Science a GoGo's Home Page Forums General Discussion General Science Discussion Forum Are we in a cosmic bubble?

Chown. Marcus, New Scientist, 15.11.2008, considers the possibility that our observable Universe might consist of a cosmic bubble within which the density of matter is lower than in the surrounding cosmos. This would mean that gravity inside the bubble would be weaker than outside.

Chown explains how this situation could explain the apparent acceleration of the Universe’s expansion, without the need to invent some mysterious dark force. “In such a low density region, the breaking pull of gravity is weaker, and so the region would quite naturally be expanding faster than the more dense area enveloping it. A bubble surrounding us, covering the volume from which light emitted over the past few billion years is just reaching us, would be just the thing to explain the supernova observations. Observing from within the bubble, but using distant supernovae as yardsticks, we would see a universe whose expansion seems to be occurring faster than it used to – without the need to invoke dark energy.”

Any comments?

I don't think that works - if it were the case that bubble of denser material should be collapsing in, due to its own gravity. We would see that at blue-shifted light in the CMB. Instead we see red-shifted light.

Bryan

The idea is that our visible universe is a bubble of lower density, not higher density, hence it being evacuated into the surrounding invisible universe. It's rather like the formation of galaxy superclusters and the voids between them.

Well, I'm not sure just how much faith we should put into Dr. Chown's idea. I did a small investigation on Google, and found several things about him. He started as a cosmologist, if I recall correctly, but he is now basically a science writer. He apparently writes some rather irreverent stories about science. So I'm not sure he is a great authority on the subject.

Now for a brief discussion of the idea expressed. I don't take New Scientist, so I don't have the actual article. Based on what Bill S. says I can think of an analogy, though I certainly don't claim that it is necessarily a good analogy.

So, here is my analogy. It seems then that our universe is something like a bubble of steam in a boiling pot. If we look carefully at the bubble we find that the matter density in the bubble is much less than the matter density outside the bubble. But to keep from collapsing the pressure inside the bubble must be higher than the pressure outside the bubble. This suggests to me that the energy density inside the bubble must be higher than the energy density outside the bubble. However, energy also has gravity, so the gravity inside the bubble must be greater than the gravity outside the bubble. And that seems to be just the reverse of what he is postulating. So I don't have much faith in his suggestion.

Bill Gill

Bill.

I no longer have the original article, but looking back through my notes I find that Chown gave an example of isotropy in the same article which I found very unsatisfactory.



I tried thinking along this line, but stuck on the evacuation mechanism.

Simply gravity, Bill, that's the proposed mechanism, given that gravity is the dominant grand scale force, and in the 'Chown scenario' there's a lot more of it outside our 'bubble' than inside. However dubious one may or may not be regarding the competence and qualifications of Dr. Chown, his idea/hypothesis is quite comprehensible, i.e. there is absolutely no need whatever for any hypothetical pressure (e.g.Dark Energy) within our visible universe to account for its accelerating expansion.


The analogy doesn't hold water (scuse pun). The essential and difference is scale. Gravity of water in the pot is a totally insignificant factor, whereas it's all conquering at the cosmic scale. Unless, of course, Dark Energy is real.

I certainly wouldn't call the competence and qualifications of Dr. Chown into question. I have read a couple of his books, and found them generally informative. My comment about the example of isotropy simply highlights my belief that even the better authors of P S can leave the reader thinking "What?". Perhaps, especially if I happen to be that reader.

Thanks for the clarification on the evacuation mechanism. That was, more-or-less, where I started, then I sort of thought myself round in a circle and had gravity doing odd things.

Goon fan?
Showing your age?

I'm not sure why the analogy wouldn't hold water (well, I guess it's the pan that holds the water, but that's ok). The bubble in the water is created by the fact that a part of the water has become steam (gaseous water). The energy in the steam is high enough so that the molecules of which it is composed are moving fast enough to knock back any lower energy molecules in the liquid water and keep them from moving in and collapsing the bubble. The pressure created by temperature in atomic/molecular fluids may be in a different from from the pressure that holds our universe open in a universe that is a void in a larger universe, but there still has to be some sort of pressure inside the void, or it would collapse from the pressure of the universe outside of the void. You want to keep in mind that energy is energy, no matter how it is expressed. In the boiling water it is expressed as the movement of molecules. On the large universal scale it is expressed as gravity. It still comes down to an energy balance.

Bill Gill

That is exactly what I meant in my last post.

The denser, "outside" universe should collapse into our smaller, less-dense "inside" universe - the net gravitational pull the "outside" universe would feel would be towards the centre of the low-density bubble - i.e. our universe (i.e. sucks to be us).

And it also wouldn't account for the acceleration we see (making some assumptions on my part). If you have an "empty" space surrounded by a roughly spherically-distributed material on the outside, the net gravitational pull inside of that sphere is zero - i.e. the gravity from one direction cancels out the gravity from the other. This is why there is no gravity on the inner surface of theoretical constructs such as Dyson spheres, and thus they need rotation to provide centripetal force. Ergo, on the galactic scale there shouldn't be a net accelerative force*

*assuming a lot - i.e. roughly spherical and even distribution of the "outside" universe, no delay effects due to travel-time of gravity (which I think occurs at 'c'), etc.

