Regarding the Big Crunch

[ericksvensson]

Greetings, PF community. I have a very simple question regarding the Big Crunch theory.

As far as I know, the Big Bang was not an explosion in space, but expasion of space itself. But how about a possible Big Crunch? Does this theory deal with the contraction of space itself, or is it just galaxies attracting eachother due to gravitational forces?

I may have a misconception, but I believe the Big Crunch could only possibly happen on a closed universe, and has to do with the overall density of the universe. But the point I'm still missing, is that I'm not sure if this theory implies a decrease in the expasion of the universe until a critical point in which space would begin to contract, or would it simply be an scenario where galaxies begin to attrack eachother and we end up with a very dense concentration of mass in an almost-empty universe.

Perhaps I'm missing some fundamental concepts here, sorry if this is too basic.

Thanks!

[nicksauce]

I'm not sure if this theory implies a decrease in the expasion of the universe until a critical point in which space would begin to contract

This is correct.

[DaveC426913]

Good point.

The universe is expanding and space along with it. There are several theories as to what is causing this, but whatever it is, it is something more exotic than the 4 fundamental forces.

So, if gravity slows and stops the expansion, and then begins to contract again, by what logic would space start to contract as well? Seems to me, the matter in the universe would start to coalesce but empty space would not; it would be "left behind".

When the Big Crunch came, it would be an event in space, not of space.

I think.

Not sure if it is meaningful to talk about empty space "out there" if there's no matter in it.


[EDIT]Upon reflection, I'm thinking expansion and compression must be symmetrical.

[korquad]

so what's really happening then?

does the dark energy cause the expansion of space directly or it acts on matter thus forcing space to create itself and overall seem like an expansion? (from my understanding, the former would be in consensus with the present theory, but i want to be sure..)

and how is gravity seen in this picture? it is said that gravity curves space.. does that mean gravity, supposing it would mash together all matter, will bend space in such manner that it would intertwine with matter and contract along with it? or the curvature of space doesn't take place that way and it would still be possible for it to exist outside matter?

(these may well be questions already asked and possibly answered on this forum, but if there is something to be blamed, that is my inability to have found those threads.. do not let this stop you from answering to my inexperienced concerns..)

[universe11]

My question: how was calculated the singularity(zero volume, infinite density) of the Universe, black holes?
Because I consider the Universe(black hole) can not have a volume less than the mass of the Universe(black hole) multiplied with the Planck density.

[korquad]

i know my question may have been so elementary that it isn't worth answering, but still can anyone show me in the right direction? just trying to put pieces together regarding our universe like anybody else (even if it is at a lower level of understanding, since i have just begun seriously studying physics..)

[Rick88]

Dark energy causes the expansion of space itself (at least this is the current understanding, but we don't really know what dark matter/energy is!)

If there will be a contraction, space itself will contract so I am not sure what you're really saying about matter, gravity and space. Try explaining it again :)

R.

[korquad]

Dark energy causes the expansion of space itself (at least this is the current understanding, but we don't really know what dark matter/energy is!)

thank you for confirming that..

If there will be a contraction, space itself will contract so I am not sure what you're really saying about matter, gravity and space. Try explaining it again :)


i was only curious about the nature of gravity and it's relation with space.. that's how i found this specific thread and the common curiosity of the OP..

you said in case of contraction 'space itself will contract'.. does that mean gravity has the same property as dark energy, only that it affects space by contracting it? or it actually has an impact only on matter, but incidentally while all the matter in the universe would converge to a point, a singularity (or however would it be properly called) due to gravity, that very fact would also 'drag' space along with it?

and the other question from my previous post was kind of the reverse of this second possibility: if gravity contracts matter, but not space, would it remain mostly empty in the end?

(some of these questions were also mentioned in the other posts before mine.. but since they remained unanswered, i wanted to update my/our concerns..)

on another note, i still don't understand why would there be a big crunch, since the universe is expanding and not only that, but also accelerating.. of course it can be said that dark energy, since we don't know its nature, might not always influence space as in this moment.. or perhaps the universe is expanding only to find out it will collide with itself at one moment.. but these are topics for other threads..

in any case, big crunch or not, the questions are more related to the way space behaves and why..

hopefully i made myself clearer..

korquad..

