Is the Big Bang Expanding into a Preexisting Void?

[nunogirao]

This doesn't actually make a difference. The thing is, space-time is not some absolute thing, so even if you have two big bang events born within some other space-time separated by some distance, those two big bang events still can never interact in any way, shape, or form. The new space is generated within each individual event, and cannot effect either the parent universe or anything else.

I don't know if the space-time was created in the BB or if it is something that ever existed (and there we go to the «when/why» it was created).
So, is the void just the space-time fabric, where BBs occour?
And, could it be possible that different types of universes (with different rules) populate the same void? Could a «neutrino type» universe exist that just transverse our universe, without influencing it?

 

[Chalnoth]

I don't know if the space-time was created in the BB or if it is something that ever existed (and there we go to the «when/why» it was created).

However it started, space most definitely expanded. It may have started from a pre-existing space-time, as I mentioned. But entirely new space is produced as the universe expands. The expansion of the new universe cannot ever possibly be observed in the old universe.

And, could it be possible that different types of universes (with different rules) populate the same void?

From what we know of high energy physics today, it definitely appears that different regions of space-time, both within a universe and between them, may well have different low-energy laws of physics. We really can't say much at all about the extent of this variation just yet. For now, the only variation from place to place that we have at least some confidence of is the electroweak symmetry breaking event, which appears to have a particular parameter that is random and varies from place to place (as in it is likely different some place far from our observable universe, but is the same everywhere we can see).

Could a «neutrino type» universe exist that just transverse our universe, without influencing it?

Well, unless you want to talk about higher-dimensional theories like string theory, it just isn't sensible to talk about another universe existing transverse to our own. In General Relativity, our space-time is self-contained and cannot overlap with any other universe.

However, in string theory, the "true" universe has 10 space-time dimensions (11 in M-theory), and we might exist on a brane with 4 space-time dimensions, which could, in principle, be close to another 4-dimensional brane, rather in the same way that two sheets of paper can be placed close to one another. Most forces are stuck on the brane, so that we could not observe the other brane through them, but gravitational interactions always remain, and we could interact with this other universe through gravity.

 

[yogi]

How do we know that the Big Bang did not expand into a preexisting void?

What is the justification for this knowledge/belief?

We don't - unless we are creatures that are simply satisfied to collect data, some degree of speculation cannot be avoided (Eddington). And when it comes to beginnings, there is no dearth of speculative theories. Most of these are based on what we observe within the confines of the Hubble sphere centered upon our observational position on Earth - but there is much evidence that leads to the notion that the actual universe of matter in the form of particles is at least 3 times the Hubble scale - this in part due to an extrapolation of redshift data that shows nebula receding from one another at velocities in excess of c.

I personally have come to speculate upon a pure de Sitter exponentally expanding background space - with neither beginning in time nor space - an interesting twist on the big bang as an initial expansion is a symmetry breaking withing some small volume of the de Sitter background space leading to an abrupt contraction of that volume into a dense volume of proto particles - followed by later expansion to its present limit defined by a de Sitter horizon.

When it comes to beginnings, some are better than others - but there is not much possibility of being proved wrong even if your ideas are as far out as mine.

 

[JDoolin]

How do we know that the Big Bang did not expand into a preexisting void?

What is the justification for this knowledge/belief?

If you go back to the original source "Relativity, Gravitation, and World Structure" you will see that Milne's model described a Big Bang expanding into a preexisting void. He reassured the reader that nothing could ever get in from outside because the outside edge of the explosion consists (now and forever) of a surface with infinite density traveling at the speed of light.

In fact it only comforts me a little, because if there were another universe in the same infinite void, it's density in its outer shell would also be infinite, and moving toward us at the speed of light. The good news is, if something hits you at the speed of light, you won't ever see it coming until it gets there.

As far as the justification for rejecting the Milne model, I have been trying to figure that out as well. So far, all I've found is that they already have whatever they want with the standard model, and they have read so much misinformation about the Milne model that they find it ludicrous. In fact, you can't even so much as print up the density function for the Milne model on Wikipedia because it is "original research." I imagine, if you quote anything by Milne and put it up under the article for Milne's Model, it will be taken down, because Wikipedia relies on "Reliable Secondary Sources." Unfortunately, the few remaining copies of Relativity Gravitation and World Structure are probably to be burned, and the idea, whether correct or not, will be forever lost.

 

[JDoolin]

If the universe is surrounded by an infinite void, it is difficult to explain why the cmb intensity is identical in every direction.

