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?

[ST88]

1. An unbounded universe can expand the same way that a bounded universe can, simply by pushing the boundaries at the unimaginable speed of 70 km per second per megaparsec.

I do not see how an unbounded universe can have a boundary to push.

When we say bounded and unbounded, we are actually talking about it in only the first three dimensions. A bounded universe has a three-dimensional boundary, an unbounded universe does not have a three-dimensional boundary. But it can still be said that there is an amount of space "within" an unbounded universe. The proof here is if we can actually attain our former position by traveling in a straight line in any given direction. Whatever this distance is can be said to be the extant size of the universe, like traversing the diameter of a circle. This means, de facto that a boundary exists.

And also don't forget that the reason the balloon expands is because matter is pushed into it.

If matter is spontaneously created, this would violate the first law of thermo.

I agree. However, "spontaneously" can mean a number of things. We can't see in a fourth dimension, and so a fourth-dimensional object passing through our universe would seem to us to be appearing and disappearing at different places -- most likely we would not be able to recognize it as one single object.

2. An unbounded universe does not necessarily need to expand "outward" - it can expand in any of a number of dimensional directions, creating the effect of expansion while also maintaining the same size-to-matter ratio that keeps it stable.

If we can look at our universe from a higher dimension then we could see that it is expanding "outward". However, from our perspective, we would simply see space growing larger.

If space is not increasing in all directions at all places, then the customary example of a balloon being blown up is not appropriate. If only certain points are expanding and also not expanding in all 3 dimensions, then it would be almost impossible to conceptually visualize. And it would be even harder to explain why it would behave that way.

There is no reason to think that even a bounded universe is expanding into the shape of a perfect sphere. We can expect that there will be resistance and slippage in various places shortly after the primodial creation event. It only follows from this that various dimensional expansions will behave in the same way (assuming there are any).

4. An unbounded universe that would produce the kinds of observations that we see from galaxies rushing away from us does not necessarily need to be expanding at all.

I agree that there could be other explanations for the redshift other than an expanding universe. But why even theorize an unbounded universe when a bounded universe is the most simple and elegant model?

Occam's Razor? It's true that the simplest solutions are the most likely, but that doesn't mean we should stop looking, because sometimes things are complicated.

Why theorize? Simple human curiosity. If there is more than just one cause that would produce a single effect, shouldn't we try to rule out some of the causes by looking at other effects?

[otseng]

But why even theorize an unbounded universe when a bounded universe is the most simple and elegant model?

Occam's Razor? It's true that the simplest solutions are the most likely, but that doesn't mean we should stop looking, because sometimes things are complicated.

Right, Occam's Razor - "Of two equivalent theories or explanations, all other things being equal, the simpler one is to be preferred."

And out of the two scenarios - bounded and unbounded, a bounded universe is simpler to understand mathematically, conceptually, and experientially.

[otseng]

Even though a bounded universe seems to be the simpler (and more likely) model of the universe, most cosmologists believe it is unbounded. Why? Because the ramifications of a bounded universe are unacceptable.

We see that the cosmic background radiation is uniform in all directions. Also, all galaxies (except the closest ones) are receding from us. And those that are farther away from us are receding faster. (Of course, this is if you believe the redshift is evidence of receding bodies)

However, if we are in a bounded universe, this would strongly imply that we are at (or near) the center of the entire universe. (Perhaps the Catholic Church wasn't so far off after all.)

[ST88]

Even though a bounded universe seems to be the simpler (and more likely) model of the universe, most cosmologists believe it is unbounded. Why? Because the ramifications of a bounded universe are unacceptable.

We see that the cosmic background radiation is uniform in all directions. Also, all galaxies (except the closest ones) are receding from us. And those that are farther away from us are receding faster. (Of course, this is if you believe the redshift is evidence of receding bodies)

However, if we are in a bounded universe, this would strongly imply that we are at (or near) the center of the entire universe. (Perhaps the Catholic Church wasn't so far off after all.)

Equal cosmic radiation doesn't necessarily imply that we are at the center of the universe.

