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Superluminal recession speeds

  1. Aug 24, 2003 #1


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    I found an article by Tamara Davis and Charley Lineweaver called
    "Superluminal Recession Velocities"

    http://astro-ph/0011070 [Broken]

    the date shown is Jan 2001, though the number suggests late 2000.

    It is 4 pages so easy to download and it is specifically on that topic-----Lineweaver's longer 2003 general survey is actually better as a source of information but not as focused on that particular topic.

    Basically it is just the difference between the highly local "special relativity" (where there is that speed limit) and GR (where there isnt) and there is no inconsistency-----SR only applies in a local coordinate patch and forbids one thing PASSING BY another at any speed over c. They explain this adequately in the first paragraph entitled "Two Kinds of Velocity"
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  3. Aug 24, 2003 #2


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    BTW they also mention the same fact I did in another thread, that if any object (a quasar) is observed with redshift 3 or more, then the light we are now receiving was emitted when the object was receding from us at faster than the speed of light

    they say explicitly and graphically (using the swimmer in the current analogy) that the light was initially swept AWAY from us and they explain, as best one can using words to talk about GR, how the light can nevertheless eventually reach us

    so to speak by gradually attaining regions where the Hubble expansion flow is not as swift

    the paper contains nothing new but is a helpful clarification.
  4. Aug 24, 2003 #3


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    light from 90% of the observable universe started off backwards


    One of the fun cosmology things on the web is ned wright's "CosmoCalc" online calculator

    He teaches the graduate course in cosmology at UCLA and has a real good cosmology textbook out. He also is a co-leader of the WMAP project now transmitting CMB data.

    His calculator lets you calculate what volume percent of the observable universe is such that when the light coming to us was emitted it initially was carried backwards, away from us.

    This is the case with over 90 percent.

    Here's how you find that out. by putting in high redshift (z) you see that the vol of the observable is 12,000 units
    (the units are cubic gigaparsecs but it doesnt matter which units one uses)

    But Davis and Lineweaver note that from anything you see with redshift 3 or more (conservatively) the light now arriving here from it was emitted by it while it was receding faster than c.

    Put 3 in the calculator and you learn the volume of space with z less than or equal to 3-----it is less than 10 percent of the observable volume.

    Actually a stronger result is true based on current data. The breakpoint is not z = 3 (which was a conservative rough guess made around year 2000). A sharper bound, from Lineweaver's 2003 paper, is z = 1.4

    The volume of space with redshift less than or equal to 1.4 is only some 3 percent of the total observable volume. Again using Wright's "CosmoCalc".

    So one may say that if light is coming to us today from some random point in the observable universe then TYPICALLY that light came from something that was receding at greater than c at the time it was emitted and therefore that light was initially losing ground------"aimed at us" so to speak, but the distance initially increasing from us to it.

    It's interesting because counterintuitive. But implicit in the "teardrop" shape lightcones we've been seeing and hearing about for some time
    Last edited: Aug 24, 2003
  5. Aug 25, 2003 #4


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    Are you attributing in GR superluminal recessional velocity or any other sort of superluminal velocity to something other than cosmological expansion?
  6. Sep 17, 2003 #5


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    I found the article on Superluminal Recession Velocities at: HERE

    However, the case they gave for the superluminal recession velocity equating to a high z specifically indicated that the photons were emitted at the Big Bang. There was no boundary timeframe given for that epoch, leaving me to wonder if their logic also applies to later time periods.
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