A problem with Time and Space

[ecells]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Well, the reason I didn't call it immediately is because there is a context in which your statement that "the expansion of space does not need to be FTL in order to achieve 'real' diameters larger than the observed universe" might make sense, but it all depends on your definition of "real" diameters and the "observed" universe! This is why I asked what you meant by those terms. We are back to my old favorite, the angular-diameter distance. One interpretation is to take the angular-diameter distance of an object, the distance that object was from this coordinate when the light we see was emitted, and use only those distances to define the "observed" universe. This differentiates the "observed" from the observable.The most distant galaxies we see, as defined by their angular-diameter distance, are the galaxies we see at the edge of our Hubble sphere, where objects were receding at the speed of light when they emitted the light we now detect. These galaxies have a redshift of around z=1.4 and emitted their light only 9.1 billion years ago, but they were around 5.7 billion light-years away at that time.We have received no electromagnetic radiation that was originally emitted at a distance larger than 5.7 billion light-years away, so that can be considered to be the radius of our observed universe. We see objects with higher redshifts, but these were closer than 5.7 billion light-years to us when they emitted their light.So I like to think of the size of the universe that we have actually observed as being only 5.7 billion light-years in radius. Sure, those z=1.4 galaxies are thought to be a lot further away by now, but they have only been observed to be 5.7 billion light-years away. In that sense, this is a "real" radius for the observed universe, and so indeed is the maximum light-travel time of 13.7 billion light-years. The question is whether those two distance measures can be compared in any meaningful way. <br /> Posted by SpeedFreek</DIV></p><p>&nbsp;</p><p>Thank you. You basically answered the OP question for me.<img src="http://sitelife.livescience.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-cool.gif" border="0" alt="Cool" title="Cool" /> </p>