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The Gravo-Thermal Catastrophe

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Re: The Gravo-Thermal Catastrophe

#21

Post by Nesco »

trhway wrote: post #20
Nesco wrote: post #10

Earlier quoted context omitted.

This mass to event horizon radius relationship is a property of a Schwarzschild spacetime geometry, globally the universe has a FLRW spacetime geometry

The light has no chances of getting out of the 13.7B ly bubble due to Hubble expansion. Sounds a lot like black hole.

The universe has no center, a black hole has one. The limits of the visible universe is an horizon on your frame of reference

In nerdspeak, the geometries are not the same, one is isotropic the other anisotropic

Re: The Gravo-Thermal Catastrophe

#22

Post by pfdietz »

leephillips wrote: post #18
pfdietz wrote: post #11

As I understand it, those simulations did not include three-body interactions that could leave particle pairs bound. If this happens, those binaries can now inject energy into the cluster as a whole, keeping it inflated and preventing collapse. Of course, the binaries' orbits shrink over time, so this doesn't go on forever.

What is a three-body interaction in classical gravity? If you calculate the force on each particle from every other particle, what’s left out?

I mean, they did not calculate interactions on a sufficiently short time scale that cases where three bodies come together and two come out bound would occur.

Re: The Gravo-Thermal Catastrophe

#23

Post by hnuser123456 »

Nesco wrote: post #21
trhway wrote: post #20

Earlier quoted context omitted.

The light has no chances of getting out of the 13.7B ly bubble due to Hubble expansion. Sounds a lot like black hole.

The universe has no center, a black hole has one. The limits of the visible universe is an horizon on your frame of reference In nerdspeak, the geometries are not the same , one is isotropic the other anisotropic

When you are in a black hole, you are always heading towards the center.

The universe is expanding faster than the speed of light beyond a distance of 13.7b ly away, and the comoving diameter of the universe is already something like 93b ly. If you could teleport to another part of the universe, you would have a whole new local environment, but it would still be flying apart faster than you could ever hope to catch up to the furthest objects (without another teleport), and the furthest objects you can reach are getting more and more sparse as they are always flying outside of your observable universe. I.e., inside a black hole, where all possible directions only point further inward, and yet also "away from everything else", as anything that fell in before you, gets pulled in faster and faster "away from you, towards the singularity", and anything that falls in after you has increasingly no hope of catching up to you.

Re: The Gravo-Thermal Catastrophe

#24

Post by mikhailfranco »

This idea has been explored by Julian Barbour in his book The Janus Point.

http://www.platonia.com/books.html

The related math and modelling goes under the name Shape Dynamics:

https://en.wikipedia.org/wiki/Shape_dynamics

Shape Dynamics - An Introduction

https://arxiv.org/abs/1105.0183

Re: The Gravo-Thermal Catastrophe

#25

Post by Nesco »

hnuser123456 wrote: post #23
Nesco wrote: post #21

Earlier quoted context omitted.

The universe has no center, a black hole has one. The limits of the visible universe is an horizon on your frame of reference In nerdspeak, the geometries are not the same , one is isotropic the other anisotropic

When you are in a black hole, you are always heading towards the center. The universe is expanding faster than the speed of light beyond a distance of 13.7b ly away, and the comoving diameter of the universe is already something like 93b ly. If you could teleport to another part of the universe, you would have a whole new local environment, but it would still be flying apart faster than you could ever hope to catch u…

Inside a black hole, the center is in your future, it’s a point in time where geodesics end

Time doesn’t seem to end in the universe at large…

Re: The Gravo-Thermal Catastrophe

#26

Post by xbar »

pdonis wrote: post #7
keskival wrote: post #5

I suppose in the real world such stars would collide in the center of the sphere and possibly form a black hole before achieving the required density approaching infinity, and also catapult stars out so that they leave the system by exceeding the escape velocity without encountering an elastic wall returning them to the system.

> in the real world such stars would collide in the center of the sphere and possibly form a black hole Yes, the article mentions that towards the end.

“Things only get messier from here.”

Re: The Gravo-Thermal Catastrophe

#27

Post by trhway »

Nesco wrote: post #25
hnuser123456 wrote: post #23

Earlier quoted context omitted.

When you are in a black hole, you are always heading towards the center. The universe is expanding faster than the speed of light beyond a distance of 13.7b ly away, and the comoving diameter of the universe is already something like 93b ly. If you could teleport to another part of the universe, you would have a whole new local environment, but it would still be flying apart faster than you could ever hope to catch u…

Inside a black hole, the center is in your future , it’s a point in time where geodesics end Time doesn’t seem to end in the universe at large…

>Inside a black hole, the center is in your future, it’s a point in time where geodesics end

we're going to end up in one of those super-large black holes in like 20 or 50 billions of years. One can say that we're already falling into that blackhole. And the geodesics of our local Universe end in the core of that super blackhole. The core of that blackhole can be considered the center of our local Universe the same way like the core of any blackhole is considered the center for everything falling into the blackhole.

Re: The Gravo-Thermal Catastrophe

#28

Post by drudd »

jessriedel wrote: post #8

Is this right?: * Although you can make the enveloping sphere as large as you want, the (anti-)equilibration process requires a sphere of some finite radius because if you wait long enough a few stars eventually get launched at escape velocity, and if these actually escaped they would effectively cool the remaining stars. * Therefore, the characteristic time scale for this process (i.e., the timescale on which the av…

I was asked this question on my grad candidacy exam (which was quite a while ago, so my memory is hazy), and I believe stars tend to form binary systems which can halt the runaway gravitational collapse (the potential energy in a hard binary can be a very significant fraction of the total energy of a relatively loosely bound globular cluster).

Re: The Gravo-Thermal Catastrophe

#29

Post by kazinator »

Something doesn't add up, obviously.

For one thing, squeezing the sphere smaller against pressure requires work. That's an external energy input. The system is not closed if some agent is available that can squeeze the sphere smaller.

Re: The Gravo-Thermal Catastrophe

#30

Post by jessriedel »

drudd wrote: post #28
jessriedel wrote: post #8

Is this right?: * Although you can make the enveloping sphere as large as you want, the (anti-)equilibration process requires a sphere of some finite radius because if you wait long enough a few stars eventually get launched at escape velocity, and if these actually escaped they would effectively cool the remaining stars. * Therefore, the characteristic time scale for this process (i.e., the timescale on which the av…

I was asked this question on my grad candidacy exam (which was quite a while ago, so my memory is hazy), and I believe stars tend to form binary systems which can halt the runaway gravitational collapse (the potential energy in a hard binary can be a very significant fraction of the total energy of a relatively loosely bound globular cluster).

In real life this can happen, but I think only by flinging out stars on escape trajectories. That’s the thing explicitly prevented by the reflecting sphere in the model discussed in the post.

Basically, the issue is that you can’t end up in a stable equilibrium of binaries (and binaries of binaries) in a bounded phase space because the dynamics are time-reversible. The only way you get coarse-grained irreversible behavior is with an unbounded phase space where there are no recurrences.

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