There is a famous passage in A Study in Scarlet where Sherlock Holmes explains to Watson why, whenever he is told a fact about astronomy, he does his best to forget it.
“But the solar system!” Watson exclaims.
“What the deuce is it to me?” Holmes interrupts. “You say that we go round the sun. If we went round the moon it would not make a pennyworth of difference to me or to my work.”
I would guess that most Europeans felt the same way during the great debates over the Copernican and Ptolemaic theories, and that even today most laymen have a similar indifference toward the debates over contemporary models of the universe. No branch of physical science is more remote from the practical. What does it matter to you and me whether spacetime is infinite, or finite and closed like the surface of a sphere? What difference does it make if the universe expands forever until it dies from the cold, or if, after many billions of years, it starts to contract? Who cares whether the contraction will end in a black hole or whether the universe will bounce back into existence, as foretold by Hindu myths, to start another cycle in an endless round of cosmic rebirths?
Well, astronomers, physicists, and philosophers care, and it has always been impossible for them not to consider such questions. Why does a chicken cross the road? Because it’s there. Why do astrophysicists build models of the universe? Because the universe is there, and because they have in their heads the materials and the mathematical tools for constructing such models.
Indeed, the materials and tools are available in an abundance far exceeding that of earlier centuries. By “materials” I mean, of course, the entire body of scientific knowledge—never certain, always changing, but steadily improving in its power to explain and predict the peculiar behavior of the outside world. There are, therefore, excellent reasons to believe that today’s models of the universe “fit” reality better than the old ones.
Modern cosmology began with Einstein’s model of a finite yet unbounded universe. Although it cannot be visualized, it is easy to understand. A straight line is infinite and unbounded, but bend it through a space of two dimensions and it can form a circle. This “closed universe” is still one-dimensional, with no boundaries, but now it has a finite size. A plane is infinite and unbounded. Bend it through “three-space” (i.e., three-dimensional space) and it can be the closed surface of a sphere. Perhaps, said Einstein, our three-space bends through four-space to form the “surface” of a hypersphere. Like the circle and the sphere’s surface, such a space is unbounded in the sense that you can travel through it as far as you like, in any direction, and never reach an end. Nevertheless it is finite in size. To prevent gravity from collapsing the universe, Einstein posited an unknown repulsive force that preserves the cosmos in static …
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