I believe the community consensus (based on surveys) is that there aren't enough microlensing events to explain the amount of dark matter we see, using MACHOs.
I’d be skeptical of such a claim. We have very limited ability to detect dark black holes. The range of possible masses for said black holes is broad if primordial black holes exist.
And here's the relevant part. It's fair to say that Necib is not very bullish on BH's being the explanation for dark matter.
(SC = Sean Carroll of Caltech, LN = Lina Necib, then at Caltech, now on faculty at MIT)
SC: But it could be black holes, right?
LN: Yes it could be primordial black holes. There has been a lot of work, especially recently, on trying to understand what is the possibility or what is the parameter space so if the theory of primordial black holes explaining dark matter is still possible, there is some disagreement within the community of how much of it is ruled out. The theory is not completely ruled out but quite a few chunks of it are. It depends on the measurements and there are some very small windows for example. I think 15 to 30 solar masses is still possible, something like that so it is not a completely ruled out theory.
...
SC: [...] Black holes, you seem to be saying, if I can summarize it - if there is some mechanism for making black holes early on, they could be some of the dark matter but it’s hard to make them be all of it. Is that fair?
LN: That’s right, yes. It’s much more difficult to make them pure, to make them all completely the dark matter. It could be a small fraction, that has not been ruled out, yes.
A lot of these folks are working on the assumption that Hawking Radiation would have evaporated all small primordial black holes (the kind that could traverse the earth with little to no damage).
However Hawking Radiation is a testable prediction of a theory that has yet to be tested. I dislike that physicists tend not to speak of Hawking Radiation in such terms since it is not a fact.
In particular, I would argue that the Black Hole Information paradox isn't a paradox, it's an evidence against Hawking Radiation which in turn pushes back on quantum mechanics. Most of dark matter may very well be explained by small primordial black holes we're not (yet) well equipped to detect.
An observation, it seems a lot of the big theories in physics are mostly math and thin on observational data. I understand the necessity. It's pretty hard to observe a black hole and run tests on one after all.
It took us to the space age to start getting a lot of practical observational proof of relativity for example. Now its all confirmed relativity for now but for a lot of these other things we are sorely lacking. Hawking radiation is a good example of that as you pointed out. How do you even get observational data on hawking radiation without the starship Enterprise anyways?
Here's the thing about dark matter: basically everyone would rather have almost any answer other than undetectable WIMPs. They've tried. Very hard. And yet, here we are.
Here's the thing about science: reality doesn't actually owe us elegant or in any way aesthetic explanations of anything. We've gotten so many of them that we've started taking them for granted, started feeling entitled. But it's only a feeling.
> But the smart money is not on MACHOs, to say the least.
That might be because it's easier to dream up and get funding for crazy axion detection equipment ... and there's not much you can do to make more microlensing get observed.
I hate videos that instead of getting to the point give me all kinds of historical background nonsense. I just wanted to know about this new black hole, I don't need a lecture on something I already know. There are videos for that.
Oh god, followed by “welcome back! Before the break we learned about the various types of black holes and paradigms this discovery may turn on their heads. To recap…”
That seems basically impossible. Black holes are either so small (in diameter) that you'd have trouble hitting them if you tried or so massive they're going to noticeably throw off your navigation well before you reach the event horizon. Or both.
Looking at black holes with known mass ranges: the realistic mitigation strategy would be to just to leave the Solar System.
Even if a black hole missed the Earth, just flying through the Solar System would be enough to completely disrupt all planetary orbits.
Any mitigation strategy to alter the course of the black hole itself would require so much mass/energy to the point that we'd be making our own black holes for the purpose of space travel, so the Solar System wouldn't matter too much in the grand scheme of things anyway.
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But as we look at smaller and smaller black holes outside known mass ranges (e.g. primordial black holes, artificial black holes, etc...), things start to change.
A black hole with the mass of anywhere smaller than a star to the mass of a planet is more or less still going to result in the same effects as a stellar mass black hole.
A dwarf planet mass black hole on a collision course with Earth would destroy Earth. If we knew it was coming say a 400-1000 years in advance, we may be able to accelerate a large asteroid into it resulting in a different trajectory for the black hole causing it to miss the Earth, but the tidal forces would still rip the Earth apart or at least send it into a different orbit.
An asteroid mass black hole would strike the Earth causing a massive amount of gamma radiation cooking everything on the surface as it entered the Earth's atmosphere only for it to continue through the planet, emerging on the other side with another bout of radiation cooking before continuing off to shoot back out into space. It would be about the same as two small moons crashing on either side of the Earth.
A tennis ball mass black hole going close to the speed of light would go right through the Earth without too much fuss. The black hole's radius would be too small (smaller than an electron) to cause any damage.
