expansion of universe vs looking direction

[SpeedFreek]

It's along the same idea that if one were to keep heading for the "edge" of the universe, one would end up on the other side of the universe, which, as we know today, is absurd.

Sorry I'm late! The notion is only "absurd" due to the rate of the expansion of the universe precluding the detection of such a topology if the fundamental domain of the universe is larger than a certain size. Of course, it is absurd to think we might be able to traverse the whole universe as we cannot beat the rate of expansion, but topologically it is probably the most likely scenario unless the universe is infinite in extent.

http://arxiv.org/abs/astro-ph/0604616

Extending the WMAP Bound on the Size of the Universe - 2006

What is the shape of space? While this question may have once seemed more philosophical than scientific, modern cosmology has the chance to answer it using the oldest observable light in the Universe, the Cosmic Microwave Background radiation (CMB). NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) has made a detailed map of the CMB sky which has been used to provide answers to many age-old questions about the nature of the Universe.

While it is certainly possible that the Universe extends infinitely in each spatial direction, many physicists and philosophers are uncomfortable with the notion of a universe that is infinite in extent. It is possible instead that our three dimensional Universe has a finite volume without having an edge, just as the two dimensional surface of the Earth is finite but has no edge. In such a universe, it is possible that a straight path in one direction could eventually lead back to where it started. For a short enough closed path, we expect to be able to detect an observational signature revealing the specific topology of our Universe.

It seems however, that we cannot find a short enough closed path. This does not necessarily mean the universe is infinte, it might just mean the universe is too large to have such a closed path, so the question is still very much an open one.

In simple terms, there are real problems like the ones Gödel describes if the universe has a boundary, so the universe is either infinite, or if it is finite it "wraps around" on itself topologically such that there is no boundary. So a straight line either goes on for ever, or comes back to where it started, or passes back through regions it has already traversed but on a different path (like a line drawn at an angle around a torus, for instance), but it never reaches an edge.

 

[FlatEarth]

It seems however, that we cannot find a short enough closed path. This does not necessarily mean the universe is infinte, it might just mean the universe is too large to have such a closed path, so the question is still very much an open one.

In simple terms, there are real problems like the ones Gödel describes if the universe has a boundary, so the universe is either infinite, or if it is finite it "wraps around" on itself topologically such that there is no boundary. So a straight line either goes on for ever, or comes back to where it started, or passes back through regions it has already traversed but on a different path (like a line drawn at an angle around a torus, for instance), but it never reaches an edge.

Not so fast with the conclusions! There are other possibilities. One that I find more plausible is that spacetime does end at some point. A theoretical craft capable of exceeding the expansion rate of spacetime would arrive at a point where it could go no faster than the expansion rate. There's no going beyond spacetime because there is nothing beyond it! If spacetime stopped expanding at some point, so would the craft.

Not long ago I believed that a “hall of mirrors” effect was possible, but it doesn't actually make sense. There's nothing in science to support such a phenomenon, so I'm with yevaud on this point. I think it's most unlikely to be true.

 

[SpeedFreek]

I don't think it plausible that space-time comes to an end, and I have never heard of such a model.

I agree with Joseph Silk, Savilian Chair of Astronomy at the University of Oxford.

Is the universe infinite or finite? (you can also read back a few pages at that link)

 

[FlatEarth]

I don't think it plausible that space-time comes to an end, and I have never heard of such a model.

I agree with Joseph Silk, Savilian Chair of Astronomy at the University of Oxford.

Is the universe infinite or finite?

That's an interesting article, but it really amounts to a thought experiment IMO. There is no way to know about the overall configuration of the universe, only that what we can detect appears flat (for some reason, I kind of like that description). As long as observations are honored, any ideas that are consistent with those observations are possible, but I personally like a simpler explanation. The point is that there are more possibilities than you described in your earlier post.

 

[SpeedFreek]

As long as observations are honored, any ideas that are consistent with those observations are possible, but I personally like a simpler explanation. The point is that there are more possibilities than you described in your earlier post.

The simplest explanation for a flat universe is the 3-Torus, there is no simpler finite topology. No model with an "edge" where spacetime ends is consistent with any observations, nor would it have a simpler topology than the 3-Torus (think of the boundary conditions at your "edge").

Have a look at that link from page 188 onwards:

The topological choices simplify in a flat universe. There happen to be just 18 distinct types of flat spaces. All of these, except for the analogue of the infinite plane, are multiply connected; that is to say, closed loops are possible.

