Build-A-Verse #2: The Case for Dark Matter

Pre-post Comment: Remember back in September when I said I’d do a series of posts detailing solved, partially solved, and unsolved problems in Cosmology? Well, I can’t believe I’m actually following through with it. So here’s part two. 

(Click here for part one.)

The vastness of the Universe is something that never fails to astound me. Considering its incomprehensible size, our lack of information regarding some of its aspects is not entirely surprising. Even so, you would expect that if something makes up about eighty percent of the universe’s matter content, we would know at least something substantial about it. And yet we have Dark Matter: something that is five times the amount of normal matter (i.e., you, me, Trump, chocolate cake), and about which we have a surprisingly little amount of information. In fact, we know more about what it is not, than what it is.

Dark Matter doesn’t interact with electromagnetic radiation aka light. This means we can’t see it as opposed to normal-matter objects like stars and galaxies. If we can’t see it, how do we know it’s there? Well, if eighty percent of your saved-for-later chocolate cake goes missing, you’d notice, won’t you? Our Universe would behave very differently if we didn’t have dark matter. And the only way to explain the way it does behave is to include ‘some form’ of dark matter.

Let’s rewind a bit. Dark matter doesn’t interact with light but it does interact gravitationally (since it has mass). Meaning, it exerts a gravitational pull on the objects that surround it. So even though we can’t “see” dark matter itself, we can collect evidence for its existence by studying how the things around it behave. So for example, light traveling near a dark matter halo[1] would “bend”, as it would if it passes any kind of massive object. Everyone with me? No? Let’s make it even more concrete.

The Bullet Cluster:

bulletcluster_comp_2048
Source
This is the Bullet Cluster: so called because, well, it looks like a  bullet and physicists are oh-so-creative with names. The bullet cluster is interesting because it gives the strongest indirect evidence for the existence of Dark Matter. (Also, because it’s literally a picture of two colliding galaxies clusters and if that’s not interesting, nothing is.)

What happens when Galaxy Clusters collide?

There are different things that contribute to the mass of a galaxy cluster. There are stars and such obviously, but more importantly, there is a hot gas of ionized particles. Most of the “baryonic mass” (a fancy way of saying the mass which originates from the standard-model matter content aka normal matter) comes from this ionized gas. So when two clusters collide, most of the stars pass right through. The particles in the gas of one cluster, however, collide with the ones from the second cluster and radiate energy. We can detect this radiated energy in the form of X-Rays.

What we can also do, is figure out where all the mass of a cluster is centered. Since mass bends light, we can study distortions of light from the surrounding clusters and model the mass distribution of a cluster. This process is known as gravitational lensing. 

If there is no dark matter, the mass of the bullet cluster should be centered where most of the gas is. This is the pink region in the picture above. When we calculate the mass distribution through gravitational lensing, what we actually find is that it’s centered at two places – the blue regions in the picture.

This discrepancy is fairly easy to explain using Dark Matter. Since by definition dark matter doesn’t interact, when the clusters collided, dark matter just kind of slipped through. The baryonic matter, on the other hand, did all sorts of crazy things, collided and bounced and ended up producing the X-rays that we see today.

The bottom line? Most of a galaxy cluster is actually made of dark matter. (The ratio is approximately 5:1.)

There is a plethora of other evidence which I won’t get into here. Despite it, however, we are still clueless as to the particle nature of dark matter[2]. We haven’t been able to detect it through direct-detection experiments[3], nor have we been able to produce it at colliders like the LHC[4]. The quest to pinpoint the exact properties of Dark Matter is a long one. The experimental results are disheartening, more often than not. But it’s insanely, mind-bogglingly compelling and you’ll have a hard time getting me to shut up about it.

Until next time.

Post-Post Comment: Questions? Thoughts? Comment below!

[1] Because of it’s gravitational nature, Dark Matter condenses to form structures which we refer to as halos.

[2] I’m currently working on a particular class of these Dark Matter models which is based on the assumption that dark matter couples to the Higgs. Exciting stuff.

[3] The one-line summary of these experiments is this: We build super-cool large underground tanks full of target particles and wait for a Dark Matter particle to collide with them which gives us an ‘event’ that we can study. (The idea is simple, the execution, however, is arduous).

[4] Producing, as in we crash two super-fast beams of normal “standard-model” particles. According to the Dark Matter model you’re considering, there are different small but finite probabilities for these particles to annihilate into Dark Matter particles. Since our detectors wouldn’t see these dark matter particles, we’d have “missing energy”. (We’d see 2 particles going in, none coming out).

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