Is the universe bounded or unbounded?

[otseng]

In the Light, stars, and creationism thread, I proposed a theory to reconcile a young earth with being able to see stars that are billions of light years away. The theory assumes that the Big Bang is true, however, it also assumes that the universe is bounded. In typical cosmology, it is assumed that the universe is unbounded.

Bounded means that the universe has a boundary to it. There exists an "edge" to the universe in which beyond this boundary, our universe does not exist.

In an unbounded universe, there is no "edge". The universe "wraps" around itself. So, if you are to go in any direction in a straight line, you will eventually come back to the starting point.

This is hard to conceptualize, but can be explained like a surface of a sphere. On the surface of a sphere, if you start at any point and then go in a straight line, you will eventually come back to the starting point. Now, instead a 2-D surface on a sphere, the universe is a 3-D topology that curves in on itself.

The ramifications of either of these two assumptions make for drastically different cosmological conclusions.

So, the questions are:
1. Is the universe bounded or unbounded? Why?
2. What are the ramifications of whether it is bounded or unbounded?

[otseng]

Sorry, I didn't mean to say moving at all. I meant to say that their behavior could be more akin to Brownian motion than to directional movement, bombarding us from all sides roughly equally. All sides of a boat get wet equally despite the fact that all water molecules are not shooting directly at it.

Ah, I see your point now. But, that is also assuming then the universe is not expanding. If the universe is not expanding, then yes, objects in the universe could conceivably be moving in a Brownian type fashion. But, if the universe is expanding, on the macro level, objects are moving away from each other. Though there might be some non-expanding movements (Brownian) on the micro level.

[ST88]

But, that is also assuming then the universe is not expanding. If the universe is not expanding, then yes, objects in the universe could conceivably be moving in a Brownian type fashion. But, if the universe is expanding, on the macro level, objects are moving away from each other. Though there might be some non-expanding movements (Brownian) on the micro level.

This brings up a different point. If the expansion of the universe really is like the expansion of a 3-dimensional balloon, then the borders would be expanding faster than the middle. I don't think this is the case. There is nothing about space as we know it or about the primordial Creation Event that says matter need not be present at the universe border between it and nothingness. Now, I realize that "border" in this case doesn't make much sense. Assuming we could travel to this phenomenon faster than its own speed, would we be able to travel past it into absolute nothingness? The mind reels.

1) What if "matter" were just the expression of whatever is beyond the universe, in a pre-existing form, suddenly being subsumed into it?

2) What if the fourth-dimensional hole through which the universe is expanding from is actually letting in matter -- or creating it.

But aside from all this, what I mean to say is that it ain't necessarily so that space itself is expanding between all points. The balloon example is primarily a metaphor, and the balloon itself doesn't necessarily need to be strictly three-dimensional. It may be, for example, that space is only expanding between galaxies (due to gravitational forces or whatnot), and that the objects not a part of the fabric of space are merely riding the waves of expansion.

[otseng]

If the expansion of the universe really is like the expansion of a 3-dimensional balloon, then the borders would be expanding faster than the middle.

If the universe is unbounded, there would not be a border or a middle.

If the universe is bounded, we should see that objects near the boundary should be moving away from the center faster than objects closer to the center of the universe.

This is what Hubble discovered in 1929:

In 1929 Hubble compared the distances of the galaxies to the speed at which they were moving away from Earth, and he found a direct and very consistent correlation: The farther a galaxy was from Earth, the faster it was receding. This relationship was so consistent throughout the 46 galaxies that Hubble initially studied, as well as in virtually all of the galaxies studied later by Hubble and other scientists, that it is known as Hubble’s Law. Hubble concluded that the relationship between velocity and distance must mean that the universe is expanding.

Source: Encarta

It may be, for example, that space is only expanding between galaxies (due to gravitational forces or whatnot), and that the objects not a part of the fabric of space are merely riding the waves of expansion.

But why is it only space that has no matter at that location expanding? Also, gravitational force is extremely weak. Suppose it was just a speck of dust in outer space. Would not the space that this speck occupy expand if the universe was unbounded?

[ST88]

If the universe is unbounded, there would not be a border or a middle.

If the universe is bounded, we should see that objects near the boundary should be moving away from the center faster than objects closer to the center of the universe.

We agree on this.

But why is it only space that has no matter at that location expanding? Also, gravitational force is extremely weak. Suppose it was just a speck of dust in outer space. Would not the space that this speck occupy expand if the universe was unbounded?

Do not discount the effects of the main forces of matter. We do not fully understand gravity. If superstring theory is correct, for example, there is a rational explanation for why only the vacuum of space would be expanding -- <vast oversimplification> only those areas without attachment would have the opportunity for expansion.</simplification> If the graviton model is correct, it would follow that expansion would cause gravity forces to be weaker.

