Today I attended this colloquium on the self-interacting dark matter, and I find the topic very interesting. I will try my best to explain it here in simple terms.

1. Basic concepts

First of all, what is dark matter? As the name suggests, dark matter is some strange matter that’s “dark”, meaning that we know they are there but we just can’t see it. Sound mysterious isn’t it? Well, you may ask, if we can’t see them, how do we know they exist? The idea is very simple. Imagine that you are holding one end of a long rope that has the other end hidden behind a closed door. In general, we don’t know if there is anything at all behind the door, but if there is a force pulling on the rope, we know that something must be there, better if we know roughly how strong the pulling force is, then we will have some ideas of what might be pulling it, just like if the pulling force is weak, it’s probably not an elephant that’s pulling the rope. The same concept applies to gravity. If there is a gravitational pull, we know that something must be there, no matter if we can see it or not. With a good measurement of how strong the pulling force is, we can figure out how massive the object is that pulls us. In the case of our galaxy, if we look at some outermost stars in our galaxy, we can measure the gravitational pull that they experience by observing their motions, and to people’s surprise, the pulling force that we estimate based on the motion of these outer most stars is much stronger than we expect from looking at the visible matter! In our rope example, if we feel a tremendous pulling force but only see a kitten in the room, something is wrong! An invisible monster is hiding in the room! This is how surprising the discovery of dark matter is to scientists.

2. Rotation curves

We briefly mentioned that by studying the motion of a star, one can deduce the gravitational force that the star experiences and hence the mass of the matter that exerts the force. How exactly is it done? This brings us to the concept of rotation curves. The idea is very simple, imagine that you are trying to swing an object in a circular motion in the air through a string, daily experiences tell us that to make the object rotates faster, we have to pull the string harder. Similarly, stars rotate faster with stronger gravitational pull. In other words, how fast a star rotates about the center of our galaxy tells us how much matter there is inside the galaxy that attracts it. What’s more interesting is that if we look at how fast stars rotate at various distances away from the center of our galaxy, we get a good understanding of how matter is distributed in our galaxy. This profile of how fast stars rotates across a variety of distances away from the galaxy is a good characteristic of the distribution of matter in the galaxy and is given the name “rotation curve” of the galaxy.

3. Self-interacting dark matter

The self-interacting dark matter model is one of the many dark matter models that can explain some of the observed features in the rotation curve. In simple terms, the strength of the self-interaction between dark matter particles determines many observable features in the rotation curves. Reversely, by matching the rotation curves of different galaxy systems, we can estimate how strong the self-interaction is between the dark matter particles within the galaxy system. This is extremely useful as in one can now study the properties of dark matter at various interaction strengths by looking at different types of galaxies. What do I mean by that? Take atoms as an example, when atoms interact through low energy bombardment, an atom behaves like a solid sphere, but when atoms interact through high energy collisions, the inner structure of atoms, such as nucleus and electrons, emerge and affect the observable physics. The same may be true for dark matter particles, at different strengths of interaction, the dark matter particles may behave differently, showing their underlying structures. Therefore, to be able to study dark matter particles at different strengths of self-interaction through looking at different types of galaxies is a great way to figure out what dark matter really is.

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