Of course this is way above my pay grade.
> For example, perhaps the strongest constraints on primordial black holes come from microlensing searches [...] In these efforts, astronomers monitor bright but distant sources, waiting to see if a dark object passes in front of them. These searches have long ruled out an evenly dispersed population of small black holes.
> But if primordial black holes exist at a range of masses, and if they’re packed into dense, massive clusters, those results could be less significant than researchers thought, García-Bellido said.
I heard about this most recently on Sean Carroll’s podcast:
What would they have been made up? Do quarks have mass, and there was enough of them? Or does energy through E=MC^2 mean that enough pure energy density can cause a black hole?
I'm not sure exactly when primordial blackholes would have formed but I think it would have been sometime after inflation, though perhaps before baryogenesis. Here's the reasoning: I think it would be after the GUT epoch so gravity would have splintered off from the other forces and I think it would have been after inflation started so that inflation had a chance to exaggerate the scale of quantum fluctuations to create the necessary size & scale of density fluctuations for blackholes to form.
Energy alone is enough to form a blackhole. However, it's not the energy density per se. That's sort of a necessary but not sufficient condition since you also need the surrounding spacetime to be at a low enough density relative to the region where you are expecting a blackhole to form.
This, incidentally is a common source of confusion about the big bang. "Why didn't it just form a blackhole?" The answer is because ALL of the spacetime was at the same density.
I could say that a small sphere is denser than a large sphere because the distances between points are smaller on average, but that requires the spheres to be embedded in something with a distance metric(Euclidean 3-dimensional space).
What is the thing that spacetime is embedded in that provides a pointwise distance metric?
A technical explanation of the expansion involves the inflaton field.
(There's also energy density of the gravitational field.)
In other words, the number of concentrated masses intercepting rays of light from distant background sources would be given by the number of BH clusters as opposed to the number of BHs.
The objects we're looking for are large masses in the space between us and the distant light source. Typically the light source would be in the LMC or in our own galactic center, and the BHs would be, for example, in our own galactic halo.
We wouldn't expect to be finding BHs in a distant galaxy by microlensing.
I'm describing (or trying to describe!) what seems to be the astrophysics consensus. Sean Carroll is a Caltech astrophysicist who was interviewing another astrophysicist, and my quote from the article gives the explanation from a third. Am I getting this wrong?
And now I wonder the same as holler: firstly, can you differentiate a black hole made of matter from one made of antimatter? The resulting energy couldn't leave it, AFAIK?
Secondly, if there is any difference, what would happen to Earth in case of a collision?
I'm not an expert, but isn't that because we don't observe gamma rays from matter-antimatter annihilation in between the galaxies?
Not as far as I'm aware. A black hole is defined solely by its mass, charge and spin. What goes into it becomes irrelevant once it crosses the event horizon, except for those three factors.
(Though one does wonder, what happens if an object with negative mass enters a blackhole? But negative mass may not even exist.)
> Secondly, if there is any difference, what would happen to Earth in case of a collision?
None in practical terms.
But I'm just guessing here.
Correct, the photons produced from annihilation should match the inputs.
To decrease you'd need to add negative mass or negative energy, which aren't things we've found to exist yet.
We don't actually know _what_ will happen in the singularity of a black hole, to answer the question of if annihilation will happen there. As far as I know we don't have any model of what happens in a singularity.
Perhaps they are so point-like that they can have sufficient density averaged out to account for the observed gravitational effects but a small enough cross-section that they contribute negligibly to light absorption?
"Hawking estimated that any black hole formed in the early universe with a mass of less than approximately 10^15 g would have evaporated completely by the present day."
> The original idea dates back to the 1970s with the work of Stephen Hawking and Bernard Carr. Hawking and Carr reasoned that in the universe’s first fractions of a second, small fluctuations in its density could have endowed lucky — or unlucky — regions with too much mass. Each of these regions would collapse into a black hole. The size of the black hole would be dictated by the region’s horizon — the parcel of space around any point reachable at the speed of light. Any matter within the horizon would feel the black hole’s gravity and fall in. Hawking’s rough calculations showed that if the black holes were bigger than small asteroids, they could plausibly still be lurking in the universe today.
This theory proposes the black holes would be big enough to still be around.
Or would black holes of smaller size not be able to capture enough matter to sustain themselves?
The Schwarzschild radius of Vesta is 396.1 nm ; which means it a black hole with that mass would intersect 6.281 cm^3  of material if it went through the center of the Earth.
I have no idea why it might look like a cosmic ray. The surface gravity would be absolutely insane, so I would naïvely expect something like this look like an earthquake. 
It would leave a trail of wreckage behind it that, on a micro-scale, would look somewhat similar to that of a super-high-energy cosmic ray. A hole that large is large enough to swallow large numbers of atoms, and wrench the remaining ones out of position; that would look not too different from the damage you'd get if you sat in the way of a particle accelerator, at least very close to the path of the hole.
I'm not sure what would happen to you if one fell straight through you, but I suspect you'd be fine. (Unlike the particle accelerator, there wouldn't be much secondary radiation.)
Don't quote me on that, though. And don't try it at home, either.
If anyone out there does, in fact, manage to try this at home, please send a letter about it to the following address:
P.O. Box 5232, SE-102 45 Stockholm, Sweden
Street address: Sturegatan 14, Stockholm
"Bugorski understood the severity of what had happened, but continued working on the malfunctioning equipment, and initially opted not to tell anyone what had happened."
