Yale’s Astro 160: From Epicycles and Back

With Yale’s Astro 160 (Frontiers and Controversies in Astrophysics) I’ve now completed my first semester at Yale, so to speak. It’s the fifth class I’ve taken by podcast, and in my day five classes was a full load. It’s been a semester full of good courses (all reviews under the Yale tag). Moreover, they’ve been free of tuition and with no homework or reading required. I did miss the parties though.

Like Philosophy 176, Astro 160 tackles some very deep questions, albeit of a physical as opposed to a metaphysical sort. Also like Philosophy 176, the subject of Astro 160 is specific yet motivates an enormous amount of the broader field (physics) upon which it relies. The ostensible subjects of Astro 160 are three rapidly developing areas of astronomy: the search for and discovery of (1) planets outside the solar system (exoplanets), (2) black holes, and (3) dark energy.

Professor Charles Bailyn, an expert in black hole discovery, approaches these topics by reviewing major milestones and controversies in the history of work in these areas. His first class begins with the failed theory of Ptolemaic epicycles in which the Earth is the center of the universe and the motion of celestial objects about it are modeled with circular motions embellished with ever more complicated wiggles and variations in speed. The theory eventually collapsed from its own weight and was replaced with the far more sensible heliocentric model.

In summarizing the evolution of the frontier of astrophysics over the centuries, Bailyn draws out the stylized lessons–parables really–that scientific culture has drawn from each controversy and breakthrough. The parable of the epicycle is that of Occam’s razor: simple models are better. The class more-or-less continues in this way right up to present day. Bailyn succeeds in explaining the current state of astrophysics while avoiding much of the technical and mathematical formality that would otherwise make recent work inaccessible to a non-specialist. That’s not to say the class is free of technicalities or mathematics. It has both (expect a lot of calculations in scientific notation), but in simplified form with no calculus, and hardly any trigonometry. Anyone comfortable with algebra can follow it.

I’m a former physics geek, but I hadn’t encountered material such as this in 15 years. (Back in the day I worked a summer for David Wilkinson, a pioneer in the study of cosmic microwave background radiation. He and his work get brief mention in Astro 160.) This time around I viewed it (listened to it) with mature eyes (ears). I was struck by just how much could be known–is known–about the most distant objects and about the most distant moments of the past. With our reason, a few empirical techniques, and some ever-increasingly precise equipment we can judge the distance and composition of stars, the size and orbital shape of exoplanets, the presence of black holes, and ultimately the origin, evolution, and fate of the universe. Amazing!

The third section of the class covers dark matter, which is mysterious enough, and dark energy, the properties of which are so unknown it sounds like a joke. It was in these lectures that I missed the visual most. Complex transformations of lines on graphs were occurring that I couldn’t fully follow aurally. (Video, lecture notes, and transcripts are available online but I didn’t look at them.) The class ends with a plot in which there is some significance of three lines intersecting at a single point. I didn’t get it in detail. But, in some sense neither does Bailyn or anyone else. The axes are the density of dark matter and the density of dark energy.  In Bailyn’s words,

So, what we are plotting in this wonderful plot where everything works out so nicely is the density of something we don’t know anything about versus the density of some other thing that we don’t know anything about. And so, in a certain sense … the fact that we’ve got these two axes means we don’t have any idea what’s going on.

Bailyn goes on to suggest that the lack of understanding of dark matter and dark energy smells of epicycles. In a sense, astrophysicists have invented these two ideas so that observations fit theory. But the theory doesn’t predict much because we don’t understand the ideas. They’re really just fudge factors, like the wiggles on circles of a geocentric model of the universe. A new idea, a breakthrough, either in theory or observation, is needed to move astrophysics forward. If it comes during his professional lifetime, no doubt Bailyn will discuss it in class.

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