results. Over the past decade, astrophysicists gradually divided into two camps. One believes the difference is significant and one thinks it might be due to measurement errors.It could be that mismatches are caused by errors, which would support our fundamental model of the universe. Another possibility is a thread, which, when pulled, suggests that some fundamental new physics is required to put it back together. The 'Hubble tension' has resulted from the fact that each piece of evidence has seensawed back and forth over the years.Wendy Freedman is a renowned astronomer and John and Marion Sullivan University Professor Astronomy and Astrophysics. She made some of the first measurements of the expansion rate of space that led to a higher Hubble constant. Freedman provides an overview of recent observations in a review paper that was accepted to the Astrophysical journal. Her conclusion is that the gaps are closing.This means that there may not be any conflict at all and that our standard model does not need to change.The Hubble constant is the rate at which the universe expands. It is named after Edwin Hubble (SB 1910, PhD 1917), who is credited for discovering the expansion in 1929. Because the Hubble constant refers to the age and evolution of the universe, scientists are keen to pinpoint this rate.In the last decade, results from two of the main measurement methods started to diverge. This was a significant problem. Scientists are still debating whether this is a significant mismatch.The cosmic microwave background, which is very faint light leftover from the Big Bang, can be used to measure the Hubble constant. This can be done in space as well as on the ground using facilities such the UChicago-led South Pole Telescope. Scientists can use these observations to build a'standard model' for the early universe. They can then run the model forward in time to determine what the Hubble constant should look like today. They get a result of 67.4 kilometers per megaparsec.Another method is to observe galaxies and stars in the near universe and measure their distances. This will allow us to see how fast they are moving away. Freedman is a well-known expert in this field. In 2001, her team performed one of the most important measurements with the Hubble Space Telescope to photograph stars called Cepheids. Their result was 72. Freedman and her colleagues have continued to measure Cepheids over the years, reviewing more telescope data every time. However, in 2019, they published an answer that was based on a completely different method, using stars called red giants. It was an idea to cross-check the Cepheids using an independent method.Red giants, which are large and bright stars, always reach the same peak brightness before quickly fading. Scientists can measure the intrinsic peak brightness of red giants and then calculate the distances to host galaxies. This is an important but challenging part of the equation. How accurate these measurements are is the key question.In 2019, the first version of this calculation used a single galaxy from very close to calibrate the luminosities of red giant stars. Freedman and her colleagues have been running the numbers over the past two-years for several galaxies, and different star populations. Freedman stated that there are four ways to calibrate the red giant luminosities and they agree within 1%. This indicates that this is a very good method of measuring distance."I wanted to carefully examine both the Cepheids as well as the red giants. Freedman said that he is familiar with their strengths and weaknesses. "I've come to the conclusion, that it doesn't take fundamental new physics to explain differences in local and distant expansion rates. They are consistent, according to the new red giant data.Taylor Hoyt, a University of Chicago graduate student, has been measuring the red giant star clusters in the anchor galaxies. He added that "we keep measuring and testing red giant branch stars and they continue to exceed our expectations."Freedman's Hubble constant is 69.8 km/s/Mpc, which is almost the same value as that derived from the cosmic microwave backgrounds experiment. Freedman stated that no new physics was required.Although the calculations with Cepheid star numbers still yield higher numbers, Freedman's analysis suggests that this difference might not be too significant. She explained that the Cepheid star calculations have been more complex and noisier than usual. They are young stars located in active regions of galaxies. This means that there is potential for contamination from other stars or dust to alter your measurements.She believes that better data can resolve the conflict.Scientists will start collecting these new observations next year when the James Webb Space Telescope launches. Freedman and her collaborators have been given time on the telescope to conduct a major program that will allow them to measure more of both Cepheid stars and red giants. She stated that the Webb telescope will provide us with higher sensitivity, resolution, and data will be better very, very soon.In the meantime, she decided to look closely at all the data and found that a lot of it was actually consistent.Freedman stated, "That's how science progresses." "You can kick the tires to check if it deflates. So far, there have been no flat tires."Scientists who had hoped for a fundamental mismatch may be disappointed. Freedman finds excitement in both answers.She stated that there was still room for new physics. However, even if it wasn't, it would prove that the standard model is fundamentally correct. This is a significant conclusion to reach. Science is not a science we can predict. We are learning as we go. It's an exciting time to work in the field."###Citation: "Measurements the Hubble Constant - Tensions in Perspective" Wendy Freedman is the Astrophysical Journal.
