# Different scenarios

1. Oct 10, 2009

### Dmitry67

I assume that current experimantal data allows to exclude only some scenarios, for example, closed Universe: Big Bang->Big Cranch

We know that Universe is almost flat, but we don't know if it is closed or open. We also don't know how 'strong' the Dark Energy is: is it just a Cosmological constant or Quintessence.

It gives 4 scenarios compatible with the current experimantal data:
1. Closed but expanding forever
2. Closed, ends in a finite time by Big Rip
3. Open, expanding forever
4. Open, ends by Big Rip.

What is your favourite one you believe in?

P.S. I know there is a special case with a flat universe, but I don't know, in case of Quintessence or Cosmological constant if it flat now is it flat forever, or curvature becomes negative when universe expands and becomes more and more dominated by the Dark Energy?

2. Oct 11, 2009

### Chalnoth

Not necessarily. We don't yet know the makeup of the dark energy, and until we do, we can't say much definitive about the future expansion of our universe.

However, the trend so far is that it seems likely to continue expanding forever, driven by the dark energy, and thus any small deviations from flatness won't matter one bit.

Well, we know very accurately how "strong" it is. What we don't know is how it changes with time.

Belief in these matters is silly. The real world is a certain way. We don't yet know all of the precise details of that. But attaching ourselves to one or another option before the evidence is in just biases the results.

The curvature doesn't change with time. It's just a part of the initial conditions. But its effect is diluted by the expansion as $$1/a^2$$. If the dark energy continues to dilute more slowly than this, as it has done so far, then the effects of the curvature will continue to decrease with time.

3. Oct 11, 2009

### Dmitry67

No, like Einstein said "I dont believe that God plays dice"
Was it silly?

4. Oct 11, 2009

### cristo

Staff Emeritus
It is silly to draw an analogy between an out of context comment and the scientific method. As Chalnoth says, belief does not come into science: the 'correct' theory is the one that matches the observational evidence best, not the one that you 'believe' most.

5. Oct 11, 2009

### Chalnoth

Pretty much, especially since he was fundamentally wrong on the question of reality to which he was referring at the time.

6. Oct 11, 2009

### Dmitry67

Ha! He called the cosmological constant his biggest mistake - et voila - he was actually right, it is non-zero!

He said "I dont believe that God plays dice" - and Copenhagen Interpretation is less and less popular, in QM we are discussing here Bohmian (deterministic) or MWI (deterministic), but you can find only few threads about the CI

He believed in the Universe existed for eternity, and now look - we are discussing the eternal inflation/cosmic darwinism scenarions.

7. Oct 11, 2009

### Chalnoth

Except it still was a pretty darned big mistake on his part, because he attempted to use the cosmological constant to balance the expansion so that there could be a steady state universe. This approach was roundly disproven by Hubble's observation of the expansion. The fact that the cosmological constant appears to be non-zero (we don't yet know for sure) doesn't make this a non-mistake.

Except here he was, if I remember correctly, attempting to find a hidden variable theory, an attempt which wholly failed.

Look, Einstein was a really great physicist, who completely transformed our view of how the world works. But like every scientist, he made mistakes. Taking somebody at their word just because of who they are or what they have done is a huge mistake.

Except it still was a pretty darned big mistake on his part, because he attempted to use the cosmological constant to balance the expansion so that there could be a steady state universe. This approach was roundly disproven by Hubble's observation of the expansion. The fact that the cosmological constant appears to be non-zero (we don't yet know for sure) doesn't make this a non-mistake.

Eternal inflation is eternal into the future, not the past. Cosmic Darwinism is generally considered to be extremely unlikely. In any case, we just don't know whether or not the universe is eternal into the past, and it's foolish to hold too strongly to one or another option until the evidence is in.