Joel R. Primack, a cosmologist at the University of California at Santa Cruz, notes that the big bang involves physical processes quite unlike those of everyday experience. For that reason, people often find it quite difficult to grasp what astronomers mean when they refer to an Rexpanding universe.
One common misconception, Primack says, is "that the big bang is an explosion that occurred at some point in a preexisting static space--which is not a picture in accord with our modern theory of space-time and gravity." He explains why this image of the big bang as an explosion in space, like the detonation of a bomb, is incorrect:
"According to modern cosmological theory, based on Einstein's General Relativity (our modern theory of gravity), the big bang did not occur somewhere in space; it occupied the whole of space. Indeed, it created space. Distant galaxies are not traveling at a high speed through space; instead, just like our own galaxy, they are moving relatively slowly with respect to any of their neighboring galaxies. It is the expansion of space, between the time when the stars in these distant galaxies emitted light and our telescopes receive it, that causes the wavelength of the light to lengthen (redshift). Space is itself infinitely elastic; it is not expanding into anything."
The lengthening, or redshifting, of light that Primack describes was first observed by Edwin Hubble in 1929. This phenomenon is often referred to, incorrectly, as a Doppler shift. A Doppler redshift results from the expansion of light emitted by a receding object. Cosmological redshifts result from the expansion of space (and the light moving through that space) between us and a distant galaxy or quasar. Space is expanding everywhere, so the more distant an object is, the more rapidly it appears to be moving away.
Primack then considers another aspect of the reader's question: What lies beyond our cosmic horizon, the visible "edge" of the universe?
"Every observer in the universe is surrounded by a sphere beyond which nothing can be seen: the observer's cosmic horizon (the point at which the apparent recessional velocity equals the speed of light). Because it is the expansion of space rather than the high velocity of distant galaxies that prevents us from seeing beyond our cosmic horizon, there is no reason to suppose that galaxies outside it are any different from those inside it. Indeed, the extreme isotropy, or smoothness, of the cosmic background radiation (it appears to have the same temperature in all directions, to about one part in 100,000), together with the law of General Relativity and other physical assumptions that seem reasonable, implies that the universe must remain pretty much the same out to a considerable distance beyond our horizon.
"The theory of cosmic inflation, a recent elaboration of the big bang, suggests that at still greater distances, the universe is very different from the way it is locally. Although we cannot check this prediction directly, other predictions of inflation are being tested by new observations, especially those to be made by the astronomy satellites that NASA and the European Space Agency plan to launch in the next five to 10 years. There are many books that discuss current theories of the expansion of the universe and related topics. Of these, my favorite one is Cosmology, by Edward R. Harrison (Cambridge University Press, 1981)."
Takamasa Takahashi of St. Norbert College adds a few comments:
"Many scientists do not ask 'What came before the big bang?' because it is beyond the scope of our physical theories. Usually we consider the big bang to be the beginning of time and space, and so it is meaningless to ask what existed before or what lies beyond the expanding universe. Because space itself is intimately connected with matter in the universe, as matter was created in the big bang, so was space. There is no 'empty space' that the universe is expanding into.