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Author Topic:   Question About the Universe
cavediver
Member (Idle past 3725 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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GDR
Member
Posts: 6203
From: Sidney, BC, Canada
Joined: 05-22-2005


Message 17 of 373 (506984)
04-30-2009 5:48 PM
Reply to: Message 11 by cavediver
04-30-2009 2:19 PM


GDR writes:
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?
cavediver writes:
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.
I understand that at as galaxies expand further away from us the rate of expansion increases to the point at which it exceeds the speed of light. At that point in time light is no longer able to reach us, (cosmological horizon) however light that it had previously emitted, (as we perceive it) would continue its journey enroute to our telescope.
My question is this. It seems to me that seeing as how, (as you say, if we have everything right), that what we are seeing is something that is as far back in time as it is possible for us to view. We are viewing it as it was 13 billion years ago. Does this mean then that from our perspective there is no part of the universe that we can't view, even though much of the universe including this particular star has since moved beyond the cosmological horizon.
I hope that makes better sense.
Thanks

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Michamus
Member (Idle past 5239 days)
Posts: 230
From: Ft Hood, TX
Joined: 03-16-2009


Message 18 of 373 (507029)
05-01-2009 5:29 AM
Reply to: Message 17 by GDR
04-30-2009 5:48 PM


GDR writes:
It seems to me that seeing as how, (as you say, if we have everything right), that what we are seeing is something that is as far back in time as it is possible for us to view.
It as far back as we are able to view currently.
GDR writes:
We are viewing it as it was 13 billion years ago.
Not necessarily, as bluejay posited, it is possible that the expansion of spacetime at sub-light speeds would increase the amount of time it would take for the light to reach us.
Think of it like trying to throw a football to your friend who is running away from you, while you are running backward. It will take the ball more time to reach your friend, than it would if you were simply both stationary.
GDR writes:
Does this mean then that from our perspective there is no part of the universe that we can't view
No, it doesn't mean that, unless we were at the center of the universe, which we already know we aren't from red shift. Heck we aren't even the center of our galaxy.
Using basic geometry, we can determine that anything beyond the "center of the universe" (if there is even one), cannot be seen, as the light is still in transit to us, due to the fact that the universe is younger than the distance any object beyond the center of the universe is from us. That is, if we are 13.7BLY away from the center of the universe.
Edited by Michamus, : diagram didn't show like i wanted it to

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Taq
Member
Posts: 10158
Joined: 03-06-2009
Member Rating: 4.7


(1)
Message 19 of 373 (507118)
05-01-2009 4:28 PM
Reply to: Message 14 by Blue Jay
04-30-2009 2:50 PM


Re: Expansion and the Movement of Light
It also seems that, if each kilometer is thought of as expanding over a period of time, wouldn't much of the expansion between us and the other star be happening behind the light as it traveled? To my mind, this means that any light we're seeing didn't actually travel the entire distance from there to here.
You need to think of this in terms of quantum mechanics. The probability wave of the photon doesn't collapse until it is observed. Once the photon is observed only then can you make sense of it's path. IOW, it doesn't make sense to talk about space expanding behind the light.
The speed of light is not a law. It is THE law. All observations change in order to keep this law in place. This is why you get redshift. The frequency of the light changes so that the velocity of light and the energy of light is conserved. The way in which you observe the passage of time and distance all change in order keep the observed velocity of light the same.

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lyx2no
Member (Idle past 4798 days)
Posts: 1277
From: A vast, undifferentiated plane.
Joined: 02-28-2008


Message 20 of 373 (507152)
05-02-2009 9:13 AM
Reply to: Message 14 by Blue Jay
04-30-2009 2:50 PM


