What if there's no dark matter?

Of course there’s interaction between dark matter and regular matter, if there were not such interactions then we wouldn’t have any reason to suspect there was any such thing as dark matter. The interaction we’re observing is gravity. Maybe there are other interactions, but we don’t know anything about else about dark matter except that there’s some sort of extra mass in galaxies that hasn’t been detected in any way except gravitationally. It might be that the gravitational observations aren’t really detecting dark matter, but some other effect, but if so that’s a giant can of worms, so the most likely explanation is some sort of particles that have mass but don’t interact electromagnetically. We know there are particles that don’t interact electromagnetically, so it isn’t like finding neutrinos with mass will break physics.

Well, yeah BESIDES gravity. I thought that part would be obvious.

Neutrinos oscillate, so we’re pretty sure they have mass.

I’m pretty sure that he meant “with significant mass”.

Though one of the proposed dark matter candidates, the axion, is probably considerably lighter than neutrinos. It’s just that there are purported to be a heck of a lot more of them.

Actually there were a couple of observations made this year that implied dark matter interacting with itself with another force than just gravity.

I think I’m right in saying this appears to be a weak force acting over a large distance.
What I wonder is whether we have enough data now to rule out more short distance and powerful interactions?

Why does Dark Matter form halos around galaxies? If its not effected itself by gravity then shouldn’t it just shoot around randomly? If it is effected by gravity why isn’t it pulled closer to the center of galaxies or into the central black hole that most galaxies have?

It is affected by gravity: That’s the whole point. And it is pulled towards the center: That’s exactly what gravity is. It doesn’t reach the center for the same reason we don’t, because it’s in orbit.

Ok maybe my question wasn’t clear. Why is most of the dark matter in a halo orbiting galaxies in such a convenient way that it explain the discrepancies we see in spiral rotation? Why isn’t it distributed more uniformly in various orbits closer in (which would tend to cancel out its effects if it was more uniform as I understand it).

Dark matter forms halos around galaxies because it’s a bunch of particles that mostly don’t interact with other matter through anything other than gravitation. It doesn’t stay in the center of the galaxy because it passes right through any dust or solid objects that would slow regular matter down without interacting with it, so there’s nothing to make it clump up or get stuck. Some tiny fraction of it would have hit the central black hole directly, but black holes, even supermassive black holes, are tiny compared to the galaxy, so the vast majority of it just orbits. The halo is not a ring like an angel’s halo, it’s a big basically spherical blob that’s denser closer to the middle, and generally uniform in a single galaxy. (Galactic collisions can alter the shape).

On your second post, being uniform doesn’t cancel it’s effects, and having more density towards the center of the galaxy is what’s needed to explain the faster rotation of the outer stars of spiral galaxies. I’m not sure your ‘why’ question makes sense; things exist in a way that ‘conveniently’ fits what we observe because that’s how we form models of them in the first place. There’s no mysterious structure to dark matter, the distribution is what we expect from something with mass that doesn’t interact with EM starting from a uniform distribution and evolving along with galaxies and other structures. Basically scientists look at how what we can see moves, note that it doesn’t move like we expect but seems to have a bunch of mass that we can’t see, then work out what that mass is and how it behaves.

So think about what would happen if you had a fairly uniform cloud of ordinary matter. If there are any regions of the cloud that are denser than the others, gravity will pull them together into clumps. The particles bang into each other but don’t bounce off perfectly, they lose momentum and start to stick together. And so you have the formation of stars in this way. A hydrogen atom falling into a mass concentration gets slowed down by hitting the other hydrogen atoms, until you have a giant dense cloud of hydrogen.

