Universe always expands???

[Pradeepkumar]

I have read that Universe will go on expanding(Big Bang Theory).At one stage the expansion stops and the universe will shrink again(Big Crunch theory).Is it true ???

[ajb]

Probability not given what we now know about cosmology. For instance dark energy and the near flatness of the universe.

[pioneer]

Dark matter and dark energy poses an interesting consideration. Let us assume it exists, since we don't fully understand it and its properties, the question one asks is what happens when it interacts with light energy? Does it create a doppler shift looking affect? The doppler shift calculations were from a time before we even knew there was dark stuff. The dark stuff came later due certain anomalies. Maybe there is cause and affect. Gravity will contract light so anti-gravity should expand it, which would look similar to doppler. The most distant stuff goes through the most filter, so it should be more shifted. It is not clear what percent this might be.

[Klaynos]

We don't know... it might expand forever it might just stop and do nothing eles it might collapse...

[Arch2008]

At the present rate, the universe would expand forever while gradually slowing, but never stopping. However, if the effect of Dark Energy continues to grow, then in approximately fifty billion years DE would overcome the Strong force locally and all the atoms would fly apart (the Big Rip). For the universe to “crunch” in the distant future, DE would have to “decay” into something with normal mass.

[Kyrisch]

I thought recent experimentation showed that the universe was expanding at an accelerating rate.

[Arch2008]

http://www.ringnebula.com/is_the_universe_flat.htm

 

If the expansion of the universe continues to accelerate, due to DE, then you get the Big Rip.

I was trying to address the three possible futures.

[Klaynos]

I thought recent experimentation showed that the universe was expanding at an accelerating rate.

 

But until we know the nature of dark energy we don't know how long this will last so maybe it'll end "soon" :S

[Edtharan]

Dark matter and dark energy poses an interesting consideration. Let us assume it exists, since we don't fully understand it and its properties, the question one asks is what happens when it interacts with light energy?

Umm...

 

The reason it is called "Dark Matter" is that it doesn't interact with light. So the question "what happens when it interacts with light" has the answer: Nothing because it doesn't interact with light.

[Klaynos]

Posts about "A + B = C" moved to: http://www.scienceforums.net/forum/showthread.php?goto=newpost&t=34665

[ajb]

Dark matter in the form of WIMPs are assumed to not carry electric or colour charge and so interact directly via the weak and gravitational force.

 

What this means is that any interaction with light is not going to be via the electromagnetic force directly. However, an interaction with light could be mediated via the weak and gravitational sectors. This will be suppressed, but non-zero.

[ajb]

Well Klaynos, if you think of space-time as a 4-d manifold with a (pseudo-)Riemannian metric then nothing is expanding anywhere.

[Klaynos]

Well Klaynos, if you think of space-time as a 4-d manifold with a (pseudo-)Riemannian metric then nothing is expanding anywhere.

 

How does that work?

[ajb]

I liken it to a cone. Locally it looks like [math]S^{1} \times\mathbb{R}[/math]. Now lets imagine the circle to be "space" and the line to be "time". The circle gets bigger as we move up the line. This is the "cosmological expansion". However, the cone as a whole does not change.

 

The same thing happens when we look at 4-d space-times. The spacial parts seem to expand, but when we take into account the whole space-time nothing "happens". Thus space-time does not need anything to expand into.

[Klaynos]

I liken it to a cone. Locally it looks like [math]S^{1} \times\mathbb{R}[/math]. Now lets imagine the circle to be "space" and the line to be "time". The circle gets bigger as we move up the line. This is the "cosmological expansion". However, the cone as a whole does not change.

 

The same thing happens when we look at 4-d space-times. The spacial parts seem to expand, but when we take into account the whole space-time nothing "happens". Thus space-time does not need anything to expand into.

 

That's a nice understandable way of explaining it so even dippy people like me get it

 

I half expected something like that... thanks.

[ajb]

No problem.

 

One of the biggest misunderstandings of the big bang is the issue of if the universe needs to be expanding into something. The honest answer is that although nothing is needed, it is not really known if our 4-d universe is embedded into a higher dimensional manifold or not. String/M-theory suggest it could be. However, from a classical general relativity point of view we don't really need to worry about this.

[Klaynos]

I've always been told that there's no requirement for the universe to expand into something but this is a very nice way of explaining why not!

[Dr. Jekyll]

I liken it to a cone. Locally it looks like [math]S^{1} \times\mathbb{R}[/math]. Now lets imagine the circle to be "space" and the line to be "time". The circle gets bigger as we move up the line. This is the "cosmological expansion". However, the cone as a whole does not change.

 

The same thing happens when we look at 4-d space-times. The spacial parts seem to expand, but when we take into account the whole space-time nothing "happens". Thus space-time does not need anything to expand into.

 

This kinda thinking I like! But it drags me down to another touchy area. If we regard the whole space-time as static (as a cone/unit ball/etc.), there is no ramdomness or "random spontaneouss decay," "double slit experiment randomness," "entangled information being send instantaneously," etc.

 

Why not? It would be perfectly obvious for anyone who can observe the 4D space time what would happen. Yes? There is no room for "no the particle DIDN'T decay" if we allready know it will do so; since we can observe the whole space-time.