Re: Expansion and the Movement of Light
It also seems that, if each kilometer is thought of as expanding over a period of time, wouldn't much of the expansion between us and the other star be happening behind the light as it traveled? To my mind, this means that any light we're seeing didn't actually travel the entire distance from there to here.
This is essentially correct. See page 11 of Steven Weinberg's Cosmology for starters, but check out chapter 1.6 (page 46) for accelerated expansion. I have been attempting to calculate the distances in question that I might be able to give an answer myself, but my calculus isn't up to it yet, and I'm pretty sure you'd not want to wait around. (I'm not allowed to take calculus or physics till next year.)
I seems to me that if the velocity of expansion at the emitter is Hd, and the velocity of expansion of the absorber is 0, then the average velocity of expansion for the trip would be Hd/2. But this may be a violation of point 5.
However, I offer some corrections to statements in preceding posts:
  1. In an accelerating universe the observable universe is getting smaller as the distance at which the expansion becomes superluminal draws nearer.
  2. Don't think of it like trying to throw a football to your friend who is running away from you, while you are running backward. Firstly, that would be an effect caused by their own peculiar motion rather then cosmological expansion and there are significant differences. Secondly, lest one confuse the velocity of the two travelers as being subtracted from the velocity of the ball it's best not to. This is exactly the idea that SR flees screaming from.The photon emitter and the photon absorber would both see the photon moving at c.
  3. There can be no center of the Universe.
  4. The energy of the light is not conserved. Wave length is inversely proportional to energy. Red-shift = longer wave length = lower energy.
  5. It seems like I'm making it harder than it needs to be. Particularly bogus and non-triumphant.
Correction I: In point number 4 I wrongly understood Taq to mean that the photon retained all of its energy. I used a bad choice of words, of course the energy is conserved.
Correction II: Upon further reading I'm less certain of this, but I don't abandon it yet. In an accelerating universe atoms will eventually be torn apart by expansion. Where would that put the horizon?
Edited by lyx2no, : Grammar
Edited by lyx2no, : Consistency.
Edited by lyx2no, : Missed point 3.
Edited by lyx2no, : Bad choise of words.
Edited by lyx2no, : Correction II.

Genesis 2
17 But of the ponderosa pine, thou shalt not eat of it; for in the day that thou shinniest thereof thou shalt sorely learn of thy nakedness.
18 And we all live happily ever after.

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Replies to this message:
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GDR
Member
Posts: 6203
From: Sidney, BC, Canada
Joined: 05-22-2005


Message 21 of 373 (507338)
05-03-2009 7:11 PM
Reply to: Message 20 by lyx2no
05-02-2009 9:13 AM


Re: Expansion and the Movement of Light
I'd just like to throw this out to anyone. Cavediver writes;
quote:
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 star is 13 billion years old making it one of the byears ago it was still within the cosmological horizon, what would never have been visible to us? Wouldn't anything that had never been visible, or that is not visible to us now, have to be older than this star that is 13 billion light years away, and if this was one of the first stars formed what could be older?
Edited by GDR, : erred in writing million instead of billion as pointed out lvx2no

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Replies to this message:
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lyx2no
Member (Idle past 4798 days)
Posts: 1277
From: A vast, undifferentiated plane.
Joined: 02-28-2008


(1)
Message 22 of 373 (507345)
05-04-2009 12:21 AM
Reply to: Message 21 by GDR
05-03-2009 7:11 PM


Push-of-War
This star is 13 [b]illion years old making it one of the very first stars formed. If it is still acessible to us, in that 13 [b]illion years ago it was still within the cosmological horizon, what would never have been visible to us? Wouldn't anything that had never been visible, or that is not visible to us now, have to be older than this star that is 13 billion light years away, and if this was one of the first stars formed what could be older?
In a static universe we would expect the particle horizon to be equal to the age of the universe. However, in our expanding universe the particle horizon is more than three times that. Though, even this value is not the entire Universe. During the inflationary epoch, from 10-36 through 10-33 seconds after the Big Bang, the Universe expanded by a factor of at least 1026 dragging most of the Universe well beyond what has ever been or will ever be visible. Were our bit the size of a marble the universe proper would be the size of the planet Mars.
Once the Universe returned to rates of expansion comparable to what they are today the density of the dark matter and baryons produced a gravitational attraction that was greater then the negative pressure of dark energy. The particle horizon pushed farther and farther out. But as the Universe expanded the dark matter and baryon density decreased while the dark energy density remained nearly unchanged. About five billion years ago the tables turned in favor of the negative pressure of the dark energy and the rate of cosmic expansion began to accelerate. This acceleration has caused the once receding particle horizon to retract. It is speculated that it will reach zero in some 50 billion years.
Edited by lyx2no, : Punctuation.
Edited by lyx2no, : Tense.

