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AriAstronomer
2010-Jun-08, 09:40 AM
I was listening to a mysteries of the universe podcast by pamela and frasier, and they were talking about how dark energy seems to be the cause of the expanding universe, even though we still don't know what Dark Energy actually is.

My question, is that, if the universe is currently expanding (and at an accelerating rate), and by energy conservation laws matter can't be spontaneously created, then how come there is still debate whether the universe will end up in a big crunch or continuing to expand into infinity? If all the matter that will ever exist was present at some point after the initial KABOOM, and the matter was also as close to each other as it would ever be, and yet the universe still expanded, then what chance is there now that the expanding universe may reverse, when galaxies are much farther apart, and gravity has a much weaker effect?

Unless I am understanding this problem incorrectly, the fight between the big crunch and the infinite expansion is a battle of the titans between gravity and dark energy.

loglo
2010-Jun-08, 09:55 AM
I thought the Big Crunch scenario went out the window when dark energy was quantified to be ~75% of the mass-energy of the universe.

astromark
2010-Jun-08, 10:27 AM
Accepting of that question is as clear as it seems ... Then Yes, it is gravity being beaten by expansion. However the use of the term
Initial Kaboom... makes it sound like an explosion when a expansion is what it was ... The space that contained the universe expanded.
Initially very quickly... That the most distant regions of the known universe are eccelorating away at a ever increasing rate.

This quote; " I was listening to a mysteries of the universe podcast by pamela and frasier, and they were talking about how dark energy seems to be the cause of the expanding universe, even though we still don't know what Dark Energy actually is. " end quote... Is correct.

I can not imagine any reason why that expansion as observed would slow or stop... ever. You may not use the same words as I have,
but I see your understanding is the same as mine. Its the constant I see a lot of in these pages. Can I say becoming the mainstream view ?

Cougar
2010-Jun-08, 01:18 PM
...how come there is still debate whether the universe will end up in a big crunch or continuing to expand into infinity?

Like loglo said, I don't think anyone is debating that there will be a big crunch anymore.


If all the matter that will ever exist was present at some point after the initial KABOOM, and the matter was also as close to each other as it would ever be, and yet the universe still expanded, then what chance is there now that the expanding universe may reverse, when galaxies are much farther apart, and gravity has a much weaker effect?

About all we can say is, in the early universe the initial condition of the expansion obviously overcame the collapsing effect of gravity. But gravity was slowing down the expansion back then, and prior to learning about the acceleration, most everyone figured it was still slowing now. It wasn't that clear if it would slow to a stop and reverse or expand forever. Even then, expansion forever looked likely since we could not detect enough matter in the universe to stop the expansion.

So yeah, now, after learning of the acceleration, there is ample justification to think expansion will continue, and no justification to think the expansion will reverse and begin to contract (without making some rather wild and far-fetched assumptions).

Algol
2010-Jun-08, 01:50 PM
As the much of the nature of dark energy is unknown, there exist a possibility that dark energy will collapse into matter/radiation (similar to how inflation was stopped in the very early universe).
If this happen, we could end up with a universe with high enough density that it collapses into a big crunch.

So, until we know more about dark energy, the big crunch can not be completely ruled out.

astromark
2010-Jun-08, 07:33 PM
' Algol.' I find what you have said as completely wrong and misleading. I can not agree with any of that.
BUT also note that the understanding of the force 'Dark Energy' is an unknown force. Your conclusion is valid.

Algol
2010-Jun-09, 09:33 AM
Dark energy is not necessary a force, In the Friedmann equation it comes out as something with a negative pressure, and this will act as a repulsive force.
I am however no expert, but to my knowledge, we could still have a big crunch if the following are true (highly unlikely though):

1. If we live in a positively curved universe
2. Dark energy is some kind of exotic particle that is unstable

When the dark energy particles decay, the universe will once again be dominated by radiation or matter. If the universe is positively curved, its density is high enough to cause the expansion to stop and the universe to collapse in on itself.

A similar thing happen during the inflation, when the universe was expanding at exponential rates (the same as dark energy is causing). Once what caused the inflation-expansion decayed, the universe stopped its exponential expansion and started to behave as radiation dominated universe (and later matter dominated) and the expansion was slowed down.

ctrebor
2012-Sep-24, 04:29 AM
What I find fascinating is considering what a recollapsing universe would look like. Would it not in fact look like ours, where the rate of expansion kept increasing towards light speed, with a rapid increase right at the end, as all matter collapsed and space time stretched towards the final singularity?

tusenfem
2012-Sep-24, 06:52 AM
What I find fascinating is considering what a recollapsing universe would look like. Would it not in fact look like ours, where the rate of expansion kept increasing towards light speed, with a rapid increase right at the end, as all matter collapsed and space time stretched towards the final singularity?


ctrebor,
Please do not put your ATM ideas in Q&A. You have your own thread where you can discuss this topic.
@ All, please do not pursue this topic, take the discussion to the ATM thread.

Cougar
2012-Sep-24, 01:10 PM
As the much of the nature of dark energy is unknown, there exist a possibility that dark energy will collapse into matter/radiation (similar to how inflation was stopped in the very early universe)... If this happen, we could end up with a universe with high enough density that it collapses into a big crunch.

Space undergoing such a phase transition sounds like a vacuum metastability event (http://en.wikipedia.org/wiki/False_vacuum#Vacuum_metastability_event), which would be the ultimate ecological catastrophe, where not only life as we know it would be impossible, but the same with chemistry as we know it.


So, until we know more about dark energy, the big crunch can not be completely ruled out.

Can't argue there. It looks unlikely, but predictions about the future are always tricky.