[bmitc]

It is my understanding that, from a gravity-only standpoint, you are right. But I actually thought that black holes slowly evaporate, i.e., lose mass, from emitting Hawking radiation. It isn't clear from the article whether the vacuum energy black holes still have that property.

The article confuses me on something else. It mentions a link between black hole mass and the expansion of the universe, but then it seems to imply that the expansion causes the black holes to gain mass which in turn causes the expansion to accelerate. It doesn't seem to address why the universe is expanding in the first place. But I guess dark energy was proposed as the thing that was doing the expansion acceleration, and not the expansion cause.

[pdonis]

> I actually thought that black holes slowly evaporate

This is believed to be true, but the time scale is something like 60 or more orders of magnitude longer than the age of the universe, so (a) no evidence for this effect exists or is likely to be found any time soon, and (b) it's irrelevant for the dynamics of our current universe anyway.

[justinpombrio]

I thought we had evidence that black holes evaporate in that Earth hasn't been swallowed up yet.

A while back there were concerns (notably not from physicists) about the LHC forming black holes. I remember the response being that tiny black holes frequently form in the upper atmosphere due to high energy particle collisions, but black holes emit more radiation the smaller they are(!), so these tiny black holes evaporate nearly instantly. (Thus the same would happen if the LHC made any.) A tiny black hole that didn't evaporate would be scary because it could grow larger but not smaller.

[andrewflnr]

To be more precise, if LHC was capable of forming black holes, then they would also be regularly formed in the upper atmosphere by cosmic rays... but more likely neither of those is the case. I don't think many serious physicists actually think particle collisions create black holes.

[ISL]

Collisions of sufficient energy could potentially create tiny black holes. GP has the right idea though -- evaporation goes power-law faster the smaller the black hole.

No matter what the mechanism for protection from cosmogenic-collision black holes, if they were problematic, the Sun would have been destroyed long ago through a black-hole creation, black-hole capture, solar-collapse process with cosmic rays much higher in energy than anything humans will ever generate. So, as long as you can look outside and see the sun, you need not ever sweat the particle-collision destroys the world hypothesis, no matter whence the particles are generated.

[pdonis]

> A while back there were concerns (notably not from physicists) about the LHC forming black holes.

There were concerns, but they were not well founded in actual physics.

> I remember the response being that tiny black holes frequently form in the upper atmosphere due to high energy particle collisions

I'm not aware of any such response. The response I'm aware of was that events with higher energy than the LHC is capable of creating happen routinely in cosmic ray collisions, and no black hole formation has ever been observed in such collisions, so black hole formation is not going to happen at the LHC either. That is consistent with our best current theoretical prediction, which is that you would need an accelerator capable of reaching the Planck scale, many orders of magnitude higher energy than the LHC, for black hole production to be possible.

[opportune]

Not really, a black hole is not different orbitally than any other object with mass. You could replace all the stars in the Milky Way with a black hole of the same mass and basically nothing would change for us except not having starlight. Black holes suck everything in the same way that stars and planets do - unless you are literally heading right towards it, if the relative velocity is high, you would just move past each other.

A tiny black hole would not have enough mass to pull in a significant amount of matter and would just pass through the earth if it were coming from space. If the black hole were created on earth it would need a lot more mass than a collider could give it to do anything funky.

[bmitc]

That makes sense. I forgot about the timescales for the evaporation. Thanks!

[ikrenji]

evaporation could be relevant for small blackholes, eg the tiniest ones quickly disappear