Genesis 2
17 But of the ponderosa pine, thou shalt not eat of it; for in the day that thou shinniest thereof thou shalt sorely learn of thy nakedness.
18 And we all live happily ever after.

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 Message 21 by GDR, posted 05-03-2009 7:11 PM GDR has replied

Replies to this message:
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GDR
Member
Posts: 6203
From: Sidney, BC, Canada
Joined: 05-22-2005


Message 23 of 373 (507349)
05-04-2009 1:07 AM
Reply to: Message 22 by lyx2no
05-04-2009 12:21 AM


Re: Push-of-War
lyx2no writes:
This acceleration has caused the once receding particle horizon to retract. It is speculated that it will reach zero in some 50 billion years.
I can hardly wait.
Thanks a lot lyx2no. That clears it up for me.

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Taq
Member
Posts: 10158
Joined: 03-06-2009
Member Rating: 4.7


Message 24 of 373 (507389)
05-04-2009 3:27 PM
Reply to: Message 20 by lyx2no
05-02-2009 9:13 AM


Re: Expansion and the Movement of Light
Correction I: In point number 4 I wrongly understood Taq to mean that the photon retained all of its energy. I used a bad choice of words, of course the energy is conserved.
To be fair, this is still controversial. The following paper does show that cosmological expansion can be treated as a Doppler shift with the subsequent conservation of energy:
Link
However, they do state that:
quote:
As a result, it is widely accepted that energy is not locally conserved in general relativity3, although claims are made that energy is globally conserved during expansion. This is in stark contrast to the normal Doppler shift where, as demonstrated in the text, energy is conserved on a photon-by-photon basis.
So I am not correct in stating that energy MUST be conserved. However, I am pretty sure that the velocity of light is conserved.

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alaninnont
Member (Idle past 5518 days)
Posts: 107
Joined: 02-27-2009


(1)
Message 25 of 373 (508011)
05-09-2009 9:10 PM


Faster than the speed of light
This is off topic but I started thinking about it when the speed of light was mentioned. As far as we know, nothing moves faster than the speed of light. How about gravity? 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?

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NosyNed
Member
Posts: 9006
From: Canada
Joined: 04-04-2003


(1)
Message 26 of 373 (508012)
05-09-2009 9:55 PM
Reply to: Message 25 by alaninnont
05-09-2009 9:10 PM


Gravity
This is off topic but I started thinking about it when the speed of light was mentioned. As far as we know, nothing moves faster than the speed of light. How about gravity? 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?
It would take 8 minutes. As I recall this has been demonstrated to be true by observations of the moons of Jupiter.
The "speed of light" isn't comparable to a speed limit on a highway. It is a fact about the nature of the thing called spacetime.

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cavediver
Member (Idle past 3725 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 3725 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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Lurkey
Junior Member (Idle past 4170 days)
Posts: 11
Joined: 11-03-2012


Message 29 of 373 (678878)
11-11-2012 7:17 AM
Reply to: Message 26 by NosyNed
05-09-2009 9:55 PM


Re: Gravity
If there was another me on another earth just over the CH and he shot a satellite at me so that it outpaced my shrinking CH, would i eventually see it blue shift into existence (if i was very patient)?
Or would i never see the satellite because he just cannot chuck it fast enough?...like there is too much expanding universe between me and him 'pushing back' if you know what i mean.
EDIT: Sorry Ned, this is not a reply to your post....my bad...still learning the ropes!
Edited by Lurkey, : No reason given.

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kofh2u
Member (Idle past 3902 days)
Posts: 1162
From: phila., PA
Joined: 04-05-2004


Message 30 of 373 (679070)
11-12-2012 10:06 AM
Reply to: Message 20 by lyx2no
05-02-2009 9:13 AM


Re: Expansion and the Movement of Light
3. There can be no center of the Universe.
Do you agree there is point from which the Big Bang expansion took place, an initial but moving location which could be called the cosmic center of gravity?

This message is a reply to:
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