Does the big bang imply a finite universe

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  • #26
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matt.o said:
This is a common misconception. Galaxies receding at faster than light speeds can still be observed by us. If you think about it a little it will become clear. Consider a galaxy travelling with the hubble flow at such a distance that it is receding at faster than the speed of light. Now consider a point half way between us and the FTL galaxy. The recessional velocity here is not FTL, wrt us and the FTL galaxy. Hence light can get to this midway point from the FTL galaxy. If light can get to the midpoint, then it can also get to us. If you search for Tamara Davis and Charlie Lineweaver on google scholar or ADS, then you will come up with references on FTL recession.
Yes, but by the time the light from the FTL galaxy reached the half-way-point galaxy, the half-way-point galaxy would be beyond the cosmological event horizon as well.

Originally Posted by Mike2
Some pose that the CMB proves an initial big bang and subsequent expansion. I don't disagree. I'm suggesting that there may be more than one big bang and more than one CMB created each time the universe experiences a big rip. In other words, a big rip causes a new big bang with all the characteristics of the one we see now. Is there any observable evidence that can disprove this big rip = big bang theory? Thanks.
This is very hard to prove or disprove. I would suggest that there could be no observational evidence since the big rip occurred before the big bang (hence unobservable to us).
One thing that may end the big-rip = big-bang scenario is this: if the quantum fluctuation are not scale invariant, then the scale factor of space cannot expand forever to create big-bangs over and over again. If particles are extended objects, then this cannot happen. If particles are singularities, then this might still happen unless the path-integral formulations are not scale invariant. What do ya think?
 
  • #27
Chronos
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I wonder if a different analogy would help demystify FTL expansion. If a jet plane flew past you traveling in excess of the speed of sound, would you hear it as it receeded into the distance?
 
  • #28
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Mike2, you should just go and find the papers by Davis and Lineweaver.
 
  • #29
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Mike2 said:
Yes, but by the time the light from the FTL galaxy reached the half-way-point galaxy, the half-way-point galaxy would be beyond the cosmological event horizon as well.
matt.o is right, Mike2. The recession velocity of stellar objects makes no difference as to WHETHER the light from these objects will reach us or not; the distance and recession velocity only makes a difference to WHEN the light from these objects reaches us. If a spacetime event is beyond our event horizon then the light from that event will not reach use before the "end of time"; but all objects in a universe which is infinitely old were within our event horizon at some stage during the past, therefore we will receive light from all of those objects today.

MF :smile:
 
  • #30
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moving finger said:
...If a spacetime event is beyond our event horizon then the light from that event will not reach use before the "end of time"; but all objects in a universe which is infinitely old were within our event horizon at some stage during the past, therefore we will receive light from all of those objects today.
The first sentence agrees with me that if there is a cosmological event horizon, then we will never see objects beyond it. Or what else can "before the 'end of time'" mean?

Inflation predicts an exponential expansion rate in the early universe. This means that the universe could have approached an infinitely small singularity in the infinite past. In other words, then universe may be infinitely old. We don't know how long the universe existed before inflation. Yet we may still have an event horizon in this universe.

If distance is increasing faster than an object is moving, it will never reach us. If you roll a ball eastward at 5mph on an infinitely long train moving westward at 6mph, the ball is actually moving away from a stationary observer behind the ball at 1mph. What?
 
  • #31
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Mike2 said:
The first sentence agrees with me that if there is a cosmological event horizon, then we will never see objects beyond it. Or what else can "before the 'end of time'" mean?
I mean that even if the universe exists for an infinite time into the future then no information from a spacetime event which is beyond our event horizon will ever reach us (this is in fact simply the definition of the event horizon). However the event horizon relates to events in spacetime and NOT to objects in space (this is a common confusion). If the universe has existed for an infinite time in the past then there will be a time somewhere in the past when events from a particular object were inside our event horizon, no matter how far that object was from us then or is now, therefore we will see events from that object from that time.

Mike2 said:
Inflation predicts an exponential expansion rate in the early universe. This means that the universe could have approached an infinitely small singularity in the infinite past. In other words, then universe may be infinitely old. We don't know how long the universe existed before inflation. Yet we may still have an event horizon in this universe.
If it is infinitely old (in the past) then this simply means that there will be some events which are within our current event horizon from all objects in the universe.

Mike2 said:
If distance is increasing faster than an object is moving, it will never reach us. If you roll a ball eastward at 5mph on an infinitely long train moving westward at 6mph, the ball is actually moving away from a stationary observer behind the ball at 1mph. What?
This is also a common misconception. Even if recession velocities exceed the speed of light, we can still receive light from the objects concerned.

Check out http://arxiv.org/abs/astro-ph/0310808 for a very useful series of explanations.

