This story has been a long time coming. This is not the final version, but I like it enough and I drank too much coffee to be able to sleep tonight so I’m gonna update my blog with it. Then, I’ll play some Dota. And then, maybe the sun will come up.
How Common are the Magellanic Clouds?
Deep in the southern sky are two hazy blobs of light. Early explorers adrift the dark seas saw them night after night–called them clouds. Though we now know that they’re galaxies, satellites of the Milky Way, the name still stuck. The Magellanic Clouds, the larger of which is only one hundredth the size of our galaxy, are among the farthest objects the eyes can see.
Now they’re clues to a great cosmological mystery: just what is our universe made of? The best guess, following decades of data, is a mixture of ordinary matter, cold dark matter, radiation, and dark energy. So to test this, scientists, with the aid of supercomputers, made universes.
The procedure is straightforward enough: Load these ingredients into an early universe soup, in their correct proportions. Give them a set of rules to obey, namely our known physical laws. Then, let it go.
For the most part, what they get is what we see, a universe with stars and gas and galaxies and clusters of galaxies. But some things weren’t quite right. For one, the Magellanic Clouds weren’t there.
Or rather, they just weren’t there usually. Of all the Milky Ways that formed in a typical simulation, only a few percent had satellites as large and as close as these. “So what?” one might ask. Sounds like a minor thing. But it was by minor things, historically, that theories came undone. If the models had made no prediction about large, close satellites, that would be one thing, but they did. They said, most likely, there should be none, and we had two.
Comb the sky, was one scientist’s idea. Risa Wechsler leads an astrophysics research group at Stanford University. She and her team saw an opportunity to test the standard model of cosmology, all they needed was more Milky Ways. And they would find them–22,000 galaxies which matched the brightness and size of our own–among the cataloged objects of the Sloan Digital Sky Survey.
Atop a mountain in New Mexico, the 2.5 meter telescope dedicated to the Sloan Survey has been sweeping the sky for the past decade. Its massive catalog of deep space objects has nearly a billion entries, but most objects, especially dim ones, are without redshift information.
Redshift is what astronomers use to calculate distance. As space expands, light, a wave–a series of peaks and troughs in space–stretches out with it and becomes redder. The reddest light, therefore, has traveled the longest so comes from the farthest away galaxies. Astronomers, by reconstructing an object’s original colors, can calculate its redshift, as long as it gives off enough light to work with. But most don’t.
Without distance data the cosmos is a mat on which stars and galaxies are pinned one on top of another. How do you tell a satellite from a background galaxy a billion light years away?
The answer from Wechsler’s team: you don’t, not for a single object at least. Which galaxy has which satellites, that’s information they didn’t need. Their question was “how common are the Magellanic Clouds?”, and its answer was there, somewhere in the data, they just had to dig.
Around each Milky Way they drew a small circle. Instead of sizing up the objects in it one by one, they counted everything–foreground, background, satellites and all. They compared this number to what they counted in a random section of the sky of the same size. The second number was, on average, slightly smaller than the first.
The difference, of course, was the satellites, and the difference was very small. Eighty percent of Milky Ways had no large satellite at all. Eleven percent had one, they found, and only three percent, like us, had two. Some scientists, based on this result and others, have started to question whether the Magellanic Clouds are even bound to us, or if, in some cosmic close encounter of sorts, they’re merely passing through.
Wechsler’s team is happy to leave that to the astronomers. As for them, what their simulations predicted exactly matched what they saw. A victory for the current cosmological model, no doubt, but a message closer to home as well: we are an oddity after all.
Here’s the final version. Thanks John for the edits!