Take a look around you. Anything you can see, the walls of the room you are in, the computer (or phone) you are looking at, even you, everything in you, is made of atoms. Little bundles of quarks and electrons. Lets zoom out a little. Look at the moon, atoms, look at the sun (don’t really, that is dangerous!), parts of atoms, everything you can see is made of atoms, or a least parts of atoms. So let me tell you something pretty amazing. Those atoms, and every other particle we have every discovered are only 4% of all the matter in the universe.
4 percent! That is a tiny number. So where is all the rest of it? Well, as it turns out, it just seems to be floating about the universe. The more complicated question is what it is. In fact, I chose to write about this because actually, we aren’t quite sure, but I will get to that later. Before that, let me explain how physicist realized that everything that we had been observing for the whole of human existence, was only a fraction of all there was out there.
Like the majority of physics discovery, it started by someone staring up into the sky. In this case, it was a man called Jan Oort. He was studying the little place called the Milky Way, the galaxy that holds not only our solar system but between 200-400 billion stars. And he noticed something about the galaxy: namely that it was spinning far to fast than it should for the mass that it had. This was noticed because of a couple of things we have spoken about before, kinetic energy and gravity.
When galaxies spin, everything within them has kinetic energy. In fact, kinetic energy is just the word for when something is moving. However, kinetic energy is a vector (it only goes in straight lines) and you will notice that a star in a spinning galaxy is at no point going in a straight line, it is continually turning. This is where gravity becomes important. The only thing that keeps the stars from flying out into space is their attraction to the matter inside of them, closer to the center of the galaxy.
So far so good, the simple idea is that stars don’t fly away because they are pulled back. Now it becomes a little more complicated. You see, at the center of the galaxy there are usually more stars in a smaller space. This means that stars closer to the center (as gravity decreases over distance) will feel a much larger gravitational force than those far out in the galaxy. Now, this situation is the reverse of the one from before. The stars now have to move a lot faster in order to avoid falling into the center of the galaxy.
So where does this become important? Well, it was found that at the edge of the galaxy, stars had a much larger amount of kinetic energy than expected. In fact, this graph shows what the velocity at different parts of the galaxy should have been (A) and what it actually turned out to be (B).
If our basic understanding of the physics behind the behavior was to be applied here, the stars on the outer edges of the galaxy would simply start flying out, as gravity could not contain them.
This was the problem that was noticed. Everything we observed did not fit together as simply ask we would expect. In fact, this sort of discovery can be related back to a fantastic man and the father of modern physics, Isaac Newton. Newton and everyone else in his time saw then when they pushed an object, it would stop moving after a while. Anyone else would have seen this and concluded that the universe doesn’t allow motion without a push being sustained. However, Newton took observations made by Galileo and calculations of his own and realized this was not the case, it was air resistance that was stoping the movement. Similarly, it would be easy to look at a galaxy and take the obvious route but we now have observations that show us that this obvious route doesn’t work.
So, how did we deal with this? We made something up, of course! You see, in physics, it isn’t necessarily a bad thing (though sometimes it is) to look at a situation, see there is missing, and simply place something in there. As you make more observations, you can define this something. So this is where Dark Matter emerged. Some of its properties could be defined already, we obviously couldn’t see it, so it didn’t interact with electromagnetic radiation (light would simply pass straight through it) but its main property was that it had mass. The only way we could detect it was simply on the effect it had in these galaxies. The Dark Matter just floated around, adding in mass in those outer parts of the galaxy and maintaining the gravitational force that was need to contain the stars.
The most popular idea of Dark Matter we have, is that is made of WIMPs, or Weakly Interacting Massive Particles. If these really do exist, there would be millions passing through us every second. How do we detect these then? As the name suggests, they don’t interact very much with anything. Well it is similar to a undetectable particle we have looked at once before: The Neutrino. There are two main ways to detect naturally formed WIMPs or we can try create them ourself.
Detecting natural WIMPs is simple in theory but difficult in practice. You cool a substance to an incredibly low temperature and then wait. It may take a while, but when a WIMP eventually hits into one of the atoms of substance, it warms up a miniscule amount. This can be detected, and is noted.
Another method that may be used is creating our own WIMPs. We know that mass is just energy, from Einstein’s E=MC2, and Dark Matter has mass. So we put enough energy into one place and hopefully we will create some Dark Matter. Or at least that is what we are trying to do at the LHC in CERN, Geneva. They hit two particles together really hard, which releases a lot of energy that turns into particles. We won’t be able to detect WIMPs directly, but if we see that there is some mass is missing after we have looked at all the other products of the collision, then we know that some where produced.
After the Higgs Boson discovery earlier this year, Dark Matter could be the next big discovery in physics, so watch this space!
I hope you’ve enjoyed this post! If you did then please check out our last two posts:
What is the abc conjecture? And why does its recent proof matter? - The abc conjecture is a problem that was once described by Dorian Goldfeld as “the most important unsolved problem in Diophantine analysis”. So what really is it?
What is Tupper’s Self-Referential Formula? A mathematical magic trick, semantics and meaning…- what is Tupper’s Self-Referential Formula, how does it work, and is it really as amazing as it seems?