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New On-line Cosmology Calculator

  1. Sep 21, 2003 #1

    marcus

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    An astronomy prof who teaches at a University in Iowa
    maintains a resource page of online astro java applets
    for students and general public to use
    and one of the applets is "cosmology calculator"
    http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html

    Here is her homepage
    http://www.earth.uni.edu/smm.html

    If you try out the "cosmology calculator" and want it to give mainstream consensus answers similar to Ned Wright's and so on, then leave the Hubble parameter 70 (the default)
    and put 0.27 in for the matter density ("omega")
    and put 0.73 in for the cosmo. const. ("lambda")

    Then every time you put in a redshift z (like z = 6.4 for a recently observed quasar) and press "calculate" it will tell you
    how far away the thing was when it emitted the light we are now receiving from it

    how far away it is now

    how fast it was receding from us when it emitted the light we are now receiving
     
    Last edited: Sep 21, 2003
  2. jcsd
  3. Sep 21, 2003 #2

    marcus

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    A nifty rule of thumb for recession speeds

    For z in the range 1.5 to 6, good rule of thumb for recession speeds is this:

    add one to the redshift and multiply by 0.4
    this will give the speed that the thing was receding
    at the moment it emitted the light that we are seeing
    and it will give the ("then") recession speed as a
    multiple of c.

    So redshift 1.7 implies 1+z = 2.7 implies it was receding at just over the speed of light

    And redshift 5 implies 1+z = 6 implies it was receding at 2.4 times speed of light

    This is an approximate, not exact, rule. It seems accurate to two significant digits which often gives a good enough idea, and saves having to use one of the online calculators or (godforbid) do an integral.

    So that quasar observed in 2002 with z = 6.4 was retreating at almost 3 c when it sent us its light.
    It might occur to someone to ask how it happens that the light ever got here. It must have begun its journey by actually losing ground to the Hubble flow and being swept back away from us---how can it eventually arrived here? Davis and Lineweaver discuss this in an explanatory paper "Superluminal Recession Velocities" that I believe you can find by google as well as by the arxiv search function.
     
    Last edited: Sep 21, 2003
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