Black holes ruled out as universe’s missing dark matter • Hacker News Board

Re: Black holes ruled out as universe’s missing dark matter

#21

Post by flukus » Wed, Oct 03, 2018, 3:51 AM UTC

DougWebb wrote: post #8

sulam wrote: post #4

This spawned a question for me that seems like it must be nonsensical but I don’t know the answer. Why did the Big Bang not just produce a Big Hole? IOW, if all the current mass of the universe was in a small volume wouldn’t it have promptly created a black hole? I’m assuming the answer has something to do with mass not actually existing in its current form until the universe expanded enough. However I don’t have any…

I'm absolutely not an expert, but I believe there wasn't any mass at first; it was all energy. It had to slow down to become mass, at which point it was spread out enough. Or something along those lines. Another possibility is that black holes are places where lots of mass accumulates in the same place in space, while the big bang was a rapid expansion of space itself. Even if there was enough mass in one spot to be…

You can make black holes with massless particles: https://en.wikipedia.org/wiki/Kugelblitz_%28astrophysics%29

Re: Black holes ruled out as universe’s missing dark matter

#22

Post by InclinedPlane » Wed, Oct 03, 2018, 3:53 AM UTC

oldandtired wrote: post #13

sulam wrote: post #5

Earlier quoted context omitted.

We have observed their x-ray jets directly and have observed their creation (recently). Pretty sure they exist. :)

Let's change this a little bit. We have observed specific phenomena but not that which created such phenomena. As far as "Black Holes" as per theoretical descriptions are concerned, they cannot exist in a finite time universe as far as we (the observers) are concerned. The theoretical models used all have as a basic assumption that increasing gravity (whatever that may be) will cause a slow down in time as observed a…

We have observed matter falling into a black hole. We've watched "hot spots" in orbit of a black hole in the accretion disk shine their light at us as they orbited the black hole. We've watched as the orbit got tighter and tighter and the light was dimmed by gravitational red shifting, then we've watched the light blink out and disappear as the blob of gas fell below the event horizon.

The evidence for the existence of black holes is incredibly strong at the moment. In fact in some ways it's stronger than the evidence for the existence of atoms about a century ago. Nobody except cranks or extreme contrarians seriously disputes the theory these days.

Re: Black holes ruled out as universe’s missing dark matter

#23

Post by pdonis » Wed, Oct 03, 2018, 4:01 AM UTC

sulam wrote: post #4

This spawned a question for me that seems like it must be nonsensical but I don’t know the answer. Why did the Big Bang not just produce a Big Hole? IOW, if all the current mass of the universe was in a small volume wouldn’t it have promptly created a black hole? I’m assuming the answer has something to do with mass not actually existing in its current form until the universe expanded enough. However I don’t have any…

> if all the current mass of the universe was in a small volume wouldn’t it have promptly created a black hole?

No, because it was expanding too rapidly. The usual intuitions about how much mass in how small a volume it takes to make a black hole assume that the mass is stationary, or nearly so.

Re: Black holes ruled out as universe’s missing dark matter

#24

Post by waterhouse » Wed, Oct 03, 2018, 4:04 AM UTC

twiceaday wrote: post #11

DougWebb wrote: post #8

Earlier quoted context omitted.

I'm absolutely not an expert, but I believe there wasn't any mass at first; it was all energy. It had to slow down to become mass, at which point it was spread out enough. Or something along those lines. Another possibility is that black holes are places where lots of mass accumulates in the same place in space, while the big bang was a rapid expansion of space itself. Even if there was enough mass in one spot to be…

Doesnt E=mc^2 form an equivalence? I would expect energy to warp space-time just as mass does.

I believe it does. If you have a bottle full of chemicals with potential energy, and then you make them react and get the energy from them (via, I dunno, heat), while all the particles remain in the bottle, then I think the bottle weighs less afterwards—though probably by an undetectably small amount. I think I read that the change in mass is detectable after, say, nuclear fission.

Re: Black holes ruled out as universe’s missing dark matter

#25

Post by pdonis » Wed, Oct 03, 2018, 4:04 AM UTC

bpchaps wrote: post #10

sulam wrote: post #4

This spawned a question for me that seems like it must be nonsensical but I don’t know the answer. Why did the Big Bang not just produce a Big Hole? IOW, if all the current mass of the universe was in a small volume wouldn’t it have promptly created a black hole? I’m assuming the answer has something to do with mass not actually existing in its current form until the universe expanded enough. However I don’t have any…

My naive (and likely wrong) understanding is that cosmological inflation was caused by a "false vacuum" of the higgs field. Think of it as if the energy pressure of a system is higher than a vacuum, but has similar pressure properties of a vacuum. That had the effect of gravitational pressure pushing matter outwardly, instead of pulling it inwardly like we have it today (to answer your question). That caused enormous…

Your description of what happened during inflation is basically correct. However, in order to explain why a black hole didn't form as soon as inflation ended and gravity started behaving "as we know it", you have to include the fact that all of the matter that was formed at the end of inflation was expanding very, very rapidly. (The universe was actually vacuum during inflation--at the end of inflation all of the energy in the false vacuum got transferred to the Standard Model fields, creating matter and radiation at extremely high temperature and expanding.)

