No one knows for sure because astronomers can’t see even as far as the farthest star. The sphere of space around Earth that we can see keeps growing as astronomers use new technologies. Right now astronomers can see about 14 billion light-years in any direction. That whole distance is filled with stars — in huge star islands called galaxies — all the way out. Astronomers are pretty sure that more of the same continues a few billion light-years beyond that.
A popular theory of the universe is called “inflation.” If it’s correct, our universe is enormously larger than what we’ve seen so far. Inflation theory also suggests that, in the first fraction of a second of time, a tiny chunk of space and matter blew up, or inflated, into everything we know — our universe. If that’s so, then other tiny chunks might also have inflated — forming other universal “cells” where the laws of physics as we know them might not exist.
So, one way to answer your question — what’s beyond the farthest star — might be to imagine whole other universes — side by side with ours.
Inflation theory is now 20 years old and about as well accepted as any theory that can’t be directly tested. The numbers in the radio script are averages of the different values that result from variations of the standard inflationary model.
I should quantify here some of the numbers for the radio program. When I say that the Inflated Universe is “enormously larger” than the Observable Universe, the actual scaling factor is at least e^60, which gives a volume 10^78 greater. This is considered a very conservative estimate. It could be much larger. Also, the “fraction of a second” after which inflation began is when the universe was 10^-35 seconds old. This may well give this program the distinction of dealing with the largest and smallest numbers ever on Earth and Sky.
There are some non–standard inflation theories where, instead of happening once, during the Big Bang, inflationary events can happen at any time. If that is true, our universe could be entirely surrounded by one mother universe instead of a crowd of siblings.
You may be wondering what keeps these universes apart. It’s all very Star Trek, actually. The borders are topological anomalies called Domain Walls that behave in a very strange fashion.
For more detail please see these websites:
- AN EXPOSITION ON INFLATIONARY COSMOLOGY by Gary Scott Watson. It is a lengthy discussion of Inflation at an Physics undergraduate level.
- The Inflationary Universe. (Journey Through Astronomy) This is a more accessible page that explains some of the benefits of Inflationary Theory.
- A non-standard Inflationary Theory is vividly described in an interview with its creator at: Big Bang Bust (Wired.com)
- or you can read Dr. Fred Adam’s forthcoming book Origins of Existence where he deals with this subject in some depth.
- Inflation for Beginners (University of Sussex School of Biological Sciences)