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Author Topic:   Do We Live in an Infinite Universe?
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 4 of 60 (334517)
07-23-2006 1:11 PM
Reply to: Message 1 by GDR
07-23-2006 11:56 AM


Gotta be brief, so just to start:
What is the definition of a universe?
I would go with EVERYTHING, i.e. the sum total of any multi-verse type idea. The Universe from the Big Bang could be all there is to it. Or the Big Bang could actually be a "nucleation" from a parent universe, and there may be an infinite number of such nucleations occuring in parallel. Or the universe could be cyclical in an oscillating universe type of thing. But it's only worth considering the largest "covering"-verse.
{Aside: Though there may be an infinite regress of "covering"-verses!!! Or even better, a cyclical set of "covering"-verses... there was an awesome cartoon in one of the old Humour threads with an astronaut who crash-lands on his own head }
Edited by cavediver, : No reason given.
Edited by cavediver, : No reason given.

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 6 of 60 (334523)
07-23-2006 1:31 PM
Reply to: Message 3 by nwr
07-23-2006 12:55 PM


Re: It doesn't matter
But the predictions are all within that 200 light year sphere that I mentioned... Since the model was designed to work well for our space-time neighborhood, this is expected
Our model of the Universe is based upon observations of isotropy and homogeniety, both observed in the billions of lyr range... And the "stars" of greatest test of GR, the binary pulsar measurements, are substantially further away than 200 lyrs
In summary, it really doesn't matter.
If we were only worried about things that "mattered", cosmology would have died long ago!

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 Message 7 by nwr, posted 07-23-2006 1:36 PM cavediver has replied

  
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 8 of 60 (334527)
07-23-2006 1:43 PM
Reply to: Message 7 by nwr
07-23-2006 1:36 PM


Re: It doesn't matter
The photons, by virtue of which we are able to make such observations, were already within that 200ly radius at the time I was born.
I thought that must be what you meant... I would say that believing what those photons show is one of our safer extrapolations, unless you really think God invented them all 6000 yrs ago

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 11 of 60 (334552)
07-23-2006 4:45 PM
Reply to: Message 10 by nwr
07-23-2006 4:24 PM


Re: It doesn't matter
But expecting to deduce that there must have been a creator
We're not though, are we? As far as I am aware, this thread is purely for discussing the possibility and implications of an infinite universe. I think I've stated my belief before: that physics can only push out the bounds of any potential creator, never prove (or even provide evidence for) such a creator.

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 13 of 60 (334734)
07-24-2006 4:39 AM
Reply to: Message 12 by GDR
07-23-2006 5:36 PM


I still have trouble conceiving of a universe where there was point when T=0 being a universe of infinite time
I'm not sure what you mean here. Do you mean a universe where there is no T<0? In that case, the universe can only be semi-infinite in time, i.e. has an infinite future. This is how our universe appears at the moment. If our universe was closed and there was no cosmological constant, then the universe would also have an end: the Big Crunch, and then the universe would be truly finite in extent both spatially and temporally.
However, that says to me then that when scientists talk about parallel universes
This is still the same universe as I would define it.
or when they talk about every now being an eternal universe
This is going a bit far for this discussion. This is talking about the nature of time way beyond GR. I would leave this alone for now, as we are getting to conjecture upon conjecture. Let's just stick with conjecture

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 15 of 60 (334812)
07-24-2006 11:12 AM
Reply to: Message 14 by GDR
07-24-2006 10:45 AM


The tear drop would represent time as we know it starting and ending just once but flowing back into infinite time as represented by the circle.
Ok, I got you. The way I would look at it is that time flow is just something we experience. If anything, it is almost the definition of conciousness. From the Universe's POV there is no flow. There is just all of time, just as there is all of space. You can look at that as a circle if you like, but nothing is going around the circle; it is simply fixed. The problem comes in joining the ends, becasue although it would be nice to think of the end of time simply flowing back into the beginning of time, there is the problem of entropy. The universe is in a very different state at the BC than at the BB.
The problem as I see it with that scenario is that appears to me to require the Big Crunch.
True.
I thought however that as the expansion is accelerating the theory of the BC has been pretty much discounted with or without a cosmological constant.
The acceleration is driven by the CC. With no CC, there is no acceleration, and we are back to the good old days of the three BB scenarios: closed, flat, and open, of which the latter two have semi-infinite time, just as with the accelerating universe.
However as the universe expands gradually all of the other galaxies will expand beyond the event horizon for our galaxy. As gravity moves at light speed we will no longer be in the gravitational effect of any other galaxy. Does that matter to us?
Awesome question I've never herad that one before! The answer is yes and no. Any new perturbations to the local gravitational field will propegate out at c and never reach the other galaxies. But the original curvature is still there. You could say that the gravitational potential is always felt, but changes to the potential can be hidden.