If WE, and everything we see came from the same point and the same time, then wouldn't it make sense to expect it all to look the same in every direction?

 

[twofish-quant]

If WE, and everything we see came from the same point and the same time, then wouldn't it make sense to expect it all to look the same in every direction?

Only if you happen to be lucky enough to be near the center of the explosion, and that is a very weird coincidence. If all of the places in the universe that you happen to end up, how is it that you ended in the middle?

 

[twofish-quant]

If you go back to the original source "Relativity, Gravitation, and World Structure" you will see that Milne's model described a Big Bang expanding into a preexisting void.

And if you go back to that era, you will find that it's only one of about a dozen ideas that people had that made perfect sense with the data that they had available. Something that I like students to do is to look at old scientific papers and see what they were arguing about at some decade. The reason for this is that the way that we look at people in the 1930's is how people are going to look at us in the 2050.

Personally, if I had been around in 1960, I would have been a strong opponent of big bang because its much less elegant than steady-state. Looking the arguments, I probably would have considered the first measurements of CMB in 1965 to be experimental errors (and there were lots of reasons to think that they would have been wrong) and I probably wouldn't have been converted until the early 1970's.

I wouldn't be terribly surprised if five years from now, someone quotes me bashing Webb's results on the changing fine structure constant which will be standard knowledge at that time.

As far as the justification for rejecting the Milne model, I have been trying to figure that out as well.

It doesn't fit the data. People have been extremely patient explaining why it doesn't fit the data.

Unfortunately, the few remaining copies of Relativity Gravitation and World Structure are probably to be burned, and the idea, whether correct or not, will be forever lost.

If it's right, then people will stumble on it in the end. Continental drift and black holes were two ideas that were dormant for decades before someone observed something. Also scientists are more open minded that I think you give them credit for.

 

[twofish-quant]

For now, the only variation from place to place that we have at least some confidence of is the electroweak symmetry breaking event, which appears to have a particular parameter that is random and varies from place to place (as in it is likely different some place far from our observable universe, but is the same everywhere we can see).

This particular parameter is the fine structure constant which happens to be set to 1/137.(something) which is a number that looks pretty random. There are a number of experiments trying to see if the fine structure constant varies over space and time. One group has reported yes. Everyone else has reported no, and the details are in another thread on this forum.

 

[Chalnoth]

This particular parameter is the fine structure constant which happens to be set to 1/137.(something) which is a number that looks pretty random. There are a number of experiments trying to see if the fine structure constant varies over space and time. One group has reported yes. Everyone else has reported no, and the details are in another thread on this forum.

Actually, no, I wasn't talking about the fine structure constant, but rather the weak mixing angle. I don't think we have any indication that the fine structure constant could be different from place to place, but as I understand it the weak mixing angle is expected to be a result of a spontaneous symmetry breaking event, which would produce different angles in different locations (though it is definitely the same everywhere within our observable universe).

 

[twofish-quant]

When it comes to beginnings, some are better than others - but there is not much possibility of being proved wrong even if your ideas are as far out as mine.

Which is why I find this sort of thing surprisingly uninteresting. What happens at t=0 is rather uninteresting to me because you can basically make up anything and there is nothing that can prove you wrong.

This is not true for events at t=3 million years or t=3 minutes. At t=0 you can argue that some giant multidimensional space bird laid an egg that turned into the universe. At t=3 minutes, the temperatures are those that we can simulate in nuclear reactors. So you have to ask why you don't see space birds popping out of fusion reactors. At t=3 million years, you have to ask why you don't see space birds everywhere. There is a boundary point right now, at which if there is a giant space bird, then you such see it with the LHC.

At t=0, you don't know when something strange is happening because everything is strange. That's not true for t=3 million years, so when you see bricks start levitating themselves, then you know something odd is going on.

I'm a little puzzled why people are so fascinated with t=0, and its a religious, cultural and historical thing. Personally, I'm more interested in the "dark ages."

 

[Chalnoth]

I think it's precisely because it's rather wide-open that people want to talk about it so much: people don't have to actually know anything to say something about what's going on. But when you get into the later universe, we know quite a lot, and just pulling random ideas out of your backside starts to be directly contradicted by experiment. Actually presenting something potentially interesting requires knowledge and serious thought. So I think most people get rapidly discouraged when they get into the areas where we do have ample experimental knowledge.

Of course, we do regularly get people who are genuinely interested in the areas where we know more as well. So it isn't quite so bad as all that.