[Siberion]

I just want to make clear that, as far as I understand, dark energy tends to increase the rate of expansion of the universe. It is not the primordial source of expansion.

I think the question was more directed to a lack of understanding of how the Big Crunch would really work. When I tried to picture this event, I came up with an scenario in which gravitational force would overcome universal expansion, just like galaxies close enough to eachother attrack eachother in the galaxy clusters scenario. My first guess was that empty space would be simply left behind, but upon thinkin I think that Dave is right when he states that contraction and expansion must be symmetrical.

[korquad]

empty space would be simply left behind, but upon thinkin I think that Dave is right when he states that contraction and expansion must be symmetrical.

where does that 'must' come from? (since i'm sure you're not talking about the aesthetics of symmetry..)

[Siberion]

where does that 'must' come from? (since i'm sure you're not talking about the aesthetics of symmetry..)

Sorry if I did not make myself clear. The must in my previous statement was a mere representation of my lack of understanding of the subject on a more deep context, according to my current general definition of the Big Crunch.

Perhaps I should rewrite my sentence as, 'As far as I understand, the Big Crunch proposes an scenario where the metric expansion of space eventually reverses. If that would be the case, there would be no point in talking of metric expansion reversing if the space is simply left behind. Therefore contraction should be symmetrical to expansion'.

Of course this is merely intuitive, and I'm pretty sure that intuiton does not help much when we talk about these kinds of topics. Perhaps someone more expert in the subject can correct me, since its very probable that I'm missing some concepts here.

[Rick88]

thank you for confirming that..



i was only curious about the nature of gravity and it's relation with space.. that's how i found this specific thread and the common curiosity of the OP..

you said in case of contraction 'space itself will contract'.. does that mean gravity has the same property as dark energy, only that it affects space by contracting it? or it actually has an impact only on matter, but incidentally while all the matter in the universe would converge to a point, a singularity (or however would it be properly called) due to gravity, that very fact would also 'drag' space along with it?

and the other question from my previous post was kind of the reverse of this second possibility: if gravity contracts matter, but not space, would it remain mostly empty in the end?

(some of these questions were also mentioned in the other posts before mine.. but since they remained unanswered, i wanted to update my/our concerns..)

on another note, i still don't understand why would there be a big crunch, since the universe is expanding and not only that, but also accelerating.. of course it can be said that dark energy, since we don't know its nature, might not always influence space as in this moment.. or perhaps the universe is expanding only to find out it will collide with itself at one moment.. but these are topics for other threads..

in any case, big crunch or not, the questions are more related to the way space behaves and why..

hopefully i made myself clearer..

korquad..

Ok, let's talk a bit about the background of the big crunch theory because there are some confused ideas here.

If one studies Friedmann's equation: \dot{R}2 = \frac{8}{3}\piR2 - k + 1/3\LambdaR2, where R is the scale factor of the universe and depends on time, k is the curvature of the Universe and \Lambda is a constant to represent the presence of dark matter.
From this equation, some models of the Universe can be extrapolated by letting one term (in the RHS) dominate over the others.
To obtain the Big Crunch, we need to consider large values of R. That is pretty much now. So not small values of R when the Universe was still "small" because of the Big Bang.

This would make the Lambda term the dominant one, and we would therefore obtain that R = e(1/3\Lambda)^1/2.
If we let \Lambda < 0, we obtain a negative acceleration.

Therefore when the Universe reaches a large value of R, say now, the term due to dark matter will firstly slow down the Universe expansion and then reverse it to a deceleration. This would culminate into a singularity, the Big Crunch. Therefore the whole Universe (or space) would contract due to gravitational interactions between matter (dark and non). Gravity doesn't really come into play.

As a final note, this model is *incorrect* as it has been observed that the Universe is undergoing an acceleration due to dark matter.

But it was a fine theory.

[Siberion]

...

Thanks for the reply, but when you refer to dark matter, do you really mean it, or should I replace for 'dark energy', in all of the sentences? Or perhaps only in a few of them?

