What astronomers do

Bad Astronomy
By Phil Plait
Sep 27, 2006 7:13 AMNov 5, 2019 6:51 AM


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I'm working on a huge project for my day job right now, and it involves writing a lot of basic material about dark energy. I'm supposed to be brief, but how do you concisely describe dark energy? You can't. Even at 900 words I think I'm giving it too short a treatment. I'll forgo the details about dark energy is for now because it's not directly related to what I want to say here --and anyway, when the project is done I'll be able to link to it. In the course of reading up on DE so that I understand it well enough to explain it simply (ha! Sure!), I came across a technical paper called Measuring Cosmology with Supernovae. It's written by Saul Perlmutter and Brian Schmidt, two astronomers. But not just any two astronomers! They were the heads of their respective teams, both of which independently figured out that the expansion of the Universe is accelerating. The reason I'm bringing this up is because, for one thing, I wouldn't have expected them to write a paper together. They don't hate each other or anything-- in fact, they're both very nice fellows. But in the years leading up to 1998, their two teams were rivals, and the competition was fierce. But in science, even rivals for such a big goal -- and figuring out the fate of the Universe is maybe the biggest -- can be on friendly terms. But there's another reason why I'm writing this. Back then (and still, today) they were both trying to observe very distant exploding stars, technically called Type Ia supernovae. This particular flavor of star, it so happens, explodes with a total energy that is almost entirely predictable. Well, "predictable" is the wrong word. Really, when you observe how the brightness of the explosion changes over time -- days, weeks, and months -- you can calculate the total energy released. This in turn tells you how bright the supernova really is. When you compare that to how bright it appears through the telescope, you can determine the distance-- and that's one of the Holy Grails of astronomy. These things can be seen at tremendous distances, billions of light years. This in turn can be used to figure out all sorts of interesting things about the Universe at large, like its overall shape, what it's made of on the grand scale, and how it's expanding. In the late 1990s, both teams were bagging lots of supernovae. When they compared the distance they got for the supernovae versus how bright they were, they got a surprise: the supernovae were all too faint. It was as if they were farther away than expected. As they so succinctly put it in their paper:

Both samples [from the two teams] show that [supernovae] are, on average, fainter than would be expected, even for an empty Universe, indicating that the Universe is accelerating.

Don't be fooled by the dry declarative nature of that sentence: a huge amount of work went into being able to say it. They had to eliminate a host of other things that could be making the supernovae look fainter, like dust between us and them, possible chemical differences between supernovae we see locally and ones that are far away, and lots of other esoteric causes. After much work, they scratched everything else off the list one by one, until they were left with the weirdest idea of all: the Universe was not just expanding, but accelerating. Weird. But even that is not why I'm writing this entry. In the paper Brian and Saul wrote, there is some math that involves relativity and such. It goes on for a while, relating such things as the expansion rate, the density, and the pressure -- which, believe it or not, is fairly standard stuff for cosmologists. Finally, though, the equations are put together in a way that tells you something very important... and in the paper, Saul and Brian make a statement so astonishing that when I read it, I exclaimed out loud (using words I won't write here). I literally stood up from my desk and had to tell everyone in my office about it. The authors said:

Combining Eqs. (4 - 6) yields solutions to the global evolution of the Universe.

Read that again. "Combining Eqs. (4 - 6) yields solutions to the global evolution of the Universe." This is a review paper, so they skipped the several hundred steps to get to equations 4, 5, and 6. But still, in the end, this is what they are saying: you can write down some relatively simple equations, and from there determine the ultimate overall state and fate of the Universe. How frakking cool is that? The whole Universe, from end to end, stem to stern, port to starboard, the whole enchilada, the sum total, the whole nine yards, the complete package, the whole kit and caboodle... and if you're smart enough, work hard enough, and have enough smart and hard-working people before you lay the groundwork, you can calculate the global evolution of the whole shebang! That's I'm writing this entry. Science! Man oh man. I love this stuff.

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