# B Expansion propagation

1. Sep 8, 2017

### Grinkle

Is there any theory to say if expansion started with a finite seed and propagated out at a finite speed vs somehow happening everywhere at once? I know the layman's phrasing is that expansion happened everywhere at the same time. That might just mean that the very small part of the early universe that today is our observable universe expanded all at once as nearly as we are able to observe, or it might really mean that somehow a dense universe of infinite expanse expanded everywhere all at once.

2. Sep 8, 2017

### phinds

Not only is there no such theory, it would go against all empirical observations. It's a non-starter.

3. Sep 8, 2017

### Grinkle

Which? All at once, or propagated?

Edit - Also I think I should be saying "inflation" where I am saying "expansion".

Last edited: Sep 8, 2017
4. Sep 8, 2017

### phinds

I read "finite seed" as meaning a point in space, which I now realize is likely a misinterpretation of what you mean. if by "seed" you mean "a hot dense plasma of indeterminate size and shape, possible infinite", then to say that it "propagated" out from that is another poor choice of terms. Things receded from each other.

Irrelevant to the correctness of your statement since it would apply to both.

5. Sep 8, 2017

### Fervent Freyja

Not all at once, speed would be a definite constraint. Expansion does not occur with the same rates at some locations in space. The Hubble parameter is an average only, and the rate cannot be applied just anywhere in space. The rate varies depending upon the presence of mass and it's distance between other massive systems. It's just observed that lower-density large regions of space typically continue expanding. Space between some galaxies or large voids expand much faster. Some configurations of matter in space, like your hands or the earth, prevent any significant expansion from occurring.

6. Sep 8, 2017

### phinds

That's a good point (at least as regards expansion, possibly not true for inflation). We always (well, maybe I should say *I* always) talk about expansion in cosmological scale terms but you are right that as matter congregates into galaxies, expansion inside the gravitationally bound regions slows and stops.

7. Sep 8, 2017

### Grinkle

I mis-spoke. I mean to be asking about the brief period of inflation - very rapid expansion that happened early in the life of the universe, not the on-going expansion that we see today.

That is what I was picturing.

I am getting into my usual problem of not being able to properly pose a question that goes well beyond my understanding of GR / QM. Re-trying -

Our observable universe is modeled (at least by someone) to be this big at the start of inflation -

http://www.historyoftheuniverse.com/index.php?p=hadronEpoch.htm

"we know the observable Universe is between 17 centimeters (for the 10-35 second version) and 168 meters (for the 10-30 second version) in size at the start of the hot, dense state we call the Big Bang."

I think that our observations of the observable universe today support that inflation was something that happened everywhere at once, and contradict that inflation started in one spot and from there propagated. But we are only able to observe the equivalent of 17cm - 16800cm in diameter of pre-inflation stuff. I wonder if we can be as precise with our conclusions as to conclude that inflation cannot have had a greater than 0 propagation speed rather than just give some bound on it - eg, if inflation moved any slower than 10c, we would observe those effects and we don't observe them or some statement like that.

Also I wonder do we have any theoretical model that provides a mechanism whereby inflation can happen at all places in an infinite universe simultaneously, and not just in a part of the infinite universe that when inflation occurred was 168 meters across, give or take? Not that it makes sense for something to be simultaneous across 168 meters but NOT across 168km or some larger possibly infinitely large distance - I don't mean to imply that.

Last edited: Sep 8, 2017
8. Sep 8, 2017

### Fervent Freyja

I've come across many inflationary theories. There is really only consensus on the basic idea of inflation and very little on any particular theory. I don't see what you are suggesting about inflation happening in an infinite universe simultaneously. There is a beginning and end point where the event and the rate slows down, therefore the particles within the inflation field may be moving outwards but away from each other at speeds relative to nearby particles, but the speeds throughout the process will have changed (implying it hadn't been simultaneous). I accept the idea of inflation, but current theories seem iffy to me. The SM of particle physics doesn't seem to give any explanation for the event, I would love for it to give a better description! I mysteries.

9. Sep 8, 2017

### stoomart

Sounds like something ekpyrotic/cyclic models describe, which I understand are alternatives to inflation models.

https://en.wikipedia.org/wiki/Ekpyrotic_universe
https://en.wikipedia.org/wiki/Cyclic_model
https://en.wikipedia.org/wiki/Inflation_(cosmology)#Alternatives.2Fadjuncts

10. Sep 9, 2017

### Jorrie

I suppose you mean that after inflation, the expansion rate (H) decreases. During expansion, the simplest model has a constant H, giving exponential expansion.

The simplest answer to the riddle of the 'size' before inflation is probably that we do not quite know. It is normally taken that if we find evidence for a spatially infinite universe today, it must have started spatially infinite, because finite things cannot grow into infinite things.

We have both theoretical and observational support for spatial flatness, but I don't think it is certain. It could be just-just positively curved, which makes it finite according to our best model. But, as has already been said by @phinds, Occam's razor points towards flatness.

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