Expert: James Gort - 7/20/2011

Question
An article in the current Scientific American says that we can see out to 42 billion light years. Since the universe is only 14 billion years old, how can we see things 42 billion light years away.

Many thanks,

Steve

Answer
Hi Steve,

A "good" explanation can be found at http://en.wikipedia.org/wiki/Observable_universe

That wiki talks about comoving distance.  So please see http://en.wikipedia.org/wiki/Comoving_distance  As mentioned there, comoving distance is defined to be equal to the proper distance (the D in Hubble's Law) at the present time.  So if you just talk about the present time (and not how distance changes over time), just use Hubble's Law to get the comoving distance (today).   The comoving distance doesn't change, so if you want to know where the object will be in 10 years (the proper distance that Hubble's Law would give you, if you observed the object 10 years from now), then you need to use the formula for 'chi' in that wiki page.  The confusion many people have (justifiably!) is between "proper distance" and "comoving distance".

That said, I want to remind you that the origin of the universe by a "Big Bang" is very much still a theory, and there are alternative theories.  Some observations do not support an expanding universe or the Big Bang.  There are several references concerning this, but perhaps the most authoritative person is Halton Arp, a leading astronomer and researcher on galaxies, who wrote "Seeing Red".  That book is highly recommended to get an alternative view.  Or read "A Different Approach to Cosmology" by Hoyle, Burbidge, and Narlikar.  Another great book which gives a scientific view on how the universe has always been in a steady state.

So regardless of what Scientific American says, please be aware that there are alternative theories (by well known cosmologists), so keep an open mind.

Cheers,

Prof. James Gort