[Rick88]

Good point, it should be dark energy.
But the difference is subtle, so it doesn't really matter.

R.

[korquad]

thank you for replying.. my question lied just in what you said right here (although you might have meant something else in the second phrase, since, from what you first said, gravity does come into play..):

Therefore the whole Universe (or space) would contract due to gravitational interactions between matter (dark and non). Gravity doesn't really come into play.

so space is contracted indirectly by wrapping itself around matter, 'shrinking' along with it..

[Rick88]

Yes, i would say so.

R.

[korquad]

all right.. now i should ask how come space would completely curl around matter making it impossible to exist on its own and also be filled with 'nothing' (only with the possibility to 'shelter' matter).. but that one i guess is a question i should put on hold until i'll gather more understanding of physics and its underlying math..

[Rick88]

Well, the thing is that "nothingness" can't exist. There is just no such thing.

Try with this interesting thought: Imagine a room. Nothing can be outside of it. The room is your Universe.
The room is full of objects. Maybe you can't see them, because they could just be particles such as electrons, protons, neutrons and photons.
Now take everything away, every single particle, even light.
Can you imagine the room filled by emptiness? I can't. :)

[korquad]

Well, the thing is that "nothingness" can't exist. There is just no such thing.

Try with this interesting thought: Imagine a room. Nothing can be outside of it. The room is your Universe.
The room is full of objects. Maybe you can't see them, because they could just be particles such as electrons, protons, neutrons and photons.
Now take everything away, every single particle, even light.
Can you imagine the room filled by emptiness? I can't. :)

why not? given your analogy, by removing every trace of matter from that room, wouldn't make the room disappear.. it has the property of harboring matter, but it happens to be empty.. thus by filling it again with matter (let's say through the crack in the wall which suddenly emerged because some god bashed his head on the other side) it would be a process which takes place differently, governed by different math, than the first time when the room had to be created piece by piece..

little edit: just to be sure you know what i mean although i think you got it: that 'nothingness' i'm talking about is not related to that in which some people thought the universe is expanding into..

[Rick88]

you are assuming that space can exist without the presence of matter.

[korquad]

you are assuming that space can exist without the presence of matter.

yes.. and that assumption started my inquiry on this thread.. to find out if it's plausible or not given our present understanding of the universe..

[Rick88]

space without matter is useless.
You can't interact with it, because there is no matter.
you can't even see it, because there is no matter.

Also the fact that "nothing" is inside it would make it not exist.
(it's a weird concept, i know. but there is just no such thing as "emptiness")

[Naty1]

This covers current terminology and concepts:

http://en.wikipedia.org/wiki/Big_crunch

When you use the terminology BIG CRUNCH you necessarily imply the contraction of space itself.
But that is not generally what is believed will happen as Hubble's observations imply an increasing rate of expansion, NOT a slowing as required for any kind of crunch....gravitational or otherwise....

[Naty1]

you are assuming that space can exist without the presence of matter....]

There is no evidence to the contrary; GR tells us SPACETIME is an entity, not just space nor time independently.

However, quantum mechanics assures us there are vacuum fluctuations in any spacetime...and hence there is NO perfect "emptiness". Spacetime is never truly "empty" and the common terminology "vacuum" does NOT mean nothingness....

Another way to think of these entities is that vacuum fluctuations emanated from the big bang along with spacetime....so there is some fundamental connection between them we do not yet understand.....just as there is dark matter and other forms of energy, the four fundamental forces, all which were apparently "unified" at the moment of the big big....

[korquad]

When you use the terminology BIG CRUNCH you necessarily imply the contraction of space itself.


well, yes.. but the curiosity was related on how would the contraction of space have taken place..

]
However, quantum mechanics assures us there are vacuum fluctuations in any spacetime...and hence there is NO perfect "emptiness". Spacetime is never truly "empty" and the common terminology "vacuum" does NOT mean nothingness....


indeed i heard about that.. but that would constitute a 'nothingness' in a sense.. from what i recall, those fluctuations are seen as constant instances of emergence and annihilation of virtual particles.. and the 'vacuum' also possesses a certain energy, which in the absence of matter, would drop to zero, so to speak, not having anything to compare it with.. so overall 'vacuum' might be considered empty in the absence of matter..