This is a problem for a bounded universe, because we have to assume that the microwaves that make up the cosmic background radiation and are coming at us equally from all directions behave the same as light particles. And light particles are not coming at us from all directions, so why would microwaves be any different?

The question can be turned on its head. If the photons of microwave radiation behave like light particles, why can't we see light from all directions?

The truth is, the isotropism that we see in the cosmic background radiation is only on a large scale. Matter is not evenly distributed. And there are fluctuations in the cosmic background radiation depending on where in the sky you look. Now, assuming the Earth is in the center of a bounded universe is silly, but assuming that cosmic radiation would necessarily be equal at that same center of the universe (wherever it is) is also silly. There is nothing that says that there even should be a cosmic radiation "center" or any reason that the radiation we see would not be all around us no matter where we were in the universe.

Those same studies that show virtual similarity in all directions also acknowledge "miniscule" differences. If, as Douglas Adams put it, the Universe is so mind-bogglingly big as it is, these differences would be miniscule, wouldn't they?

[otseng]

The truth is, the isotropism that we see in the cosmic background radiation is only on a large scale. Matter is not evenly distributed. And there are fluctuations in the cosmic background radiation depending on where in the sky you look.

Matter not distributed evenly would certainly account for the fluctuations. But, suppose the universe was bounded and we are not at the center of the universe, we should see differences in intensity in different directions. We should see a greater intensity of radiation towards the center of the universe since more matter would be in that direction. And we should see less intensity in the opposite direction since less matter would be in that direction.

The line of thinking would be the same for redshifts. If the universe was bounded and we are not at the center, redshifts would not be an accurate measurement of distances. We should see drastic redshifting from stars on the opposite side of the center the universe. And we should see far less redshifting of stars that are between us and the closest edge of the universe.

Now, assuming the Earth is in the center of a bounded universe is silly

No, it's not an assumption, but simply a conclusion if the universe is indeed bounded.

[ST88]

suppose the universe was bounded and we are not at the center of the universe, we should see differences in intensity in different directions. We should see a greater intensity of radiation towards the center of the universe since more matter would be in that direction. And we should see less intensity in the opposite direction since less matter would be in that direction.

The line of thinking would be the same for redshifts. If the universe was bounded and we are not at the center, redshifts would not be an accurate measurement of distances. We should see drastic redshifting from stars on the opposite side of the center the universe. And we should see far less redshifting of stars that are between us and the closest edge of the universe.

This isn't necessarily true if only because the Earth itself is moving. We are hurtling along with the rest of the Milky Way Galaxy at something like 600km/second through what you might call "space", and the redshifts and blueshifts of the background radiation on either side of our field of vision reflect this. Apparently there is still some debate over why exactly the earth is moving faster than the background radiation, and I plead ignorance here.

But still, any effect that might show us that the radiation on the other side of the universe is redshifted even more than other areas of the sky would be desensitized by this effect. Why there would be blueshifts in this scenario is, again, a mystery. Why would the Earth be moving faster than microwave radiation? Surely the microwaves aren't moving directly towards and away from us in opposite directions.

I suppose it's possible that, having cooled to 3 degrees Kelvin, the photons really are moving at a slower pace -- the speed of light particles has been demonstrated to be variable depending on the temperature of the medium through which it passes.

And as far as moving towards or away from different areas of the sky, In an expanding bounded universe, we would see objects that are moving away from us as different varieties of red and blue shifts no matter where they were. And we can't assume that all objects are radiating out from a "central" point either. It has been, what, 20 billion years since the Big Bang? We would expect streams of matter to be shooting out from the direction of the the center, but we would also expect three-dimensionall eddys and other effects of mass movement, not to mention gravitational pull, to send objects fly in all different directions. What if the earth were not moving away from the center of the universe but instead on a kind of spherical line from one point to the other -- or even on a curving trajectory due to the rotation of the Milky Way, or even was rotating around the center?

