In the past I have done a week-long series on certain topics. I don’t do them all the time because they take more work than one-off posts, but they tend to be rather popular. So far the series have been fairly broad in scope, covering the quantum revolution, or science fiction vs. science fact. But this time I’m trying something a bit different: cover one topic in detail. Background, proposed models, observational evidence, and why we support one theory over the alternatives. We’ll start with dark matter. It was first proposed in the mid 1900s, and since then its existence has become both more confirmed and more bizarre.
Line of Perseus
The Perseus cluster is a massive galactic cluster consisting of thousands of galaxies. It is often a focus of study because it is both massive and reasonably close (about 240 million light years away). Recently we’ve discovered some interesting x-rays coming from the region. The results have been published the Astrophysical Journal, showing there is an unexplained emission line in the x-ray spectrum.
Electric Boogaloo
As I’ve noted before, the idea of dark matter isn’t invoked just to make the standard theories work. Despite our incomplete understanding of dark matter, there is significant evidence to support it. But how do we know that regular matter isn’t enough to account for the unseen mass in the universe? What if it were something like dark mode plasma?
A Difficulty with Dwarfs
Most of the predictions of cold dark matter agree very well with observation, which is why it is a dominant part of modern cosmology. But there are aspects of the dark matter model that don’t agree well with observation. Take, for example, the clustering of dwarf galaxies around the Milky Way and Andromeda galaxies.
Not Like the Others
The nature of dark matter is one of the greatest mysteries in modern astrophysics. Actually that isn’t quite true. We do know of one type of dark matter, and we’ve even detected this dark matter in experiments on Earth. This particular type of dark matter is more commonly known as neutrinos.
Mine Over Matter
Minnesota’s Soudan Underground Mine State Park is a former iron mine. Because of the orientation of the hematite, the mine had to go deep, and by the time of its close as an active mine in 1962 they were mining more than 2000 feet below ground level. Given its depth, and the geology of the region, the lower levels of the mine are well shielded from cosmic rays, which makes it a perfect location for sensitive experiments such as the search for dark matter. This is why it’s the location for the Cryogenic Dark Matter Search (CDMS).
Dark Star
So it is tempting to wonder what effect dark matter might have on the formation and evolution of stars themselves. The problem is we can’t be sure without an understanding of type of dark matter which exists. That hasn’t stopped some astronomers from speculating, however.
Dark Matters
You’ll notice the graph above is fit to the dark matter model, but not any alternative model. This was done intentionally, and it helps to feed the fires of sensationalism in science reporting. Unfortunately it makes it much harder to convey what’s really going on. These results were hyped, intentionally, and they shouldn’t have been.
When Dark Matter Collides
In the ongoing search for dark matter particles, the most popular are efforts to detect them directly here on Earth. Another way to look for dark matter particles is to look for the by-product of their collisions with each other. A recent paper posted on the arxiv has done just that, and they think they’ve found a signal.