Is dark matter made of primordial black holes?
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I recently went to a talk from the author of https://arxiv.org/abs/1709.06576, which claims that PBH's can't make up the majority of the dark matter because the merger rate would be much higher than what LIGO observes.
Try these lectures on for size:
[0]: linked directly to primordial black holes
https://youtu.be/vPHfvMQd73A?t=31m39s
[1]: previous lecture
What the authors are suggesting is that this "dark matter" effect is actually the result of many small black holes orbiting galaxies. That they aren't exotic particles, just something we already know about in an unexpected place.
Is the unexpected element of this that it is weird to think that very small black holes interact with each other? What does it mean that they don't collapse into each other and rather stay individual?
So objects usually need to be captured into mutual orbit first via 3-body interactions and then slowly move closer to each other via further perturbations or gravitational waves.
I think it has to do with the number of black holes that would be required, the rate that black holes evaporate naturally(?), and our current best theories of the history and mechanisms of the universe. Like maybe in a universe where all these black holes form, we wouldn't expect to see [something we definitely do see]. So you need to adjust more variables to make that fit the puzzle, but now more things we do see don't make sense.
It's a bit like a big Sudoku, but with way more constraints!
You’re talking billions of billions of billions of years before most of them even start shrinking.
We assume that our gravitational model is correct and that an additional item is needed to bring the model into agreement with the observations made. When a model seems to work so well, we are generally unable to go back and look at the basis (and assumptions) on which our theories are made. This applies in all areas of our lives, including all of our science. That is the nature of who people are.
The odd part about Star Wars is the relative weakness of their weapons. Star Trek have "warp core breaches in a bottle" AKA the photon torpedo, which is much better at damaging starships than ramming.
According to https://what-if.xkcd.com/129/ a moon-mass black hole would be as big as a grain of sand.
I don't know how far out LIGO can detect small-mass black-hole mergers, nor how likely these mergers would be. Nor do I know how detectable micro-lensing events would be.
But as far as we know there are no "anti stars" out there that could collapse and form a black hole. We haven't observed any at least (don't ask me how we could tell an antimatter star from a regular one - but we could tell if one met a regular one).
edit: someone pointed out the gas in the galactic halo is not dense enough to form accretion disks around PHBs, so, no. We wouldn't see anything glowing.
The black hole as dark matter theory is also largely discredited (MACHO theory for MAssive Compact Halo Object) because gravitational lensing studies don’t support it.
While you can argue they are still flawed, they do clearly show that the electrons have fairly large probabilities of being observed pretty much anywhere inside the "radius" of the atom.
At this point, arguing about "are atoms mostly empty space" becomes a futile semantic argument. The model you picked in order to defend this statement is indeed useful in some limits, but it has been proven wrong. Imagining electrons as clouds has proven to be much closer to reality.
I have to concede, though, that "empty" is not a particularly meaningful physical concept, quite the opposite. Disregarding the Aether theory, only to replace it with fields of potentials, that is kind of running in circles.
A model is fundamentally wrong. That's what it means to have a model. At that "clouds" is no more helpful then "empty". The fact of the matter is that no one has ever seen these things, no nucleus, no electrons, as far as I know.
Yet, you haven't pointed out what was actually wrong with "Those are simply the electron orbitals which describe the probability of finding the respective election at a given point".
Edit: Perhaps you intended to imply that this description is overly idealized (not to say simple).
If you have too much external pressure (gravity), those smeared electron clouds come too close to the nucleus, react with it, and the only thing left is neutrons.
As a non-astrophysicist, just scanning the wikipedia entry [1] it appears to me that gravitational microlensing studies must be fiendishly difficult, you probably need massive amounts of data and of compute power to get to highly confident conclusions. Is my impression accurate?
If that is so, than the MACHO theory should not be discredited that quickly. It is after all the only theory that does not require any new physics in order to explain the missing mass.
[1] https://en.m.wikipedia.org/wiki/Gravitational_microlensing
No gamma ray signals as hoped/expected from evaporating PBH’s.
LIGO is not seeing PBH mergers at a rate that would support the numbers required.
Micro lensing studies don’t support it.
Is it’s possible, but it’s much less likely than other theories which allow for less fine tuning of parameters.
There’s a very good graphic that shows what I’m talking about. https://astrobites.org/wp-content/uploads/2017/08/constraint...
From: https://astrobites.org/2017/08/31/could-dark-matter-be-black...
As such to say that the electron is "smeared" around the atom is not saying anything meaningful about the electron. All we can say is that we don't know where it is. To attribute a dispersal property to the electron (or any other particle for that matter) is not doing anything helpful to discover or understand what an electron is.
It's the direct result of the Geiger–Marsden experiments.
That leaves a rather small window in between were black holes as dark matter works, but nobody has postulated a plausible mechanism to only generate black holes in a narrow mass range, so the idea can be made to work, but fell out of fashion because the wimp miracle looked theoretically more attractive.
The wave function may be smeared out over space, but that does not mean that there is an electron everywhere. Electrons are never at two places, never has anyone looked and found the same electron at two different places at the same time.
What happens if nobody looks is up for debate, whether you think the electron has no position, whether you think it is guided along its path by a pilot wave, whether you think it split into zillions of copies in different universes, or whatever.
But the electron or many electrons filling out its entire orbit is not compatible with any of that or any experimental evidence. At least as far as I know, I am not a physicist.
On the contrary, imagining the electron as a smeared cloud (a complex-valued wave function with a bunch of specific properties), is pretty much the only way to explain chemical bonds.
The confusion stems from semantics. Sure, nobody claims that an electron has been detected at two spots simultaneously, not because it does not happen, rather because to say that you need the premise that the electron is a point particle with a position.
Sure, it is useful to claim that the electron is a point particle - in plenty of circumstances it does look like one. But modern physics pretty conclusively has demonstrated that it is actually something described by a wave function, that is most certainly smeared over space.
I am a physicist, so I will leave the discussion of whether "smeared" is a good word for it to the philosophers and linguists. It is a good enough word for me, when I try to derive properties of atoms.
Sure, but the crucial points is, quantum fields are just mathematical tools, the universe is filled with particles, not with quantum fields stretching across all of spacetime. The fact that it is under some circumstances convenient to mathematically treat a large collection of particles as a quantum field does not make that quantum field a real thing, just as describing the traffic flow on roads with fluid dynamics as a mathematical modeling tool does not mean there actually exists a car field spanning across all the roads.
[...] yet others are happy to just shut up and use the model to do calculation that just work.
This certainly seems to works in many circumstances, but I think it is not really good enough to treat the point raised in this thread. When you are discussing whether an atom is mostly empty space, you have to be more careful. Just because your mathematical model uses objects that fill the entire atom does not necessarily mean that this is true for the actual atom, it could just be an artifact of your mathematical model.
However, please also accept that for many people (me included) the model of quantum fields is much more real, and the model of point particles is the one that is just a mathematical illusion.
We invented quantum fields to make locality obvious but this is not possible without making the description redundant and so we just declared that one physical state corresponds to an entire equivalence class of mathematical states.
So I really don't think this is a matter of philosophy, they are not equivalent models.
[0] https://www.sciencenews.org/article/dozen-new-black-holes-fo...
Even if they remained "dark" - not emitting any light of their own - they would block or otherwise interfere with the light coming from everything behind them.
The reason PBHs can be a dark matter candidate is because they can compress enormous amounts of matter into a very small space, and are difficult to detect when they're not interacting with ordinary matter.