There are no more possibilities for a flat universe - either it is infinite, or closed loops are possible, as I said. You should know by now that I always post the mainstream view.

 

[FlatEarth]

The simplest explanation for a flat universe is the 3-Torus, there is no simpler finite topology. No model with an "edge" where spacetime ends is consistent with any observations, nor would it have a simpler topology than the 3-Torus (think of the boundary conditions at your "edge").

In concept, you can't get much simpler than space that ends at a boundary. The boundary condition would be...interesting. Of course there are no observations of this, nor are there observations of your preferred configuration. Observations indicate a flat universe, but anything beyond that is speculation.

Anyway, it seems to me that the 3-Torus model has curved space, but space is actually measured to be dead nuts flat (technical jargon) in all directions. I skimmed the text, but Dr. Silk talked about normal routes and shortcuts between two points, a characteristic that would create double images of the same object. That doesn't occur. I do admit I need to reread it!

The topological choices simplify in a flat universe. There happen to be just 18 distinct types of flat spaces. All of these, except for the analogue of the infinite plane, are multiply connected; that is to say, closed loops are possible.

There are no more possibilities for a flat universe - either it is infinite, or closed loops are possible, as I said. You should know by now that I always post the mainstream view.

I know you are careful not to make statements without supporting documents, but did you ever consider that you are limiting yourself with the vision of others? I believe there are more possibilities than Dr. Silk described, and one is a finite universe with an outer boundary that is ever changing. Observation and theory do not preclude this possibility!

 

[SpeedFreek]

The simplest explanation for a flat universe is the 3-Torus, there is no simpler finite topology. No model with an "edge" where spacetime ends is consistent with any observations, nor would it have a simpler topology than the 3-Torus (think of the boundary conditions at your "edge").

In concept, you can't get much simpler than space that ends at a boundary. The boundary condition would be...interesting.

...and not at all simple.

Anyway, it seems to me that the 3-Torus model has curved space, but space is actually measured to be dead nuts flat (technical jargon) in all directions. I skimmed the text, but Dr. Silk talked about normal routes and shortcuts between two points, a characteristic that would create double images of the same object. That doesn't occur. I do admit I need to reread it!

The 3-Torus has 4 dimensional geometry and is totally flat (i.e. the internal angles of a triangle add up to 180 degrees, which is not the case for a 2-Torus (a 3 dimensional torus, or donut!)). The double images would only exist if you could observe a closed loop, but it seems there are no small enough closed loops as the universe is too large. That we do not observe the closed loops simply means our observable universe is smaller than the fundamental domain of the whole universe so either it is infinite, or it is finite but much larger than our observable part of it.

I know you are careful not to make statements without supporting documents, but did you ever consider that you are limiting yourself with the vision of others? I believe there are more possibilities than Dr. Silk described, and one is a finite universe with an outer boundary that is ever changing. Observation and theory do not preclude this possibility!

I am careful that my statements reflect the mainstream view in cosmology (anyone can find "supporting" documents - ask Harry!). Mathematically, there are only 18 possible topologies, full stop. This is the accepted state of affairs - the consensus view. There are no other possibilities using our current understanding. The finite universe with an outer boundary is not one of the possible shapes for the universe, it is as simple as that. You will not find one viable cosmological model with an edge where everything stops - they are simply not considered as a possibility nowadays.

 

[orienteer]

Earth has been traveling for the past 4 billion years give or take, but has not gotten anywhere. If I take a piece of fiber optic cable and look in the end, I will see what ever is at the end and it will appear to be in front of me, in other words my vision bends without me realizing it. When we look through a telescope, if the photons were to be circling the 'center' of the universe, is it not possible that we would never see the edge of the said universe? After all how many grooves are on a record album? Perhaps the 13 billion years number is merely the length of the longest groove. Of course, by that definition space travel would become plausible since all of the groove on the album is no more than twelve inches away

 

[FlatEarth]

... There are no other possibilities using our current understanding. The finite universe with an outer boundary is not one of the possible shapes for the universe, it is as simple as that. You will not find one viable cosmological model with an edge where everything stops - they are simply not considered as a possibility nowadays.

