3 ms·

Why should it stay there? Imagine you had a sphere of particles with an initial velocity of 0 that interact only through gravity. At first, these particles woul

Why should it stay there? Imagine you had a sphere of particles with an initial velocity of 0 that interact only through gravity. At first, these particles would all accelerate towards the center of gravity. Once they reach the center of gravity, they would have accumulated some velocity, and so continue to travel away from the center of gravity (but still be accelerating towards it). Eventually, the acceleration towards the center of gravity would 0 out their velocity again, and they would start falling back towards it. The end result is that your original sphere oscillates indefinantly, but always returns to its original state. As your initial configuration becomes more random, this discernible time structure begins to fade, and you end up with a voluminous cloud. What allows regular matter to collapse into dense structures is that it interacts with itself to shed velocity.

Causality1 8y ago
So then why is its structure and location correlated with galaxies at all? Shouldn't it be relatively even throughout the universe?
- improbable22 8y ago
  I think this is roughly the question why are there galaxies (and clusters, and stars) at all? And the answer is that the primordial soup wasn't perfectly smooth, and gravitational attraction has the tendency to make clumpy things more clumpy. Precisely how un-smooth the soup was, and how clumpy galaxies are today, is quite a large part of the information we have about the history of the universe -- the CMB is a snapshot of the soup right when it came out of the kitchen. If dark matter was just like matter, then you'd expect it to go along for the ride, and exist now mostly in the same places (and at the same contrast) as normal matter. If it were something else, then its distribution could be much less clumpy, and I think this is a constraint on what kinds of things are plausible candidates.
tabtab 8y ago
I thought in practice it appears to form halos or donut-like formations. It doesn't seem to like the gravitational centers, doesn't like the far reaches of space, but instead is a kind of Goldilocks force that hovers on the outskirts of galaxies.
- gizmo686 8y ago
  I should probably qualify that I am not a physisist, but that also strikes me as expected. In theory, a dark matter particle is in orbit of the center of gravity [0]. However, an object is fastest when it is at the lowest point in its object. This would mean that no individual particle is hovering on the outskirts, but each particle spends most of its time there. As a result, at any given time, that is where the majority of the particles are. I have no idea how elliptical their orbits are, so it is also possible that their orbits just never take them near the center, in which case my initial condition of 0 velocity would be innacurate. (Which is good news unless we want to reinvent the Ether) [0] I practice, this is probably a nasty n-body problem as the mass is so diffuse, so "orbit" may be misleading here.

Causality1 8y ago

So then why is its structure and location correlated with galaxies at all? Shouldn't it be relatively even throughout the universe?

improbable22 8y ago
I think this is roughly the question why are there galaxies (and clusters, and stars) at all? And the answer is that the primordial soup wasn't perfectly smooth, and gravitational attraction has the tendency to make clumpy things more clumpy. Precisely how un-smooth the soup was, and how clumpy galaxies are today, is quite a large part of the information we have about the history of the universe -- the CMB is a snapshot of the soup right when it came out of the kitchen. If dark matter was just like matter, then you'd expect it to go along for the ride, and exist now mostly in the same places (and at the same contrast) as normal matter. If it were something else, then its distribution could be much less clumpy, and I think this is a constraint on what kinds of things are plausible candidates.

tabtab 8y ago

I thought in practice it appears to form halos or donut-like formations. It doesn't seem to like the gravitational centers, doesn't like the far reaches of space, but instead is a kind of Goldilocks force that hovers on the outskirts of galaxies.

gizmo686 8y ago
I should probably qualify that I am not a physisist, but that also strikes me as expected. In theory, a dark matter particle is in orbit of the center of gravity [0]. However, an object is fastest when it is at the lowest point in its object. This would mean that no individual particle is hovering on the outskirts, but each particle spends most of its time there. As a result, at any given time, that is where the majority of the particles are. I have no idea how elliptical their orbits are, so it is also possible that their orbits just never take them near the center, in which case my initial condition of 0 velocity would be innacurate. (Which is good news unless we want to reinvent the Ether) [0] I practice, this is probably a nasty n-body problem as the mass is so diffuse, so "orbit" may be misleading here.