 

[twofish-quant]

I don't think we have any indication that the fine structure constant could be different from place to place, but as I understand it the weak mixing angle is expected to be a result of a spontaneous symmetry breaking event, which would produce different angles in different locations (though it is definitely the same everywhere within our observable universe).

My (possibly incorrect) understanding is that the fine structure constant also arises from the same spontaneous symmetry breaking event as the weak mixing angle, and so it's likely to be random for the same reasons. This is why people have suddenly got interested in anthropic arguments because with the current state if HEP, the fine structure constant is basically a totally random value.

Also the reason that people like cosmic inflation is that it provides an explanation for why the universe seems to look the same. What happened was that during and after the SSB event, the universe expanded so much that places with different values of the fundamental constants become unobservable.

 

[Chalnoth]

My (possibly incorrect) understanding is that the fine structure constant also arises from the same spontaneous symmetry breaking event as the weak mixing angle, and so it's likely to be random for the same reasons.

Hmm, that's conceivable.

Also the reason that people like cosmic inflation is that it provides an explanation for why the universe seems to look the same. What happened was that during and after the SSB event, the universe expanded so much that places with different values of the fundamental constants become unobservable.

Indeed. In fact, it seems that inflation expanded the universe so much that even defects that would have occurred from such a spontaneous symmetry breaking event are so far unobserved (namely, cosmic strings).

 

[twofish-quant]

Hmm, that's conceivable.

Did some more research on this. It turns out that the fine structure constant is believed to result from broken symmetry at the energies that strong and electroweak forces unify. This is different from the weak mixing angle which happens when EM and the weak forces unify. The latter are energies we can do direct experiments on.

The other interesting thing is that it turns out that we can do lab experiments to show that the fine structure constant isn't constant. As energy increases there are vacuum effects that change the value of the fine structure constant.

Indeed. In fact, it seems that inflation expanded the universe so much that even defects that would have occurred from such a spontaneous symmetry breaking event are so far unobserved (namely, cosmic strings).

One consequence of inflation is that the unobserved universe is a lot, lot bigger than the observed universe. You can get a limit for the size of the unobserved universe. You figure out how many cosmic strings you are likely to generate, you see how much you have to inflate the universe so that you don't see any. That gives you a bound as to how much of the universe is unobserved.

 

[Chalnoth]

Did some more research on this. It turns out that the fine structure constant is believed to result from broken symmetry at the energies that strong and electroweak forces unify. This is different from the weak mixing angle which happens when EM and the weak forces unify. The latter are energies we can do direct experiments on.

That makes a lot of sense, and it's why I wouldn't go so far as to say we (yet) have good reason to believe that the fine structure constant varies from place to place in the universe. Granted, I think it's highly likely, I'm just not so sure that we're there yet in terms of observation.

The other interesting thing is that it turns out that we can do lab experiments to show that the fine structure constant isn't constant. As energy increases there are vacuum effects that change the value of the fine structure constant.

It's been a little bit since I've looked into this, but from what I understand, this variation is largely understood, and doesn't constitute an actual variation of the coupling constant, but instead some "effective" variation. I'm not entirely certain what this means, but I gather that you can wrap some of the terms in higher-energy interactions back into the strength of the interaction, allowing coupling constants to run with energy.

The effect of this, it turns out, is that at some rather high energy, the variation of these effective coupling constants for the strong, weak, and electromagnetic forces tend towards close to the same value. If we add supersymmetry to the mix, the alignment between the coupling constants at high energy is much better.

One consequence of inflation is that the unobserved universe is a lot, lot bigger than the observed universe. You can get a limit for the size of the unobserved universe. You figure out how many cosmic strings you are likely to generate, you see how much you have to inflate the universe so that you don't see any. That gives you a bound as to how much of the universe is unobserved.

Well, at lower bound, at least! I don't think you could, even in principle, obtain an upper bound from this because this only estimates the amount of expansion after the symmetry breaking event, while there could in principle have been quite a lot of expansion before that event.

 

[JDoolin]

Only if you happen to be lucky enough to be near the center of the explosion, and that is a very weird coincidence. If all of the places in the universe that you happen to end up, how is it that you ended in the middle?

Quite the contrary. That is no coincidence at all. Every particle in the system is, in its original trajectory, in the center of the explosion.

What is your velocity? In your own reference frame, your velocity is zero. The particles around you have velocities anywhere from zero to the speed of light. Therefore no matter which particle you pick, it is going to be approximately in the center.

