[Editorial note: This is taken from the book The Tyrannosaurus Prescription by Isaac Asimov, an essay anthology of his.]
The word universe comes from Latin words meaning "turning as one." It is everything, treated as a unit. It is all the matter and energy that exists.
We have the disadvantage of studying the Universe from within. We can see those portions near to us, but the farther portions become progressively dimmer, progressively blurred. Even with all our instruments, much of the Universe remains too distant and dim to see at all—let alone in detail.
From what we see, though, we can come to conclusions. So suppose we imagine we are viewing the Universe from outside under such conditions that we are aware of the whole thing. (This is impossible, of course, since there is no such thing as "outside the Universe," but let us imagine it anyway.)
The Universe would look like a three-dimensional mesh of fine strands of light, with empty spaces between. There would be a great many small empty spaces, a smaller number of larger ones, a still smaller number of still larger ones. As for the lines of light, they would gather here and there in small knots or clumps of light, with a smaller number of brighter knots, and so on.
The Universe would most resemble a sponge built of light. The curving lines and sheets of light are built up of about one hundred billion dots of light (some considerably brighter than others). Each of these dots is a galaxy.
The Universe as we view it would be most notable for its stillness. Nothing whatever would seem to be happening to it. The reason for this is that no progressive change, large enough to be noticeable under our Universal view, can possibly take place at faster than the speed of light. The speed of light (186,282 miles per second) may seem unimaginably fast to us, but on the scale of the Universe as a whole, light may be considered virtually motionless.
Suppose, for instance, that as a result of some unimaginable event, the central point of one of the galaxies of the Universe ceases to emit light. It grows dark. Suppose that a wave of such darkening spreads outward from that central point in all directions at the fastest possible speed, that of light. We, watching from without, might see the galaxy (visible to us as a dot of light) begin to grow slightly dimmer, but it would take tens of thousands of years before the galaxy would blank out completely. It would take hundreds of thousands of years for the darkening to extend to other, neighboring dots. It would take some 12 billion years, at the very least, for the entire Universe to darken.
If we began watching at any stage of this Universal darkening we would see absolutely no change in the course of a lifetime, and very little in the course of a hundred lifetimes. (The same would be true, by the way, if the Universe were dark to begin with and began to grow light from some central point, the influence spreading outward at the speed of light.)
We, ourselves, are as much a prisoner of our place and time as everything else is. We cannot, under any circumstances we know of, go faster than the speed of light. At that speed, it would take us about 160,000 years to go to the far end of our own galaxy and back, and 4,600,000 years to travel to the Andromeda galaxy, our nearest large neighbor, and back. To be sure, at the speed of light, Einstein's relativity tells us, the rate of time-passage will sink to zero and, for us as we travel, no time will seem to be passing. Back on Earth, however, when we return to it, we would find that 160,000 years had irrevocably passed while we were visiting the far end of the Galaxy or that 4,600,000 years had passed while we were flashing by the Andromeda and returning.
It is not likely that we will be able to go at the speed of light, however. The greatest practical speed may prove to be no more than a fifth the speed of light, in which case the relativistic slowdown of time for the traveler is insignificant. It would then take 800,000 years of the astronaut's real time to visit the other end of the galaxy and return and 23,000,000 years to visit the Andromeda and return.
It may be, then, that with the best will in the world, any man in his own lifetime may be able to do no more than visit the very nearest stars, and from the Universal view that travel distance will be essentially zero.
Suppose, though, that, as we view the Universe as a whole, we overcome its motionlessness, by imagining that we speed up time a million-fold. Or, alternatively, we can imagine that some kindly superbeing has taken a detailed photograph of the Universe every hundred thousand years and that now we have the opportunity to run the film through a projector at the usual sixteen frames a second.
At this speed, the galaxies undergo rapid changes. Each one spins rapidly about its center. If it is a spiral, the spiral arms may disappear and reappear. None of these changes would be visible from our Universal view, of course. The dots of light would remain just dots of light.
At this speed, also, some galaxies will be exploding in a sudden burst of light, some will develop black holes that will grow enormously and devour millions of stars in a matter of seconds. Other galaxies will collide and produce incredible showers of radio waves and other radiations. None of this will be particularly visible either. Some of the dots of light in our Universal view may brighten slightly and others may dim slightly, but we probably won't notice it happening without careful measurements, if then.
In that case, will even speeding up time do nothing to remove the Universe's changelessness? Not so. There is one change that is the overwhelming fact about the Universe.
As we watch the film run we will notice that the Universe is visibly expanding. The holes in the spongy structure will slowly grow larger, and the curves and swoops of light will slowly thin out and spread apart, so that the intensity of light in any one spot will dim. In short, the Universal sponge will grow larger and larger and dimmer and dimmer.
We might also run the film backward. In that case the Universe will be visibly contracting. The holes in the spongy structure will slowly grow smaller, and the curves and swoops of light will slowly thicken and tighten. In short, the Universal sponge will grow smaller and smaller and brighter and brighter.
If we continue to run the film in the normal direction indefinitely, the Universe may expand and dim indefinitely until it is too dim to see at all. If, however, we continue to run the film in the backward direction, there is a limit to the length of time we can continue to do so, for eventually the Universe must shrink to nothing.
In fact, if we start at the present and run the film backward at 100,000 years every one-sixteenth of a second, then in about two hours the Universal view will be seen to have contracted itself into a tiny dot that is unbearably bright (though not in visible light) and unbearably hot, and then it will blink out into nothingness.
If we start at that point of nothingness and run the film forward, the dot will appear with its unbearably bright heat and quickly expand and cool. That is the "big bang," in which, astronomers now suspect, all the matter and energy of the Universe was formed out of nothing, in accordance with the peculiar rules of quantum theory.
This big bang presents astronomers with a fascinating problem. At the moment of the big bang, that original point of light must have been homogeneous. Everything in it must have been completely mixed. As it expanded, it should have stayed completely mixed. The whole Universe today ought to be just one large, ever-expanding, ever-thinning gas, which would always be the same everywhere in the Universe.
Instead, from the Universal view, we see a terribly uneven Universe. Matter and energy have coagulated into the dots we call galaxies, and these have, in turn, collected into lines and curves of light that give the Universe a spongy appearance. How can the Universe have gone from a featureless dot of light to a sponge? Cosmologists are still arguing over it and trying out various theories.
Another problem is this: Will the Universe expand forever?
The Universe is expanding against the pull of its own gravity, and, as a result, its rate of expansion is slowing. But is this braking effect of gravity sufficient to bring the expansion to a complete halt some day and start a contraction instead?
That depends on the quantity of matter in the Universe, for matter is the source of the gravitational pull. At the moment, it seems that the amount of matter we can detect is not more than about 1 percent of the quantity needed to stop the expansion someday. Yet there are some indications that the expansion will stop some day. If that is so, it means that there is at least a hundred times as much matter in the Universe than we can detect so far.
This is called "the mystery of the missing mass," and cosmologists are arguing over this heatedly.