Monday, March 2, 2015

Does the Universe have an edge?

I don't think anyone really knows. There are lots of ideas, though. If there has only been one Big Bang (which in itself seems unlikely to me), then the Universe has an average radius, approximately the age of the universe times the speed of light, ignoring the possibility of stuff that travels faster than the speed of light. Intergalactic space is thought to have an average density of something like 1 hydrogen atom per cubic meter [citation needed] although recent news from the big telescopes suggests that there are stars and planets also outside of galaxies. 1 atom/m^3 is pretty empty.  If the universe is alone, I suspect there is no way we will ever see its edge, as it is receding from us at or faster than the speed of light.

There are cosmologists who believe in all seriousness that our universe is expanding like a bubble in a foam of universes, in which case it is possible, I suspect, that we will some day detect the boundary, where it impinges on another universe. Does the edge slow down? Is there a shock wave? Please forgive me for being so Newtonian about this, but I can't imagine a foam of universes where all of them can expand indefinitely at or above the speed of light. Not, at least if they all share the same dimensions*. Then the question begs to be asked, does the foam have a boundary? What would be beyond that?

"It's turtles all the way down."

*What if our three familiar dimensions are actually dimensions #38576027184365756392,  #38576027184365756393, and #38576027184365756394, and other universes are also three dimensional but occupy other dimensions than our particular three of length, width, and height? (I am ignoring the 9, 11, 17 or whatever dimensions are required by string theory.)