What is the role of dark energy in the potential occurrence of a Big Crunch?

[Nugso]

Questions about "Big Crunch"

http://map.gsfc.nasa.gov/universe/uni_shape.html

From the picture above, it looks like the universe has three kinds of shapes. ( Density>1, density<1, density=1) So how the density of the universe will be(if it'll ever) be smaller than 1? How does it change? What 'affects' it?

Also, what will happen happen at the Big Crunch? What will happen when Big Crunch starts to happen? Will the stars burn out? Since the universe will be getting smaller, will the Earth and other planets etc be flat? If not, how will they be affected by it? Will we(or the ones who'll be able to live) witness to it? Or will it be instant?

I guess I asked a lot of questions but I felt like I have to know them all. Thanks in advance.

 

[timmdeeg]

http://map.gsfc.nasa.gov/universe/uni_shape.html

From the picture above, it looks like the universe has three kinds of shapes. ( Density>1, density<1, density=1) So how the density of the universe will be(if it'll ever) be smaller than 1? How does it change? What 'affects' it?

Also, what will happen happen at the Big Crunch? What will happen when Big Crunch starts to happen? Will the stars burn out? Since the universe will be getting smaller, will the Earth and other planets etc be flat? If not, how will they be affected by it? Will we(or the ones who'll be able to live) witness to it? Or will it be instant?

I guess I asked a lot of questions but I felt like I have to know them all. Thanks in advance.

Ω isn't the density, it describes the ratio energy density / critical density. According to current data this ratio is very close to 1. This means that the Big Crunch is rather unlikely.
A hypothetical Big Crunch would end up in a big crush.

 

[Nugso]

Ω isn't the density, it describes the ratio energy density / critical density. According to current data this ratio is very close to 1. This means that the Big Crunch is rather unlikely.
A hypothetical Big Crunch would end up in a big crush.

Thanks for the correction. Um, so do you mean it's unlikely for the radio energy density to change? If so, why?

Edit: Also, will it then be instantenous? Or will we be able to witness?

 

[timmdeeg]

As the value of Ω is so close to 1, most cosmologists think that the geometry of the universe is flat. Ω = 1 means flat, see the picture in your link. In this case the universe should expand forever. But as the nature of the dark energy is unknown, nobody can predict the final fate of the universe with certainty.

Big Crunch means that the expansion of the universe will stop and it then starts to contract. Like a stone which you throw upwards achieves its maximum height and then falls down.

 

[Nugso]

As the value of Ω is so close to 1, most cosmologists think that the geometry of the universe is flat. Ω = 1 means flat, see the picture in your link. In this case the universe should expand forever. But as the nature of the dark energy is unknown, nobody can predict the final fate of the universe with certainty.

Big Crunch means that the expansion of the universe will stop and it then starts to contract. Like a stone which you throw upwards achieves its maximum height and then falls down.

So it won't be instantaneous. It'll take millions of years(not exactly, I've just made it up) for big crunch to happen.

 

[Tanelorn]

Could there be more than one big crunch? What I mean is that everything smaller than a small galaxy cluster eventually collapses and everything larger than a small galaxy cluster expands due to D.E.?

 

[Nugso]

Um, could there be more than one big crunch? I don't what you mean by this. Will those so-called collapses and expansions happen instantaneously?

 

[phinds]

So it won't be instantaneous. It'll take millions of years(not exactly, I've just made it up) for big crunch to happen.

Uh ... "millions" is low by MANY orders of magnitude (if it did happen, which I don't think it will).

 

[Nugso]

Uh ... "millions" is low by MANY orders of magnitude (if it did happen, which I don't think it will).

Well, at first I was going to write thousands of years. :shy: Why do you think it won't happen what's going to happen instead? Because of the ratio thing timmdeg mentioned above?

 

[timmdeeg]

Could there be more than one big crunch? What I mean is that everything smaller than a small galaxy cluster eventually collapses and everything larger than a small galaxy cluster expands due to D.E.?

I don't think so. Once the pull of gravity exceeds any repelling energy the universe as a whole will start to contract, provided it is homogeneous and isotropic as assumed by the Einsteinian models.

 

[phinds]

Well, at first I was going to write thousands of years. :shy: Why do you think it won't happen what's going to happen instead? Because of the ratio thing timmdeg mentioned above?

There is zero evidence for it in our current understanding of cosmology. The universe is expanding at an accelerating rate and there is no evidence that it will stop doing so. What's going to happen is that galactic clusters will eventually make up their entire observable universe. That is, each galactic cluster will exist in an observable universe that has nothing else in it.