In truth, our current understanding does not include what spacetime is, and it also doesn't include how it behaves at the farthest reaches of the universe. You are preaching scientific dogma here, and as I said, you are limiting yourself by adhering to concepts established by others who themselves are only making guesses. That is your prerogative, but you shouldn't make claims and state them as facts, because clearly they are not.

 

[FlatEarth]

Earth has been traveling for the past 4 billion years give or take, but has not gotten anywhere. If I take a piece of fiber optic cable and look in the end, I will see what ever is at the end and it will appear to be in front of me, in other words my vision bends without me realizing it. When we look through a telescope, if the photons were to be circling the 'center' of the universe, is it not possible that we would never see the edge of the said universe? After all how many grooves are on a record album? Perhaps the 13 billion years number is merely the length of the longest groove. Of course, by that definition space travel would become plausible since all of the groove on the album is no more than twelve inches away

WMAP (Wilkinson Microwave Anisotropy Probe) determined the curvature of space to be within 1% of flat. Is it possible that we can be fooled by the geometry of space to think this is true, and in reality we are looking into a curving medium? That can only be guessed at, but observations don't support that idea.

http://map.gsfc.nasa.gov/

 

[SpeedFreek]

In truth, our current understanding does not include what spacetime is, and it also doesn't include how it behaves at the farthest reaches of the universe.

Due to the cosmological principle, spacetime is expected to behave in a similar way everywhere in the universe.

You are preaching scientific dogma here, and as I said, you are limiting yourself by adhering to concepts established by others who themselves are only making guesses. That is your prerogative, but you shouldn't make claims and state them as facts, because clearly they are not.

I never went as far as to claim them as facts, if you check my wording would should see I simply claim they are the only viable scientific possibilities using our current scientific knowledge of the universe. Science rarely claims to deal in facts except when dealing with laws - everything else is subject to falsification.

All I am saying here is that, counter to common intuition, a universe that has an edge where space-time stops is not the simplest option. Scientifically and mathematically, it not considered a viable option using our current knowledge. If that is dogma, so be it, but I thought this was a scientific discussion rather than a philosophical one, so when you said I should hold my horses and that there are other possibilities, I felt I should point out that current mainstream science does not agree with you.

 

[SpeedFreek]

WMAP (Wilkinson Microwave Anisotropy Probe) determined the curvature of space to be within 1% of flat. Is it possible that we can be fooled by the geometry of space to think this is true, and in reality we are looking into a curving medium? That can only be guessed at, but observations don't support that idea.

http://map.gsfc.nasa.gov/

Yes, it is entirely possible that we are looking at a universe with a curvature so large that it would, to all intents and purposes, look flat within our observable universe.

 

[ramparts]

Indeed, that's a possibility we should be quite used to, having seen the Earth's surface as flat only to find out (due to its tremendously large radius of curvature) that it is, in fact, not

 

[FlatEarth]

Due to the cosmological principle, spacetime is expected to behave in a similar way everywhere in the universe.

Yes, right up to where it ends.

I never went as far as to claim them as facts, if you check my wording would should see I simply claim they are the only viable scientific possibilities using our current scientific knowledge of the universe. Science rarely claims to deal in facts except when dealing with laws - everything else is subject to falsification.

All I am saying here is that, counter to common intuition, a universe that has an edge where space-time stops is not the simplest option. Scientifically and mathematically, it not considered a viable option using our current knowledge. If that is dogma, so be it, but I thought this was a scientific discussion rather than a philosophical one, so when you said I should hold my horses and that there are other possibilities, I felt I should point out that current mainstream science does not agree with you.

Perhaps you should have phrased things differently, because you stated your opinions as absolute, with no other solutions possible, when clearly there are. This is a discussion of science, and as such, one should not impose unproven limits. This is more a criticism of scientists like Dr Silk, who would like us to believe they can answer questions that they actually can not.

 

[FlatEarth]

Indeed, that's a possibility we should be quite used to, having seen the Earth's surface as flat only to find out (due to its tremendously large radius of curvature) that it is, in fact, not

This is heresy!

 

[SpeedFreek]

This is a discussion of science, and as such, one should not impose unproven limits. This is more a criticism of scientists like Dr Silk, who would like us to believe they can answer questions that they actually can not.

There are a lot of scientists like Dr Silk!

http://curious.astro.cornell.edu/questi ... number=274

The discussion above accepts that we cannot really answer the question as it is outside of science, and then goes on to say that, because of that, current scientific theory precludes an edge - "All we do know is that based on our current understanding of theoretical cosmology, the universe does not have a boundary - it is either infinite or it wraps around itself in some way". :?

