Expert: Philip Stahl - 8/28/2009

Question
If all the universe is expanding, as if we're on the surface of a bubble, then why is there an even(generally speaking) distribution of galaxies in every direction?  It seems as though when we peer "out" of the bubble(toward the edge of space-time), our vista should be relatively empty, or at least vastly different from all other directions.

Answer
Hello,

First of all, the observational fact of the cosmos' expansion is not in any way conjoined with being on "the surface of a bubble". Those are two different things, and it must be emphasized the latter is an *analogy*. It is also a very flawed analogy for a number of reasons:

i) expansion is of space *and* time not just space - so a simple "bubble" as a 3-D entity is inadequate to explain or represent it

ii) The rate of expansion is not uniform, and we know there have been periods of inflation (early on) during which very rapid expansion occurred, and now (what appears to be) accelerated expansion, based on (Type Ia supernovae)  data that show the presence of dark energy.

iii) It is inappropriate to assert we are "on the surface of a bubble" as if to mean or indicate we are at the literal *boundary* of the universe. No, we are not. What one needs to do here is to discriminate between the actual universe and the *observable* entity - or what we can detect.

Since light travels at the finite speed of 300,000 km/s this means we can only see the light that has managed to reach us. This is the light that has been propagating for 13.7 billion years in every direction.

However, this is *not* the limit of the actual universe's boundary. (Which is actually much, much greater for complex reasons to do with the cosmos' topology)

iv) Given the preceding, it is simply not possible to say that we can "peer out" as if from the edge boundary of the real universe and see only emptiness. We cannot do so, because we are embedded within a 4-D space-time manifold in which the entire expansion of the cosmos unfolds. To assert an ability to "peer out" as if from the edge, would imply we are in a superior observational position and we are not.

Perhaps a better analogy than a simple "bubble" (though still not perfect by any means) is that embodying an expansion that has no defined locus. No point of central reference, if you will.

Think then of raisins expanding on a (baking) loaf of bread pudding, or ink dots on an inflating balloon. No single raisin (or ink dot)enjoys a privileged position of "center" to the particular expansion. All expand outward in relation to every other. To make the analogy a bit more realistic, one can incorporate a rapid initial inflation (say of the balloon) and a much later accelerated inflation - to correspond with the current accelerated expansion arising from dark energy.

Hopefully, this answer makes some sense, but if not - there is an excellent book written at a very general level that can help:

'The Big Bang Theory - What It is, Where It Came From, And Why It Works',
by Karen C. Fox, John Wiley & Sons, 2002.


If you are subsequently more ambitious, try to get:

'The First Three Minutes', by Stephen Weinberg, 1977.

The Appendix of his book is particularly useful, and requires no more than
High school algebra to get through