[yor_on (OP)]

I'll cite this "  if the density exceeds a certain critical value, the gravitational pull of each mass on every other mass will slow down the expansion of the universe "

My own take is that the gravitational pull should decrease with distance. Even though it is defined as infinite you should be able to assume that with a infinite distance between suns etc the 'space' between them becomes 'flat'.

so how did we reach this conclusion?
If you know the history of it it's even better, because I've searched for it without finding it.
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the question comes from a argument in where a expansion should slow down, to finally reverse and contract, due to the mass density becoming diluted (suns etc, more spread out inside a universe) which I sincerely doubt. Mass/energy bends space, the other way to describe it as in a Big Bang being close to a critical point, in where nothing can expand due to too much energy inside a confinement, makes a lot more sense to me. That one should be able to connect to the Schwarzschild radius, defining a black hole too.
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Hmm, rereading myself I see that it can be understood as I'm asking about what I thought of as the main stream history. But the question is about the one in where this mass density spreading out, at some critical point, leading to a contraction.

( And I saw it, will not say where, expressed by a physicist, one that usually makes a lot of sense.. Which actually made me assume that this was accepted main stream science that everyone except me knew about, which lead me to formulate this question the way I did  :)

[Petrochemicals]

Dunno, but if the big bang is anything like a conventional chemical explosion it will. As far as i understand the universe is accelerating in its expantion, possibly because the energy component is still very dense, as this decreaces and space enlarges, just like a conventiona explosion, it will slow. If it follows this form the projectiles will fall from the explosion and the pressure wave will collapse.

[yor_on (OP)]

The idea of energy dissipating makes sense Petrochemical, but a 'perfect' vacuum has no friction. And if the energy amount for a universe stays the same, assuming us not to 'gain' energy from 'outside' in a expansion, then why should less gravity over a volume lead to a expansion reversing?

[evan_au]

if the density exceeds a certain critical value, the gravitational pull of each mass on every other mass will slow down the expansion of the universe

This was considered a possible outcome of the Big Bang before Dark Energy was discovered. Depending on the initial density of the universe, there were 3 possible outcomes:
1. If the density was very high, the universe could expand for a while, but gravity would slow it down until expansion stopped, and then gravity would pull it together in a "Big Crunch" (or "Gnab Gib", as one cosmolgist described it).
2. If the density was very low, the universe could expand, and gravity would slow it down slightly, but it would still keep expanding forever. As yor_on says, this is possible because the strength of gravity is reduced as the universe expands.
3. If the density were "Just Right", the universe would keep expanding forever, and gravity would slow it down a lot, but not quite enough to ever stop it.

This is analogous to the situation with a spaceship leaving Earth; there are 3 possible outcomes:
(1) if it is below escape velocity, it will travel some way, but gravity will bring it back to Earth;
(2) If it exceeds escape velocity, gravity will slow it down, but it will keep speeding away forever (As yor_on says, this is possible because the strength of gravity is reduced as the distance increases);
(3) If it exactly matches escape velocity, gravity will slow it down a lot, but it will keep traveling away.

A number of cosmological surveys tried to measure the density of the universe, and the rate of expansion. Unfortunately, the results were too close to model 3 to be able to clearly define which model was correct.

The searches that discovered Dark Energy were using bigger telescopes, and hoping to refine the previous measurements, and perhaps resolve which one of the 3 models was correct. However, they found something quite unexpected:
- The rate of expansion was actually increasing over time
- Something that none of these three models suggested
- This led to the Dark Energy hypothesis, sometimes expressed as a non-zero value for Einstein's Cosmological Constant.
See: https://en.wikipedia.org/wiki/Big_Crunch

[yor_on (OP)]

Thanks Evan, that one do make sense, that's how I think of it too. Still, the one I'm wondering over is when you get a spread out universe due to a expansion and at some 'critical point' of dilution of matter this leading to the expansion reversing (contracting). It may have been poorly written but that was what it stated to me.
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And this is just a question of the mass density, as far as I got it. No other energies involved. If that one really exist the history of its development should be fairly interesting. And there was nothing in this argument that I saw, defining it from a proposed overwhelming mass density. So it might have been poorly written by its author, but it got me really confused.

