# Future of the universe

1. Oct 26, 2014

### wolram

Some thing i have been wondering, this from wiki.

is the universe heading towards a Big Freeze, a Big Rip, a Big Crunch, or a Big Bounce? Or is it part of an infinitely recurring cyclic model?

2. Oct 26, 2014

### phinds

The current consensus is that it is headed for the big freeze. The big rip seems to have been pretty much ruled out, although I don't know that that's conclusive. All the others have been promoted by various folks at various times but don't have any empirical evidence.

3. Oct 26, 2014

### bapowell

I think we can reasonably propose that the big crunch is unlikely, because it requires a closed universe that is not vacuum-dominated. The others, bounce (as perhaps part of a cyclic model), freeze, or rip at this time are all perfectly plausible.

4. Oct 26, 2014

### Doug Huffman

The Value of the Cosmological Constant http://arxiv.org/abs/1105.3105
John D. Barrow, Douglas J. Shaw
Abstract (Submitted on 16 May 2011)
We make the cosmological constant, {\Lambda}, into a field and restrict the variations of the action with respect to it by causality. This creates an additional Einstein constraint equation. It restricts the solutions of the standard Einstein equations and is the requirement that the cosmological wave function possess a classical limit. When applied to the Friedmann metric it requires that the cosmological constant measured today, t_{U}, be {\Lambda} ~ t_{U}^(-2) ~ 10^(-122), as observed. This is the classical value of {\Lambda} that dominates the wave function of the universe. Our new field equation determines {\Lambda} in terms of other astronomically measurable quantities. Specifically, it predicts that the spatial curvature parameter of the universe is {\Omega}_{k0} \equiv -k/a_(0)^(2)H^2= -0.0055, which will be tested by Planck Satellite data. Our theory also creates a new picture of self-consistent quantum cosmological history.

I believe Leonard Susskind derives it during one of his lectures, certainly to demonstrate its small positive magnitude.

5. Oct 26, 2014

### Mr.CROWLER

Yes most scientists agree that it will most likely be a big freeze where galaxies are so far apart, but are still intact. Eventually stars stop being made the last ones left eventually burn out and then the only thing left to govern the universe will be black holes. The universe won't grow big enough to produce enough dark energy for the big rip. Another possibility for the end of the universe as we know it is a phase change which I'm trying to learn more about right now.

6. Oct 26, 2014

### phinds

I have to disagree with a couple of your points. First, the size of the universe is not, I think, relevant to the big rip scenario, and second, the universe will continue to grow and the amount of dark energy will continue to increase forever (that's in the big freeze scenario that you describe, which as I have already said, is, I believe, the consensus opinion on what will happen.

The big rip in not contingent so much on the amount of dark energy but whether or not it ever takes hold inside gravitationally bound systems. Since the amount of dark energy in the neighborhood of gravitationally bound systems is not going to increase (the amount per unit volume of space is roughly constant over time), it seems unlikely that this could ever happen, since it hasn't already, but some people do believe that it will, or at least could (but most seem to think it unlikely).

7. Oct 26, 2014

### Chalnoth

It was never ruled out. It's just that it was never taken very seriously. The big rip scenario has all sorts of theoretical problems that make it very implausible.

8. Oct 27, 2014

### Mr.CROWLER

Well from what I read my understanding is when the universe is twice as big there will be twice as much dark energy and the universe will expand faster when the amount of dark energy increases.

9. Oct 27, 2014

### phinds

Yes, but that is, as I said, irrelevant to local considerations. The amount of dark energy will increase because the size of the universe will increase. The amount of dark energy near galactic clusters will NOT increase because of that. You need to reread my post.

10. Oct 27, 2014

### Chalnoth

No.

The rate of expansion is proportional to the energy density. As the universe expands, the density of normal matter and dark matter go down, while the density of dark energy stays at least close to the same (perhaps exactly the same). So overall, the rate of expansion drops slowly, approaching a constant (or nearly constant) value set by the density of dark energy.

The reason why this is called an "accelerated expansion" is because with a constant expansion rate, the distances between objects accelerates.

11. Oct 27, 2014

### Mr.CROWLER

So technically normal matter and dark matter have been losing density since the creation of dark energy? If that's the case then thanks for clearing that up so basically it's almost like a star where gravity wants to crush it and fusion is pushing out. The density of normal matter and DM essentially is trying to pull things closer, but without the density dark energy could push things away from each other pretty much without any resistance.

12. Oct 27, 2014

### Chalnoth

No. Normal matter and dark matter have been losing density as long as our universe has been expanding. They lose density because in an expanding universe, galaxies get further apart. Same amount of matter, bigger volume = lower density.

This part is more or less accurate. Normal matter and dark matter act to slow the rate of expansion.

13. Oct 27, 2014

### Mr.CROWLER

What is the less part, so I can fully understand...I appreciate your help.

14. Oct 27, 2014

### Chalnoth

I'm not sure I can get much more accurate without also being a lot more confusing. The super-short version is that fusion doesn't actually push out (fusion keeps the star hot: in order to collapse, the star has to cool, but fusion prevents the star cooling), and the fact that the universe is expanding while the star isn't leads to other things that cause the analogy to break down.

But in super-simplistic terms, matter pulling inward while dark energy is pushing outward is good enough to go along with.

15. Oct 27, 2014

### Mr.CROWLER

Fair enough thanks buddy

16. Oct 27, 2014

### phinds

Dark energy is believed to have been around since the earliest times in the universe, it just wasn't able to overcome gravity until about 5 or 6 billion years ago. That is, it has always had an effect, just not one that was as noticeable as it is now that it has become dominant.

17. Oct 28, 2014

### julcab12

Which one is more data friendly? Are there any oddities on the metric parameter -- density of matter and the cosmological constant.

18. Oct 28, 2014

### bapowell

The relevant data is about the dynamics of the current dark energy-fueled expansion. It is not yet known whether the energy density causing the accelerated expansion is decreasing (potentially Big Crunch, but unlikely; most likely Big Freeze), increasing (Big Rip), or staying constant (Big Freeze). The empirical quantity of interest is $\dot{w}$, the rate of change of the equation of state parameter, $w = p/\rho$, giving the ratio of pressure to density of the fluid driving the expansion.

Cyclic models are harder to address empirically, because we are only observing one cycle. These tend to involve looking for evidence in the early universe that might resemble a bounce (typically based on assumptions of the theory underlying the bouncing/cyclic cosmology).

19. Oct 28, 2014

### julcab12

Ah, Thanks. I did a little skimming and found out that the current best measurements so far put w at −1 but with an uncertainty of 5%. ΛCDM still the simplest and favors freeze for now. What ever happens to quintessence?

20. Oct 28, 2014

### bapowell

The best fit value of $w$ is only part of the story. We need to know how it is changing in order to project what the future holds. Quintessence may still be in the running -- importantly, it requires a changing $w$. Hence the importance of this measurement!