Homework 2: The "Hubble tension" -- answers

  1. Find a recent paper, published in 2026, by authors Cai and Wang, which features the phrase "Hubble tension" in its title.
    1. In what journal was this published?
            2. Research in Astronomy and Physics
    2. What are the volume number and page number for this paper in that journal?
            3. Volume 26, page 084011

  2. Read the "Introduction" section of the paper (you can skip the long paragraph starting with "Hubble resolutions" -- it's mostly a long list of references) and answer the following questions.
    1. How many "Hubble tension" controversies have there been over the years?
            2. Three
    2. One of these fights was settled by measurements made by the Hubble Space Telescope. What were the extreme values of H0 in question at that time?
            3. The second Hubble tension was a fight between those who thought H0 was about 50 km/s/Mpc, and those who thought it was close to 100 km/s/Mpc.
    3. How did those extreme values compare to the value determined by Hubble himself in 1929?
            4. Hubble's value in 1929 was about 500 km/s/Mpc, so both extremes in the 1970s and 1980s were far smaller.
    4. What are the extreme values in the most recent "Hubble tension?"
            5. The current (third) fight is over the difference between the Early Universe values of around 67 km/s/Mpc and the Late Universe values of about 73 km/s/Mpc.

  3. Figure 1 shows several techniques used by astronomers to measure the "Early Universe" value of the Hubble constant.
    1. Pick two abbreviations shown in color on the figure. Write out the full names of the techniques for which they stand.
            2. (for example) CMB = Comic Microwave Background, BAO = Baryon Acoustic Oscillations, BBN = Big Bang Nucleosynthesis

  4. Figure 2 shows several techniques used by astronomers to measure the "Late Universe" value of the Hubble constant.
    1. Pick two abbreviations shown in color on the figure. Write out the full names of the techniques for which they stand.
            2. (for example) TRGB = Tip of the Red Giant Branch, SBF = Surface Brightness Flucuations, JWST = James Webb Space Telescope, TF = Tully Fisher, CSP = Carnegie Supernova Project, SARM = SpectroAstrometry and Reverberation Mapping, etc.