This model would also suggest that we occupy a rather special space in the "inner" universe; one relatively close to the centre. Otherwise, we should see uneven degrees of spreading depending on the direction we look - we do not.

Bryan

Bryan, I thought I was going round in circles thinking along that line.

Looked at another way, would it not be that gravitational attraction between bodies in the denser area would be greater than in the less dense area, so the net force would be outward from the less dense bubble?

Yes, we would have to occupy a special position, but there might be numerous bubbles, so a near central position in one of them would not be that special.

In the Dr Chown scenario, why should there be pressure from outside the void, Bill, sufficient to overcome the gravity of those 'outside' regions. This is what I'm failing to see in the viewpoints of both Bryan and yourself. This is the very thing that has been puzzling cosmologists about the accelerating expansion of our visible universe, and which has lead them to the tentative conclusion re Dark Energy. So what do you propose as the nature of the force providing "pressure of the universe outside of the void" - and why do you think it would have to exist? That's what I'm asking.

Consider the immense voids that exist within the 'foam' of galaxy superclusters in our visible universe; by your reasoning, would one not expect that gravity would be insufficient to prevent some force from dispersing the material of that foam into those voids, resulting in a more or less uniform array of stars (or most likely no stars at all)?

Ah, but I'll have know I missed the radio series, Bill! Only caught up with it when it reached TV, together with the very first Dr. Who on Saturday afternoons.

The position at which I eventually arrived was something like this:

Outside; greater density of matter leading to greater gravitational attraction.

Inside; lower density of matter leading to lower gravitational attraction.

Matter tends to move from area of lower attraction to area of higher attraction, so dense areas will become more dense, while less dense areas become even less dense.

With increasing density outside, relative to inside, outward motion will accelerate.

Posts so far suggest that reaction to this will be mixed.

Bill S., as you've no doubt noticed, that's how I see it too. In a scenario without Dark Energy, it seems to fit all observations re gravity vs other forces at the cosmic scale. Even so, I'd very much like to hear more from Bryan and Bill Gill.

Rede. I too look forward to comments from the "other side".
You will appreciate that as one who was a teenager for the original Goon Show, time is of the essence for getting all these things sorted out.

As you all know, I’m not terribly bright…but I have spent the last couple of years focused on this very issue.

As a matter of fact, I think that this subject is; to the macroscopic universe what quantum mechanics is; to the microscopic universe.

It occurs to me that this topic is related to another thread entitled “The universes expansion acceleration solved”.

I find it curious that no one has mentioned “Dark Flow” yet since it may relate to this issue and is well documented.

I think that this subject intertwines with, Hubble’s Equation, Hubble’s constant, gravity (and its math), gravity as warped space, the physical characteristics of the “bubble”, Dark Flow and infinite progression/regression. (Oh, and what happens to pi when you quantize it.)

I think that the most glaring issue with gravity as the expander is Hubble’s linear equation vs. gravity’s parabolic equation. Except for some anomalies like Dark Flow and the Great Attractor, observation demonstrates that the acceleration is linear and increasing. With only a few exceptions, the parabolic influence of a supposedly dominant force is nowhere in sight.

As a cause and effects type of person it seems compelling to discard the notion…that a very real impasse has been reached. The universe as a whole seems to behave like there’s a different force at work…Dark Energy perhaps.

It may surprise you to know that I am a proponent of gravity as the accelerant.

Once again, the devil is in the details. It does seem to me that despite the overwhelming observational evidence that there is a combination of circumstances that allow gravity to be compatible with observations.

Should I go on?

Ok, I knew if I just let it ferment for a while I would come up with something more to say.

Assume that our universe is a bubble of low density in a much larger universe which has a much higher density. Just for grins lets call that the bigverse. Then, since there is much less mass in the universe than in the bigverse there will be a reduced attraction on the side of the bigverse that bounds the universe. That means that the matter at that contact point will be more strongly attracted away from the universe. So then matter in the bigverse will start to clump up. And, as we have seen in the universe that means that the bigverse will start forming the equivalents of stars and galaxies. So that the bigverse will no longer have anything like a consistent density. So most of it will consist of low density areas and there won't be any bubbles such as our universe. Most of the bigverse will consist of a gigantic collection of low density regions with galaxies scattered through it.

Bill Gill

And then I didn't carry that thought on through. Since the bigverse has a very high density, when it starts forming stars they will be massive stars. So they will go through into black holes rather quickly. That means the bigverse will be full of supermassive black holes. Then the lower density matter left will form normal stars that will clump into galaxies around the supermassive black holes. Sounds like the universe we live in doesn't it?

Bill Gill

Bill, that sounds like a logical development; it will need some thought. In the meantime, what difference would it make if the "bigverse" is still at the stage of forming its stars and galaxies?