The biggest controversy in cosmology is coming to an end?

The biggest controversy in cosmology is coming to an end?


Our universe is expanding. In 1929, based on the redshift of spectral lines, Edwin Hubble obtained convincing evidence that distant galaxies are moving away from us. The farther galaxies are away from us, the faster they move away from us.

How fast is the universe expanding? To know the answer to this question, we need to accurately measure the size of the Hubble constant (H₀) . However, the two main methods used to measure the expansion rate have produced different results. In the past ten years, the difference in the measured values ​​of the Hubble constant has gradually divided astrophysicists into two camps: one believes that the difference between the two is significant; the other believes that the difference may be caused by measurement errors. of.

If it can be proved that this difference is caused by errors, it means that our existing standard model of cosmology on the operation of the universe should be correct; and if it can be determined that this difference is significant, it means that new The physics principle is used to make up for the missing part in the existing cosmological model. In recent years, every new result obtained from astronomical observations has allowed this controversy to continue.

Recently, a new review papers had just been received, “Astrophysical Journal”, the famous astronomer Wendy Freedman of the University of Chicago (Wendy Freedman) of observations in recent years have been outlined, she obtained The conclusion is: the latest observations are closing the gap between the results obtained by the two methods. In other words, the difference in expansion rate may not exist at all, and we do not need to make major changes to the existing standard model of the universe.


Since Hubble confirmed that the universe is expanding, scientists have tried to accurately measure the Hubble constant because it is closely related to the age of the universe and how the universe evolves over time. However, when the two main measurement methods gave divergent results, the scientists realized that they had encountered a substantial problem. This extends the controversy that continues to this day.

The first method used to measure the Hubble constant is to observe the very faint light left by the Big Bang—the cosmic microwave background (CMB) . Such measurements have been carried out many times both in space and on the ground. After obtaining the measurement results, astronomers can input these observations into the “standard model” of the early universe, and then predict what the Hubble constant should be today. In this way, the answer they got was 67.4 km/s/Mpc .

Another method used to measure the Hubble constant is by observing stars and galaxies in the nearby universe, and then measuring their distance from us and their speed away from us. Friedman has been an expert in using this method to measure the Hubble constant for decades. In 2001, she and her collaborators used the Hubble Space Telescope to measure Cepheid variable stars and obtained a landmark measurement value of 72 km/s/Mpc .

Cepheid variable stars are a class of stars whose brightness changes periodically, and their brightness or dimming is related to their intrinsic brightness. Astronomers will use the parallax method to calculate the distance of Cepheid variable stars in the Milky Way, and then look for Cepheid variable stars in neighboring galaxies, compare the brightness of these Cepheid variable stars with the brightness of Cepheid variable stars in the Milky Way, and estimate the proximity The distance between the galaxy and us.

Since 2001, Friedman has been measuring Cepheid variable stars and can analyze and check more observational data each time. However, in 2019, in order to test the results obtained by Cepheid variable stars, she and her colleagues published a result based on a completely different method-they measured stars called red giants .

Red giants are very large and bright stars whose brightness always reaches the same peak brightness before rapidly dimming. If scientists can accurately measure the intrinsic brightness peaks of red giants, they can measure the distance between us and our host galaxy. This is an important but difficult part, and the key question is how accurate these measurements can be.

Friedman wanted to carefully observe the difference between Cepheid variable stars and red giant stars. She knew the characteristics of these two types of stars. In 2019, Friedman and her collaborators used a very close galaxy to calibrate the brightness of the red giant. In the past two years, she and the team have performed calculations on several different galaxies and constellations. According to her, there are now 4 independent methods that can be used to calibrate the brightness of red giant stars, and the errors of these methods are all within 1%. Such an error range shows that this method of measuring distance is very reliable.

By measuring red giant stars, Friedman calculated the value of the Hubble constant to be 69.8 km/s/Mpc, which is consistent with the value measured by CMB .


Although the number calculated by Cepheid variable stars is still higher, Friedman’s analysis believes that we may not need to worry too much about this. Because Cepheid variable stars are a relatively noisy, complex, and difficult to fully understand stars, they are young stars in the star-forming regions of active galaxies, which means that there may be dust and other things from other stars that will interfere with our measurement results .

In her view, as we use more advanced measurement methods and better telescopes to collect data, these problems may eventually be alleviated. For example, if the Webb Space Telescope (JWST) with higher sensitivity and resolution is successfully launched, astronomers will be able to collect updated observations.

At the same time, Friedman also carefully checked the existing data and found that most of the data are consistent. So Friedman concludes: “We don’t need new physics to explain the difference between local and long-distance expansion rates. The new red giant data show that they are consistent.”

Such results may disappoint scientists who support “we need new physics.” But in fact, no matter what the result is, it is an exciting answer. As Friedman said: “New physics still has room for development, but even if it doesn’t, it shows that our existing standard model is basically correct. This is also a profound conclusion that needs to be drawn. This is what makes science interesting: we don’t know the answer in advance. We learn as we walk. This is a very exciting time.”

Reference source:

This article comes from WeChat public account : Principle (ID : principle1687) , author: L. Lerner

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