So, when you say that a discussion of science should not impose unproven limits, that applies to your applying an unproven limit to the universe - an edge where everything stops! Either we don't know, or using what we do know tells us the universe doesn't have an edge.

 

[FlatEarth]

There are a lot of scientists like Dr Silk!

http://curious.astro.cornell.edu/questi ... number=274

The discussion above accepts that we cannot really answer the question as it is outside of science, and then goes on to say that, because of that, current scientific theory precludes an edge - "All we do know is that based on our current understanding of theoretical cosmology, the universe does not have a boundary - it is either infinite or it wraps around itself in some way". :?

So, when you say that a discussion of science should not impose unproven limits, that applies to your applying an unproven limit to the universe - an edge where everything stops! Either we don't know, or using what we do know tells us the universe doesn't have an edge.

There is a difference between unproven rules that limit ideas, and ideas of a limited universe, but we both know that. The subject is outside of science, and that's what draws me to it. I have a problem with scientists like Dr. Silk trying to impose their ideas on us without adequate qualifiers. Again, that's not directed at you because I know you like to stick to mainstream science, but on this topic I believe mainstream science is on shaky ground.

Actually, I don't think the universe just stops. Based on observations, I tend to think it is expanding, with a moving boundary that cannot be reached due to the rate of expansion, and of course, never crossed (I don't mean to imply that a boundary is suggested by observations, only that if there were one, it would not be stationary). This is probably more closely aligned with the concept of an infinite universe, but I have trouble with an infinite universe because we know it isn't infinitely old. In a practical sense, however, it may as well be considered infinite.

 

[SpeedFreek]

Actually, I don't think the universe just stops. Based on observations, I tend to think it is expanding, with a moving boundary that cannot be reached due to the rate of expansion, and of course, never crossed (I don't mean to imply that a boundary is suggested by observations, only that if there were one, it would not be stationary).

Why do you think the boundary cannot be reached due to the rate of expansion, when the rate of expansion would surely be the same there as it is everywhere else? From our point of view, the distance where a galaxy apparently recedes at the speed of light is 14 billion light-years away, and the assumption of the cosmological principle is that the same would be true for any observer in the universe. Your idea introduces a direction in which an observer cannot travel away from their own galaxy, if that direction is towards the edge - why is this? :?:

 

[FlatEarth]

Why do you think the boundary cannot be reached due to the rate of expansion, when the rate of expansion would surely be the same there as it is everywhere else? From our point of view, the distance where a galaxy apparently recedes at the speed of light is 14 billion light-years away, and the assumption of the cosmological principle is that the same would be true for any observer in the universe. Your idea introduces a direction in which an observer cannot travel away from their own galaxy, if that direction is towards the edge - why is this? :?:

Actually, this goes back to a discussion we had a while back. I think it's likely that spacetime expands faster than the rate we observe the universe expanding, because objects in the universe are subject to the effects of gravity. It's the same rationale that says space expands all around us, but gravity keeps the solar system, Milky Way, and Local Group gravitationally bound. Space expands, but objects in a gravity well of a given strength overwhelm the effects of expanding spacetime, and do not recede from each other. Taken a step further, the effect of gravity between more distant objects is much less, however it still exists. It's logical to conclude that spacetime must be expanding at a greater rate than we measure by observing the rate at which distant objects recede. The rate of spacetime expansion may be much higher, and could be as high or higher than the speed of light. :!:

 

[Woggles]

Wow then if this is true, then there would be no end to the expansion? And if so then at some point in time our universe would just go dark. It is hard to grasp a never ending expansion, human point of point lol. Of course this leads my think to what is our universe expanding in? Wait perhaps that just too simple thinking, perhaps it's not expanding in something as more expanding in space-time? And Space-time is consider a dimension (4th I think). ?

 

[SpeedFreek]

But FlatEarth, can you not see that you are giving the expansion a direction with that idea? I think you are treating the expansion like a river flowing past things, dragging them along with the current, but with the objects floating down the river slower than the actual river is flowing. Is that the kind of thing you mean?

