## Bouncing Brane-Balloons Cosmology

Discussions ranging from space technology, near-earth and solar system missions, to efforts to understand the large-scale structure of the cosmos.

### Bouncing Brane-Balloons Cosmology

'Colliding branes' and 'bounce cosmology' are terms that we have read about a lot in the pop-sci literature of the recent past, but you may not have heard of "brane-balloons". This is not surprising, because I have just made it up - seriously! So what is the point of posting it in this sub-Forum?

Well, the old 'balloon analogy' has done an excellent job of giving us some grasp of the basics of an expanding universe, so why not see if it could give us some degree of grasp of the basics of 'branes' and 'bounce' cosmology' as well. Here is an elementary graphic that I will try to utilize for such a daunting task. ;-)

Imagine that what you see above is the surface of a double-skinned hollow sphere with a gap between the two skins. The skins are two membranes (this is where the sci-jargon "brane" comes from) that are expanding like a blown double-skinned balloon with some air trapped in the gap between the two skins.

Also imagine that the outer surface of the inner skin is a 2-D rendering of the total 3-D space of our universe, with the observable portions indicated by a dark blue circle on that surface, with a radius equal to the Hubble radius RH.

The purple radius R is labeled Hyperradius, because it is the radius of a 3-D rendering of a 4-D spatial sphere (note, not 4-D spacetime). R is indeterminate is size, but from modern observations it is at least 10 times larger than the Hubble radius RH. Confusing? Yes, but don't despair, there is more confusion to come, but possibly with more clarity (hopeful, wishful thinking?) at a later cosmic time.

Radius R increases with time at a rate dR/dt where dt is a small increment in cosmic time. The rate of expansion is proportional to the Hubble constant, the latter given by H = dR/(R dt) in normalized cosmological units, i.e. per billion year.

The outer skin (Brane B) represents another 4-D (spatial) universe, very close to and parallel to our own, that also expands, but possibly at a different rate than ours. Note that '4-D spatial' here means 5-D spacetime, 4 spatial and one time dimensions.

Being 3-D spatial creatures, we cannot directly observe anything in the parallel universe, but indirectly we may just have the possibility of observing its effects on our own universe. If it exists at all! Interesting, isn't it?

Being already such a cosmologically large chunk to chew, let's break it here for today. Please don't ask very deep questions (if any), because I probably won't be able to answer them.

PS. This is not a personal theory, just a personal effort to try and wrap my own brain around branes - if you would excuse the irresistible pun.

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### Bouncing Brane-Balloons Part II

BurtJordaan » 30 Oct 2019, 22:31 wrote:Please don't ask very deep questions (if any), because I probably won't be able to answer them.

I thought I'll jump the gun by posting some Q&A's to myself, ones that I can at least answer to some degree. ;-) For ease of reference, here is that graphic again:

Here are the first five Q&A's

Question 1: On the graphic, where is Earth? In the central dot, just above the word "observable". Remember that all the black and blue ink sits on the surface, with only the pink ink going inside the balloon. By definition of the "observable universe", we must always be at its center. We are obviously not at the center of our universe, because the surface goes all the way around...

Question 2: So what is going on inside the balloon? Unknown things live there - perhaps superstrings, other branes, quantum weirdness - but I have no clue. On the outside, brane theories definitely postulate at least one more brane - brane B in the graphic, i.e. the parallel universe that I mentioned in my prior post.

Question 3: What good is brane B for? Very simple - to bounce brane A off from and so create a Big Bounce that doesn't have to start from R=0. The theory goes that the last such bounce happened some 13.8 billion years ago en there may have been more in the very, very distant past (like a trillion years ago, maybe). And there may be another bounce some trillion years into the future.

Question 4: Yea, OK, but so what? The interesting thing is that this brand of parallel universes may explain all that we currently know of cosmology, plus a few that we don't have explanations for at present. Like the origins of dark matter and dark energy - possibly - because each brane can theoretically affect the other ones spacetime. And neither side's astronomers will be able see the other side's matter, so might reckon that some of the observable effects come from dark matter. And, hang on to your chair, it may perhaps eliminate the need for dark energy partially, or even altogether!

