Expert: Philip Stahl - 2/16/2009

Question
QUESTION: Philip,
According to current theory, what happens to the light traveling outward past the edge of the expanding universe?
Thanks,
Paul

ANSWER: Hello,

Th notion of light "travelling past the edge of the universe" is a very tricky concept, especially when people conceive it in a more or less literal way. (As light somehow exceeding the limit of the known universe)

Though the most distant objects (in the CMB or cosmic microwave background) emitted their light 13.7 billion years ago, they have since condensed into galaxies and those have now been displaced to about 46.5 billion lY distance. So, the observable universe must be about 93 billion lY across. (For more details on this, consult the special Cosmology issue of ASTRONOMY magazine: May, 2007, p. 28)

(This seems to violate Einstein's special relativity but bear in mind his theory only applies to setting the limiting speed on light itself, it doesn't apply to the expansion of space)

What astronomers call the "edge" of the universe is the "light horizon" - bounded by the speed that light travels 300,000 km/s). Thus, the 13.7 billion lY limit.  As time goes on, and the cosmos expands-  more of the (46.5 blY dia.)  universe enters into our visible "horizon".

Thus, it is more true to say that the already expanded (to 46.5 blY) cosmos is now "phased" into our available light sphere, than that light has "travelled outward past the edge of the universe".

The latter is, of course, correct, provided one knows what that means when he uses the phrase!



---------- FOLLOW-UP ----------

QUESTION: Philip,
Thanks for the quick answer, and I appreciate you bearing with me asking in terms of my understanding.
Are you basically saying that the universe is expanding faster than the speed of light (and getting faster), so that light can never catch up with the expanding "edge" of the universe?
Thanks,
Paul

Answer
Hello,

Yes.

Basically, I am saying that *space* is expanding at a rate v > c, and as I noted this expanding space is not subject to relativity's constraints. So, true, light can never "catch up" to the actually expanded space. There will always be a much vaster region of the latter (currently estimated at about 93 billion lY across) than the accessible light visual "horizon" available to us (currently 13.7 billion lY radius now).