WHAT DOES EXPANSION MEAN?

The essence of the big bang cosmology is an expanding universe. The redshift of the light from galaxies is proportional to their distance (as inferred from brightness). No cause of galaxy redshift other than a velocity away from the observer was considered plausible, so Hubble's result was taken to mean that, the farther away from us a galaxy is, the faster it moves away from us. Hence, the overall universe had to be expanding.

Of course, the redshift still might be caused by something other than velocity. The only way to be sure is to perform observational tests. When considering tests for expansion, it is important to know what expansion really means in the big bang theory. The three Friedmann models described ways in which the expansion would appear the same from everywhere within the universe. But if this expansion meant that all matter in the universe was at one time located at a point in space, then the universe would have a center and an edge. That would make every point in it "special" with respect to the origin point and with respect to the void beyond the edge. The view would not be the same from everywhere.

To understand expansion in the big bang theory, we are asked to visualize an expanding balloon as a 2-dimensional analogy of our 3-dimensional universe. Every point on the surface of the balloon gets farther away from every other point as the balloon expands. Yet no point on the surface serves as the center, and there is no edge. The expansion is slowed by gravity and may eventually halt and begin to contract back to its origin point; or the expansion rate may be too high to ever halt the expansion. It is up to observations to tell us which kind of Friedmann universe we inhabit by allowing us to measure the cosmic deceleration parameter, q.

But all these Friedmann universes are very different from the kind of expansion one would get if the universe originated in an explosion into pre-existing empty space. This is because the big bang is an explosion of space and time, not an explosion into space and time. A recent paper by Harrison explains: "From a purist point of view one cannot help but deplore the expression 'big bang', loaded with inappropriate connotations ..., which conjures up a false picture of a bounded universe expanding from a center in space. In modern cosmology, the universe does not expand in space, but consists of expanding space. And this correct picture leads naturally to a distinction between the redshift-distance and velocity-distance laws."² Odenwald and Fienberg state the point in more detail:³ "This [cosmological] redshift, which again is not a Doppler shift, arises from the expansion of space-time itself. Light waves literally stretch as the universe expands between the time the light was emitted and today, when it finally reaches us." ... "Now galaxies are located at fixed positions in space. They might perform small dances about these positions in accordance with special relativity and local gravitational fields, but the real 'motion' is in the literal expansion of the space between them." ... "This is not a form of motion that any human being has ever experienced, in that it does not involve travel through space. So it is not surprising that our intuition reels at its implications and seeks less radical interpretations."

So the big bang postulates that the cosmological expansion occurs, not because galaxies move apart through space, but because more space is being continually added between them. This continual creation of space ex nihilo⁴ is an integral part of the theory. Without it, the cosmological principle would be violated.

"And in the beginning there was nothing. And God said 'Let there be light.' And there was still nothing, but now you could SEE it!" -- Anonymous