MF :smile:
 
  • #32
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moving finger said:
...If the universe has existed for an infinite time in the past then there will be a time somewhere in the past when events from a particular object were inside our event horizon, no matter how far that object was from us then or is now, therefore we will see events from that object from that time.
What you are saying is that nothing can cross the cosmological event horizon to be seen no more. However, it is my understanding that things can cross the event horizon so that we loose touch with them. Furthermore, as I understand it, the universe is accelerating in its expansion. And when the expansion rate changes, the distance to our event horizon changes. With acceleration, the event horizon is contracting ever closer to us as time passes. This can mean nothing more than that objects presently within our event horizon will slip behind it when it contracts to that distance. And as we've agreed, we cannot see what is going on with objects behind the event horizon. That doesn't mean that we may never be able to see it again, only that we will never see what it's doing from the time it does slip behind the event horizon to the time it reemerges, if the universe were to decelerate and the event horizon should grow.

Check out http://arxiv.org/abs/astro-ph/0310808 for a very useful series of explanations.
MF :smile:
Yes, I've read it, a couple of times. They could revise it to make it a little more clear between things that were once in our event horizon and things that are just now slipping behind it.
 
  • #33
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could we slow an object down and send it back to the 'center of origin' of the universe

the universe does not have a event horizon

it has an avarage one, but only the molecules the molecules have one

each object has its own event horizon, because we can slow one down so it's EvHr
is futher than another objects
 
  • #34
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Mike2 said:
What you are saying is that nothing can cross the cosmological event horizon to be seen no more. However, it is my understanding that things can cross the event horizon so that we loose touch with them. Furthermore, as I understand it, the universe is accelerating in its expansion. And when the expansion rate changes, the distance to our event horizon changes. With acceleration, the event horizon is contracting ever closer to us as time passes. This can mean nothing more than that objects presently within our event horizon will slip behind it when it contracts to that distance. And as we've agreed, we cannot see what is going on with objects behind the event horizon. That doesn't mean that we may never be able to see it again, only that we will never see what it's doing from the time it does slip behind the event horizon to the time it reemerges, if the universe were to decelerate and the event horizon should grow.


Yes, I've read it, a couple of times. They could revise it to make it a little more clear between things that were once in our event horizon and things that are just now slipping behind it.
Sorry, Mike I think you still misunderstand the meaning of event horizon. I think may people misunderstand because in everyday language an horizon normally relates to a physical distance and is not dependent on time or on spacetime events, and objects are either within the horizon or not.

However, the cosmological event horizon is an horizon in 4D spacetime and relates ONLY to events in spacetime, and not to objects in space. Whereas objects may move in space, an event in spacetime is fixed in 4D spacetime, it is not moving with respect to the spacetime axes. Therefore an event which is within our event horizon will always be within our event horizon, it will not move outside of or cross our event horizon, ever.

What does happen is that objects move, which means that even though we may have seen events from those objects in the past, future events related to those objects may not be accessible to us because those future events may be beyond our event horizon.

Yes, our 4D event horizon is contracting as space expands, which means that events from more and more objects in the universe are outside of our event horizon as time progresses. But that does not mean those objects disappear from view, it just means that the events we do observe from those objects come from distant times in the past (if you go far enough back in time you will find events from those objects which are within our event horizon, even though the “present day” events from the same objects may be outside of our event horizon).

MF :smile:
 
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  • #35
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lawtonfogle said:
could we slow an object down and send it back to the 'center of origin' of the universe

the universe does not have a event horizon

it has an avarage one, but only the molecules the molecules have one

each object has its own event horizon, because we can slow one down so it's EvHr
is futher than another objects
see my previous post.

Event horizons relate to events in 4D spacetime, not to objects in 3D space. Events do not move, objects do. Therefore it makes no difference what you do to an object prior to or after an “event” relating to that object – the event is either within our event horizon or it is not.

MF :smile:
 
  • #36
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once the Deep Space telescope is launched(don't recall the exact name), in what is it, 2014? By NASA. It is supposed to be able to see all the way back to the time of the big bang, and we'll be able to see the radiation era. Will we get any answers from this? any assumptions? Will we ever be able to prove factually that the universe is finite or infinite?
 
  • #37
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TheUnknown said:
once the Deep Space telescope is launched(don't recall the exact name), in what is it, 2014? By NASA. It is supposed to be able to see all the way back to the time of the big bang, and we'll be able to see the radiation era. Will we get any answers from this? any assumptions? Will we ever be able to prove factually that the universe is finite or infinite?
current cosmology suggests that we will not be able to see any events before the recombination time, about 300,000 years after the Big Bang, because prior to this the universe was an ionised and pretty uniform "plasma". The CMB is the "relic" of this plasma, hence the CMB (which we can see and study today even without satellites) is probably the "oldest" thing we can study in the cosmos.