Furthermore, the way objects move appears differently to us depending on which direction they move:

Consider a star cluster in the shape of a perfect, fixed ring of stars that are moving in parallel directly towards us. If we take multiple snapshots of the ring as it moves toward us, all the stars seem to be moving outward from a central point at the same rate. Similarly, if the ring is moving directly away from us, all the stars seem to be moving towards a central point at the same rate.

If the ring is moving across our line of sight, at the left far horizon it appears as a circle that grows bigger and more elliptical as it approaches. When it passes directly in front of us, the ellipse momentarily flattens into a straight line of stars, and as it passes away from us the ring grows smaller and less elliptical, until it is a circle again at the right far horizon. The stars nearer to us in the ring move across the sky slightly faster than those farther away.

[1.0] The Discovery Of The Stars

There are also the problems of gravitational redshifting and wave absorption, which are quite beyond me.

[otseng]

This isn't necessarily true if only because the Earth itself is moving.
We are hurtling along with the rest of the Milky Way Galaxy at something like 600km/second through what you might call "space", and the redshifts and blueshifts of the background radiation on either side of our field of vision reflect this.

But, when the effects of the Earth moving is taken out, the background radiation is fairly uniform in all directions.

I was thinking more about the Earth's movement. The movement could simply just be the movement within the Milky Way Galaxy, rather than our galaxy hurling through space. Is there evidence that our galaxy is actually on some path through space?

And we can't assume that all objects are radiating out from a "central" point either.

If the universe is bounded, then there was a central point. If it is unbounded, then there would be no concept of a central point.

There are also the problems of gravitational redshifting and wave absorption, which are quite beyond me.

Certainly interesting that it claims that the "expanding universe is the greatest mathematical deception in 20th century physics".

[ST88]

But, when the effects of the Earth moving is taken out, the background radiation is fairly uniform in all directions.

This would also be true if the photons themselves were not moving at all. Picture this type of radiation as a 3-dimensional ocean, and we are sailing on it. Even if it were moving just slightly, it could be in any direction, such as water molecules are generally moving with the currents, but at any specific point, there would be omnidirectional movement.

I was thinking more about the Earth's movement. The movement could simply just be the movement within the Milky Way Galaxy, rather than our galaxy hurling through space. Is there evidence that our galaxy is actually on some path through space?

Yes, in fact, the motion and direction of this galaxy is coordinated with other galaxies and space objects towards a common point, called "The Great Attractor". This little bit of weirdness in the behavior of all of these common objects (including the Andromeda galaxy), might be attributable to the gravity of The Great Attractor

The mass of the Great Attractor truly is great. Whereas our galaxy contains the equivalent of 10EXP11 solar masses, the Great attractor is estimated to be on the scale of 10EXP17 solar masses; a million times heavier than the Milky Way. If the Milky Way were a piece of gravel, the Great Attractor would be a truck. It's attraction is so strong that we are being sucked into it at the rate of 600 km/s. In comparison, the earth moves around the sun at the relatively pokey rate of 30 km/s and rockets escaping the earth's gravitational pull barely move at 11 km/s.

(A student site, but also has interesting links)

Not only is the galaxy moving through space, the solar system, as a unit, is rotating around the center of the galaxy within one of the arms, and it takes
230 million years to make one full rotation.

And we can't assume that all objects are radiating out from a "central" point either.

If the universe is bounded, then there was a central point. If it is unbounded, then there would be no concept of a central point.

Not quite what I had in mind. What I meant was, not all objects would be moving away from this central point, like particles in a blast explosion. We should expect that gravity and the normal forces of current-like behavior would alter the course of many objects since the primordial creation event.

[otseng]

But, when the effects of the Earth moving is taken out, the background radiation is fairly uniform in all directions.

This would also be true if the photons themselves were not moving at all.

I still don't understand your point. There are numerous places stating that the background radiation is uniform in all directions (1, 2, 3).

[ST88]

But, when the effects of the Earth moving is taken out, the background radiation is fairly uniform in all directions.

This would also be true if the photons themselves were not moving at all.

I still don't understand your point. There are numerous places stating that the background radiation is uniform in all directions (1, 2, 3).

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.