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Author Topic:   Question About the Universe
cavediver
Member (Idle past 3634 days)
Posts: 4129
From: UK
Joined: 06-16-2005


(1)
Message 4 of 373 (506844)
04-29-2009 6:45 PM
Reply to: Message 1 by GDR
04-29-2009 3:46 PM


Hi GDR,
1/ Doesn't this mean that what we are now seeing is that star as it was at a time very shortly after the Big Bang?
Yes (on the order of several hundred million years after the BB)
2/ I also understood that there is an "event horizon" because of the expansion of the universe that has moved galaxies beyond our ability to perceive them. However here we have a star that has been here since nearly T=0 and we're still able to view it. Why is that?
We are seeing the "star" before the cosmological expansion carries it over the cosmological horizon. If we continue to observe it over time, we will see it receed and redden to the point that it fades from view.
3/ Using the penny on the balloon analogy shouldn't this mean that we are looking virtually all the way around the balloon, (the universe), and if we looked in another direction that we could see the same star very much closer?
No - even if the Universe is closed as in the balloon analogy, this is on a scale unimaginably larger than the distance to the "star", because of inflation. In an inflated closed universe, the observable universe is a tiny fraction of the whole universe.
Edited by cavediver, : No reason given.

This message is a reply to:
 Message 1 by GDR, posted 04-29-2009 3:46 PM GDR has replied

Replies to this message:
 Message 5 by GDR, posted 04-29-2009 7:37 PM cavediver has replied

  
cavediver
Member (Idle past 3634 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 11 of 373 (506964)
04-30-2009 2:19 PM
Reply to: Message 5 by GDR
04-29-2009 7:37 PM


If as RDK says, the universe is 13.87 billion years old, wouldn't this make it among the very first stars to be formed?
Aboslutely - unless we have something very wrong!
As I understand it, we are seeing the light that left that star 13 billion years ago even though, from our perspective, that star has since that time inflated beyond the cosmological horizon.
Not "inflated" - inflation is reserved for what happened in the first 10-34sec of the Universe - but "carried by the expansion of the Universe" would be fine.
If then, we can look out into space and see things as they were 13 billion ago then what is left to have inflated beyond our ability to perceive?
I'm not sure I understand your question. Most of the Universe has never been visible to us and never will. This "star" was relatively very close to us for it to be visible at all.

This message is a reply to:
 Message 5 by GDR, posted 04-29-2009 7:37 PM GDR has replied

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cavediver
Member (Idle past 3634 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 12 of 373 (506965)
04-30-2009 2:29 PM
Reply to: Message 6 by Blue Jay
04-29-2009 11:45 PM


Re: Expansion and the Movement of Light
If the universe has been expanding since the Big Bang, wouldn't this mean that a star 13 billion lightyears away was less than 13 billion lightyears away at a time closer to the Big Bang?
Yes - much much closer. If you imagine the sphere on the sky centred on us with surface so far away that it contains this star, the actual surface area of that sphere is relatively quite small - much smaller than many of the concentric spheres that are much closer to us! Think of standing on the North Pole, and being surrounded by the lines of latitude. The Arctic Circle is obviously smaller than the Tropic of Capricorn, and the Equator, but by the time we reach the Tropic of Cancer, the surrounding circles are growing smller, down past the Antarctic Circle and finally we reach the South Pole as a point.

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cavediver
Member (Idle past 3634 days)
Posts: 4129
From: UK
Joined: 06-16-2005


(1)
Message 13 of 373 (506967)
04-30-2009 2:35 PM
Reply to: Message 8 by Blue Jay
04-30-2009 9:16 AM


Re: Expansion and the Movement of Light
If the expansion of the universe is slower than the speed of light
Doesn't make sense. Expansion is measured in how much distance appears in so much time in so much existing distance. So, 10m/sec per km is an expansion rate. Every second, any distance of 1km becomes 1.01km. This is not a speed or velocity. You could say that across 1km, space is expanding at 10m/sec and you'd sort of have a point. Is this faster than light? Of course not. But what about this same expansion across 5x108km? Across that distance, you could say that space is expanding at 5x109m/sec, which is significantly faster than light. Any uniform expansion rate, over a sufficient distance, creates a "superluminal" expansion.

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 Message 8 by Blue Jay, posted 04-30-2009 9:16 AM Blue Jay has replied

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 Message 14 by Blue Jay, posted 04-30-2009 2:50 PM cavediver has replied

  
cavediver
Member (Idle past 3634 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 16 of 373 (506982)
04-30-2009 5:14 PM
Reply to: Message 14 by Blue Jay
04-30-2009 2:50 PM


Re: Expansion and the Movement of Light
is the expansion rate uniform, or close enough to it to get away with saying it is?
We assume it is spatially, and the CMBR suggests this. It is not uniform in time. We used to think the rate was decreasing with time, but we now know that the rate appears to be increasing.
wouldn't much of the expansion between us and the other star be happening behind the light as it traveled?
It's not a well-defined question, as the answer is very observer-dependent. To the photon itself, there is no distance between its point of emission in the star and its point of absorption in the telescope that is being used for the observation. It does not experience any "expansion".

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cavediver
Member (Idle past 3634 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 27 of 373 (508043)
05-10-2009 5:33 AM
Reply to: Message 25 by alaninnont
05-09-2009 9:10 PM


Re: Faster than the speed of light
If, hypothetically the sun vanished at this instant, would the pull of gravity it exerts on us cease instantly or would it take eight minutes as light would?
As Ned has answered, gravitational radiation propegates at the speed of light - this is derived from General Relativity, and calculations based on observations of binary pulsars experimentally confirm this.
However, your proposed experiment is not actually meaningful. For the Sun to just "vanish" would mean that our understanding of physics is very wrong, and so it is not possible to say what would or would not happen as a result of this!

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cavediver
Member (Idle past 3634 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 28 of 373 (508048)
05-10-2009 6:31 AM
Reply to: Message 26 by NosyNed
05-09-2009 9:55 PM


Re: Gravity
As I recall this has been demonstrated to be true by observations of the moons of Jupiter.
It was actually observations of Jupiter passing by the line of sight of a quasar, but the method has been dismissed as erroneous - except by the original authors We're really only left with the binary pulsar calculations, where the rate of change of orbital period (caused by the the emiited gravitational radiation) depends upon the speed of propegation of the gravitational waves.

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cavediver
Member (Idle past 3634 days)
Posts: 4129
From: UK
Joined: 06-16-2005


(1)
Message 40 of 373 (679696)
11-15-2012 10:16 AM
Reply to: Message 33 by Lurkey
11-14-2012 6:24 PM


Re: Cosmological Horizon
If we’re in a pocket, then isn’t every other point in the universe in overlapping pockets?
Yes
Don’t we shift from one pocket to another just by moving through our own pocket?
No - each "pocket" as you call it is defined by a point in space *and* time. You are not free to move to any other pocket, only those that can be reached by you travelling at less than the speed of light.
Someone in a neighbouring pocket has areas of the Universe within his horizon that are not within yours. He can potentially reach these regions - you cannot. But for him to do so, he needs to set off *now*, otherwise those regions will be lost to him. By the time you manage to get to where he is now, those regions will be long gone. Make sense?
Oh, and just to correct some common confusion - we're not talking about the Cosmological Horizon (or Particle Horizon) here but the (Cosmological) Event Horizon.

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Replies to this message:
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