One possible exception may be the sources of large-scale structure (which sources may pre-date the recombination time) - we cannot study these directly but we can study their present-day effects in terms of the large-scale structure of the universe.

MF :smile:
 
  • #38
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moving finger said:
Sorry, Mike I think you still misunderstand the meaning of event horizon. I think may people misunderstand because in everyday language an horizon normally relates to a physical distance and is not dependent on time or on spacetime events, and objects are either within the horizon or not.

However, the cosmological event horizon is an horizon in 4D spacetime and relates ONLY to events in spacetime, and not to objects in space. Whereas objects may move in space, an event in spacetime is fixed in 4D spacetime, it is not moving with respect to the spacetime axes. Therefore an event which is within our event horizon will always be within our event horizon, it will not move outside of or cross our event horizon, ever.

What does happen is that objects move, which means that even though we may have seen events from those objects in the past, future events related to those objects may not be accessible to us because those future events may be beyond our event horizon.

Yes, our 4D event horizon is contracting as space expands, which means that events from more and more objects in the universe are outside of our event horizon as time progresses. But that does not mean those objects disappear from view, it just means that the events we do observe from those objects come from distant times in the past (if you go far enough back in time you will find events from those objects which are within our event horizon, even though the “present day” events from the same objects may be outside of our event horizon).

MF :smile:
I think we are on the same page here. Yes, we can still see what things were like in the past. But at some time in the future, we will not see distant things that have crossed the event horizon. They crossed the event horizon some time after the distant past in which we see them now. So in the future we will see them cross the event horizon and disappear from our sight. If the universe should start to decelerate and the event horizon expands, then some distant objects will cross back within the event horizon, and at some time in the future (13 billion years or so) we will see them reemerge.

I wish they would update their paper on "Expansion Confusion" to make these distinctions clear.
 
  • #39
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As I understand it, the CMB is a uniform energy that permeates space at 2.7K. It is left over from the energy of the big-bang. We know that there cannot exist a temperature below 0K (because temperature is after all the kinetic energy of atoms). But when you think about it, an infinite area implies that the CMB must exist at 0K. Furthermore, if the Universe was infinite - there would be an infinite distance between any two atoms. You cannot get finite distances from an infinite expanse.

Some might argue that although the matter density is finite, space itself is infinite. This would go in contradiction to relativity; there is no absolute 'space', it is rather defined by matter and the paths of light rays. Space, and any other definition of area, is a convenient abstract concept to decribe the relationships between matter. If matter exists in a finite amount (like at the big bang), then so does space.
 
  • #40
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Symbreak said:
As I understand it, the CMB is a uniform energy that permeates space at 2.7K. It is left over from the energy of the big-bang. We know that there cannot exist a temperature below 0K (because temperature is after all the kinetic energy of atoms). But when you think about it, an infinite area implies that the CMB must exist at 0K. Furthermore, if the Universe was infinite - there would be an infinite distance between any two atoms. You cannot get finite distances from an infinite expanse.
a finite temperature for the CMB, and a finite distance between atoms, is consistent with a spatially infinite universe containing an infinite amount of matter.

Symbreak said:
Some might argue that although the matter density is finite, space itself is infinite. This would go in contradiction to relativity; there is no absolute 'space', it is rather defined by matter and the paths of light rays. Space, and any other definition of area, is a convenient abstract concept to decribe the relationships between matter. If matter exists in a finite amount (like at the big bang), then so does space.
there is nothing to suggest that the amount of matter in the universe is necessarily finite

MF :smile:
 
  • #41
Chronos
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If we had a neutrino telescope, we could 'see' relic neutrinos released about 1 second after the big bang. It would not, however, be much to 'look' at - about as interesting as watching channel 1 [in the US] on your television. MF gave a fine explanation of why any object we can currently see, will always remain within our event horizon. Our event horizon expands along with the rest of the observable universe and all objects on this side of it are forever stuck on this side of it.
 
  • #42
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Chronos said:
If we had a neutrino telescope, we could 'see' relic neutrinos released about 1 second after the big bang. It would not, however, be much to 'look' at - about as interesting as watching channel 1 [in the US] on your television. MF gave a fine explanation of why any object we can currently see, will always remain within our event horizon. Our event horizon expands along with the rest of the observable universe and all objects on this side of it are forever stuck on this side of it.
There wouldn't even be any talk about an event horizon if nothing ever crossed it. We would simply say that there ARE no other galaxies farther then some point. The distance to the cosmological event horizon is fixed by the rate of expansion, it is not fixed with respect the the galaxies themselves. The farther away things are, the faster they are moving away from you. At some distance space itself is receding faster than light away from us. Light emitted beyond that point we will not be able to see. Space itself and any galaxies fixed in it are crossing that event horizon determined by the rate of expansion. For as closer space moves away, it picks up speed until it is receding faster than light and crosses the event horizon. So things cross the event horizon.
 
