What is the universe expanding into?

This is a popular question, and it comes about because astronomers have identified the universe as being of an infinite 'Big Bang' type with a definite origin to space and time at 13.7 billion years ago. Also, the circumstances of its birth are summarized by inflationary cosmology, which makes predictions about what the universe was doing when it was a trillionth of a trillionth of a trillionth (10^^-36) of a second old. The basic idea is that space and time were 'created' at the big bang at the earliest computable time, 10^^-43 seconds, by some as-yet mysterious process - quantum tunneling, quantum fluctuations, it goes by a variety of names. What came out of this was a vast arena of space, but undergoing quantum fluctuations in gravitational curvature and energy probably throughout its entire plenum - we may never know for certain. This plenum, which contained highly curved and changing space curvature, expanded in time (a quality that now had meaning) until the universe got to an age of about 10^^-35 seconds. Then one of the quantum patches in this vast dilating and stretching sea of fluctuating curvature underwent inflation.

It expanded from 10^^-24 inches to several inches across, yet it was still connected to the even vaster plenum that had emerged from the Big Bang. We don't know what this vast plenum was doing at that time. Some other parts of it might also have reached the inflation stage and dilated too, but perhaps for shorter or longer periods. This plenum could literally have been infinite in the mathematical sense, or simply 'very very big'. Today, the universe continues to expand, and it is still connected to this vaster plenum that emerged from the Big Bang which is stupendously huge but whose limits we have no real way of deciding. Our part of the universe is that region that Inflated way back at 10^^-35 seconds, and today compared to the size of the visible universe (13 billion light years in radius) this larger region could be 10^^50 or 10^^100 times bigger. The universe is expanding because the underlying, even vaster plenum is still expanding. Its limits are beyond our visible universe, and beyond even that huge patch that inflated, and encompasses all those other patches, which perhaps inflated too, to make slightly different universes 'way out there'. But, even this larger universe isn't expanding by 'conquering' or displacing already existing space. In a sense it is expanding by literally creating space as it goes. Beyond its limits could be pure nothingness lacking space, time or dimension.

This answer was updated in 2011. See my books: The Astronomy Cafe (1998) and Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011

Return to Ask the Astronomer