This is basic relativity. Think about it.

 

[Chalnoth]

Quite the contrary. That is no coincidence at all. Every particle in the system is, in its original trajectory, in the center of the explosion.

What is your velocity? In your own reference frame, your velocity is zero. The particles around you have velocities anywhere from zero to the speed of light. Therefore no matter which particle you pick, it is going to be approximately in the center.

This is basic relativity. Think about it.

Why do you continue to claim this is in any way reasonable? We've already shown that the Milne cosmology only works if the universe is completely empty. It isn't, so it's wrong.

 

[JDoolin]

If it's right, then people will stumble on it in the end. Continental drift and black holes were two ideas that were dormant for decades before someone observed something. Also scientists are more open minded that I think you give them credit for.

If you're trying to make me feel better, it's working.

But what I'd like is, even if it's wrong, for it to be properly understood, and the reasons for rejecting it to be based on actual experiment, rather than misinterpretation. I'd like to see why it's wrong.

 

[JDoolin]

Why do you continue to claim this is in any way reasonable? We've already shown that the Milne cosmology only works if the universe is completely empty. It isn't, so it's wrong.

No. You've asserted that the Milne cosmology only works if the universe is completely empty. And that was based on claiming that Milne's metric was not the Minkowski metric, which I already told you was not true.

 

[Chalnoth]

No. You've asserted that the Milne cosmology only works if the universe is completely empty. And that was based on claiming that Milne's metric was not the Minkowski metric, which I already told you was not true.

Um, no, it wasn't. It was based on the fact that the Einstein tensor vanishes with the Milne metric. The Einstein tensor also vanishes with the Minkowski metric. This changes nothing.

 

[TrickyDicky]

No. You've asserted that the Milne cosmology only works if the universe is completely empty. And that was based on claiming that Milne's metric was not the Minkowski metric, which I already told you was not true.

You seem to be muddled on this.

Let's see Minkowskian and Milne spacetimes(they are the same spacetime with a change in coordinates that shouldn't affect the physics) are both empty, meaning there is no gravity sources therefore no gravitational field. So the Milne cosmology is defined that way, as empty, and that has nothing to do with anyone's claims.

You are yourself admitting that Minkowski and Milne metrics are equivalent so you are implicitly admitting that the Milne universe is empty, so I don't know exactly where you disagree.

 

[JDoolin]

You seem to be muddled on this.

Let's see Minkowskian and Milne spacetimes(they are the same spacetime with a change in coordinates that shouldn't affect the physics) are both empty, meaning there is no gravity sources therefore no gravitational field. So the Milne cosmology is defined that way, as empty, and that has nothing to do with anyone's claims.

You are yourself admitting that Minkowski and Milne metrics are equivalent so you are implicitly admitting that the Milne universe is empty, so I don't know exactly where you disagree.

While Milne was attempting to show how ridiculous Eddington's ideas were, he gave an equation which would map comoving world-lines to world-lines that were moving away from a single event at a constant velocity. The equation was nonsense, and Milne's point was that it was nonsense.

However, because his point was also that Eddington's ideas were ridiculous, the Eddington followers latched onto the very equation that Milne was describing as nonsense, and began calling it The Milne Model.

I admit that the Minkowski metric and the real Milne metric are equivalent.

ds^2=dt^2-dx^2-dy^2-dz^2​

However when you map in the nonsense equation, and use the metric given on Wikipedia for the Milne Model:

ds^2 = dt^2-t^2(dr^2+\sinh^2{r} d\Omega^2)​

where

d\Omega^2 = d\theta^2+\sin^2\theta d\phi^2​

... this metric is no longer equivalent to the Minkowski Metric.

 

[Chalnoth]

And we're back to my previous question: why are you so absurdly opposed to a simple change of coordinates?

 

[TrickyDicky]

However when you map in the nonsense equation, and use the metric given on Wikipedia for the Milne Model:

ds^2 = dt^2-t^2(dr^2+\sinh^2{r} d\Omega^2)​

where

d\Omega^2 = d\theta^2+\sin^2\theta d\phi^2​

... this metric is no longer equivalent to the Minkowski Metric.