 

Maybe I just had a dozen too many beers, but ... Bite me!

[ajb]

We still have causality. If that is what you are alluding to?

[Dr. Jekyll]

We still have causality. If that is what you are alluding to?

 

Wouldn't causality (as we know it) be "non existent" for anyone who can observe the whole space time? Like, e.g., spontaneous decay seems random. But, if we draw a Feynman diagram it seems as we have causality. Wouldn't the same apply to an space-time observer?

[ajb]

Basically you can still thing of things as being in the past or the future of a point. In effect this is causality. I am not sure if a proper description of this would be helpful as it requires some knowledge of differential geometry.

[Klaynos]

Thank you Snail for that.

 

I agree with your points.

 

And would like to add that the reason that ajb's explination sounds 2 (or 3 surely as it's a code) dimensional is because the human mind can't picture 4D or higher dimensions easily at all... lots of people think it's impossible to truely picture these things in your head, we we're forced into using lower dimensional explinations for things that are much higher...

[ajb]

The picture I gave is two dimensional embedded in three dimensions. The only reason for that is because as Klaynos points out it is far easier to visualise what is going on in two dimensions. In no way to I imply that space-time is two dimensional nor that it is embedded in a higher dimensional space. Also, in my simple picture I took "space" to be compact, again I do not imply that this is the case in the four dimensional space-time of general relativity. (Though both of what I said could be the case.)

 

On small technical point, I assume the cone to be a smooth manifold. In order to do this I need to either smooth-out the conic singularity or simply cut it out. I will chose the latter. Also, the cone comes equipped with a metric which is inherited form the embedding and the Euclidean metric on [math] \mathbb{R}^{3}[/math].

 

For the "realistic" case of the FRW solutions one needs to do something similar with the big bang singularity. Cutting it out is the natural thing to do. What I should add, is that one usually consider the metrics in general relativity as not comming from embeddings. One considers the space-time manifold it its own right and then you solve the Einstein Field equations.

 

So in a lose way the cone "mimics" what is going on in big bang cosmologies. We have a singularity and "cosmic expansion".

[Air]

I have read that Universe will go on expanding(Big Bang Theory).At one stage the expansion stops and the universe will shrink again(Big Crunch theory).Is it true ???

 

There are three alternative scenarios for the future fate of the Universe. If the expansion continues forever then the Universe is destined for the Big Freeze, gradually pulled apart into a cold desolate wasteland of dying stars and black holes. If space stops stretching and springs back on itself, it will shrink until galaxies start colliding in the Big Crunch - terminating in the mother of all black holes. However, there is a more peaceful option. The final scenario involves the Universe gradually slowing down to a halt. With the whole Universe balanced precisely, cosmic catastrophe would be averted and space would be saved. At least for a while. Eventually the Universe would succumb to the Big Freeze, it would just take a lot longer.

 

The fate of the Universe is basically a battle fought between the inward pull of gravity and the outward push of expansion. So astronomers are trying to calculate the strength of these forces. The amount of gravity the Universe has to wield against this expansive onslaught depends on how much stuff there is out there in space. Anything with a mass has its own gravity. Even you yourself have a gravitational force that attracts everything else around you, including other people. The bigger you are, the stronger this force is, and so, the Earth, being the most massive thing around, completely overwhelms the tiny forces that we personally possess. So to calculate the fate of the Universe, we must weigh it to find its density.

 

Antarctic IceIn astronomy, the density of the Universe goes under the symbol [math]\Omega[/math] (or Omega), the last letter of the Greek alphabet, meaning 'the end'. The precise amount of matter needed to gradually halt the expansion of the Universe is known as the critical density, where [math]\Omega=1[/math]. If [math]\Omega[/math] is [math]1[/math] then the future of the Universe is a gentle, serene stop. Smaller than [math]1[/math] and we're heading for the quick Big Freeze, a universal ice age. Larger than [math]1[/math] then it's Crunch time. So our destiny depends on our density. Recent results show that [math]\Omega[/math] is at least [math]0.3[/math]. That figure includes the mysterious dark matter that lurks invisibly in space and has yet to be fully understood.

 

It has also been proving difficult to measure the precise expansion rate of the Universe. Recently it's been discovered that this expansion is accelerating under the grip of a previously undetected force, called dark energy, that is helping to pull the Universe apart. Until both these dark questions are answered, the fate of the Universe still hangs in the balance.

 

An end for EarthHowever, although the end of the Universe may look pretty black, this impending cosmic catastrophe is not our most pressing concern. In about [math]4[/math] billion years the Sun will expand and engulf our planet Earth. Also around the same time, our nearest galactic neighbour, Andromeda, will start to crash into our own galaxy, the Milky Way. Life on Earth must escape into space if it is to survive.

[Dark matter]

At the present rate, the universe would expand forever while gradually slowing, but never stopping. However, if the effect of Dark Energy continues to grow, then in approximately fifty billion years DE would overcome the Strong force locally and all the atoms would fly apart (the Big Rip). For the universe to “crunch” in the distant future, DE would have to “decay” into something with normal mass.

 

This doesn't make sense for me; "At the present rate, the universe would expand forever while gradually slowing, but never stopping." how would that be? I know the Universe is Infinite but for some reason that just stuck out.