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 17 of 60 (334825)
07-24-2006 12:24 PM
Reply to: Message 16 by GDR
07-24-2006 12:01 PM


I thought that it is pretty much conceded that the expansion of the universe is accelerating.
Yes, it is. I was talking from the theoretical model perspective.
However, I gathered that there could still be a CC that is limiting the acceleration.
Ok, we've got to be more precise here then. The acceleration is driven by a field, we'll call A. If A is positive, the universe gets pulled in on itself; if it is negative it accelerates the universe apart, as observed.
Now A could be made up of several things, but we'll just consider the important two: the Cosmological Constant,L (should be lambda but I'm too lazy to put in Greek) and some dark energy field, D.
So A = L + D. Now L can be positive, zero or negative. D can be positive, zero or negative. But we can't tell them apart at the moment. So L could be doing the accelerating, or D or both. So there's often little point separating the two concepts. When I say CC, I mean sun of the two, which is an abuse of terminology on my part.
Are you meaning then that the expansion could still at some point stop accelerating and start to collapse.
Could do. The thing about L is it is constant. But D doesn't have to be, and could be reducing so that at some point the acceleration may cease, and the universe could even recollapse.
If that were to happen wouldn't that have a profound impact on the flow of time.
It was once thought that it would, but entropy again seems to rule that out.
What would happen when the rate of collapse exceeds light speed in the way that expansion does now?
Nothing much. Only towards the end would it become seriously noticable, and by then it would be the last thing on your mind
Although I can't begin to understand it your suggestion that time and space is just a function of what is perceived by consciousness
That's not really what I was saying... although it is a discussion for another day. What I was saying was that the "flow of time" is a construct of conciousness.
Isn't it also possible though that the gravitaional field could collapse in that scenario, resulting in Big Disintegration?
No. There is no gravitational field in that sense. There is curvature. It is the Universe, as a whole, and doesn't have much respect for horizons and c which are local concepts.

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 Message 16 by GDR, posted 07-24-2006 12:01 PM GDR has replied

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 Message 19 by GDR, posted 07-24-2006 2:17 PM cavediver has replied

  
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 20 of 60 (334978)
07-24-2006 6:47 PM
Reply to: Message 19 by GDR
07-24-2006 2:17 PM


Wouldn't any change in D likely be caused by the fact that... the dark energy field is becoming more diffuse, thus reducing its gravitational pull?
It's not exerting a gravitational pull exactly, but it may certainly reduce. Then again, it could get stronger as time goes on, which would give you the Big Rip scenario. It all depends on the particular dynamics of the field, and we are a very long way from determining that. This kind of field, outside of current Standard Model physics, is the sort of thing you expect from String Theory and similar. But don't think of it like a dust which thins out as such. It is more akin to an electromagnetic field through space-time, just with different properties.
If this is the case then wouldn't we expect that instead of starting to contract that we would arrive at a point where A is neutral and there would be a state of equilibrium which might leave us in a state of infinite time.
That would be cool: an asymptotic Einstein Static Universe. Such a model may well have been devised in the literature. It maybe worth a search. But again, the dynamics of D are far from determined.
the universe is contracting then doesn't that mean that entropy is decreasing.
A bit before my time, Hawking and others thought this, which led to the time running in reverse picture (and the awesome Backwards episode of Red Dwarf ) Physically, this would be like an end to time at maximum expansion. But we're fairly sure now that entropy would just keep increasing in such a situation.
you saying that time is just the way that we perceive change as Barbour would say?
No, that's too deep for today.
If space and time are both illusions it sure becomes difficult to sort out reality from from perception.
True, but it's not what I'm saying (today). Merely that the flow of time is an illusion, not time itself. Imagine my good old globe, BB at North Pole, etc. There is the universe in all its glory: all time, all space, no flow of anything, no beginning, no end... just is. But if you look closely, you will see these little human threads, and you can see "time" ticking along them, starting at an end furthest north where these humans are born, and stretching south to where they die. They're not even all aligned. Two threads next to each other could have each "time" at a different point on the thread, so one thread thinks its having a 5th birthday in 1975, and the thread next to it thinks that it's getting married in 2012.
With an expanding universe it would seem to me that the field would be weakened.
Yes, in a sense. As the universe expands, the curvature decreases. But galaxies moving out of causal contact with each other is not going to make any difference to the gradual decrease in curvature. GR doesn;t really care how localise dthe mass is, it just wants to know how much mass there is in a particular volume. It's not a problem if you let that volume of space increase in size as the universe expands.

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 23 of 60 (335367)
07-26-2006 4:30 AM
Reply to: Message 22 by RAZD
07-25-2006 8:49 PM


An interesting side is what Einstein thought about "ether"
Yes, and this is why I always jump in if someone says that there is no ether, no space-time background. Einstein found the ether in the form of the metric of General Relativity, it is just of substantially different character to the usual concept of ether held at that time.
Today, with our everyday use of fields rather than particles or objects, it is simply taken for granted.
He has also IIRC said that we cannot disprove that an ether exists.
I've not seen that. But we know of this ether only through General Relatvity (and related metric theories) and our other field theories. I guess it stands or falls on the merits of such theories. Even ideas of your own, such as adding another dynamical particle/field to account for dark matter, would simply be incorporated into this picture. You would need to deviate substantially from current ideas of space, time and particle physics to shake the idea of an "ether".
Edited by cavediver, : No reason given.

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 26 of 60 (335416)
07-26-2006 9:57 AM
Reply to: Message 25 by nwr
07-26-2006 9:50 AM


The original concept of the ether was discarded as unnecessary. It was never actually proven to not exist.
True. The Lorentz-Fitzgerald Contraction was actually first proposed as a way to explain the null result of MM, before it found itself at the heart of SR.
Will get back to Percy's comments later.

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 46 of 60 (338939)
08-10-2006 11:45 AM
Reply to: Message 45 by Percy
08-10-2006 10:10 AM


Re: infinite universe
In that vein, there's a recent book that might interest you, The End of Science by John Horgan, in which he argues that we *are* reaching the limits of knowledge. As I've already said, such predictions have a very poor track record
So very true. Even Hawking (amongst others), back in 1980, thought that it was all sown up with N=8 Supergravity. The naivity is astounding in hindsight, but shows we can all get caught out...

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