What Gravitational Waves Can Say About Dark Matter
symmetrymagazine.org
symmetrymagazine.org
If dark matter were to interact with itself (as regular matter does), wouldn't we expect it to clump together and form the same structures as regular matter does, especially given that there's supposed to be much more dark matter to interact together than the regular stuff? AIUI one of the defining features of dark matter is that it doesn't clump together.
[1] https://en.wikipedia.org/wiki/Navarro%E2%80%93Frenk%E2%80%93...
Like a gravity Kugelblitz.
Sorry if this is a bit naive and tangential, but I've always stumbled at the thought of how does gravity-information about the interior of a black hole propagate out of the event horizon? ...Gravitons/gravity waves travel at, c?
Instead the black hole has mass, charge, and momentum (three kinds of momentum actually, but that’s not important). Whatever is going on beyond the event horizon, whatever that might be, has no effect that anyone can detect. Matter is accreted “onto” the event horizon which then expands in proportion to the mass of the volume of the hole. Maybe it’s destroyed beyond that point, or maybe it goes to another universe, but we can never know. The event horizon can also shrink if the surroundings are sufficiently cold (really really really cold) and the horizon is sufficiently hot.
Still, all of this is surface phenomena, like dropping a bowling ball into a tub of water. The water only “knows” about the surface of the ball, which which gets properties from the whole ball without exposing the center. A bowling ball in water creates waves, but the interior isn’t interacting with the water anymore than the black hole interior interacts with space (assuming an idealized perfectly rigid bowling ball). In the same way gravity waves or fractions would be a function of how the space just beyond the event horizon is warped.
Does that help?
I find this concept of a black hole's surface having the contents inscribed on it to be really difficult for me to imagine; but I can take people's word for it, and it does explain away the paradox.
This reminds me that from some vantage points the universe seems so arbitrary sometimes. This is probably just my human intuition, but personally it just appears to me that nature is not really always elegant, but rather has these work-arounds and different layers to it to keep it working. Not unlike my terrible code.
- The event horizon is a two-dimensional sphere and, being two-dimensional, has zero mass and cannot exert any gravitational force.
- The black hole within the horizon is a three-dimensional massy object and can and does exert a lot of gravitational force.
- Assume at equilibrium our black hole is somehow exerting gravitational forces on its surroundings which are what you would predict if you accurately knew the black hole's actual mass.
- Assume the black hole moves, e.g. because of inertia.
- Now it should be exerting more force than previously on one half of the universe (the half it moved toward), and less force on the other half.
- Say it moved toward you. After a speed-of-light delay, you should actually perceive more force on yourself towards the black hole. But this can't be because a messenger particle was transmitted from the black hole to you. How can it be?
Assuming this shows that black holes cannot move seems unsatisfactory, given the recession of galaxies from one another, observations believed to show black holes colliding, etc. Where are my mistakes?
Followup: one black hole collides with another black hole of roughly ten times its size. Is it necessarily the case that the center of mass of the new, combined black hole ends up at the point that was the center of mass of the small-hole/big-hole system just as the small hole crossed the big hole's event horizon?
Stop right there. A two dimensional surface can have mass if it has infinite density. And infinite density makes as much sense as any other sort of singularity...
I have some naive questions too. This is basically just me rephrasing the question I understood wallace_f to be asking: ... - The black hole within the horizon is a three-dimensional massy object and can and does exert a lot of gravitational force.
The black hole includes the event horizon, which marks the point at which we stop knowing anything or have theories to predict anything. We really have no idea what’s beyond the event horizon, and almost anything you can imagine has been conjectured as being there, from firewalls to elder gods. We’re talking about a region which can’t be properly described by he theories we have, where the manifold ceases to well behaved. Everything beyond the event horizon is causally disjoint with the rest of the universe, and may as well not exist for anything that isn’t falling past the event horizon.
- Assume at equilibrium our black hole is somehow exerting gravitational forces on its surroundings which are what you would predict if you accurately knew the black hole's actual mass.
Right, mass is one of the “hairs” a black hole has along with charge and momentum.
- Assume the black hole moves, e.g. because of inertia. - Now it should be exerting more force than previously on one half of the universe (the half it moved toward), and less force on the other half.
It sort of does, this is the basis of frame dragging when the hole is spinning. The hole warps spacetime around it, dragging reference frames in the direction of its motion.
- Say it moved toward you. After a speed-of-light delay, you should actually perceive more force on yourself towards the black hole. But this can't be because a messenger particle was transmitted from the black hole to you. How can it be? Assuming this shows that black holes cannot move seems unsatisfactory, given the recession of galaxies from one another, observations believed to show black holes colliding, etc. Where are my mistakes?
The theory of gravity we actually have doesn’t involve bosons, it’s a geometric theory describing a continuous manifold. How that squares with theories containing gravitons is well above my pay grade, sorry. The classical theory says that the warping of spacetime is continuous, and so the hole moves like something being dragged through water, including a wake and bow wave. Since the hole can’t move at c, being massive, there is always an acceptable delay for the light-speed propagation of disturbsnces in spacetime to reach you first.
Followup: one black hole collides with another black hole of roughly ten times its size. Is it necessarily the case that the center of mass of the new, combined black hole ends up at the point that was the center of mass of the small-hole/big-hole system just as the small hole crossed the big hole's event horizon?
They end up merging like two legs of a pair of pants meeting at the crotch, with the new center of mass at the barycenter of the previous orbiting pair. I loved these questions by the way, I can tell you put some real thought into them.
Lisa Randall has written a provocative book about this which will answer your question in depth: Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe.
Is it generally accepted that the large-scale structure of the universe (presumably including dark matter) reflects quantum-scale structure before inflation?
"Dark matter seems to interact with normal matter only through gravity, but, based on the way known particles interact, theorists think it’s possible that dark matter might also interact with itself. "