(or perhaps not..)

[sri sharan]

I think there might be a possibility that matter might 'drag' space along it in a big-crunch scenario if by that time the entire matter (or at least most of it) in the universe is in the form of black holes.(which might be possible considering the fact that many galaxies have super-massive black holes).
1)Black hole rotate around themselves.
2)When they rotate they wrap space time around them.
3)Now as the black holes start moving towards each other in the big crunch scenario,they drag space time along with them,just as a spinning top on a cloth will wrap some cloth and also drag it.

So,is it possible or am I wrong anywhere?

[Naty1]

the 'vacuum' also possesses a certain energy

yes

, which in the absence of matter, would drop to zero, so to speak,not having anything to compare it with.. so overall 'vacuum' might be considered empty in the absence of matter..

no evidence that is so....

[bcrowell]

space without matter is useless.
You can't interact with it, because there is no matter.
you can't even see it, because there is no matter.

Also the fact that "nothing" is inside it would make it not exist.
(it's a weird concept, i know. but there is just no such thing as "emptiness")

Naty1 makes a good point in #24.

Also, a gravitational wave is a detectable phenomenon that carries energy, but it occurs in space without matter.

[Naty1]

I think there might be a possibility that matter might 'drag' space along it

note that frame dragging IS a theoretical phenomena; do not know if there is any experimental/obserbational evidence...

...in a big-crunch scenario if by that time the entire matter (or at least most of it) in the universe is in the form of black holes.(

too much speculation here.....but the potential could, I guess, exist for everything to eventually end up as black holes, and over eons those smaller ones coalesce together.....but as I understand things, that still doesn't collapse all space in the universe into a crunch from what is currently known....

On the other hand things/views/theories/insights change so I never say never...

[Rick88]

Naty1 makes a good point in #24.Indeed. i was being careless in the words used.

[LBrandt]

To put it quite simply, if we assume that the Big Bang concept is correct, then space (and time) were created as a result of it. Space isn't a separate entity that simply is "out there". Space is a function of mass. Without the Big Bang, there is no space. Without mass, there is no space. Therefore, if one wants to postulate a "big crunch", it has to include the elimination of everything, mass, space, energy...everything.

[sri sharan]

note that frame dragging IS a theoretical phenomena; do not know if there is any experimental/obserbational evidence...
Hmm looks like there is some evidence for it after all.Have a look at the below link.
http://web.mit.edu/newsoffice/1997/blackholes.html


too much speculation here.....but the potential could, I guess, exist for everything to eventually end up as black holes, and over eons those smaller ones coalesce together.....but as I understand things, that still doesn't collapse all space in the universe into a crunch from what is currently known....

On the other hand things/views/theories/insights change so I never say never...
i know but that it sounded good(it still does) when i first thought of it.thanks anyway

[DaveC426913]

Space isn't a separate entity that simply is "out there". Space is a function of mass.

While I don't disagree with anything else you said, the above does not follow. You went from space to mass.

[BrainVoid]

Excellent name for a candy bar....on that I'm sure we all agree.

[Chronos]

Space without 'space' is illogical to me. I fail to see how 'quantum fluctuations' can arise in the absence of 'space'. I'm a fan, but, not a huge fan of 'creation ex nihilo'. But 'eternal space' is not a satisfactory alternative. There must be a better alternative.

[Naty1]

Sri...good find from MIT:

They showed that the matter's orbit in the accretion disk would wobble, much as a child's top wobbles when it slows down. The frequency at which it would wobble, based on their calculations, turned out to be the same frequency as the actual oscillations in intensity of the x-ray emissions previously measured by other researchers. They theorized that this wobble is evidence of frame-dragging, because the matter's orbit can only wobble if the space and time in which it exists are being dragged.

a possible verification of frame dragging?? So they think....

[universe11]

Maybe the accretion disk is the gamma rays burst in young galaxies...