 

[Nugso]

There is zero evidence for it in our current understanding of cosmology. The universe is expanding at an accelerating rate and there is no evidence that it will stop doing so. What's going to happen is that galactic clusters will eventually make up their entire observable universe. That is, each galactic cluster will exist in an observable universe that has nothing else in it.

Oh thanks very much. I've read an article saying that the acceleration of the universe is positive, not negative hence the universe, at least according to what today's physics says, will always expand.

 

[Naty1]

the acceleration of the universe is positive, not negative hence the universe, at least according to what today's physics says, will always expand.

That's not quite right. The acceleration of the universe IS currently increasing, yes, but whether that will continue forever, 'always', as already noted, is not known.

A big issue is whether the negative gravitational pressure, or cosmological constant, or
'repelling energy of the universe' [from a post above] is constant or not. This aspect of vacuum energy, the energy of free space, so far seems to maintain a constant density, so as distances increase there is more and more energy to power accelerated expansion . But if this energy density actually gets smaller over billions and billions of years, then the expansion of the universe could slow and eventually begin to contract. Such theories are often referred to a cyclic cosmology...meaning repeated expansions and contractions.

So in terms already posted, our best measures show the value of Ω [omega] close to 1, very close to a flat universe. If it stays that way, expansion continues and the universe remains about flat and eventually becomes cold, dark and empty. Nothing more happens. But whatever the end is, don't worry about that, our sun will die and destroy our solar system billions of years before any potential end of the universe.

 

[Nugso]

That's not quite right. The acceleration of the universe IS currently increasing, yes, but whether that will continue forever, 'always', as already noted, is not known.

A big issue is whether the negative gravitational pressure, or cosmological constant, or
'repelling energy of the universe' [from a post above] is constant or not. This aspect of vacuum energy, the energy of free space, so far seems to maintain a constant density, so as distances increase there is more and more energy to power accelerated expansion . But if this energy density actually gets smaller over billions and billions of years, then the expansion of the universe could slow and eventually begin to contract. Such theories are often referred to a cyclic cosmology...meaning repeated expansions and contractions.

So in terms already posted, our best measures show the value of Ω [omega] close to 1, very close to a flat universe. If it stays that way, expansion continues and the universe remains about flat and eventually becomes cold, dark and empty. Nothing more happens. But whatever the end is, don't worry about that, our sun will die and destroy our solar system billions of years before any potential end of the universe.

Thanks for the answer. But as it stands, the universe is currently expanding and will expand until the thing you mentioned before happens. Is there any evidence of it? How can energy density change? What changes it?

 

[Naty1]

Is there any evidence of it? How can energy density change? What changes it?

I believe there was a recent discussion on this..I'll post a link if I find it...We are still searching for answers as to what comprises about 95% of the content of
the Universe. Over 70% of the mass-energy content of the Universe is in form of the unknown vacuum energy called “dark energy”. Over 80% of the mass is in the form of the mysterious “dark matter”. A negative pressure dark energy is the same thing as a cosmological constant,
but we still have a lot to learn

The existence of DE is inferred strictly from its effect on accelerated expansion of the universe. Where it came from [other than a 'bang'] why it's here, where it's going is theoretical...not much experimental, observational detail yet.

Up until the later 1990's, it was assumed that attractive gravity was slowing down the rate of expansion and that eventually it would either reach a steady state and just go on at a constant rate forever, or much more likely would reverse direction and contract in a Big Crunch. Bad assumption: When the first measurements were made, much to everyone's surprise, it was found that not only is the rate of expansion NOT slowing down, it is ACCELERATING.

 

[Nugso]

I believe there was a recent discussion on this..I'll post a link if I find it...


We are still searching for answers as to what comprises about 95% of the content of
the Universe. Over 70% of the mass-energy content of the Universe is in form of the unknown vacuum energy called “dark energy”. Over 80% of the mass is in the form of the mysterious “dark matter”. A negative pressure dark energy is the same thing as a cosmological constant,
but we still have a lot to learn

The existence of DE is inferred strictly from its effect on accelerated expansion of the universe. Where it came from [other than a 'bang'] why it's here, where it's going is theoretical...not much experimental, observational detail yet.

Up until the later 1990's, it was assumed that attractive gravity was slowing down the rate of expansion and that eventually it would either reach a steady state and just go on at a constant rate forever, or much more likely would reverse direction and contract in a Big Crunch. Bad assumption: When the first measurements were made, much to everyone's surprise, it was found that not only is the rate of expansion NOT slowing down, it is ACCELERATING.

Thanks for explanation Naty1. I guess that's ( being unable to explain D.E) the reason it's called Dark Energy. Looking forward to reading the comments on the thread you're going to post(if you find it). It will sure clear up a few things on Big Crunch and stuff for me.