# Expansion then Contraction...an infinite Elastic Collision?

1. Oct 17, 2015

### halpmaine

Wouldn't the explanation that fits fundamental laws (e.g. Conservation of E) while making the least asumptions be an ongoing bang-crunch-bang...scenario? Why couldn't expansion be eventually reigned in by forces, dark matter and the like - ultimately leading to contraction?
Why shouldn't matter/energy behave similarly on a Universal scale as it does on an atomic or more 'local' level? Couldn't 'everything' be viewed in the context of a kind of infinite Elastic Collision?
-Halp

2. Oct 18, 2015

### Orodruin

Staff Emeritus
The Universe could have been like that, but observations tell us it is not. Note that even if the Universe did not contain dark energy, it would not be true that the Universe would eventually collapse. Compare this with throwing a ball up - if you threw it at a speed larger than the escape velocity, it would not return to the ground (neglecting air resistance - what goes up does not have to come down).

Also, conservation of energy is not as fundamental as you might think. In a general relativity setting, it is not even certain that you can define the total energy of the Universe.

3. Oct 18, 2015

### timmdeeg

Whereby - if I understand you correctly - this implies that dark energy and cosmological constant exist independently from each other. Which according to the data isn't compelling.

4. Oct 18, 2015

### Orodruin

Staff Emeritus
No, this is not a correct interpretation of what I said.

5. Oct 18, 2015

### Chronos

6. Oct 18, 2015

### rootone

Maybe infinite expanding/contracting or some other cyclic scenario is the case although nothing strongly suggests it is, but it's possible.
If we somehow determined that this is the way the Universe works, we would still want to figure out why it is that way and not some other way.
The ultimate answer really might be 42, but that is just a good approximation.
https://en.wikipedia.org/wiki/42_(number)#The_Hitchhiker.27s_Guide_to_the_Galaxy

Last edited: Oct 18, 2015
7. Oct 19, 2015

### timmdeeg

In my opinion it is true that the universe will collapse in case there is no dark energy. It is even true that it will collapse in case the cosmological constant is less than critical.

8. Oct 19, 2015

### Orodruin

Staff Emeritus
Then your opinion is wrong. It is perfectly possible to have an eternally expanding universe without dark energy, this is not a matter of opinion. If you would zoom in on the $\Lambda = 0$ line in figures like this

you would see that the line between "expands forever" and "recollapses eventually" is below $\Omega_\Lambda = 0$ when $\Omega_M < 1$. In other words, if you have an open universe containing only matter, it will expand forever.

9. Oct 19, 2015

### timmdeeg

Yeah, I was disregarding the fact that negative curvature is possible with matter only. Thanks for correcting.

10. Oct 19, 2015

### halpmaine

Thanks Orodruin/All!

I think I've been guilty of thinking of the Universe in terms of a closed, isolated system - albeit a NEARLY infinite one - to which the fundamentals would apply..??

I guess I was 'hoping' that while mathematically complex, the solution to the ultimate question would be simple...even if not elegant; everything has always been there and always will in one form or another, in one dimension or another...

Accepting that there really may not be an 'absolute' can be...metaphysically-challenging, if you will.

Time to read some more Hugh Howey and 'escape' : )