<html><head></head><body><div style="color:#000; background-color:#fff; font-family:HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:16px"><div id="yui_3_16_0_1_1456273990345_7703" dir="ltr">Many thanks, your explanation is very good and the notebook even better.<br></div><div id="yui_3_16_0_1_1456273990345_7820" dir="ltr"><br></div><div id="yui_3_16_0_1_1456273990345_7806" dir="ltr">But I have another question because I think the problem arose because I misunderstood something fundamental here. They quoted in their paper that "present day recession velocities are shown" (defining it in a later paper as "a(z) = a(0)"). So is present day recession velocity the "current" recession velocity (62c) or the currently measurable recession velocity of light that reaches us (3.3c)?</div><div id="yui_3_16_0_1_1456273990345_8390" dir="ltr"><br></div><div id="yui_3_16_0_1_1456273990345_8391" dir="ltr">If that's beyond the scope of the mailing list, I apologize.<br></div><div id="yui_3_16_0_1_1456273990345_7960" dir="ltr"><br></div><div id="yui_3_16_0_1_1456273990345_8011" dir="ltr">Thanks again,</div><div id="yui_3_16_0_1_1456273990345_8058" dir="ltr">Michael<br> </div><div class="qtdSeparateBR"><br><br></div><div style="display: block;" class="yahoo_quoted"> <div style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;"> <div style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;"> <div dir="ltr"><font face="Arial" size="2"> Thøger Emil Rivera-Thorsen <trive@astro.su.se> schrieb am 1:35 Mittwoch, 24.Februar 2016:<br></font></div> <blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; margin-top: 5px; padding-left: 5px;"> <br><br> <div class="y_msg_container"><div id="yiv0524361279"><div>
    Sorry, that did not come out very clear.<br clear="none">
    <br clear="none">
    The two different values you find are the recession velocity at the
    time of emission and the recession velocity now. Since the Universe
    has expanded by a lot since then, a current value of 62 c seems
    reasonable.<br clear="none">
    <br clear="none">
    I have a notebook demonstrating some key Astropy features for
    students at our department, including this very calculation, here:
    <a rel="nofollow" shape="rect" class="yiv0524361279moz-txt-link-freetext" target="_blank" href="http://nbviewer.jupyter.org/urls/ttt.astro.su.se/~trive/teaching/tools-package/Astropy-intro.ipynb">http://nbviewer.jupyter.org/urls/ttt.astro.su.se/~trive/teaching/tools-package/Astropy-intro.ipynb</a>
    (Scroll to the bottom)<br clear="none">
    <br clear="none">
    <div class="yiv0524361279moz-cite-prefix">On 02/24/2016 01:14 AM, Thøger Emil
      Rivera-Thorsen wrote:<br clear="none">
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    <blockquote type="cite">
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      62 c at z=1000 does not sound completely off for the *current
      value*. I suppose the values between 2 and 4 c you get gggfrom
      Davis & Lineweaver is velocities at the time of emission?
      These two differ *strongly*.<br clear="none">
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      <div class="yiv0524361279moz-cite-prefix">On 02/24/2016 12:25 AM, Michael
        Seifert wrote:<br clear="none">
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          <div id="yiv0524361279yui_3_16_0_1_1456269798172_2296">
            <div id="yiv0524361279yui_3_16_0_1_1456269798172_2297">Hi,<br clear="none">
              <br clear="none">
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            today I tried to determine cosmological recession velocities
            based on the paper of Davis, Lineweaver, 2001 (<a rel="nofollow" shape="rect" class="yiv0524361279moz-txt-link-freetext" target="_blank" href="http://arxiv.org/pdf/astro"></a><a rel="nofollow" shape="rect" class="yiv0524361279moz-txt-link-freetext" target="_blank" href="http://arxiv.org/pdf/astro">http://arxiv.org/pdf/astro</a>- ph/0011070v2.pdf)

            with astropy.cosmology.<br clear="none">
            <br clear="none">
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          <div id="yiv0524361279yui_3_16_0_1_1456269798172_2323">But I'm not sure
            what I'm doing wrong (or if I just misunderstood their
            reasoning). But they said the recession velocity is just
            "v_rec = da(z)/dt * comoving distance(z)" and "da/dt =
            H(z)*a(z)" so the recession velocity should be with "cosmo =
            astropy.cosmology.WMAP9" and redshift "z=1000":<br clear="none">
            <br clear="none">
          </div>
          <div id="yiv0524361279yui_3_16_0_1_1456269798172_2324">v_rec = cosmo.H(z) *
            cosmo.scale_factor(z) * cosmo.comoving_distance(z)<br clear="none">
          </div>
          <div id="yiv0524361279yui_3_16_0_1_1456269798172_2371"><br clear="none">
          </div>
          <div id="yiv0524361279yui_3_16_0_1_1456269798172_2380">but this gives me 62
            times the speed of light where it should only be between 2
            and 4 (based on the paper). But just using<br clear="none">
            <br clear="none">
            v_rec = cosmo.H0 * cosmo.comoving_distance(z)<br clear="none">
            <br clear="none">
          </div>
          <div id="yiv0524361279yui_3_16_0_1_1456269798172_2381">gives me 3.25 * c
            which seems to fit their calculations. Is my formula or
            reasoning wrong or is there something strange in the
            comoving distance calculation of WMAP9?<br clear="none">
            <br clear="none">
          </div>
          <div id="yiv0524361279yui_3_16_0_1_1456269798172_2383">I hope this question
            is not too basic but I'm not very deep into cosmology and it
            just seemed strange to me.<br clear="none">
            <br clear="none">
          </div>
          <div id="yiv0524361279yui_3_16_0_1_1456269798172_2384">Many thanks in
            advance,<br clear="none">
          </div>
          <div dir="ltr" id="yiv0524361279yui_3_16_0_1_1456269798172_2385">Michael</div>
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