If so, our observations tell us that the expansion cannot work like that, unless that "flow" is actually directed away from Earth (I think we covered this before), putting the Earth at the centre of expansion as if the Earth was in a very special place in the Universe. Perhaps you could try considering the idea that the expansion only happens between things, as if it flows away from all objects towards the gaps in between them, causing increasingly large puddles of space in those gaps.

Or, conversely, if the expansion were a force, it would be pushing on us with the same strength from all directions rather than flowing past us, and pushing all objects (at the largest scales) away from the centre of the gaps in between those objects, equally, everywhere.

 

[FlatEarth]

Wow then if this is true, then there would be no end to the expansion? And if so then at some point in time our universe would just go dark. It is hard to grasp a never ending expansion, human point of point lol. Of course this leads my think to what is our universe expanding in? Wait perhaps that just too simple thinking, perhaps it's not expanding in something as more expanding in space-time? And Space-time is consider a dimension (4th I think). ?

Woggles, please keep in mind that my views are not shared by mainstream science! If you are looking for accepted concepts, Speedfreek, ramparts, and the Mods are the ones to listen to! My ideas amount to conjecture and thought experiments!

According to the Big Band Theory, the universe is not expanding in something that is bigger. It was created at the Big Bang, and spacetime, which is part of the universe, expanded ever since. Spacetime consists of three dimensions plus time, the forth dimension. This is not conjecture!

 

[FlatEarth]

But FlatEarth, can you not see that you are giving the expansion a direction with that idea? I think you are treating the expansion like a river flowing past things, dragging them along with the current, but with the objects floating down the river slower than the actual river is flowing. Is that the kind of thing you mean?

If so, our observations tell us that the expansion cannot work like that, unless that "flow" is actually directed away from Earth (I think we covered this before), putting the Earth at the centre of expansion as if the Earth was in a very special place in the Universe. Perhaps you could try considering the idea that the expansion only happens between things, as if it flows away from all objects towards the gaps in between them, causing increasingly large puddles of space in those gaps.

Or, conversely, if the expansion were a force, it would be pushing on us with the same strength from all directions rather than flowing past us, and pushing all objects (at the largest scales) away from the centre of the gaps in between those objects, equally, everywhere.

Speedy, I understand the concept of inflation, and I know it is not directional. It is unidirectional, expanding everywhere at an equal rate. We also know that matter does not move with the inflation of spacetime on a 1 to 1 ratio. Gravity tends to keep all matter together, while spacetime expands at its own rate, as evidenced by our solar system and galaxy. There is no observable expansion of spacetime occurring within the Milky Way, yet we know spacetime is expanding here. Taken to the scale of the observable universe, if we assume that the gravity slows the recession rate of distant objects relative to the expansion of spacetime, then it's easier to picture that spacetime may well be leaving gravitationally bound objects behind! Picture a God's eye view of the universe, with spacetime expanding at a certain rate, and a zone of gravitationally bound objects expanding at a slower rate. We actually do not know how fast spacetime expands. We can only measure the rate at which objects recede from us. Does this help to clarify the idea?

 

[ramparts]

How do we know spacetime is expanding here, FlatEarth?

 

[FlatEarth]

How do we know spacetime is expanding here, FlatEarth?

Because.

http://www.astro.ucla.edu/~wright/cosmology_faq.html

Why doesn't the Solar System expand if the whole Universe is expanding?

This question is best answered in the coordinate system where the galaxies change their positions. The galaxies are receding from us because they started out receding from us, and the force of gravity just causes an acceleration that causes them to slow down, or speed up in the case of an accelerating expansion. Planets are going around the Sun in fixed size orbits because they are bound to the Sun. Everything is just moving under the influence of Newton's laws (with very slight modifications due to relativity). [Illustration] For the technically minded, Cooperstock et al.computes that the influence of the cosmological expansion on the Earth's orbit around the Sun amounts to a growth by only one part in a septillion over the age of the Solar System. This effect is caused by the cosmological background density within the Solar System going down as the Universe expands, which may or may not happen depending on the nature of the dark matter. The mass loss of the Sun due to its luminosity and the Solar wind leads to a much larger [but still tiny] growth of the Earth's orbit which has nothing to do with the expansion of the Universe. Even on the much larger (million light year) scale of clusters of galaxies, the effect of the expansion of the Universe is 10 million times smaller than the gravitational binding of the cluster.

I must admit, I do have a hidden agenda, ramparts. Because you are a proto-physicist of sorts, you could one day base your thesis on this subject! What do you think?