“Sometimes I don’t want to see the puppeteers,
sometimes I just want to see the magic therein,
and sometimes I just want to pry open the atoms
and know why they spin.” –Glen Sutton
But it isn’t just the atoms — the minuscule building blocks of matter — that spin. It’s also the individual galaxies, collections of some mind-boggling number (like 1068) of atoms, that spin.
Messier 95, above, is just one such example. But how did these galaxies get to be this way?
To answer this question, we have to go all the way back to the early stages of the Universe, to when it was almost perfectly smooth.
If we go all the way back to when the Universe was just a few hundred-thousand years old, we see that some regions of space appear slightly colder (bluer) than others, while some are slightly hotter (redder). Why is this?
Because some places in space have slightly more matter than others, while other places have slightly less.
The Universe is actually filled with the same radiation in all of those places, but the places with more matter have a slightly deeper gravitational well for that radiation to climb out of. Compared to the average, that temperature appears slightly colder. Conversely, the places with slightly less matter than average have a shallower well, as so that radiation appears hotter.
The hot spots we don’t care so much about, because over time, they’re going to be lousy at attracting more matter. But the cold spots are a different story. Being ever-so-slightly denser means they do an ever-so-slightly better job at attracting more and more matter towards themselves than to other regions of the Universe.
The result is that — over hundreds of millions of years — these regions gain more and more mass, and eventually grow into galaxies.
And depending on what you pull in, and what your initial conditions are, you will start with some small amount of angular momentum, or overall rotational motion about some axis. And just like a figure skater who starts off spinning slowly who then pulls her arms in, a collection of matter that starts with some small amount of angular momentum will collapse under the influence of gravity.
And when it does, that angular momentum remains. Since your object is now the same mass but much more compact in size, it rotates much more quickly.
Now, as far as we can tell, there shouldn’t be any overall angular momentum to the galaxies in the Universe. In other words, if we took a giant survey of spiral galaxies, we would expect to find the same number of galaxies rotating “clockwise” from our point of view as we found rotating “counterclockwise” from our point of view.
Telescopes, at long last, are finally good enough that we can start doing this with some degree of accuracy.
So we do; we look across a huge swath of the sky, at individual galaxies strewn across the Universe and at huge clusters of galaxies clumped together, like the Hercules Galaxy Cluster, below.
And what do the first results of a study like this show? Well, if you listen to the University of Michigan’s press office, they’d have you believe that there is a preferred direction of rotation for galaxies in the Universe. In fact, they say that the direction our galaxy rotates is 7% more common than the other way around, and that
the chance that the excess could be a cosmic accident is something like one in a million.
Are you skeptical? You should be. Because I went to the paper (access restricted). First off, they use only a sample of galaxies (about 15,000) in the nearby Universe, and it isn’t clear whether this is a fair sample or not, or whether it’s biased in some way. Second off, the odds that this measurement would occur at random — in other words, if there were really no preferred direction — isn’t one in a million, it’s 0.08%, or about your odds of rolling a Yahtzee on the first throw. And finally, the statistical significance of this result is not even at four-sigma.
In other words, this would be such a revolutionary discovery — overturning our long-held idea that the Universe is the same, on average, in all directions — that it really would require some extraordinary evidence. What’s been seen so far is suggestive and interesting, in other words, it looks like further studies should definitely be done, but it would take a much more compelling study to convince me that galaxies prefer to spin one direction over the other in the Universe.
So that’s why galaxies spin, and why we really expect them to spin with no preferred orientation in the sky! But I’ll definitely be keeping my eye on this one.
P.S.: For those of you who’ve been hankering for a technical review of modern cosmology, you may want to check out this new paper (access not restricted), as it’s an excellent summary for the budding specialist. Enjoy!