Edit: Added "going close to the speed of light" to the tennis ball mass black hole explanation. If it was not going fast to the speed of light, the black hole would evaporate before it could travel through the Earth. The evaporation would probably also cause so much heat it would cook the planet.
> An asteroid mass black hole would strike the Earth causing a massive amount of gamma radiation cooking everything on the surface
I'm ignorant on the matter, could you explain why this would happen? A black hole with a mount Everest mass would be a tiny fraction of a micrometer in diameter. Even assuming a huge asteroid we would be talking about micrometric objects so I don't understand how could they create such a dire scenario if traveling through Earth.
For comparison an Earth sized black hole would be ~9mm in diameter if my Google-fu is correct.
Hawking radiation. A black hole emits hawking radiation at a wavelength about equal to its diameter, and with power (ie energy/time) inversely proportional to its diameter. So the smaller the black hole, the higher the energy of the photons it emits and also the more energy it emits per second. A black hole with the mass of Mount Everest (~10^14 kg) would, according to a hawking radiation calculator[0], have a schwarzschild radius of 10^-23m, and a proportional wavelength on the order of extremely intense gamma rays or cosmic rays, with a temperature of a billion degrees kelvin. Low-mass black holes aren't black at all.
It's important to note that Hawking Radiation is not a confirmed phenomenon, and unless we actually find such a small black hole, it will never be confirmed (stellar mass black holes are too cold compared to the microwave background to emit Hawking radiation, and will continue being so for an extraordinarily long time.
Really it depends on what you consider a mass of an asteroid to be.
If it's large enough, the black hole is going to be releasing xrays. If it's smaller, it's going to be releasing gamma rays.
The smaller the black hole, the faster it evaporates. The faster it evaporates, the more energy it puts out.
The black hole at this scale is still going to have too small a cross section to suck up any matter and the radiation might end up blowing away any matter before it gets to the black hole in the first place.
At these masses, there's no real danger in the black hole passing through the Earth itself. The only danger is how much radiation is going to be shot out as it passes through the atmosphere on either side of the planet.
Excuse me if this is a silly questions, but you got my imagination going.
>A tennis ball mass black hole would go right through the Earth without too much fuss. The black hole's radius would be too small (smaller than an electron) to cause any damage.
Is it possible that tiny black holes of this size could be travelling through the earth at a regular basis? Do we have the means to reliably detect black holes that small?
1. The smaller the black hole, the brighter and shorter lived it is. To get the black hole to live for any decent amount of time, it'd have to be travelling close to the speed of light for the effects of time dilation to extend it's life time from our perspective.
2. Detecting gravitational effects on a small scale is stupidly hard because of how weak gravity appears to be (compared to the other 3 forces: strong nuclear, weak nuclear, electromagnetic).
If you were to shine a laser and the tennis ball massed black hole travelled through the path of the laser, you wouldn't see any light missing because the wavelength of light would be larger than the black hole.
If you stopped the black hole (so it's not moving close to the speed of light), and tried to shine the laser at it for a greater effect: not only would you not see less light, you would actually see light shooting out of it so bright it'd be like putting your eye against many many atomic bombs because the Hawking Radiation would be so strong. (It'd also destroy itself in a blink of an eye)
You can increase the mass of the black hole to make it cooler and live longer. A 1,000 metric tonne black hole would live for around 46 seconds shedding radiation at an ever increasing amount before exploding into nothingness again. Assuming you could filter out the light coming from the black hole, you might see the phase of the laser light shift, but it'd all still end up at it's destination.
The greater problem would be trying to figure out how to shield the Earth from the amount of light shooting out of it.
Note that, without a consistent theory of quantum gravity, we don't know if black holes of such sizes can actually exist; and unless we happen to detect some, we don't actually know if Hawking radiation exists - it's not something that can be observed for a stellar-mass black hole in realistic time frames (a stellar mass black hole is predicted by the theory to evaporate after ~10^64 years - so the evaporation is far too slow to be observable; further, in the current age of the universe, the temperature of the cosmic microwave background is too high compared to the temperature of the Hawking radiation emitted by a stellar mass black hole for it to be observable, even theoretically).
Yeah we could shoot a black hole at the right mass (this is tunable) so that it evaporates on the target.
There's some other approaches too:
- Move a larger black hole into the target planet (this would require so much energy that other approaches would be better).
- Send a device to the target planet that generates a black hole massive enough to live long enough to oscillate through the planet back and forth, consuming the planet from the inside. It would take 600 quadrillion metric tonnes of mass to form a black hole 1 nanometer across, so this could be difficult to start.
