Purpose/1. The Universe/Introduction/The Universe/The Expanding Universe
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Two competing theories quickly emerged to explain the expanding universe phenomenon: the Steady State theory and the Big Bang theory. The first postulated that the universe is self-sustaining, and claimed that what we see today is what will be seen in billions of years time; galaxies move apart, but the total picture remains the same. According to the Steady State theory, the stars that are moving outwards are being replaced by new ones that are perpetually being formed out of cosmic dust. This dust is being created from atoms, the theory went, that themselves are being continuously created from cosmic energy. (We cannot observe this creation, because the theory predicts that it need only occur at a rate of about one atom per 500 cubic meters, every 1000 years,[12] which is far too slow to be detected.)
In contrast, the Big Bang explanation stated that the universe began with an explosion, and that everything within has been moving apart since that moment, with the intervening space expanding and cooling[13] as it proceeds. In this theory, everything was created during, or shortly after, “the bang,” rather than slowly and continuously, atom by atom.[14]
I remember reading some of the discussions reported in the press when these theories were announced, shortly after the second world war. They provided a stimulating intellectual alternative to the dreary task of becoming formally educated. At that time, I favoured the Steady State theory because I couldn’t understand how everything could come from nothing in one big burst. Creation seemed slightly more feasible if it happened very slowly, and I thought that perhaps the atoms being created came from energy released as the matter in receding galaxies became stretched further and further apart.[15] The Steady State theory also seemed more appealing philosophically, because if the universe continued forever, one would never have a beginning to explain.
However, three observations eventually dismissed the Steady State theory. First, if the universe did start with a bang, then there should still be some trace evidence of this explosion to be found. And there is; it has been heard since radio astronomy first began. Everywhere one searches, in addition to the electromagnetic information received from stars and galaxies, there is a constant hiss of background radiation. This hiss comes from energy that remains (unconverted into matter) from the originating Big Bang.[16]
Second, the universe does not remain the same over time, as the Steady State theory requires; it changes as it becomes older. This was discovered when quasar locations were established upon a four-dimensional map of the universe. This showed them to exist only at great distances from our system, signifying that they were present billions of years ago but no longer exist today.[17] Astronomers also find (by searching far and near in distance, and so effectively far and near in time) that galaxies tend to change their shape as they age.
Third, scientists have calculated the variety and abundance of chemical elements that should have been formed following the Big Bang, and their calculations predict exactly the ratios that are found to exist in space. Furthermore, the particular mixture predicted by the Big Bang theory (and corroborated by observation) is quite different from the mixture that the Steady State theory predicts.
Various other kinds of evidence support the Big Bang theory, and it rules the roost today.[18] The creating Bang is calculated to have occurred about 13.7 billion years ago.
While no cosmologist doubts that our universe is expanding, there has been much debate about whether it will continue to expand forever. Should this be the case, our universe will end up as a diffuse, dark, frigid, dead junkyard, with no energy differences left to power change. If, however, the attractive gravitational forces are strong enough, the universe’s current expansion will be slowed down, stopped, then reversed. This reversal would cause everything to be pulled tighter and tighter together, and it would all eventually be gathered into one gigantic black hole, presumably to continue shrinking until it reverted back to whence it came.
This uncertainty about the universe’s future may have been resolved by measurements taken over the last five or so years. Measurements of the brightness and red shift of distant supernovae (see next section) yield the recession speed of the universe when it was young. Comparisons of that speed with the recession speed shown by nearby galaxies reveals that the universe’s rate of expansion is increasing, not slowing down.[19] (It is currently thought that the slightly repulsive gravitational force of the “vacuum energy”[20] of empty space may be causing this acceleration.)
