Expert: Philip A. Stahl - 3/18/2009

Question
QUESTION: Philip,
If the universe is expanding faster than the speed of light, how does any light get back to us, at least from other galaxies?
Paul

ANSWER: Hello,

The key here is that the contents of the cosmos itself (e.g. galaxies and galaxy clusters) are NOT expanding faster than the speed of light, but rather the *space* is expanding faster than the speed c. Thus, there will always be a visible portion defined by (and within)the "light limit" and we will be able to see out to its "edge" near 13.7 billion LY.

However, the expanded *space* os what makes the actual diameter - call it D, something like 93 billion LY across.

As to the differential diameter: D - d, there is nothing in there we can ascertain or detect because the light limit has not attained beyond 13.7 billion LY as yet.

Again, the excellent article I referenced earlier (in an answer you seem to have read) is a great place to learn more about expanded space, and why it is so much larger than the visible light limit to the cosmos.

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

QUESTION: Philip,
Thanks for the answer, and please bear with me as I am still confused.
I thought that the expansion of space is defined/measured by receding galaxies.
Thanks for your time.
Paul

ANSWER: Hello,

No, the expansion of space to which I referred was the "metric expansion of space" which is quite apart from the physical expansion defined by the recession of galaxy clusters etc.

As I do not have drawing capabilities in this answer format, I refer you to the basis of metric expansion here:

http://en.wikipedia.org/wiki/Metric_expansion_of_space


Hopefully this will clarify issues!

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

QUESTION: Philip,
Wow, thanks for the speedy reply!
I had read the Wik article you cited (and others) before sending my last question.  
I went there again just now, and although much of it is difficult for me to understand, the first sentence is part of what led to my last question:  "The metric expansion of space is the averaged increase of metric (i.e. measured) distance between objects in the universe with time."
I'm missing something here, and still hoping you can help me out.
Thanks for your patience,
Paul

Answer
Hello,

Alas I don't think I can be much more help because getting into metric space and intervals based on it would mean getting into exactly WHAT a metric space is. Have you ever taken topology courses, or even algebraic geometry? Without some background in those it is extremely difficult to explain.

(EXAMPLE: let x be a set and d a mapping of the set X x X into the set of non-negative real numbers. The mapping d is called a "metric space" for X iff (if and only if) for each triple (x, y, z) in X:

a) d(x, y) = 0 iff x = y

b) d(x, y) + d(y, z) >= d(z, x)

where d(x,y) denotes the image d(x,y) of (x,y) under d.

The pair (X, d) is then the "metric space" and the number d(x, y) is called the *distance* between x and y)



Basically, think of the intervening space between actual objects (e.g. galaxy clusters) being stretched.

If you place ink dots on a balloon then blow it up, what happens? Well the intervals ("metric spaces") between the ink dots, increase. This is a rather crude analogy to the situation. Thus, if one discounts the objects themselves (ink dots) and just focuses on the stretched space, it is much much larger than the simple displacement of the objects suggests.

Hope this helps.