But now suppose the matter isn’t ordinary matter, and doesn’t interact electromagnetically, it’s “dark matter”. So it’s attracted gravitationally to a slight concentration of mass. It falls towards the concentration. What happens next? It falls in and then swoops right back out. It doesn’t get stuck in the clump like baryonic matter because it passes right through baryonic matter. Maybe dark matter would have subtle interactions with itself, but we don’t really know yet. So the dark matter particles orbit around the mass concentrations, but don’t clump. This is why they have “halos” rather than forming dark matter concentrations. And while there might be a small concentration of dark matter around stars, most dark matter is not bound to a particular star.

And if you think about how orbits work this makes sense. Suppose a particular dark matter particle starts to fall towards a star. It falls faster and faster and faster. Then it reaches perihelion and starts falling back out. If it was in interstellar space and falling from effective infinity, it’s going to reach escape velocity from the star and fall back out into interstellar space. It will have a hyperbolic orbit, not an elliptical orbit.

Normal matter is much more likely to get trapped in the solar system because it interacts with and is slowed by other particles. If a falling particle smacks into a planet, it gets stuck on the planet. If it gets close to the star it hits the star’s electromagnetic field and could get captured. Dark matter wouldn’t interact this way, it doesn’t get affected by the friction of the relatively dense (compared to interstellar space) cloud of matter that is a solar system.

Good post Lemur.

But doesn’t the fact that there IS some concentration of dark matter around regular matter imply some sort of non gravitational interaction? Though it might be pretty damn weak.

Not necessarily. You can get captures gravitationally, too, it’s just a lot harder. The process is sometimes called “dynamical friction”, but it’s really not fundamentally any different from ordinary drag, just based on a different force.

It makes sense that a solar system could capture extra dark matter. A particle falling towards the sun gets close to Jupiter, gets a slingshot and happens to end up in an elliptical orbit.

And the galaxy as a whole is the same way. And it seems that most of the dark matter we know about is captured in galaxies. But it doesn’t clump into stars and nebula like ordinary matter does, it’s a lot more thinly and evenly distributed in the galaxy. The dark matter in galaxies was very likely captured gravitationally in the way described above, just on a vaster scale. Galaxies tend to collect dark matter from the intergalactic background. Just as interstellar space is a lot more crowded with regular matter than intergalactic space, it is a lot more crowded with dark matter. Just that solar systems don’t seem to have a lot of captured dark matter compared to interstellar space.

You may enjoy the story of the Magic/More Magic switch.

I think the term “halo” is pretty misleading and gets folks thinking in unhelpful directions. To me a “halo” in 3-space is a hollow shell surrounding and wholly external to whatever the object of interest is. The Oort cloud is an example of a halo from the POV of the 8-planet Solar system.

Dark matter isn’t arranged like that. So “halo” is a crappy word to describe how it is arranged.

In essence the dark matter is a very diffuse “gas” throughout the universe. Powered only by gravity, it “clumps” ever-so-slightly into higher concentrations in high density areas like galaxies. And it clumps ever so slightly in smaller areas like star/planet systems. What it doesn’t do is stick to itself or stick to baryonic matter. So while the “gas” can form areas of higher gas density, it can’t form the equivalent of liquids or solids.

So far our tools aren’t good enough to see the minuscule concentration in our solar system. It may n fact be below the thermal noise floor and undetectable even in principle.

But we can detect it pretty readily as a galactic-scale concentration of gravitational potential. And “gravitational potential” is (with some small arm waving) another word for mass.


Well, I guess you could say that dark matter doesn’t talk much, but “discrete” is much more likely to be what Pantastic meant.


Well, you can’t see it or hear it, so…

Ok thanks, this cleared things up for me, the halo term gets used a lot and I agree its very misleading. But if dark matter is distributed all throughout the galaxy and its 75 percent of the mass in the universe then shouldn’t there be large amounts of it near our solar system? Eg shouldn’t there be observable influences on orbits in our solar system that can’t be explained by the normal matter mass we can see? Or at least observable in some motions of nearby stars not matching what we expect.

Why does this 75 percent of mass only effect things on a galaxy wide scale and not on smaller scales?