[yor_on (OP)]

So the question stands.

How can one define it such as with a lower mass density, stars galaxies etc more spread out due to a accelerating expansion, there will be a 'critical point' in where this 'thinning' leads to the opposite, aka a contraction?
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I'm not saying it's impossible, but I think you need something more than just gravity for it to happen?
And this should be when we define it such as the magnitude of energy existing stay the same, only transforms from useful to non-useful .

Assuming us to 'gain energy' by a expansion is something entirely different, no matter if you define it as keeping a equilibrium, or not. That one will question relativity, as with relativity you only have a 'inside', as far as I understands it. Or you have now redefined the universe using it.

[Janus]

Thanks Evan, that one do make sense, that's how I think of it too. Still, the one I'm wondering over is when you get a spread out universe due to a expansion and at some 'critical point' of dilution of matter this leading to the expansion reversing (contracting). It may have been poorly written but that was what it stated to me.

If you are referring to this:
"  if the density exceeds a certain critical value, the gravitational pull of each mass on every other mass will slow down the expansion of the universe "

There is nothing in that sentence that supports that interpretation.

[yor_on (OP)]

I know Janus, I quoted it assuming that such a possibility was accepted science, it's not from the guy I read but from another source as I tried to see how such an idea would work. Because the idea went against everything I thought I understood. But I've been away for a while so maybe I missed something new, or old for that matter. Thinking of it I would have formulated the question differently if I hadn't assumed the statement to be already accepted science. That you by diluting the mass of a universe through expanding its volume, reach a contraction. I've seen a lot of outlandish ideas, but that one made me confused.
=

You might say that I read a very authoritative source, and so accepted the statement, although still not understanding how it was possible. Therefore the question. But as it seems that no one recognize this I will presume that it was written assuming the science I already knew. Which means that it lacked a definition of what mass density he meant for this universe to contract.

[evan_au]

Einstein's Cosmological Constant

This "Cosmological Constant" has a central part in this story, as indeed the quoted sentence was accepted science from about the 1930s (when Hubble discovered the expansion of the universe) until the 1990s (when Schmidt & others discovered the accelerating expansion of the universe).

The Cosmological Constant falls out of Einsteins field equation, which describes the whole universe (at a very broad scale)
- The Cosmological Constant is represented by the Greek letter Lambda (Λ)
- The density of the universe is represented by the Greek letter Omega (Ω)
- It was the optimistic view of cosmologists from antiquity until the 1930s that the universe beyond the planets was unchanging. To match this prevailing world view, Einstein originally set Λ to be a magical value that would allow the universe to be static (he later described this as his greatest mistake).
- The Belgian priest Lemaitre suggested that a different value for Λ would allow for a beginning to the universe
- Once Hubble showed the expansion of the universe, cosmologists just assumed that Λ=0
- Once Schmidt showed the accelerating expansion, cosmologists assumed that Λ>0, but small.
- Ongoing studies are trying to refine the value of Λ and to find if Λ has varied over time.

See: https://en.wikipedia.org/wiki/Cosmological_constant

if the density exceeds a certain critical value, the gravitational pull of each mass on every other mass will slow down the expansion of the universe

Even with today's recognition of an accelerating expansion, it has been observed (with even better telescopes) that up to about 5 billion years of age, gravitation dominated over Dark Energy. So the expansion of the universe did slow down (but not stop).
After 5 billion years, the density of matter in the universe (Ω) was low enough that Dark Energy dominates, and we see an accelerating expansion today.

See the graph here: https://en.wikipedia.org/wiki/Expansion_of_the_universe#Theoretical_basis_and_first_evidence
Ω > 1 is the scenario where the universe collapses again
Ω = 1 is the scenario where the universe has no matter, and expansion does not slow due to gravity
Acceleration is the situation we see today

[savemars]

I'll cite this "  if the density exceeds a certain critical value, the gravitational pull of each mass on every other mass will slow down the expansion of the universe "