- Create a black hole to power a ship's thrusters (Kugleblitz drive) to smash into a target at speeds approaching the speed of light.
> A tennis ball mass black hole going close to the speed of light would go right through the Earth without too much fuss. The black hole's radius would be too small (smaller than an electron) to cause any damage.
How would such black holes exist in the first place? I thought black holes are formed if the mass of the star is at least 3 times the mass of our Sun.
Primordial black holes are another possible way (besides star collapse) that BHs could form.
The theory is that during the immediate aftermath of the Big Bang, the distribution of mass-energy became chaotic, with local hotspots where mass-energy was more dense, and other spots where it was less dense.
GR says that if ANY amount of mass-energy comes to occupy a small enough area, then that is a black hole. Star collapse BHs have a lower mass limit because their own gravity is what forces their mass into a region small enough to become a BH.
So primordial BHs could have formed with arbitrarily small mass, because they didn't have to rely on their own gravity to attain BH density. The Big Bang did it, for them.
Any primordial black holes of low mass that may have formed would have evaporated long ago through Hawking radiation. If you plug the mass of a tennis ball into the equations, it would evaporate in well under a second. (Btw, don't stand next to anything that radiates the entire mass-energy of a tennis ball in under a second. Small black holes shine really brightly)
The least unrealistic mitigation strategy would be to colonize Mars.
Note that even a near miss will cause a significant perturbation to orbits in the solar system. It's likely that a black hole passing through the solar system and not hitting anything would still render life on Earth impossible.
Earth even with massive temperature swings from today (due to wacky orbit or whatever) I think Earth would still be more hospitable than Mars. It has a magnetic field and a moon and lots of water. If all the life has died, and the atmosphere has frozen or boiled off, well, you're still no worse off than Mars, and you've still got that magnetic field and aquifers and petrochemicals to get back to space eventually.
Earth has active plate tectonics, volcanism, hurricanes/tornadoes (with enough mass in them to destroy things), land slides, floods, lightning, and maybe worst of all other humans.
If we're living in a closed-ish habitat anyways, I think Mars might even manage to be easier (provided there was a critical mass of humans living on it). It seems like the only unpredictable natural disasters there are dust storms. Especially if you consider that any post apocalyptic habitat scenario is going to leave a lot of people out, and they aren't going to be peaceful about that during the transition.
Now I'm a imagining a science fiction weapon: the black hole trebuchet. Just launch a small black hole at your enemies planetary system and they're doomed one way or the other. If the gravitational signature is too much of a dead giveaway, then launch two from exactly opposite sides and their force will cancel out - until they collide in what I imagine would produce a catastrophic explosion and either blow up or consume the enemies host star.
David Brin uses this in one of his books, playing a bit with some of the same kinds of ideas that gave us the Dark Forest theory as an explanation for the Fermi Paradox: intelligent species are out there, but nobody wants to be spotted because they’ll be killed preemptively.
This would take some intense pre-planning. So since you obviously can't capture a black hole to launch into a trebuchet, you'd have to do it to the original star before it collapses. Then time the launch so there's enough time for the collapse to occur during travel. You'd want to back up that launch time well before "just before collapsing" as the star's surface will have expanded to just be a tad bit unruly to handle. Better to grab it before that last bit of expansion just to keep things a bit easier to handle
> from exactly opposite sides and their force will cancel out
The gravitational force will only cancel out exactly in the center. Everywhere else, for example other planets, and when the Earth orbits, will easily detect the gravity.
> or consume the enemies host star.
It would take an infinite amount of time to do that, time dilation near black holes is intense.
The same strategy as if a massive star was on a collision course with earth. Namely, kiss your butt goodbye. But it's not likely to happen.
There's nothing magically destructive about a black hole. The free-floating ones in our galaxy have masses comparable to stars, and they have exactly the same gravitational effects as stars. If we found one within a few light-years of Earth, the only effect on us would be a bunch of ecstatic astronomers. The rest of us would (or rather, should) just call it Tuesday.
You can easily localize where the gravity originates and how strong it is. That part is easy as long as you are moving and can measure course changes as you move.
Then you look in that direction and see what you can see. If you see nothing then it's not a star. So either a planet or a black hole.
Either way, avoid it.
For that matter, avoid it if it's a star. It's a general rule: if it gravitates avoid it.
It is almost certainly less-likely than encountering another star.
The outcome of a close-encounter between our solar system and another star or a black hole would likely be comparable: Earth's orbit would be disrupted, presumably to our detriment.
Black holes of a few solar masses probably aren't any more dangerous than another star.
From a sci-fi perspective, I like the idea of there being micro black holes flying through the universe as a result of primordial black holes evaporating for billions of years.
Not WIMPs but MACHOs.