Question 5: This sounds too good to be true, so why should we fall for that? True, but there are also possible observable effects that inflation theory does not predict, while brane theory can. E.g. the "Bangs" of "bouncing branes" is not as hot and violent as is the case with inflation. This results in mush less (if any) gravitational waves from the brane-Bang being predicted than is the case with the Big Bang. No such gravitational waves have yet being detected, so the jury is still out.

There are many more questions that I don't know the answers to, but maybe we can speculate a little - in part III.

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### Bouncing Brane-Balloons Part III

BurtJordaan » 31 Oct 2019, 17:04 wrote:There are many more questions that I don't know the answers to, but maybe we can speculate a little - in part III.

OK, let's move into more speculative territory.

Question 6: What causes the branes to collide? It is postulated that gravitons (the force carriers of gravity in quantum physics) can "leak" into hyperspace and hence may mediate interaction between the skins (branes). Interaction like attracting each other, until they eventually bounce off one another and separate again, leaving a hot Big Bounce in their wakes on both sides.

Question 7: Doesn't a bounce require a spatial contraction prior to a bounce? No, a "contraction" in hyperspace is all that's needed. Normal spatial expansion can go on in a mildly modified way. Speculatively, the inner balloon may undergo a period of accelerating expansion due to the attraction of the outer balloon, until the bounce, which may 'jerk' it into a period of deceleration for some time. Just about like what we observe in terms of early decelerating and late acceleration of our cosmic expansion. Dark energy coming naturally out of the other brane?

Question 8: Could it really be as simple as that? Probably not, because there are theoretical problems with the mechanisms of attraction and separation between branes. I think it has fallen out of favor lately and on top of that, isn't it just replacing two mysteries (dark matter and dark energy) with another, bigger mystery?

With this I'll invite those that might have been scared off by my original request "please no deep questions", back to the discussion. Any questions, speculation, insights, whatever... ;-))

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### Re: Dimensions Aren't Cheap

Exploration is fun!

This object is embedded in a continuum.
What is the minimum number of its dimensions?

Does that include time?
Recall that while most people consider Flatland to be a 2D world, they also experienced time so it was actually 3D.
You mention space and hyperspace which implies bidirectional translational freedom.

Where is time in the diagram? (Einstein wants to know.)

If Flatland had 3 spatial dimensions but retained its 2D creatures, they would be unrestricted by the 3rd spatial dimension. They could move around like flat confetti in the wind. Thus, restriction of 3D creatures (i.e. us) to a 3-surface would have to be explained.

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### Re: Bouncing Brane-Balloons Cosmology

Hi FD, its 5-D, 4 space and 1 time, the latter not shown. One can however suppress one more normal space direction and show time on that axis, like we have discussed before.

And yes, hyperspace is transitionally bidirectional, because space can also contract under certain conditions.
This does not happen in our universe, as far as we know.

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### Re: Bouncing Brane-Balloons Cosmology

Hi Burt, what of black holes?
(Amazing play in Japan, btw)
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### Re: Bouncing Brane-Balloons Cosmology

dandelion » 02 Nov 2019, 20:07 wrote:Hi Burt, what of black holes?
(Amazing play in Japan, btw)

Apart from being infinitesimally small on the scale of this diagram (trillions of light years in circumference), BHs would only be relevant if the time dimension was also shown, as I remarked to FD above.

The diagram does not even purport to show spatial curvature, because the branes are actually flat sheets. The balloon analogy just makes it somewhat easier to comprehend.

I suppose one can speculate about wormholes connecting the two branes, but they would have no influence on the expansion dynamics.

Yes, the Springboks had an amazing power game this afternoon... ;-)

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### Re: Bouncing Brane-Balloons Cosmology

Great, I think I this involves some notions that I like a lot, especially philosophically.
(And yes, very powerful!)
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