I came here to discuss this theory but from a different perspective. The problem comes down to a simple problem, why is the universe expanding. I believe I have figured this out it. it will also answer your question. Think of it like this. gravity pulls/ compresses the fabric of the universe. like we where taught in school a ball on a sheet will explain. if you place a ball on a sheet it will make a simulated gravity well, but at the same time it will cause the fabric around the well to expand.  every force has an equal and opposite force, there for as gravity pulls the fabric it must also push it. the speed of expansion is equal and opposite of the force of gravity. this is also why the expansion is not the same everywhere, because gravity is not the same everywhere. this is also why I disagree with the dark matter explanation. think if you grab a sheet and compress a portion of it the rest of the sheet must expand too. the speed of expansion should be a directly related to the forces of gravity pulling on it. I am not highly educated, and am unable to understand the math, but I fully understand forces and there effects. I am just unable to put it into numbers

[Kryptid]

Think of it like this. gravity pulls/ compresses the fabric of the universe. like we where taught in school a ball on a sheet will explain. if you place a ball on a sheet it will make a simulated gravity well, but at the same time it will cause the fabric around the well to expand.  every force has an equal and opposite force, there for as gravity pulls the fabric it must also push it.

The rubber sheet analogy is just that: an analogy. It isn't that gravity is pulling or pushing on space. Gravity is, in itself, the distortion of space-time.

the speed of expansion is equal and opposite of the force of gravity

Force isn't a velocity, so I can't make sense of this statement.

[evan_au]

the speed of expansion is equal and opposite of the force of gravity

As Kryptid said, you can't directly compare a speed and a force (although a force on an object does change the speed of the object).

However, there is an "acceleration of the expansion of the universe" due to dark energy, and an "acceleration of the expansion of the universe" due to gravity.
- It is true that they have opposite signs - dark energy accelerates the expansion of the universe, and gravity decelerates it
- Both of them vary over time, due to different factors, so they are not equal in magnitude today
- Gravitational deceleration was greatest when the universe was most dense. The density of the universe has been declining ever since the Big Bang, and so the deceleration due to gravity is getting weaker and weaker over time.
- Dark Energy acceleration seems to be proportional to the volume of the universe, so as the universe expands, the acceleration due to Dark Energy gets greater and greater over time
- Gravity and Dark energy cannot be "equal and opposite" if one is getting weaker over time, and the other is getting stronger over time.
- Gravity and Dark Energy were temporarily equal and opposite for a moment in time, when the universe was about 5-6 billion years old. At this time, the universe moved from a gradual deceleration (gravity dominated) to a gradual acceleration (Dark Energy dominated).
- But the expansion did not stop - inertia of the expanding universe carried it through this tipping point, and we are now into accelerating expansion, with Dark Energy dominating.

As for the cause of the expansion, that is a matter of considerable debate among cosmologists.
- Just like condensing steam releases heat (a change of phase of water from gas to liquid)
- Some cosmologists think that there was a phase change in the universe, causing it to expand in size enormously in an instant of time
- Some have suggested that this was due to the 4 fundamental forces splitting apart, and becoming distinct as we see them today
- This period was called "cosmic inflation", and the results of it are visible today in the Cosmic Microwave Background
See:  https://en.wikipedia.org/wiki/Inflation_(cosmology)

[Kryptid]

I'll cite this "  if the density exceeds a certain critical value, the gravitational pull of each mass on every other mass will slow down the expansion of the universe "

I came here to discuss this theory but from a different perspective. The problem comes down to a simple problem, why is the universe expanding. I believe I have figured this out it. it will also answer your question. Think of it like this. gravity pulls/ compresses the fabric of the universe. like we where taught in school a ball on a sheet will explain. if you place a ball on a sheet it will make a simulated gravity well, but at the same time it will cause the fabric around the well to expand.  every force has an equal and opposite force, there for as gravity pulls the fabric it must also push it. the speed of expansion is equal and opposite of the force of gravity. this is also why the expansion is not the same everywhere, because gravity is not the same everywhere. this is also why I disagree with the dark matter explanation. think if you grab a sheet and compress a portion of it the rest of the sheet must expand too. the speed of expansion should be a directly related to the forces of gravity pulling on it. I am not highly educated, and am unable to understand the math, but I fully understand forces and there effects. I am just unable to put it into numbers

I would also like to point out that we have a section on this board where new ideas are supposed to be posted (called, of course "New Theories").