Big bang as Big Bounce:

http://phys.org/news/2016-07-big.html

They formulate the universe in quantum mechanics terms and that avoids the breakdown of physics at the minimum point.

My question is, doesn't this imply a universe with perhaps an infinite number of bounces and such, then how would such a construct ever have a start? Perhaps it is only on the first bounce? But where would you decide there was a first bounce?

Originally posted by sonhouse
My question is, doesn't this imply a universe with perhaps an infinite number of bounces and such, then how would such a construct ever have a start? Perhaps it is only on the first bounce? But where would you decide there was a first bounce?

You are making two assumptions:
1. An infinite number of bounces.
This is not actually implied from one bounce. The current understanding of the expansion of the universe is that it will continue forever. If this is actually the case, then no more bounces in future. Now if prior to the bounce was the reverse of current events, then one might speculate that infinite contraction might have preceded the bounce. So only one bounce.
If however there were multiple bounces then one needs to find some way to explain why it now appears that we are expanding infinitely. What are the differences?

2. A necessary start point.
If there were infinite bounces then no, there was no starting point. You do not 'decide there was a first bounce'.

Our intuitive discomfort with infinite time past but not infinite time future is just that - intuitive. It is based on macro experience not actual physics.

Originally posted by twhitehead
You are making two assumptions:
1. An infinite number of bounces.
This is not actually implied from one bounce. The current understanding of the expansion of the universe is that it will continue forever. If this is actually the case, then no more bounces in future. Now if prior to the bounce was the reverse of current events, then one might speculate t ...[text shortened]... finite time future is just that - intuitive. It is based on macro experience not actual physics.

Yes, I can see that but I wonder if there is some mechanism where each bounce loses some energy somehow, that would make each new universe slightly less massive and eventually that would have to stop, not enough mass available to make a decent universe.

Of course, if you lose say one part in a billion of the mass after each bounce you still get hundreds of millions of bounces and if each bounce timeline is say one trillion years, times a few hundred million, you have something already approaching infinity of time.

So I wonder if there is some loss mechanism after each of these theoretical bounces?

Like if the universe has extra hidden dimensions and some of that energy leaks into those dimensions somehow, maybe gravitational coupling or some such, there would be less energy left after each bounce. Suppose that loss was a good percentage of the total of the previous bounce, there maybe would only be a hundred or so bounces, say if each bounce cost the universe 1% of its energy each time.

Originally posted by sonhouse
So I wonder if there is some loss mechanism after each of these theoretical bounces?

Why are you looking for a loss not a gain?
The current universe appears to be likely to expand infinitely. Clearly that would imply a gain over previous versions.

Originally posted by twhitehead
Why are you looking for a loss not a gain?
The current universe appears to be likely to expand infinitely. Clearly that would imply a gain over previous versions.

But in that theory, it wouldn't expand infinitely long in time, some force would end the expansion and cause the reverse. Maybe that would infer a larger universe where our universe 'borrows' energy somehow from the larger or gives it up to that universe. Maybe that conserves the forces to balance out.

Originally posted by sonhouse
But in that theory, it wouldn't expand infinitely long in time

What theory? You are not making any sense.

You are desperately trying to invent a repeated bounce when the actual evidence suggests the opposite. Why go against the evidence?

Originally posted by twhitehead
What theory? You are not making any sense.

You are desperately trying to invent a repeated bounce when the actual evidence suggests the opposite. Why go against the evidence?

I think 'that theory' refers to the case where the amount of energy in the universe increases with each bounce. Since the energy content of the universe determines the overall fate and the universe is assumed to have existed eternally far back in time the amount of energy cannot be increasing with each cycle. By the same argument it cannot be decreasing either as there's no scope for there to have been any crunches. So I don't think a model where the energy content changes monotonically at each bounce is tenable. Since there is no indication that the Lagrangian density isn't symmetric under translations in time I'm left wondering why the energy should change anyway.

Originally posted by DeepThought
I think 'that theory' refers to the case where the amount of energy in the universe increases with each bounce.

Yes, I misread the sentence.

Since there is no indication that the Lagrangian density isn't symmetric under translations in time I'm left wondering why the energy should change anyway.
Constant energy would imply that the contraction prior to the big bounce mirrors the current expansion. But the current expansion is accelerating possibly due to the energy of free space. I suspect that contraction could therefore not have taken place. Although I am far from clear about how it all works.

Originally posted by twhitehead
Yes, I misread the sentence.

[b]Since there is no indication that the Lagrangian density isn't symmetric under translations in time I'm left wondering why the energy should change anyway.
Constant energy would imply that the contraction prior to the big bounce mirrors the current expansion. But the current expansion is accelerating possibly due to ...[text shortened]... ction could therefore not have taken place. Although I am far from clear about how it all works.[/b]

How do the big bouncers get around the idea the universe may expand forever? What mechanism do they propose to get it to reverse?