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Author Topic:   Question About the Universe
lyx2no
Member (Idle past 4706 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.

This message is a reply to:
 Message 14 by Blue Jay, posted 04-30-2009 2:50 PM Blue Jay has not replied

Replies to this message:
 Message 21 by GDR, posted 05-03-2009 7:11 PM lyx2no has replied
 Message 24 by Taq, posted 05-04-2009 3:27 PM lyx2no has not replied
 Message 30 by kofh2u, posted 11-12-2012 10:06 AM lyx2no has not replied

  
lyx2no
Member (Idle past 4706 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.

This message is a reply to:
 Message 21 by GDR, posted 05-03-2009 7:11 PM GDR has replied

Replies to this message:
 Message 23 by GDR, posted 05-04-2009 1:07 AM lyx2no has not replied

  
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