“No matter how dark things seem to be or actually are, raise your sights and see the possibilites — always see them — for they are always there. -Norman Vincent Peale
Dark matter. I talk about it a lot here for a number of reasons. These include:
- the fact that it makes up about 85% of the mass of the Universe,
- the only way (so far) that it appears to interact with anything is gravitationally, and
- from our observations, we’ve learned that it’s made up of slow-moving, massive particles.
You put what we know about dark matter into a simulation, and it tells you what type of structure you expect to get out. For example, if you run a simulation on the scale of about a billion light years, you would expect galaxies to be distributed like so. (Image credit: Martin White.)
When you compare your simulation with what we actually observe, you get an idea of whether your theory makes sense or not. Here’s a map of the observed galaxy distribution from the two-degree-field galaxy redshift survey. (Image credit: 2dFGRS team.)
The agreement is spectacular! Well, what if we go down to a smaller scale, like say, down to the size of an individual galaxy? You have a pretty good idea of what a typical galaxy looks like; after all, we’re in one!
What our simulations tell us is that — pretty universally — all galaxies should be surrounded by a huge, spherical halo of dark matter. How huge? It should extend out around a million light-years for a galaxy like ours. That is tremendous, considering that our galaxy is “only” about 50,000 light-years in radius.
What’s more than that is that our simulations tell us what the density of this dark matter halo should be like. Towards the edges, it should be very diffuse, but the density increases very quickly as you come in towards the disk of the galaxy, and then increases more slowly until you reach the center. And that’s what our simulations tell us.
Not so fast, though. Does this make sense? A new paper out this week says perhaps not. You see, at the center of each galaxy (including our own!) lives a supermassive black hole, millions to billions of times as massive as our Sun. And, like all black holes, when matter falls into it (whether it’s normal matter or dark matter), it cannot escape, and causes the black hole to grow larger and more massive.
But this doesn’t happen! So something is amiss. What gives? Do black holes simply not absorb dark matter? Improbable!
What it probably means is that something more complicated than our simulations are telling us is happening in the inner parts of these dark matter halos. We’ve had many clues (such as the rotation curves of spiral galaxies) that dark matter simulations get us close, but don’t get it quite right.
This new paper appears to point toward the same idea: our simulations of dark matter get us close to the right answer most of the time, but there needs to be more to the story. It makes sense that what you’d ask next is what’s actually going on. For example, how do these dark matter halos actually achieve a lower density in the center than we predict?
If you’re wondering that, I congratulate you! You are now where I am: on the cutting edge of cosmology, trying to answer one of the open questions we have today. My best guess is that there’s a process we haven’t yet discovered where dark matter does interact by some means other than through its gravity; that could explain both of these discrepancies, and is a property predicted by many dark matter models. (The scientific term would be that the dark matter halo becomes isothermal.)
Amazing, isn’t it? That looking at the most massive black holes in the Universe gives us clues to the nature of dark matter, but here we are! Any ideas on your end?