Expert: Philip Stahl - 4/28/2009

Question
Hello Philip,

I just finished reading this article at http://news.yahoo.com/s/space/spaceexplosionisfarthestthingeverseen where it is reported that astronomers are viewing the most distant object at around 13 billion light years distant. It is mentioned this was around the time when the entire universe was only 630 million years old.

I'm having difficulty understanding how it is that we on earth are around 13 billion light years away from the point in time and space when the universe is said to have only been around 630 million years old. I mean, how did earth and our entire galaxy for that matter, get 13 billion lights years AHEAD of our own origin 13 billion years ago so that we can now view the light rays arriving to us now 13 billion years later? I don't think it makes sense to assume we traveled faster than light so that we could beat the very light to our current position which shows our very own beginnings around 13 billion years ago. On the flip side, I also don't think it makes sense to assume the entire universe began at a size of 13 billion light years.

So how did we get 13 billion lights years away from our own origin so that we can view the light rays now finally arriving here or catching up with us from there?

John

Answer
Hello,

What you are forgetting is that the expansion of space and time commenced with the Big Bang. The 630 million light years figure marked *after* the “Bang” simply denotes a time and distance (630 million years, 630 MLy) after the primordial event. Our time-distance (for the expanding light cone within which observations can actually be made) is: ~13.7 billion years, 13.7 Bly) so there is no conflict or confusion.  We are simply existing  13.7 Bly after the big Bang (based on the Hubble constant) and ~ 13 Bly after the 630 MLy event.. (e.g. 13.7 BLy - 0.7 BLy =  13 BLy)

None of this ought to be confused with the separate expansion of space, which is a different issue from the expansion of space-time referenced in the Hubble law. The former has to do with the changing topology of the intervening space between galaxy clusters (mainly) as time progresses and notes that there is a differential (more rapid) expansion of space, than of space-time. (Which is why, referred to *space* expansion, the  cosmos is said to be ~ 93 Bly in diameter)

Rather than interpreting the data (from the article) as "earth and our entire galaxy for that matter, getting 13 billion lights years AHEAD of our own origin"- think of it as 13 billion years elapsed since that event (at 630 MLy) translating into a distance of 13 billion light years (from the object)  and 13.7 billion years elapsed (again, within our light cone) since the Big Bang. Thus, the elapsed time is inevitable as is the expansion of space-time to comport with it. The truly odd and inconsistent situation, therefore, would be if we existed at the same time as our origin.


As I believe I noted before in earlier answers, what all this means is that it is possible to ask questions which on the surface seem like common sense, but which actually may require far more background than a questioner has to be able to understand them. The issue of a differentially expanding *space*, apart from space-time (referenced in the Hubble law) is one of these.

That aside, I hope the immediate question you posed to do with the article has been addressed, answered satifactorily!