  • #43
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Chronos said:
If we had a neutrino telescope, we could 'see' relic neutrinos released about 1 second after the big bang. It would not, however, be much to 'look' at - about as interesting as watching channel 1 [in the US] on your television.
Yes thanks for the correction! I was of course assuming that we would be restricted to use existing technology when studying the universe, but of course that is not a necessary restriction. In fact we could in principle "look back" even farther if we had instruments that could tell us something about the hypothetical "inflaton" field/particles(?) that existed during the time of inflation?

Chronos said:
Our event horizon expands along with the rest of the observable universe and all objects on this side of it are forever stuck on this side of it.
oooops. I think you meant to say "all events on this side of it are forever stuck on this side of it."?

MF :smile:
 
  • #44
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Mike2 said:
There wouldn't even be any talk about an event horizon if nothing ever crossed it. .......... So things cross the event horizon.
yes, Mike2, you are quite right. Objects do "cross" the event horizon in the sense that it can happen that we can detect events from some objects from one era in the past, but at another era in the past events from the same object are not detectable by us, because those events are beyond our event horizon.

What Chronos should have said (I believe) is that "events" never cross our event horizon.

MF :smile:
 
  • #45
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moving finger said:
...Objects do "cross" the event horizon in the sense that it can happen that we can detect events from some objects from one era in the past, but at another era in the past events from the same object are not detectable by us, because those events are beyond our event horizon.
...

just for entertainment, as something to imagine if you wish to, have you ever pictured what it would look like to watch a distant galaxy disappear from our observable universe?

if we could live very long lifetimes and keep watching it might be like watching a ship disappear over the horizon except that it would be redshift-fading out I suppose, and we might see the people slowing down, if the telescope was good. and so evermore slow and ghostly until it wasnt there
 
  • #46
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Tamara Davis dissertation, that she did for Charles Lineweaver, was partly about that. and the thermodynamics IIRC, black holes crossing the horizon and entropy calculations. maybe I will have another look at it.

there is always this analogy of things falling into the event horizon of a black hole, except that it is different

I am not asserting anything here, just echoing some interest

here is her thesis in case anyone wants to take a look, the trouble is it is 154 pages so it can take 2 or 3 minutes to download
http://arxiv.org/abs/astro-ph/0402278

Part of the abstract says:
"...We then test the generalized second law of thermodynamics (GSL) and its extension to incorporate cosmological event horizons. In spite of the fact that cosmological horizons do not generally have well-defined thermal properties, we find that the GSL is satisfied for a wide range of models. We explore in particular the relative entropic 'worth' of black hole versus cosmological horizon area. An intriguing set of models show an apparent entropy decrease but we anticipate this apparent violation of the GSL will disappear when solutions..."

IIRC her thought is that since black holes carry a huge amount of entropy what happens to the total entropy in the observable universe (or within our cosmological event horizon rather) when one of those highly entropic black holes drifts across the horizon?
 
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  • #47
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marcus said:
just for entertainment, as something to imagine if you wish to, have you ever pictured what it would look like to watch a distant galaxy disappear from our observable universe?

if we could live very long lifetimes and keep watching it might be like watching a ship disappear over the horizon except that it would be redshift-fading out I suppose, and we might see the people slowing down, if the telescope was good. and so evermore slow and ghostly until it wasnt there
There are papers that talk about everything leaving our event horizon except our galaxy and Andromeda. There are also some that talk about a big rip, etc. So I hope that ends the conversation about things crossing our event horizon and the size of the event horizon changing with time. Sorry, I lost the references.

I have read things that talk about distant galaxies "freezing" as they approach the event horizon, they redshift, slow down, and then disappear. This is all very similar to what happens to objects that get swallowed by black holes. So I have to wonder if the cosmological event horizon has an entropy associated with it just as a black hole does. This would place a restriction on the entropy inside our cosmological event horizon just as there is a restriction inside a black hole.

If that is the case, then what does it mean that the cosmological event horizon is contracting with the acceleration of the universe? Does this mean the entropy inside the universe must decrease? Would life be a response to this? Is there more structure to come?

This reminds me of the verse:
"Earth and sky fled from His presence, and there was no place for them...
And I saw a new heaven and a new earth, for the first heaven and earth had passed away... I saw the Holy City, the new Jerusalem coming down out of heaven from God..."
Revelation 20-21.
 
  • #48
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marcus said:
just for entertainment, as something to imagine if you wish to, have you ever pictured what it would look like to watch a distant galaxy disappear from our observable universe?

if we could live very long lifetimes and keep watching it might be like watching a ship disappear over the horizon except that it would be redshift-fading out I suppose, and we might see the people slowing down, if the telescope was good. and so evermore slow and ghostly until it wasnt there
just like watching someone falling into a black hole?

MF
:smile:
 

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