RWTH Aachen Particle Physics Theory

In the post-truth society that we live in it is easy to fall down the rabbit hole of doubting every scientifically held belief. To wonder if NASA is hiding proof of intelligent extra-terrestrial life (they’re not), or if people at CERN are rubbing their hands plotting something nefarious (nope) or whether the Big Bang theory is a Big Bad Lie (it really…isn’t).  But don’t worry, we at TTK have got you covered. Every Wednesday we answer your questions live on Twitter and every whenever-this-author-stops-procrastinating-day we give you a more elaborate explanation of some of the most frequently asked questions.

Today on the agenda: Dark Matter — what it is and why you should be reasonably sure of its existence.

Simply stated, dark matter is a kind of matter that doesn’t interact with light. This means we can’t “see” it in the conventional sense. As you would expect, this makes studying dark matter a bit difficult. But if there is one redeeming quality in humankind, it is that we don’t shy away from the seemingly impossible. Of course, the question remains, if we can’t see dark matter and if it doesn’t interact all that much with other things, how do we know that it exists in the first place? The answer comes to you in four parts.

1. Galaxy Rotation Curves

Some of the earliest indirect evidence of dark matter comes from galaxy rotation curves. A rotation curve is a plot of the orbital speed of stars or visible gas present in a galaxy as a function of their distance from the galactic center. If we assume that the total mass of a galaxy is only composed of normal or ‘visible’ matter, the farther we move away from the center (where most of this mass is concentrated), the lower the orbital speeds should get. This is what happens in the Solar system. Since the Sun accounts for most of the mass percentage, the planets farthest from it revolve slowly as compared to the ones close by.

However, measurements of galactic rotation curves don’t agree with this prediction at all. Instead of decreasing with distance, the orbital speeds of outlying stars appear to either stagnate to a constant value or increase. This points towards the possibility of an additional contribution to the mass of a galaxy from something we can’t see.  Maybe something dark?

2. The Bullet Cluster

Bullet Cluster

Another smoking gun for dark matter is the Bullet Cluster. It is composed of two colliding galaxy clusters, the smaller of which looks like a bullet. Galaxy clusters are a busy place and when they collide, chaos ensues. The stars, far apart as they are, mostly survive the collision without a story to tell (aka pass through).  The particles present in the galactic plasma, however, smash and ricochet and radiate a lot of energy.

Galactic plasma makes up most of the baryonic (visible) mass of a cluster so we can derive a mass-profile for the cluster from this radiated energy. We can also model the mass-profile by studying the lensing effects of clusters. Because massive objects bend light, we can figure out their mass distribution by studying how they distort light from surrounding clusters. If the entire mass of a cluster is just the baryonic mass, these two mass-profiles should coincide. What we find instead, is that they are in exact opposition. In the image above, the pink regions are where the baryonic mass is present. The blue regions show where the total mass of each cluster is concentrated. The zero-overlap between the two implies the presence of a non-baryonic, invisible source of mass. Moreover, it purports that most of the mass of a cluster is non-baryonic or dark. (In fact, roughly 80% of the universe’s matter content is dark!)

Quick Side Note: Keep in mind that the colors are for purely representative purposes! The radiation emitted by the galactic plasma doesn’t fall within the visible spectrum. Similarly, the blue is where the experiments tell us dark matter is concentrated.

3. Large Scale Structure Formation

Sloan Digital Sky Survey

An interesting question to ask cosmologists is why does the universe have a structure? How do we go from a more or less homogeneous particle soup to well-defined clusters of galaxies and then even to clusters of clusters of galaxies? The simple answer to this question is fluctuations. Tiny fluctuations right after the Big Bang lead to overdensities and underdensities of matter. As the universe expands, these fluctuations also grow on account of gravity and we end up with clumps of matter which would eventually form stars, galaxies, galaxy clusters, etc. There is one small problem with this line of reasoning though. We know that the early universe was dominated by radiation (or light). And light exerts pressure. So even as the fluctuations would cause matter to clump, radiation would cause it to homogenize. In the end, the fluctuations would be nearly wiped out and we wouldn’t have the kind of structure that we see today.

Dark matter solves this problem. It is massive and it doesn’t interact with light. Formation of dark matter lumps would aid the ‘clumping’ of normal, baryonic matter and give rise to structure despite the homogenizing effect of radiation.

4. Cosmic Microwave Background

CMB

The CMB can be regarded as a picture of the baby universe. And though at first glance it might look like random splotches of paint, it provides deep insights into what the universe looked like billions of years ago. Any cosmological model that we create has to be in agreement with this map. By specifying initial conditions — for instance, percentage of matter, dark matter and radiation — we should end up with density fluctuations as observed here. The best model we currently have is the ΛCDM. As you might have guessed, the DM here stands for dark matter. It is only when we include dark matter in the model that our predictions line up with the data.

These are just a few of the reasons we believe that dark matter exists. And even though we haven’t detected anything like a dark matter particle (yet), everywhere we look the universe seems to suggest that it must be there. If you still don’t understand why you should believe in it, (and as a reward for reading these 1000+ words), here’s a (dark) analogy: