Is the universe still expanding?

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  • #51
SpaceTiger
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Mike2 said:
I think we are in agreement here. You say "beginning of spacetime".
No, I said "creation of spacetime". The distinction is important because "beginning" implies some sequential set of events, and those only happen within spacetime. Spacetime itself just is.


And we talk about "expansion". So doesn't that in and of itself imply that the universe cannot be infinite - that expansion would take forever to get to infinity?
It may be that an infinite universe can expand infinitely, remaining infinite all along. The point is that we just don't know. Scientists avoid infinities because they're difficult to deal with and because, historically, they've indicated problems in a theory. However, that doesn't mean they can't occur. A good scientist should be careful not to impose any such preconceived notions upon the universe. We've learned this lesson many times in the past.


So 60 e-folds is a scale factor of e^60?
That's right.
 
  • #52
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expansion on the local scale

The expansion of the universe is indeed a difficult concept to explain without resort to mathematics. Assuming for the moment that the expansion of spacetime is a fact then the bodies in the solar system are also receding from each other. I've seen arguments by professionals that this does not apply to the local scale because the gravitational forces acting between the bodies do not allow this, but this cannot be relevant if space itself is expanding. I wonder if it would be possible to measure this expansion within the solar system e.g. by measuring signals between the earth and orbiting satellites. Perhaps the effect is too small for modern techniques to measure. It would certainly provide undeniable proof whether expansion is occuring or not.
 
  • #53
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davstar said:
I've seen arguments by professionals that this does not apply to the local scale because the gravitational forces acting between the bodies do not allow this, but this cannot be relevant if space itself is expanding.
It's not that it's not "allowed", it's that the local forces dominate the changes in displacement. In that case, the physical separation between two objects changes more from their movement through spacetime than from the expansion of spacetime itself.


I wonder if it would be possible to measure this expansion within the solar system e.g. by measuring signals between the earth and orbiting satellites. Perhaps the effect is too small for modern techniques to measure.
It is indeed.
 
  • #54
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SpaceTiger said:
No, I said "creation of spacetime". The distinction is important because "beginning" implies some sequential set of events, and those only happen within spacetime. Spacetime itself just is.
Even from within spacetime there is a sense of a beginning in terms of the structure within. It would seem that complicated structures would have to come from the more simplistic. But if complicated structure should arise instantaneously, then all hope of tracing the sequence of events is lost. Or if complicated structure continues forever in all direction, then again all hope of tracing the sequence of events that lead to it is lost. So we must have a finite universe if there is any hope of obtaining a TOE.
 
  • #55
turbo
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Mike2 said:
Even from within spacetime there is a sense of a beginning in terms of the structure within. It would seem that complicated structures would have to come from the more simplistic. But if complicated structure should arise instantaneously, then all hope of tracing the sequence of events is lost. Or if complicated structure continues forever in all direction, then again all hope of tracing the sequence of events that lead to it is lost. So we must have a finite universe if there is any hope of obtaining a TOE.
Imagine that the Universe had NO beginning and that it is infinite in spacial extent and time. What does this do to your insistence on causality? I would think that your insistence on causality would give you a lot of trouble if you adhere to a BB model, especially a finite one. Infinite steady-state cosmologies can easily accomodate a TOE.
 
  • #56
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Mike2 said:
Even from within spacetime there is a sense of a beginning in terms of the structure within. It would seem that complicated structures would have to come from the more simplistic. But if complicated structure should arise instantaneously, then all hope of tracing the sequence of events is lost. Or if complicated structure continues forever in all direction, then again all hope of tracing the sequence of events that lead to it is lost.
I'm not sure how complexity enters into this. The discussion is about the beginning of time and whether it can begin as spatially infinite in extent. The initial conditions of the universe can be arbitrarily simple and yet still infinite in extent. In fact, some might argue that an infinite universe is simpler than one with an arbitrary scale.
 
  • #57
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turbo-1 said:
Imagine that the Universe had NO beginning and that it is infinite in spacial extent and time. What does this do to your insistence on causality? I would think that your insistence on causality would give you a lot of trouble if you adhere to a BB model, especially a finite one. Infinite steady-state cosmologies can easily accomodate a TOE.
How could you explain where spacetime itself came from if it were eternally existing and/or infinite in extent? Such a premise is a direct denial of any explanation. Such a position is the same as saying, "There is no explanation; it just instantaneously popped into existence." Or it would be the same as saying, "There is no explanation for where it came from because it has always been here." If the universe as a whole defies explanation, then why should you think any part of it could be explained?
 
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  • #58
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Mike2 said:
How could you explain where spacetime itself came from if it were eternally existing and/or infinite in extent? Such a premise is a direct denial of any explanation. Such a position is the same as saying, "There is no explanation; it just instantaneously popped into existence." Or it would be the same as saying, "There is no explanation for where it came from because it has always been here." If the universe as a whole defies explanation, then why should you think any part of it could be explained?
You are correct, IMO, in questioning both arguments. It is just as confounding to propose a universe from nothing as it is to propose a universe that always was.

Observational evidence indicates, but does not prove, our universe is temporally finite - just that the preponderance of evidence supports this conjecture. I allow, however, this may merely reflect the limits of our ability to perceive and describe our observable evidence.
 
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  • #59
turbo
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Mike2 said:
How could you explain where spacetime itself came from if it were eternally existing and/or infinite in extent?
If the Universe is infinite in all space and time, it simply exists, and asking for a creation story (Where did it come from?) is futile.
Mike2 said:
Such a premise is a direct denial of any explanation. Such a position is the same as saying, "There is no explanation; it just instantaneously popped into existence."
There is a not-so-subtle point that you are missing - a spacially and temporally infinite universe simply IS. It does not exist relative to anything else, nor can you consider that it "popped into esistence".
Mike2 said:
Or it would be the same as saying, "There is no explanation for where it came from because it has always been here."
That's a whole lot closer to the truth.
Mike2 said:
If the universe as a whole defies explanation, then why should you think any part of it could be explained?
We can certainly explain large parts of the Universe, although our models do not agree on some scales (thus the problems encountered in developing a theory of quantum gravity). This indicates that one or more of our models need to be modified or supplanted.

Like Space Tiger said above "some might argue that an infinite Universe is simpler than one with an arbitrary scale". I would take that position. As for the need to define the entire Universe relative to something else in hopes of satisfying causality, that is a losing battle. You then need to define the properties if the background against which the Universe "popped into existence" and explain why that background existed in the first place, and then you'll need a "creation story" to establish causality regarding the origin of that background, and so forth in infinite regression. It may not sound logical to you, but it is easier to contemplate the existence of a spacially and temporally infinite universe, than to play the causality game ad infinitum.
 
  • #60
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Mike2 said:
How could you explain where spacetime itself came from if it were eternally existing and/or infinite in extent?
I think you're taking "Theory of Everything" too literally. There will always be something whose origin cannot be explained. With traditional cause and effect, every cause must have its own cause, so an infinite chain is the only way out of an unexplainable beginning.

This discussion has long since left the realm of science, so perhaps you should bring up the issue in one of the philosophy forums.
 
  • #61
hellfire
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I just cannot imagine how physics can make a strict sense of any kind of infinite value. In this case an infinite amount of time from an infinite past. This would need of an infinite number of events to reach present. Does this make sense? Is present actually possible in such a model? It remembers me to the paradoxes related to the spatial infinite. For example, can a force (e.g. gravity) be acting on a body located at an infinite distance? This body will never change its position as it will always stay at infinity (oo - N = oo). It seams that the concept of position makes no sense at infinity. I have a bad feeling with this, and I think something similar goes on with an infinite past. I would be more happy with the existence of a state which is uncaused and from which the causal chain and time arise (may be as some fluctuation or break of symmetry).
 
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  • #62
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SpaceTiger said:
Remember that this "motion" that they appear to have from our point of view is only a consequence of where we're viewing them from. If we were on another galaxy, they'd appear to be moving in a different direction.
Is there a distinct point (distance from earth in every direction), where this apparent motion starts to become observable, or is the observed expansion rate very different at a particular distance in one direction than it is at the same distance in another direction?
 
  • #63
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Simetra7 said:
Is there a distinct point (distance from earth in every direction), where this apparent motion starts to become observable, or is the observed expansion rate very different at a particular distance in one direction than it is at the same distance in another direction?
It depends on the magnitude of the local motions, but it will be roughly the same no matter what direction you look. Basically, you'll see the effects of expansion when,

[tex]H_0d \sim v_{gal}[/tex]

The magnitude of the galaxy's peculiar velocity (vgal) will depend on where it's located, but on average, does not depend on direction. As for H0, it certainly doesn't depend on direction.
 
  • #64
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Does the expansion of space effectively give us an almost static view of distant objects in space? For example, the furthest observable galaxy in the Hubble deep field has been estimated to be approximately 13 billion light years away, so the light that is observed from that galaxy is from 13 billion years ago, when the universe was only an estimated 1 billion years old. At that time, the position that Earth now occupies in space would have been much closer to that particular galaxy, so an observer, (had there been anyone around at that time to observe), would have seen that galaxy at almost the same period in it's evolution as we are seeing it now. So in 14 billion years, only 1 billion years worth of the evolution of that galaxy has been observable, but it has been stretched out over the whole of those 14 billion years. Does this make any sense?
 
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  • #65
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SpaceTiger said:
It depends on the magnitude of the local motions, but it will be roughly the same no matter what direction you look. Basically, you'll see the effects of expansion when,

[tex]H_0d \sim v_{gal}[/tex]

The magnitude of the galaxy's peculiar velocity (vgal) will depend on where it's located, but on average, does not depend on direction. As for H0, it certainly doesn't depend on direction.
When the Earth's velocity around the Sun has been taken into account the Solar System is travelling at 390 +/- 60 km/sec relative to the surface of last emission of the CMB. However when the Sun's motion around the Galaxy is also taken into account this translates into the fact that the Galaxy is travelling relative to the surface of last emission of the CMB, which probably defines the C.M. reference frame of the universe, at 603 km/sec or about 0.2%c! (Nature, Vol 270, 3 Nov 1977, pg 9) Therefore, if our galaxy's velocity is 'typical', then
[tex]H_0d \sim 0.002c[/tex] or d ~ 10 Mpsc

Simetra7 said:
So in 14 billion years, only 1 billion years worth of the evolution of that galaxy has been observable, but it has been stretched out over the whole of those 14 billion years. Does this make any sense?
Yes, you are describing cosmological time dilation, otherwise detected as cosmological red shift.

Garth
 
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  • #66
Chronos
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I usually avoid these conversations. You are not seeing the 'big picture' here, Simretra7. You are making unfounded assumptions about the initial state of the universe and extrapolating them way beyond what is reasonable. It makes no sense.
 
  • #67
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Chronos said:
I usually avoid these conversations. You are not seeing the 'big picture' here, Simretra7. You are making unfounded assumptions about the initial state of the universe and extrapolating them way beyond what is reasonable. It makes no sense.
My intention was not to make any sort of an assumption but just to try to make sense out of what is a fascinating but extremely complicated subject.
Are you saying that our view of far off objects is not a "slow motion" view, or is it just my reasoning that is unreasonable.
 
  • #68
hellfire
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Simetra7 said:
Does the expansion of space effectively give us an almost static view of distant objects in space? For example, the furthest observable galaxy in the Hubble deep field has been estimated to be approximately 13 billion light years away, so the light that is observed from that galaxy is from 13 billion years ago, when the universe was only an estimated 1 billion years old. At that time, the position that Earth now occupies in space would have been much closer to that particular galaxy, so an observer, (had there been anyone around at that time to observe), would have seen that galaxy at almost the same period in it's evolution as we are seeing it now. So in 14 billion years, only 1 billion years worth of the evolution of that galaxy has been observable, but it has been stretched out over the whole of those 14 billion years. Does this make any sense?
Yes, this makes sense. Further on, objects located very near to our spatial position at an instant of time after t = 0 would have been observable to an observer located at our spatial position, but after 13.7 Gyr we would see them now exactly in the same instant of their evolution as this observer did.

This is due to the fact that in a dynamic space with an initial singularity the past light cone is not a cone but a "teardrop". In a static space, the size of the particle horizon now is the same as the size of the past light cone at t = 0. For a dynamic space this is not true, since the size of the light cone is zero at t = 0.

The term particle horizon refers to the current location of the objects which sent us a light ray at t = 0 that we are observing now. The term past light cone refers to the objects located in past which have a causal influence on us.

May be figure 1 in http://arxiv.org/astro-ph/0310808 [Broken] will help.
 
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  • #69
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hellfire said:
May be figure 1 in http://arxiv.org/astro-ph/0310808 [Broken] will help.
Could you check this link. I think it may be the wrong one.
 
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  • #70
hellfire
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The link is correct. Go to Full-Text: PDF, and open the document ("Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe"). You may take a look to Figure 1 and try to understand what happens with the light cone and the particle horizon. If you have questions, please ask.
 

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