# What causes the universe to eventually colapse?

1. Feb 18, 2008

### Alex48674

I'm sure this has been asked a million times, but I'm not quite sure, and I don't trust the dated topics. I'm going to look at that website that marcus uses when he cites papers, but it is quite heavy reading for me and I don't understand much of what goes on there. Thanks.

2. Feb 18, 2008

### Chronos

It is unclear if the universe will someday collapse [the big crunch thing]. Current observational evidence is too close to call - and slightly favoring the eternal expansion hypothesis.

3. Feb 19, 2008

### Wallace

Chronos is correct, the current best guess is that the Universe will not experience a big crunch but rather will continue to expand for eternity.

If the data suggested that the Universe were to collapse one day, this would be because we would have observed that there is sufficient matter in the Universe that its gravity is sufficient to overcome the rate of expansion. To put this simply, and to use an analogy I think I used in a similiar thread, imagine you throw a ball in the air. If you throw it fast enough it will escape the Earths gravitational pull but if you do not then it will eventually stop moving away, turn around and come back to Earth.

The Universe can be thought of to behave in a similiar way. Ignoring dark energy for a moment, there is a simple relationship between the density of the Universe at a given time and the rate of expansion. We need to introduce a concept known as the critical density that is defined as

$$critical density = \frac{3H^2}{8 \pi G}$$

where G is Newtons gravitational constant and H is Hubbles constant which you can think of as a measure of the rate of expansion of the Universe.

If this critical density is less than 1 then the Universe will expand forever and if it is greater than 1 the Universe will collapse to a big crunch. As I say this analysis becomes a little more complicated when dark energy is also considered, but that may be an unnecessary complication for you at this stage.

4. Feb 19, 2008

### kamerling

If the real density is less than the critical density then the universe will expand forever.
The critical density is about 9.5*10^-27 kg/m^3

a density parameter of density/(critical density) is often used in cosmological models, and the universe will expand forever if it is smaller than 1

5. Feb 19, 2008

### marcus

What people used to say before 1998 was that the universe would expand forever if and ONLY if that ratio did not exceed 1.
the ratio you mention was called Omega, or Omegatotal, and the would say that if Omega > 1, then the universe would eventually stop expanding and begin collapse.

But the mainstream view has changed since 1998, when acceleration was detected.
Now the prevaling opinion is that the unverse will expand forever EVEN IF Omega is slightly in excess of 1.

The "critical density" is not critical as regards the eventual fate of the universe, since you can exceed critical and still expand forever.

The "critical density" is critical as regards the finiteness of space. If Omega > 1 then space may be positive curved and finite volume---something like S3, the 3-sphere-----a 3D analog of the 2D surface of a ball.

6. Feb 19, 2008

### W3pcq

What do you guys think about the big rip?

7. Feb 19, 2008

### marcus

haven't heard much about it since around 2004-2005

it requires that the equation of state of dark energy change or else more generally it requires that the cosmological constant Lambda NOT be constant

and one reason we have been hearing less and less about Rip is that the observational data coming in has been running against it.

favoring an actual constant Lambda or a dark energy with fixed equation of state around w = -1

8. Feb 19, 2008

### Wallace

There are a lot of theoretical rules and conditions that must be violated in order for a Big Rip to occur that make it unlikely from a theoretical standpoint. As marcus suggested the observational data is also not strongly in favour of this occurring either.

I think if you take Supernovae alone they do hint very weakly at a Big Rip (w<-1) but w=-1 is still firmly within the error bars. Taking all the data together slight favors w>-1 with w=-1 still very much within the error bars.

In more detail, what w=-1 means is that as space expands the energy density of dark energy remains constant. As the density of matter on the other hand drops as the Universe expands dark energy becomes the dominant form of energy. For a big rip to occur, the DE needs w<-1 which actually means that as the Universe expands the energy density of DE increases. There are many theoretical rules that do not permit this, of course they could be wrong but it's hard to see how such a field could exist.

9. Feb 19, 2008

### Alex48674

So does this mean that the big bang was probably not caused by a previous big crunch?

10. Feb 19, 2008

### Wallace

The short (and long) answer is that we do not know. The issue of what happened 'before' the Big Bang is very murky with a lot of different theories as well as some wild speculation.

The other thing to point out is that we still have no idea what dark energy is and therefore we cannot say that it will continue continue to behave in the way it does today. Its equation of state could change in the future, leading the Universe to contract. There is no suggestion that this will occur but we really just don't know, since we have no idea what dark energy is.

To make a comparison, during inflation in the very early Universe, the Universe was dominated by something that behaved essentially the same way as dark energy does today, except that for some reason this energy source 'turned off' at some point, ending inflation. We can't say for sure at this point in time that the same thing couldn't occur in the future and that the 'late time inflation' we are experiencing at present could in principle turn off at some point. The is not evidence that this is the case but it is just one of many many possibilities.

11. Feb 19, 2008

### Alex48674

All very confusing ='[

12. Feb 19, 2008

### Wallace

But that's a good thing right? Unlike a lot of question you can ask of science we don't have a standard answer that can be wheeled out. We're still working on the problem and breakthroughs are being made all the time. This does make things confusing but also more exciting!

13. Feb 19, 2008

### Alex48674

Haha that too =], thing is if you get in a theological conversation with a religious person, it all sounds very sketchy too them lol, but it is exciting that we are constantly coming up with new answers and new ideas =]

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