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From black holes to dark matter, an astrophysicist explains

Katherine Mack, astrophysicist at the University of Melbourne, answered questions posed by the public on Reddit. The Conversation has curated the highlights.

Dark Matter

How do you explain dark matter to kids?

I tell them that when they touch the table, what they are feeling is the electromagnetic repulsion (or, you know, I use some less jargony way of saying that) between that surface and their hand, and that is what makes it feel solid and what keeps you from passing right through it. Dark matter doesn’t seem to have that force. It has gravity, but it doesn’t do electromagnetic repulsion as far as we can tell. So if dark matter were in the room (which it probably is), it would pass through you unnoticed. And we know dark matter is out there because of the way it moves things around in galaxies and clusters of galaxies, and how its gravity bends light.

Do we have evidence for dark matter?

The evidence that dark matter is a real component of the Universe – not an alteration of gravity – is pretty overwhelming at this point. We see evidence for dark matter in so many places now (motions of galaxies, motions within galaxies, hot gas in clusters, strong and weak gravitational lensing, gravitational microlensing, cosmic microwave background anisotropies, the matter distribution on large scales, and the collisions of galaxy clusters, for instance), it is really impossible to ignore. Attempts to just change how gravity works to fit the data don’t work when you take all of these observations together. There are places on small scales (like with dwarf galaxies) where there seem to be difficulties bringing the simplest dark matter theories into line with what we see in observations, but even those are not always compelling anomalies.

What progress has been made identifying dark matter?

Short answer: it is all a bit of a mess at the moment. There are a few dark-matter-detection experiments giving us different answers, so it is really hard to say. But there are a few things we are pretty sure we do know about dark matter: it is fairly cold (non-relativistic in its motions), it is probably some kind of fundamental particle (though there are certain models of very low-mass primordial black holes that aren’t yet ruled out), and it doesn’t seem to have significant non-gravitational interactions (in the sense that its only major, easily detectable, interactions with itself or anything else are via gravity).

There have been lots of really interesting hints lately of possible signals of dark matter’s particle physics effects in astrophysical observations, but it will probably be a few years yet before it all gets sorted out.

Will the LHC be able to assist in the search for dark matter? Why or why not?

The theory is that it may be possible to create dark-matter particles by colliding standard-model particles (in this case, protons) at high enough energy. For instance, if dark matter has the ability to annihilate into standard-model particles, something like the reverse of that process would work to make dark matter. Then you would look in the LHC detectors for all the detritus from the collision, and there would be something missing, as the dark-matter particles you just made would escape the detector without leaving a trace. So far we haven’t seen anything that looks like that.

Is it possible that dark matter is just as complex as the matter we interact with? As in, could there be many different particles, and their own types of forces? Could there be whole galaxies of dark matter, with planets and life?

There is no reason there can’t be a whole zoo of particles in the dark sector, all of which contribute to the total mass that we see as dark matter. But there are some strong constraints based on our observations on how much non-gravitational interaction there can be between these particles, or between dark-matter particles and standard-model particles.

There are models of self-interacting dark matter in which dark-matter particles can exert significant non-gravitational forces on each other, and sometimes these models are proposed to explain certain discrepancies between observations and the simpler dark matter models, but there’s really no compelling evidence (from what I’ve seen) that these self-interactions are really happening. Similarly, there are models of “atomic dark matter” in which dark matter can form “dark atoms,” but the limits on that are really strong, because if dark matter can form bound particles, then it can dissipate energy like regular matter can and we would see it form disks and collapse in ways we just don’t.

Based on our observations, dark matter appears to stay pretty puffy – it doesn’t do a lot of angular momentum exchange between its particles, so instead of making disks and compact objects, it makes blobby “halo” (spherical-ish) shapes or filaments, all just with slow gravitational collapse. It can’t easily condense down, so it can’t make things like planets or galaxies. (Or life.)

Black holes and the rest

Could dark matter interact with a black hole?

Yes, dark matter has mass, and black holes create gravitational wells and so they can pull in mass. But it’s hard to get a black hole to eat dark matter because dark matter doesn’t like to lose angular momentum, so you’d have to get the dark matter to be falling into the black hole pretty directly to get it to be swallowed.

What are your thoughts on the multiverse theory?

There are a few multiverse ideas that are theoretically plausible, but at the moment we don’t have any experimental evidence to suggest there are other universes (or universe-bubbles, whatever you want to call them) out there. There are some ideas to search for evidence of a multiverse.

Do you think anti-matter could ever be used as a source of energy?

Yes, but you’d have to find a way to contain it. This is really hard, because it can’t ever touch the walls of your container, which means you probably have to make some kind of magnetic bottle, and those are notoriously difficult to set up. It is likely that you would have to have some kind of mechanism that is continually creating your antimatter, and then you would probably have trouble getting it to be a process that gives you more energy than you get out.

Do black holes have any potential practical uses, or are they just a cool thing that exists?

When I was a kid I thought a little black hole might be a cool thing to use to get the light from your headlights to bend around corners when you’re driving. But of course if the black hole were small enough to give you the right kind of gravitational lensing, it would probably just shoot through the Earth at close to the speed of light and be lost forever. And if it were big, it would destroy the car.

There have been some ideas thrown around about using rotating black holes as energy sources (see Penrose Process), but I think it would be hard to make that work efficiently while not being ridiculously difficult to set up. Supermassive black holes in the centres of galaxies are often fantastic as distant beacons through the universe that let us see out to billions of light years away. Those are called quasars, and in that case it is technically the stuff falling into the black hole, not the black hole itself, that you are seeing.

Anyway black holes are immensely practical if you are a physicist or an astronomer because they let you learn about the shape of the Universe and the nature of spacetime. But they’re not going to do much to improve your car.

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