Part of the problem is that we are seeing light from so long ago that we have no idea where those objects are at the present time. This is important if simply because we know our location only in relation to these objects. If space were expanding, over every single small speck of the universe, how could we possibly be sure of the doppler pattern of light particles? That is, if space itself were expanding in every single location, the redshift would only occur if the light particles were not a part of this expansion. If they were, we might not see a shift at all. We can therefore ask that if light is not a part of this expansion, what is? We know we are not expanding, because molecules require a certain distance in order to bind with one another. So what is it that we have in common with light particles? (not to say that light is exclusively a particle, but it sure behaves like one).

Another curiosity is, if space is expanding, why are the distances between us and other solar-system objects not changing? The only observations we have for Hubble's Law is with other galaxies -- nothing within our own galaxy has this property.

[jwu]

If the universe is bounded, we should see that objects near the boundary should be moving away from the center faster than objects closer to the center of the universe.

That's correct, but from any point objects far away from it will seem to move away faster than those that are close, so Hubble's observations prove exactly nothing in regards to the question if we are anywhere near the center of the universe. Close objects are close because they have a slow relative motion to earth, far objects are far away because they have a faster relative motion to earth. This will be the same regardless if earth is still somewhere near the center of the universe or not.

[Ian Parker]

We are travelling at 600Km/s as defined by the 2.7K dipole moment. Let us do a gedanken experiment, let us move to a point which we now see as moving at c/2. Would we observe a dipole moment of c/2? No we would get an answer of the same order as 600Km/s although in a different direction. This is known as the COSMOLOGICAL PRINCIPLE.

For the cosmological principle to be true an infinite Universe is needed. This is tin fact the mathematical definition of infinity, that however far we go we can always go further. If the Universe were not infinite (or at least unbounded, the two are not synonomous) there wouuld be an edge and observers near the edge would experience c/2 or higher.

There is one consequence we might ponder. Suppose we prove that Evolution with randiom mutation is extremely improbable. An infinite universe immediately makes it possible. Many people use mutiverses or multiple universes to explain the anthorpic principle.

As far as Evolution is concerned I wonder whether "gynotropism" would not be a better word as biologists always talk about lines of daughters.

[steen]

There is one consequence we might ponder. Suppose we prove that Evolution with randiom mutation is extremely improbable. An infinite universe immediately makes it possible. Many people use mutiverses or multiple universes to explain the anthorpic principle.

Except that mutations aren't completely "random" anyway, and that the current mutation principles are rather well-documented to begin with. We are finding sub-cases where it is varying, but in general, the principles are doumented.

[otseng]

I've posted this quote elsewhere, but I wanted to post this here also since it's apropos to this thread.

Big Bang Theory:

Astrophysicist George F. R. Ellis explains: "People need to be aware that there is a range of models that could explain the observations….For instance, I can construct you a spherically symmetrical universe with Earth at its center, and you cannot disprove it based on observations….You can only exclude it on philosophical grounds. In my view there is absolutely nothing wrong in that. What I want to bring into the open is the fact that we are using philosophical criteria in choosing our models. A lot of cosmology tries to hide that."

[Ian Parker]

This is misleading. With the Earth/Solar System at the center of the Universe we could not construct self consistent models of the Universe. We assume General Relativity & the Cosmological Principle in constructing models of expansion, the meaning of 2.7K background and (when LISA is launched) the gravitational wave background which predates 2.7K.

None of this would have any meaning as different parts of the Universe would evolve in different ways. We assume that 2.7K represents a stage in the evolution of what we now call the Milky Way.

Incidentally one remark I should have made but did not and that is that an FT of 2.7K shows no repetition. This precludes light going round the Universe.

[Alien]

I've posted this quote elsewhere, but I wanted to post this here also since it's apropos to this thread.

Big Bang Theory:

Astrophysicist George F. R. Ellis explains: "People need to be aware that there is a range of models that could explain the observations….For instance, I can construct you a spherically symmetrical universe with Earth at its center, and you cannot disprove it based on observations….You can only exclude it on philosophical grounds. In my view there is absolutely nothing wrong in that. What I want to bring into the open is the fact that we are using philosophical criteria in choosing our models. A lot of cosmology tries to hide that."

Sorry, I don't understand this argument.

The point is not the philosophy of the model, but the geometry of it.

A 3D model with a centre in the 3D domain, like that postulated by Mr. Ellis, does not fit very well into the General Relativity equations of space curvature. What about distant objects that would then travel at superluminal velocities?

The curvature implies (in addition to the fact that the space is stretching rather than matter exploding) that a 3D model has its centre in the 4D domain, exactly like the balloon visualisation is a 2D model with its centre in the 3D domain (being the 2D model the surface of the balloon, and the 3D centre the geometrical centre in the balloon). Our current cosmological model could be called a 3D/4D-centred, as opposed to this strange 3D/3D-centred model.

Two considerations, please:

1) I would be very curious to see the geometrical equations of a 3D/3D-centred expanding universe that comply with Relativity
2) Note that this 3D/3D-centred model implies a singularity (the centre) whilst many people are desperately avoiding singularities in the universe. I am referring to the Big Bang singularity at t = 0.