In an expanding universe that is modeled by the FRW metric we assume that scale factor of the "present epoch" is unity which is equivalent to a zero redshift. Therefore, most observed galaxies with nonzero redshifts are in our past light cone.(adsbygoogle = window.adsbygoogle || []).push({});

But it is unclear to me how much back in time or far away in physical distance can we go to assume that scale factor is unity.

Alternatively, I need to know when back in time we can no longer assume that proper distance is equal to co-moving distance. What is the physical distance limit of "today universe"?

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# The present epoch in FRW metric

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