"The left half of Bugorski's face swelled up beyond recognition and, over the next several days, the skin started to peel, revealing the path that the proton beam (moving near the speed of light) had burned through parts of his face, his bone and the brain tissue underneath. As it was believed that he had received far in excess of a fatal dose of radiation, Bugorski was taken to a clinic in Moscow where the doctors could observe his expected demise. However, Bugorski survived, completed his PhD, and continued working as a particle physicist. There was virtually no damage to his intellectual capacity, but the fatigue of mental work increased markedly. Bugorski completely lost hearing in the left ear, replaced by a form of tinnitus. The left half of his face was paralyzed due to the destruction of nerves. He was able to function well, excepting occasional complex partial seizures and rare tonic-clonic seizures."
Unlike a black hole falling in from deep space, one made on the surface of the Earth will definitely remain inside the planet, making one pass through the core every 42 minutes.
It's a good schema to get all the baggage of Earth out of the novel.
He just says, whoops, earth got eaten, don't worry about Yosemite anymore, now here are some space monks that live in huge bubbled space trees.
Much more fun than having to explain things minutely.
However, the gravitational pull of an asteroid would not be negligible, so I guess it would have a big impact pulling things towards it.
shows a black hole of mass 1×10^29 kg (which is about 1/20 of one solar mass) has a radius of 148.5 meters which is asteroid size.
says at one earth's radius away that it has an escape velocity of 1447.5 km/s (kilometers per second), so yeah I think that will either destroy part of Earth or completely consume it, depending on how fast it's passing though.
For OP's question, whether it's detectable or not, it possibly is if it warps the light from background stars (same effect as https://en.wikipedia.org/wiki/Eddington_experiment).
First, they are big enough to be stable, they don't explosively evaporate in an instant. Second, they are much, much smaller than the large one on the video.
For the most part a dark matter particle attracted to another particle will just convert all the kinetic energy it gained during the attraction back to potential energy as it whizzes past and away from the other particle.
Normal matter particles can radiate away photons when they hit each other, ie friction, leading to a non-conservative interaction. This allows them to shed their energy and clump in a way dark matter can't.
There is also the possibility of dark matter only forces and fields that don't affect normal matter, but allow dark matter to self interact to some extent.
Of course General Relativity is not the final answer, we know that. But that does not mean we know nothing about gravity, or that we somehow can't use our current theory to make predictions.
Dark matter as a concept assumes General Relativity is good enough, and takes it as-is. Without modifying gravity we need some other way to explain the discrepancies like galactic rotation curves, and so for dark matter we add in some new, so-far unseen, particles. We also know they have to interact very weakly with normal matter besides gravity, otherwise we'd would already have noticed them in existing observations and experiments. Based on this we can make predictions.
What would happen if we have some matter that only interacts gravitationally buzzing around?
Well as mentioned in my previous post such matter can't cool down and clump like normal matter, so instead it would be spread out in diffuse blobs around galaxies. And when we model galaxies with such dark matter halos we find that indeed, that might explain galaxy rotation curves. It also makes other predictions which also seem to match other observations. So overall dark matter looks like a pretty good candidate.
The alternative of course is to assume there aren't dark matter particles, but rather that GR has to be replaced or modified in some non-trivial way. There are many people working on that.
However they're so far struggling, because it turns out to be very difficult to change or replace GR without failing to match existing observations. So that seems to indicate that GR is a pretty darn good theory of gravity.
I don't know about the idea of turbulence but modified gravity is serious science.
It is just not the preferred explanation right now. An important reason is that AFAIK, none of these theories are currently able to match the observations without the dark matter they intend to eliminate.
Two galaxies collide. The dust stops, the dark matter keeps going... it isn't looking good for MOND.
I'm no physicist, theory was developed while doing Einstein inspired imagination visualization/simulation of the big bang. I know it's probably so wrong in so many ways it's not worth a normal physicist to respond to, just thought I'd share.
If I were correct, which I'm probably not, it would mean there are no "primordial black holes" that existed since the moment of the big bang, but rather as time went on the black holes were created and increase in number (also being part of what causes this universe to expand)
If you want things to be received as a thought experiment, call it that up front. You'll get people thinking and talking about it. But the response you received is in line with what the word "theory" means in the scientific community.
Not sure how much more clear it can be... If the single word "theory" misapplied can't be seen through by such intelligences well what else is there to say than I'm sorry and I'll never mention it again. I'll stick to the theories I'm good at.
Hope was that, dark matter puzzle may bring new physics . Primordial black holes will be interesting to watch.
In fact most dark matter models do assume some form of extremely weak interactions with normal matter or dark matter decay, which is how we try to detect dark matter. The effective flux from such interactions is a lot larger than Hawking radiation would be.
Yes, "dark matter" is a kind of catch all term for a bunch of phenomenon / experimental measurements we don't properly understand. There's a large number of different models trying to describe the seen behavior and make predictions to be able to detect it. And in most of those models, dark matter is indeed matter!
Apart from that the only way it interacts with anything else is via gravity...
Not all that likely, I know, but good for an SF story.
"Dark" is descriptive, but it requires that there actually be relatively normal matter involved, which is going forward an unwarranted step. And then it gets reused for "dark energy", which is... what, energy you can't see or interacts weakly with existing sensors? That's not novel or even unusual. The phrase is bogus. "Error energy" would work.
> He wrote the 1991 book The Big Bang Never Happened, which advocates Hannes Alfvén's plasma cosmology instead of the Big Bang theory. He is founder, president, and chief scientist of Lawrenceville Plasma Physics, Inc.
he couldn't grok the Math and so he took to writing and scientific-populism.