The title of this blog, “Fireside Science,” is of course a reference to Franklin D. Roosevelt’s Fireside Chats, where the President would communicate directly with the American public via radio. They listened, nestled comfortably in their living rooms beside their radios, while FDR explained big and scary things like the banking crisis and World War II, in a comforting and accessible way. The analogy suggested by our title is going to be especially relevant to this post because there are a couple of frightening things, which you may or may not be aware of, that I would like to talk about.
(1) Everything that you’ve ever seen, heard, smelled, or felt is made up of stuff that only comprises 5% of the universe. For most scientists, that means everything you’ve ever studied- the entire vastness of knowledge and the blood, sweat, tears, triumph, and despair involved in those discoveries- can AT MOST describe 5% of the universe.
(2) All of the large objects in the universe- galaxies and galaxy clusters- are not only moving farther apart from each other, but the rate at which things are getting farther apart is increasing. Eventually, all of the stars in the sky will be extinguished. The universe will be devoid of life, light, everything. Also, no one really has any idea why this expansion is happening.
Ok, that might have been a little melodramatic. Let’s break this down.
In 1933, a Swiss astronomer named Fritz Zwicky was observing the motion of the Coma Galaxy Cluster. Based on the amount of light coming from the cluster, he estimated the total amount of matter in its galaxies. He then compared this to the amount of matter that must be present based on the speed of its galaxies. Based on the speed, the galaxies would have needed to have a mass 400 times that which could be accounted for by the visible light. Zwicky concluded that the galaxies must be held together by some kind of “dunkle Materie,” or Dark Matter, since it could not be seen visibly.
It turns out that Zwicky was very, very right. Astronomers and astrophysicists found places all over the universe where their observations could only be explained by “missing mass”- dark matter (although I contend that “Zwicky matter” would have sounded much cooler than “dark matter”).
With modern measurements, it turns out that dark matter accounts for 27% of the universe. Wow. That’s over 5 times the amount of “normal” matter—what you and I are made of.
So….what is it?
The person who answers this question definitively is going to get a Nobel Prize, so you can accurately call this the million-dollar question. There are a few things physicists could be sure of right from the start; first, it doesn’t interact electromagnetically, i.e. with light, but it does interact gravitationally. That accounts for two of the four fundamental forces—what about the other two? The strong force is responsible for keeping nucleons together, so that’s not really in play here, but physicists do think that dark matter interacts via the weak force, which is responsible for radioactive decay.
Thus, one of the primary candidates is some sort of weakly interacting massive particle—a WIMP (because we physicists also like to think we’re funny sometimes). Really, this is most likely a family of particles rather than single particle, such as the proposed supersymmetric particles. There are, however, other candidates, such as axions, as well are more exotic theories. The details are, of course, monumentally complicated, so everything here is just to give you a taste of what’s going on the dark matter world currently.
How do we figure out which of these theories is right?
The experimental search for direct detection of dark matter is really in its infancy, so you can expect to hear more and more about these experiments in the future. There are a lot of different experiments now, coming at the problem from all directions possible, so I’ll just describe a few basic methods here. One way is basically to fill a big tank with a heavy element, such as Xenon, and wait. Since the dark matter doesn’t interact electromagnetically, it won’t care about the electrons floating around, but it will eventually knock into the nuclei, releasing energy which can be detected. Another natural place to look for dark matter is in a machine specifically designed to blast detectors with particle soup where we can look for fun new physics- the Large Hadron Collider. Some people are getting a bit more creative though—such as a proposed experiment to find dark matter collisions with strands of DNA.
Ok so, 26% + 5%=31%. Where is the rest of the universe?
The rest of the universe- a whopping 69%- is made up of something called Dark Energy. The name is kind of misleading—dark energy isn’t directly related to dark matter in any way that we know of. It’s just called “Dark” because we have no idea what’s there. The pie chart of the universe might as well look like this:
Dark Energy is a mystery all unto itself, which I’ll discuss in my next post- to be continued!
Some more reading about Dark Matter:
The Art of Darkness, by Katie Mack
Dark Matter Mysteries: a true game of shadows, by Stuart Clark
Exploring the Science of Dark Matter with the Cryogenic Dark Matter Search