Re: Black holes ruled out as universe’s missing dark matter

#26

Post by pdonis » Wed, Oct 03, 2018, 4:05 AM UTC

twiceaday wrote: post #11

DougWebb wrote: post #8

Earlier quoted context omitted.

I'm absolutely not an expert, but I believe there wasn't any mass at first; it was all energy. It had to slow down to become mass, at which point it was spread out enough. Or something along those lines. Another possibility is that black holes are places where lots of mass accumulates in the same place in space, while the big bang was a rapid expansion of space itself. Even if there was enough mass in one spot to be…

Doesnt E=mc^2 form an equivalence? I would expect energy to warp space-time just as mass does.

> I would expect energy to warp space-time just as mass does.

It does. So do pressure and stresses. The full "source" of gravity in General Relativity is the stress-energy tensor, which includes all of these things.

Re: Black holes ruled out as universe’s missing dark matter

#27

Post by pdonis » Wed, Oct 03, 2018, 4:06 AM UTC

InclinedPlane wrote: post #20

DougWebb wrote: post #8

Earlier quoted context omitted.

I'm absolutely not an expert, but I believe there wasn't any mass at first; it was all energy. It had to slow down to become mass, at which point it was spread out enough. Or something along those lines. Another possibility is that black holes are places where lots of mass accumulates in the same place in space, while the big bang was a rapid expansion of space itself. Even if there was enough mass in one spot to be…

"Mass" is just another name for "rest-energy". A single photon cannot have mass because it has zero energy in the reference frame where it is at rest (zero net momentum). But a pair of non-parallel photons can have rest-mass, and indeed the mass of the photons inside a star is considerable, for example.

> "Mass" is just another name for "rest-energy".

Not if you're talking about what the source of gravity (spacetime curvature) is. Massless particles still have energy and still cause spacetime curvature.

Re: Black holes ruled out as universe’s missing dark matter

#28

Post by pdonis » Wed, Oct 03, 2018, 4:08 AM UTC

DougWebb wrote: post #8

sulam wrote: post #4

This spawned a question for me that seems like it must be nonsensical but I don’t know the answer. Why did the Big Bang not just produce a Big Hole? IOW, if all the current mass of the universe was in a small volume wouldn’t it have promptly created a black hole? I’m assuming the answer has something to do with mass not actually existing in its current form until the universe expanded enough. However I don’t have any…

I'm absolutely not an expert, but I believe there wasn't any mass at first; it was all energy. It had to slow down to become mass, at which point it was spread out enough. Or something along those lines. Another possibility is that black holes are places where lots of mass accumulates in the same place in space, while the big bang was a rapid expansion of space itself. Even if there was enough mass in one spot to be…

> I believe there wasn't any mass at first; it was all energy. It had to slow down to become mass

This is partly true. During inflation, the universe was vacuum; all of the energy was in the "false vacuum" state of whatever field caused inflation (it's usually called the "inflaton" field, which IMO is a very confusing name). When inflation ended, all of that energy got transferred to the Standard Model fields, forming matter and radiation at very high temperature and expanding very rapidly.

So the "all energy at first" part is true; but the "had to slow down to become mass" part is not. The matter and radiation was there as soon as inflation ended; it didn't have to slow down first.

Re: Black holes ruled out as universe’s missing dark matter

#29

Post by AgentME » Wed, Oct 03, 2018, 4:11 AM UTC

oldandtired wrote: post #13

sulam wrote: post #5

Earlier quoted context omitted.

We have observed their x-ray jets directly and have observed their creation (recently). Pretty sure they exist. :)

Let's change this a little bit. We have observed specific phenomena but not that which created such phenomena. As far as "Black Holes" as per theoretical descriptions are concerned, they cannot exist in a finite time universe as far as we (the observers) are concerned. The theoretical models used all have as a basic assumption that increasing gravity (whatever that may be) will cause a slow down in time as observed a…

Black holes may not form true singularities in any finite amount of time, but event horizons that are indistinguishable from the outside can be formed.

Re: Black holes ruled out as universe’s missing dark matter

#30

Post by roywiggins » Wed, Oct 03, 2018, 4:12 AM UTC

InclinedPlane wrote: post #22

oldandtired wrote: post #13

Earlier quoted context omitted.

Let's change this a little bit. We have observed specific phenomena but not that which created such phenomena. As far as "Black Holes" as per theoretical descriptions are concerned, they cannot exist in a finite time universe as far as we (the observers) are concerned. The theoretical models used all have as a basic assumption that increasing gravity (whatever that may be) will cause a slow down in time as observed a…

We have observed matter falling into a black hole. We've watched "hot spots" in orbit of a black hole in the accretion disk shine their light at us as they orbited the black hole. We've watched as the orbit got tighter and tighter and the light was dimmed by gravitational red shifting, then we've watched the light blink out and disappear as the blob of gas fell below the event horizon. The evidence for the existence…

I believe the argument is that event horizons exist but inside is not a singularity, but an eternally collapsing mass of the original star. From the point of view of the star, it does collapse all the way, but from our point of view that final singularity collapse is in the indefinite future.

https://en.m.wikipedia.org/wiki/Magnetospheric_eternally_col...