By Stacy Palen

The two main ways to measure the expansion of the universe have turned in different answers. For the last several years, astronomers have been arguing about whether this disagreement is unimportant (a result of measurement errors) or important (a result of new, unknown physics). This is a terrific example of the process of science. In June 2021, Wendy Freedman published a new review paper arguing that there is not a conflict after all.

Below are some questions to ask your students based on this article.

1). What is the Hubble constant?

Answer: The Hubble constant measures the rate at which the universe is expanding.

2). What are the two ways to measure the Hubble constant?

Answer: The first way is to look at the cosmic microwave background. The second is to measure the velocities and distances of galaxies to make the Hubble law graph.

3). What are the two values of the Hubble constant derived from these two methods?

Answer: 67.4 km/s/Mpc and 72 km/s/Mpc.

4). Historically, the distance to nearby galaxies was determined using Cepheid variables. What is the problem with these measurements?

Answer: They are noisy and more complicated, and the observations may be contaminated.

5). What other objects are now being used to measure the distance to nearby galaxies?

Answer: Freedman is using red giant stars, which always reach the same peak brightness before fading. These observations are less noisy.

6). What does this new method of measuring distances give for the value of the Hubble constant?

Answer: 69.8 km/s/Mpc.

7). Is this new method in better or worse agreement with the method that uses the cosmic microwave background?

Answer: This new method gives a value that is much closer—it cuts the disagreement in half.

8). Review the Scientific Method flowchart in Chapter 1 of the textbook. What part of the flowchart describes the science that’s described in this article?

Answer: The science in the article is on the loop on the left. Previous tests did not agree, so a new hypothesis was suggested (that Cepheid variables are subject to too much noise), and a new experiment (measuring red giants) was devised and performed. Further tests will be in the loop on the right.