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Black holes ruled out as universe’s missing dark matter

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

#111

Post by dantillberg »

Simon_says wrote: post #68
jobigoud wrote: post #40

Earlier quoted context omitted.

How is it the right order of magnitude? Did you compute that based on the size of the observable Universe? But that's arbitrary, the Universe is way bigger than that.

No. The input into the equation is the density of matter in space, nothing having to do with the size of the observable universe. On large scales in the visible universe, matter is fairly evenly distributed at a density of about 1 atom of hydrogen per cubic meter. For any constant nonzero density form of matter, if you pack enough of it together in a sphere in a flat spacetime, its Schwarzchild radius will eventually…

Thank you, you're adding more concrete verbiage to my suspicions that my calculations were too naive: I was only doing classical, pre-General-Relativity, calculations. On such large scales, the dynamic nature of spacetime itself would play an important role. Especially if, as evidence suggests, gravitational waves travel no faster than light.

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

#112

Post by cdumler »

ziftface wrote: post #109
cdumler wrote: post #98

Earlier quoted context omitted.

Yes, Dark Matter is a statement about the premise that something appears to be operating gravitationally while have no (or perhaps so little that we don't recognize it) interaction with other matter; however, it is not without reason. The primary focus on matter as the candidate is that if you don't believe that some form of matter is responsible, you begin having to explain how a lot of our Laws are incorrect. Dark…

Is it possible that it's something other than gravity? A force we don't know about because of how weak it is but has an effect on a large scale? I'm completely ignorant of the subject so I don't know how crazy that might sound.

No, that is a perfectly valid question. The answer is: science has no idea what gravity _is_. What we do know is:

The Newton/Einstein definition is that it is an effect by mass on space/time. Mass always travels in a straight line (unless acted on), but mass causes space/time to curve. So, what we see as gravitation attraction is a fake force. No different than being "pushed" into the back of your seat when a car accelerates. You are staying still, the car is moving forwards. The Newton/Einstein definition does not specify any mechanism. The description, however, is highly specific, and so far every test for the most subtle effects have been validated. Well, except for cosmic structures.

Quantum mechanics says that particles emit/receive interactions via the graviton carrier, and that it can be quantized and transmitted via waves. In the last few years, we have detected gravitational waves. It fits perfectly and exactly as we would expect over cosmic distances. Unfortunately, the amount of energy necessary to detect an actual graviton is beyond anything we can expect to be able to produce for the forceable future and likely will never have the resources. So that is out. Also, it makes no claim on how it can affect space/time.

So, just like we original argued over whether or not it was particles or waves, I think this paradox strongly suggests that we're just not looking at gravity correctly. I suspect that the real problem is "What is time?" Why do things experience different rates of time? It seems obvious to me that gravity, time, and entanglement are related. Gravity is very likely to be Dark Matter: a stand-in name until we can grasp the fundamentals.

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

#113

Post by Retric »

pdonis wrote: post #105
Retric wrote: post #61

Earlier quoted context omitted.

A second issue is in Time 0.0001 seconds the observable universe is only a 0.0001 light seconds radius which limits how much mass could form a black hole.

No, it doesn't. The fact that you can't see beyond a certain point at a certain time doesn't mean the mass beyond that point does not count.

If an object would react to mass outside it’s light cone.

That would mean you can communicate faster than light by moving a large mass.

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

#114

Post by pdonis »

Retric wrote: post #113
pdonis wrote: post #105

Earlier quoted context omitted.

No, it doesn't. The fact that you can't see beyond a certain point at a certain time doesn't mean the mass beyond that point does not count.

If an object would react to mass outside it’s light cone. That would mean you can communicate faster than light by moving a large mass.

> If an object would react to mass outside it’s light cone.

That mass was not outside its light cone in the past. (This is one of the main points given in favor of inflation models: that they solve the "horizon problem" because the inflationary expansion means that mass over a region much wider than our observable universe was within our past light cone at the end of inflation.)

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

#115

Post by T-A »

pdonis wrote: post #104
T-A wrote: post #16

Earlier quoted context omitted.

The early universe was ridiculously homogeneous. We can infer this with reasonable certainty from the extreme homogeneity of the CMB and from the gravitational instability of density fluctuations: as you expect, they attract more mass and accrete to form stars and galaxies. So if you start from the universe as it looks now and run time backwards, you get ridiculous homogeneity at the bang. And that's why you didn't g…

Homogeneity of the early universe by itself is not enough to prevent collapse. You need rapid expansion as well. A homogeneous universe that is not expanding at all will collapse.

