Someone went and got me a subscription to Scientific American, so for the past few months I’ve been covering physics articles, now with the benefit of a PhD. Perhaps it’s a way to keep in touch with my physics roots as my career has moved on to other things.
In this month’s issue, the cover article is “A Cosmic Crisis“, about a discrepancy between two measurements of the age of the universe.
Funny thing, there’s always a letter from the editor in chief where he introduces all the major articles, and here he contrasts the “cosmic crisis” with another ongoing crisis, that thing between US and Iran. Yep, this sure is an article that was written last month! FWIW, I could do with some reading that has nothing to do with COVID-19.
I thought I’d review the article in approximately the order in which I read it: pretty pictures first, walls of text last if at all. For serious, this is the correct way to read a science article, I can say that as a person with a PhD.
There’s one picture that immediately grabs me, which is a plot of Hubble Constant Estimates vs time of publication. It shows two measurements, one from “Distance ladder method using Cepheids” and one from “CMB measurements”. These measurements have gotten more precise over time, and starting in 2014 (which by scientific standards is very recent) the two estimates diverged. Right now it seems they’re about 3 standard deviations apart. There’s also a newer third measurement, “Distance ladder method using flaring red giant stars”, which now seems to disagree with each of the other two.
From this one plot, I can already pretty much tell what the article is about, because I have a bit of prior understanding of the Hubble Constant and the methods used to measure it.
- The Hubble Constant is a number that measures the current rate of expansion of the universe. Contrary to the name, it’s not actually a constant, the Hubble Constant slowly decreases over time according to general relativity. The standard model that predicts how the Hubble Constant changes over time is called the ΛCDM model. By extrapolating backwards you can figure out the age of the universe.
- The CMB is the cosmic microwave background, which is the the very oldest light signal we can observe in the universe, because it comes from the time when the universe became transparent. Small variations in the CMB in different parts of the sky are basically quantum fluctuations writ large, and somehow this can be used to measure the Hubble constant.
- I’m not exactly sure what Cepheids are, but I know they’re a “standard candle”, an object that shines with predictable luminosity (i.e. total amount of light power). If you know the luminosity, and how bright it appears from Earth, you can tell how far away they are. And their redshift can be used to measure how fast they’re receding. So that gives you the Hubble Constant.
- I’ve never heard of the flaring red giant method before, but I surmise that astronomers found another standard candle.
Where could these methods be going wrong? I’m sure a lot of astrophysicists have thought really hard about it!
So let’s move on to the text of the article. By the way, the normal reading order for the text of a paper is: abstract, introduction, conclusion, discussion, methods. But since this is a popular science article I guess they expect me to read it in order. Anyway, here’s what I learned:
- Red giants are a standard candle because dying red giants undergo a “helium flash”. The internet tells me that’s when the helium at its core starts fusing. Why does the helium flash have predictable luminosity? I have no idea. In fact, I don’t understand red giants at all. Grade school science taught me that red giants form when stars run out of hydrogen fuel, but it takes a degree in physics to realize that I don’t have any idea why that is.
Uh, well that’s about it. There were a lot of historical details, and an overarching narrative about astronomers (who make the standard candle measurements) vs cosmologists (who predict the Hubble constant from CMB measurements). But no real solutions, not even proposed solutions. I feel like I got more out of the one graph than the rest of the article. Well, it’s a really good graph!
Anyway, sounds exciting for the researchers in this field. I’m sure cosmologists are rubbing their hands over the thought of overturning the ΛCDM model.