Well they are not exactly the same if that is what you mean, if you have an aesthetic repulsion towards FRW metrics applied to Milne's model (why? maybe because you see the introduction of a scale factor as artificial or arbitrary in a spacetime that is essentially static? I could understand that, but Milne actually also introduced artificially an explosion in his special relativistic universe that gave particle tests their speeds up to c) that's OK, but you must realize that physically from the point of view of these particles(from their proper time and length)the metric with the scale factor and the Minkowski metric are indeed equivalent, the Minkowski metric is called the "private" frame in Milne's universe and the FRW metric would be the "public" view as seen from an outside point of view.

 

[TrickyDicky]

Quite the contrary. That is no coincidence at all. Every particle in the system is, in its original trajectory, in the center of the explosion.

What is your velocity? In your own reference frame, your velocity is zero. The particles around you have velocities anywhere from zero to the speed of light. Therefore no matter which particle you pick, it is going to be approximately in the center.

This is basic relativity. Think about it.

This is indeed an interesting property of Milne's model shared by standard cosmology, and shows that isotropy does not necesarily always imply homogeneity, so the cosmological principle is indeed as has been said here before, a philosophical preference that ultimately will have to be confronted empirically since isotropy without homogeneity is also a possibility.

I think the key here is that our cosmology based in GR is basically telling us that there is no center( in this forum"where is the center of the universe?" is a frequent question), so no observer can be in the center, so isotropy without homogeneity doesn't imply any privileged point of view and therefore perhaps the cosmological principle is not philosophically valid in a universe ruled by the theory of relativity.

 

[Chalnoth]

This is indeed an interesting property of Milne's model shared by standard cosmology, and shows that isotropy does not necesarily always imply homogeneity, so the cosmological principle is indeed as has been said here before, a philosophical preference that ultimately will have to be confronted empirically since isotropy without homogeneity is also a possibility.

At the very least, void models to explain the accelerated expansion without any dark energy have been ruled out already:
http://arxiv.org/abs/1007.3725

I think the key here is that our cosmology based in GR is basically telling us that there is no center( in this forum"where is the center of the universe?" is a frequent question), so no observer can be in the center, so isotropy without homogeneity doesn't imply any privileged point of view and therefore perhaps the cosmological principle is not philosophically valid in a universe ruled by the theory of relativity.

Well, pretty sure that isotropy without homogeneity does imply a privileged point of view. It's just that so far there's no reason to believe our universe isn't homogeneous, as the homogeneous models work, but the inhomogeneous ones so far do not.

 

[TrickyDicky]

Well, pretty sure that isotropy without homogeneity does imply a privileged point of view. It's just that so far there's no reason to believe our universe isn't homogeneous, as the homogeneous models work, but the inhomogeneous ones so far do not.

The context of the quoted paragraph seems to indicate you meant to say does not imply.
In that case, I agree, perhaps the problem lies not in the cosmological principle in itself, which is quite reasonable and seems to agree with observation so far, but in the interpretation some cosmology books make of the principle.

 

[Chalnoth]

The context of the quoted paragraph seems to indicate you meant to say does not imply.
In that case, I agree, perhaps the problem lies not in the cosmological principle in itself, which is quite reasonable and seems to agree with observation so far, but in the interpretation some cosmology books make of the principle.

Hmm, perhaps there was some miscommunication here, as I am saying that isotropy without homogeneity does imply a privileged location.

Now, bear in mind that the statement of homogeneity is not an absolute statement. Rather it's just a statement that there is a potential choice of coordinates for which the universe appears homogeneous. If it isn't possible to select such a coordinate system, but the universe still looks isotropic to us, then that says we live in a special location.

One way to look at this is that if you can find some small number observers for whom the universe is isotropic, then the universe is also necessarily homogeneous for some choices of observers (IIRC the minimum is three non-colinear observers).

 

[TrickyDicky]

One way to look at this is that if you can find some small number observers for whom the universe is isotropic, then the universe is also necessarily homogeneous for some choices of observers (IIRC the minimum is three non-colinear observers).

How far apart would they have to be?

 

[Chalnoth]

How far apart would they have to be?

In principle any distance would do, if we're talking about a hypothetical situation where we have perfect isotropy. Clearly this isn't the case, so you'd want them to be about as far apart as is required to smooth out the small-scale fluctuations, so I'd place them at around 80Mpc or so in our universe, at a minimum.

Obviously we can't do this explicitly, but this isn't the point I'm trying to make. The point I'm trying to make is that isotropy plus no homogeneity equals a special location. The reason being that if you have isotropy at many points, you also necessarily have homogeneity. So the only way you can have isotropy and no homogeneity is if there are only a tiny fraction of the available points that have isotropy, which means that the isotropic location is a special location.