You are answering a different question. The collapse of a homogeneous (i.e. FRW) universe produces an increasingly dense, but still homogeneous universe. Without a central singularity and an outside, it looks nothing like a black hole (as asked).

BTW, expansion alone is not quite enough to answer why the early universe didn't collapse. You can have very rapid expansion, yet a closed universe which eventually comes to a standstill and then starts shrinking. A better answer is that the universe is very close to having critical density, i.e. just enough mass to expand forever at an asymptotically declining rate, absent new drivers of expansion (i.e. dark energy).

One thing our answers have in common is fine-tuning: the universe started out ridiculously homogeneous, and ridiculously close to the critical density. Inflation provides a way to explain both those properties, but (as critics are fond of pointing out) at the cost of fine-tuning the hypothetical microphysics needed to drive it.

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

#116

Post by ziftface »

cdumler wrote: post #112
ziftface wrote: post #109

Earlier quoted context omitted.

Is it possible that it's something other than gravity? A force we don't know about because of how weak it is but has an effect on a large scale? I'm completely ignorant of the subject so I don't know how crazy that might sound.

No, that is a perfectly valid question. The answer is: science has no idea what gravity _is_. What we do know is: The Newton/Einstein definition is that it is an effect by mass on space/time. Mass always travels in a straight line (unless acted on), but mass causes space/time to curve. So, what we see as gravitation attraction is a fake force. No different than being "pushed" into the back of your seat when a car acc…

Thanks for the reply! This gave me a lot to look into.

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

#117

Post by hndamien »

FreeFull wrote: post #47
Filligree wrote: post #46

Earlier quoted context omitted.

So, are we assuming the universe is infinite? Otherwise there'd be a border to that.

The universe can be finite without a border. Think about living on the surface of a sphere or a torus (finite, no edge), as opposed to living on a sheet of paper

Part of me thinks that black holes are the border, and we might be inside one.

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

#118

Post by Simon_says »

pdonis wrote: post #107
Simon_says wrote: post #68

Earlier quoted context omitted.

No. The input into the equation is the density of matter in space, nothing having to do with the size of the observable universe. On large scales in the visible universe, matter is fairly evenly distributed at a density of about 1 atom of hydrogen per cubic meter. For any constant nonzero density form of matter, if you pack enough of it together in a sphere in a flat spacetime, its Schwarzchild radius will eventually…

> For any constant nonzero density form of matter, if you pack enough of it together in a sphere in a flat spacetime, its Schwarzchild radius will eventually exceed its radius. You can't do this in flat spacetime, because the matter curves spacetime, and the denser you pack it, the more it curves spacetime.

Obviously.

The point of the thought experiment was that you start with a universe with no other matter in it that's not expanding, both of which conditions our universe violates.

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

#119

Post by Retric »

pdonis wrote: post #114
Retric wrote: post #113

Earlier quoted context omitted.

If an object would react to mass outside it’s light cone. That would mean you can communicate faster than light by moving a large mass.

> If an object would react to mass outside it’s light cone. That mass was not outside its light cone in the past. (This is one of the main points given in favor of inflation models: that they solve the "horizon problem" because the inflationary expansion means that mass over a region much wider than our observable universe was within our past light cone at the end of inflation.)

Ahh, ok, based on context I assumed you would understand that as an actual light second in spacetime as in actual distance traveled. Not light second as a unit of distance in 3d space at t=0.001 seconds which is a rather meaningless number at that point.

It’s a meaningful difference, but still avoids the black hole problem.

PS: The math really does not say anything about t=0, after ~t=10^-30 to t=1 seconds you don’t get black holes.

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

#120

Post by pdonis »

T-A wrote: post #115
pdonis wrote: post #104

Earlier quoted context omitted.

Homogeneity of the early universe by itself is not enough to prevent collapse. You need rapid expansion as well. A homogeneous universe that is not expanding at all will collapse.

You are answering a different question. The collapse of a homogeneous (i.e. FRW) universe produces an increasingly dense, but still homogeneous universe. Without a central singularity and an outside, it looks nothing like a black hole (as asked). BTW, expansion alone is not quite enough to answer why the early universe didn't collapse. You can have very rapid expansion, yet a closed universe which eventually comes to…

Yes, these are all valid points.
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