According to the best available information concerning the galactic masses, it seems that at present the kinetic energy of receding galaxies is several times greater than their mutual potential gravitational energy, from which it would follow that our universe is expanding into infinity without any chance of ever being pulled more closely together again by the forces of gravity.
It must be remembered, however, that most of the numerical data pertaining to the universe as a whole are not very exact, and it is possible that future studies will reverse this conclusion.
But even if the expanding universe does stop suddenly in its tracks, and turn back in a movement of compression, it will be billions of years before that terrible day envisioned by the Negro spiritual, “when the stars begin to fall,” and we are crushed under the weight of collapsing galaxies!
What was this high explosive material that sent the fragments of the universe flying apart at such a terrific speed?
The answer may be somewhat disappointing: there probably was no explosion in the ordinary sense of the word.
The universe is now expanding because in some previous period of its history (which, of course, no record has been left), it contracted from infinity into a very dense state and then rebounded, as it were, propelled by the strong elastic forces inherent in compressed matter.
If you were to enter a game room just in time to see a ping-pong ball rising from the floor high into the air, you would conclude (without really thinking about it) that in the instant before you entered the room the ball had fallen to the floor from a comparable height, and was jumping up again because of its elasticity.
We can now send our imagination flying beyond any limits, and ask ourselves whether during the precompressive stages of the universe everything that is now happening was happening in reverse order.
Were you reading this book from the last page to the first some six or eight billion years ago?
And did the people of that time produce fried chickens from their mouths, put life into them in the kitchen, and send them to the farm where they grew from adulthood to babyhood, finally crawled into eggshells, and after some weeks became fresh eggs?
Interesting as they are, such questions cannot be answered from the purely scientific point of view, since the maximum compression of the universe, which squeezed all matter into a uniform nuclear fluid, must have completely obliterated all the records of the earlier compressive stages.