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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

#61

Post by Retric »

T-A wrote: post #16
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…

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…

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.

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

#62

Post by [deleted] »

Waterluvian wrote: post #59
Fordec wrote: post #58

Earlier quoted context omitted.

Treat the "Universe" that was created in the big bang like a firework that has just gone off. The "boundary" of the universe is just the size the explosion has expanded to for a given time. And today, 13.8B years after the big bang, our universe is 46.5 billion light-years in diameter. But just like a firework explosion, there isn't really a solid border to it that you can plant a stake or flag in. We have no clue wh…

How can the diameter be larger than 2 * the speed of light * time ?

[deleted]

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

#63

Post by danbruc »

Waterluvian wrote: post #59
Fordec wrote: post #58

Earlier quoted context omitted.

Treat the "Universe" that was created in the big bang like a firework that has just gone off. The "boundary" of the universe is just the size the explosion has expanded to for a given time. And today, 13.8B years after the big bang, our universe is 46.5 billion light-years in diameter. But just like a firework explosion, there isn't really a solid border to it that you can plant a stake or flag in. We have no clue wh…

How can the diameter be larger than 2 * the speed of light * time ?

The given number of 46.5 Gly is actually the radius, not the diameter. This can be larger than the age of the universe times the speed of light because the universe expands, i.e. while the light was traveling towards us the point of emission expanded away from us. So 46.5 Gly is the distance to the point of emission today but when the light we observe today was emitted there 13.8 billion years ago the distance was much smaller. Also note that the expansion of the universe is not limited to the speed of light because it is space itself expanding and not something moving through space which in turn means that there is no limit to the speed at which the point of emission can expand away from us.

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

#64

Post by danbruc »

kamaal wrote: post #60
ketralnis wrote: post #38

Earlier quoted context omitted.

New space is being created between those two things. When they move 1 meter apart from each other through space, they'd move 1 meter back again to meet up. But when space itself is expanding between them, they've moved 1 meter apart but now to get back together they have to move 2 meters. This has some other subtler ramifications. It means that the space between the two objects can increase at faster than the speed o…

>>It means that the space between the two objects can increase at faster than the speed of light Say they are two objects in the universe A and B. New space is created between A and B faster than the speed of light. There is a USB drive carrying a dump of wikipedia on B. Does this imply information is now moving faster than the speed of light?

No because the USB stick is going nowhere, it just remains at B. Imagine a tiled floor where A and B are the centers of two tiles, the USB stick also resting on the B tile. Expansion of space just means that you are constantly increasing the gaps between tiles maybe inserting new tiles every time the gaps become wide enough. The USB stick however just rests on the B tile not moving through space, i.e. reaching another tile, ever.

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

#65

Post by glandium »

InclinedPlane wrote: post #19
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…

This is an important question with a slightly surprising answer. First you need to understand that the Big Bang was not an explosion of matter into space, it was an explosion of matter and space (technically space-time). This might be hard to wrap your head around, it might be easier to think about space-time as non-infinite, say curving back on itself like the surface of a balloon (though as far as we can tell it is…

> And this is true for every single bit of volume around you

... except on the edges... but maybe there wasn't an edge and space is sphericalish? And if it was back then, why wouldn't it be now? Which brings the question: if you go far enough in one direction, do you come back where you started? Can we figure out whether that's the case?

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

#66

Post by Waterluvian »

danbruc wrote: post #63
Waterluvian wrote: post #59

Earlier quoted context omitted.

How can the diameter be larger than 2 * the speed of light * time ?

The given number of 46.5 Gly is actually the radius, not the diameter. This can be larger than the age of the universe times the speed of light because the universe expands, i.e. while the light was traveling towards us the point of emission expanded away from us. So 46.5 Gly is the distance to the point of emission today but when the light we observe today was emitted there 13.8 billion years ago the distance was mu…

Thanks. I kind of understand enough of that to know what to search for further.

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

#67

Post by kamaal »

danbruc wrote: post #64
kamaal wrote: post #60

Earlier quoted context omitted.

>>It means that the space between the two objects can increase at faster than the speed of light Say they are two objects in the universe A and B. New space is created between A and B faster than the speed of light. There is a USB drive carrying a dump of wikipedia on B. Does this imply information is now moving faster than the speed of light?

No because the USB stick is going nowhere, it just remains at B. Imagine a tiled floor where A and B are the centers of two tiles, the USB stick also resting on the B tile. Expansion of space just means that you are constantly increasing the gaps between tiles maybe inserting new tiles every time the gaps become wide enough. The USB stick however just rests on the B tile not moving through space, i.e. reaching anothe…

Explain to me how this works. A and B remain stationary while only new space gets created between them?

By geometry, the shortest distance between two points is a straight line. If A and B are two stationary objects, with a straight line being shortest distance between them. Then space is basically a twisted thread like path between A and B, More thread is created every time instant 't' than light can travel through it?

And if some one figures out a way to walk from A to B in a straight line then they can walk faster than anything that can walk on the thread?

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

#68

Post by Simon_says »

jobigoud wrote: post #40
dantillberg wrote: post #31

Earlier quoted context omitted.

Maybe the big bang did produce a black hole, and we're living inside it? Back in my undergrad physics days, I postulated that our "universe" could be a big black hole inside some "extraverse". I worked through classical gravitational calculations to determine, from an estimate of the mass density of the universe, what the Schwarzschild radius (aka event horizon radius) of that black hole would be, and it came out to…

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 exceed its radius. This is because the Schwarzchild radius is proportional to the mass, but the radius of the actual matter is proportional to the cube-root of the mass.

It's a fun thought experiment, but not physically meaningful in this universe because our spacetime is expanding, and the original calculations assume there's nothing at the edge pulling 'outward', but in our universe there is.

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

#69

Post by danbruc »

kamaal wrote: post #67
danbruc wrote: post #64

Earlier quoted context omitted.

No because the USB stick is going nowhere, it just remains at B. Imagine a tiled floor where A and B are the centers of two tiles, the USB stick also resting on the B tile. Expansion of space just means that you are constantly increasing the gaps between tiles maybe inserting new tiles every time the gaps become wide enough. The USB stick however just rests on the B tile not moving through space, i.e. reaching anothe…

Explain to me how this works. A and B remain stationary while only new space gets created between them? By geometry, the shortest distance between two points is a straight line. If A and B are two stationary objects, with a straight line being shortest distance between them. Then space is basically a twisted thread like path between A and B, More thread is created every time instant 't' than light can travel through…

Unfortunately I don't understand what you are suggesting or asking. Maybe this short video [1] with the classical balloon analogy will help you understand it better.

[1] https://www.youtube.com/watch?v=mnENgtCdObo

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

#70

Post by GuB-42 »

Filligree wrote: post #46
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…

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

The universe is infinite by definition. It may be closed, which mean it will repeat itself indefinitely, but that's still infinite.

What isn't infinite is the observable universe. But the observable universe is just the view we have, its boundaries are defined by the observer. In fact my observable universe is different from your observable universe, by an insignificant amount.

When physicists talk about the universe, it usually means the observable universe. Simply because for us, that's all we have.

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