“A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.” –Max Planck
When you look out at the night sky, with the deepest, sharpest eyes possible, what is it that you see?
Galaxies! Lit by hundreds of billions of suns each (and that’s just on average), they not only illuminate the Universe, but they also trace out the great cosmic web that composes the large-scale structure of the Universe.
How does it get to be that way? Well, we know the laws of physics, so we can put our ingredients for a Universe into a simulation and see what we get out.
And the simulations that match up with our observations the best are ones where we have about five times as much dark matter as normal (protons, neutrons, and electrons) matter.
So we can look at individual galaxy clusters, and measure their mass using a variety of methods (like gravitational lensing, below), as well as their total normal matter content.
And for every large galaxy cluster, we get (more or less) that same ratio: about 15-17% of the mass is “normal” matter, with about 83-85% dark matter.
In theory, gravity treats everything with mass equally, and once the Universe cools to the point where radiation (photons, neutrinos, etc.) is unimportant, we should form all the structure in the Universe — from the largest scales down to the smallest scales — with the same ratios of dark matter to normal (baryonic) matter.
And while large galaxies — like Messier 104 — definitely exhibit the expected ratio of dark matter to normal matter, things get a little sketchy when we start going down to smaller galaxies. Their rotation curves start to do something that, well, make us a little uncomfortable.
It looks like there’s too much dark matter! More precisely, it looks like there isn’t quite enough normal matter, as Messier 33 — the third largest galaxy (behind Andromeda and the Milky Way) in our local group — shows.
And if we start to look at even smaller galaxies, things get even more extreme.
Because dwarf galaxies, like the Fornax dwarf galaxy above (or the Sculptor dwarf galaxy below), have less than 1% normal matter, and more than 99% dark matter!
Well, just earlier this week I told you about the smallest mini-galaxy ever discovered. Let’s take a look.
With just over 1,000 stars and yet a mass of something like 600,000 solar masses, Segue 1 is the most dark-matter-dominated object found to date!
Some people contend that this discrepancy is cause for alarm, and some even reject the idea of dark matter because of it! And while it surprised me when I first learned about it just a few days ago, it shouldn’t have.
Because the dark matter/normal ratio starts out the same on all scales, but then you form stars! And if you run your simulations of the Universe and include star formation, you find something remarkable.
Forming stars for the first time not only released a tremendous amount of visible light, illuminating the Universe for the first time since the early stages of the Big Bang, it also did two other remarkable things:
- The ultraviolet radiation released re-ionizes the Universe, knocking the electrons off of protons in the intergalactic medium, as the video above shows. And…
- The radiation causes a small but non-negligible amount of pressure to permeate the Universe.
“Big deal,” you say. “Why should I care about a small amount of radiation pressure?”
Well, radiation pressure is the very thing responsible for bending this comet’s tail. In other words, it can cause atoms to accelerate, and exerts a force on them. That’s the normal matter. But photons do not interact with dark matter!
So what happens when you put a source of radiation pressure in or near something with normal matter and dark matter? Well, the dark matter stays put no matter what. But the normal matter?
It depends on how deep your gravitational potential well is! If you’ve got a lot of mass in a large galaxy (or very large cluster of galaxies), most of the normal matter stays put, as this extra pressure is insufficient to kick the normal matter out. But the smaller and lower-mass your galaxy (or dwarf galaxy, or mini-galaxy) is, the more normal matter gets expelled!
And that’s why dark matter rules mini-galaxies, and the tinier you are, the more dark matter rules you!