in the Proceedings of the National Academy of Sciences. That pioneering paper, together with further observations published in 1931, established beyond any doubt that the Universe is in a continual state of expansion. Galaxies recede at speeds dependent on their distances from us. With large telescopes, we can image objects moving away from us at speeds of an amazing 275 000 kilometers per second, which is over ninety percent the speed of light! We live in an expanding Universe.
Going backwards in time from today we would see the present outward-moving systems of galaxies contracting, and at all points in the Universe the density of matter would become higher and higher. But let us spend a few important moments dispensing with a very popular (but wrong) concept of what the Big Bang is, and what it is not. For, contrary to popular belief, the Big Bang is not a point in space from which matter explodes outwards.
The most widely accepted model of our expanding Universe is that of a "Big Bang." Unfortunately this phrase suggests that all matter once existed somewhere as a superdense ball of matter which suddenly exploded, scattering debris throughout space like an exploding bomb. But this picture is too simple. It presupposes that space already existed before the explosion and that the Big Bang was merely the mechanism that flung the stars and galaxies out into space. It suggests that before the bang there was empty space, like an empty room, with a highly concentrated dot of matter at its center. It suggests that after the bang the galaxies moved out to fill space, like furniture fills a room.
But the Big Bang is also concerned with the formation of space itself! There was no "room" within which the explosion could occur. Analogies are inadequate but it is something like representing the early Universe as an expanding balloon painted with dots; every dot represents one galaxy. At this juncture, our three-dimensional minds tend to play a trick on us: students often ask, "what lies outside the balloon?" This is an important question. In our analogy, the balloon is not expanding into a pre-existing space - the balloon represents the The Insignificance ofMan?
It was the genius of Albert Einstein who developed his general theory of relativity within this four-dimensional "space-time" framework. Initially the dots (galaxies) are relatively close to one another, but as the balloon expands they lie further and further away from each other.
So, in cosmology, the Big Bang represents an interval of time in the early history of the Universe when matter everywhere, at all points, was concentrated at immense densities and temperatures. Going backwards in time from today we would see the present outward-moving galaxies contracting, and at all points in the Universe, the density of matter would become higher. If the Universe is infinite now, it has always been infinite. If it is finite now, there is and was no "beyond" in the usual sense of the word.
If the Universe is infinite (open) now - shaped like an open saddle - it has always been infinite. If the Universe is finite (closed) now, there is and was no "beyond" in four-dimensional "space-time."
The Big Bang models therefore have a boundary in time. By convention, they start at time t = 0. But our Universe may not have a spatial boundary. For if the Universe is positively curved or "closed," it has no boundary in space. The analogy is of a person traveling in a fixed direction on the surface of the Earth; he or she never comes up against any impassable barrier or falls over any "edge." The person will eventually return to their starting point. However, present astronomical observations suggest that the Universe is "open" - actually, almost flat - and not "closed." If so, the expansion will continue forever, the Big Bang will have occurred only once, and space unfolds along an open boundary.
What can we know about the early moments of the Universe? Can we wind the clock back, right to t = 0? The cosmic microwave background radiation (CMBR) is a relic from the time of the Big Bang epoch when the Universe consisted of an exceedingly hot, opaque soup of charged particles and radiation. Now the Universe is much colder: a mere three degrees above absolute zero (or minus 270 degrees Centigrade), but this faint relic of the Big Bang still permeates our entire Universe in terms of its microwave glow. Its discovery by radio astronomers Arno A. Penzias and Robert W. Wilson provided a major piece of evidence in
Shrouds of the Night support of a hot Big Bang cosmology.
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