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.

By Stacy Palen

The Milky Way is tearing apart the Sagittarius dwarf galaxy. Gaia observations of the resulting tidal streams permit sufficient accuracy to detect the influence of the Large Magellanic Cloud on the interaction. Simulations show that the Milky Way’s dark matter halo is complex.

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

1). Prior to this study, what was known about the shape of the dark matter halo?

Answer: Not much. Different simulations found various shapes, some more symmetric than others.

2). In this study, what is new that allows astronomers to build a more detailed picture?

Answer: The Gaia satellite pinpoints the location of stars with truly unprecedented accuracy.

3). The following questions cannot be answered directly from the article but can be answered with a Google search. The paper discussed in this article was posted to the preprint server arXiv. What is a preprint?

Answer: It is a full draft that has not been peer reviewed.

4). In the process of scientific publication, what comes after the preprint stage?

Answer: The article undergoes formal peer review, and then it might be published.

5). What is the advantage of preprints?

Answer: They give access to the most up-to-date research.

6). Should you treat the results discussed here with more or less skepticism than the results published in a peer-reviewed journal?

Answer: You should treat them with more skepticism.

7). How could you know if the results held up to peer review?

Answer: You could search for the authors’ names or the topic in a database of journal articles.