The Dancing Universe: From Creation Myths to the Big Bang

9 | Inventing Universes

❍ 9 INVENTING UNIVERSES I saw a huge Wheel, which was not in front of my eyes, nor behind , nor beside, but in all places at once. This Wheel was made of water, but also of fire, and was (although I saw its border) infinite. —Jorge Luis Borges ide by side with the revolution in our understanding of the physics of the very small and of the very fast, the first three decades of the twentieth century witnessed yet another revolution: a new physics of gravity and of the Universe as a whole. In short, a new physics of the very large. Once again, the key intellectual stimulus came from the mind of Albert Einstein. For soon after he completed his seminal work on the special theory of relativity, Einstein started to wonder how it could be generalized to include not only observers moving with constant relative velocities, but also observers moving with varying relative velocities, i.e., accelerated motions. In an insight that he dubbed “the happiest thought of my life,” Einstein realized that there was an intimate connection between accelerated motion and gravity. A “general” theory of relativity, capable of incorporating accelerated motion, implied in a new theory of gravity. As with the thought experiment where he asked how a light wave would appear to an observer moving with the speed of light, Einstein’s insight into the inner workings of gravity came from a disarmingly simple image: How would someone who is falling down, say, from the top of a tall building (into a nice deep swimming pool), characterize the gravitational forces around him? S In the same way that special relativity revealed the shortcomings of Newtonian mechanics for velocities approaching the speed of light, Einstein’s new theory of gravity revealed the shortcomings of Newtonian gravity for situations involving strong gravitational fields. It was already known that gravity could be thought of in terms of fields, just as with electromagnetism after Faraday and Maxwell. Any mass would have associated with it an attractive field, “a disturbance in space,” which would then influence other masses placed close to it. But to say that Einstein simply generalized Newtonian ideas to include stronger gravitational fields is a gross understatement. The new theory of gravity, or general theory of relativity as it is known, provided a radically different conceptual framework to deal with the age-old question as to why objects attract each other. As opposed to the absolute space and time of Newtonian physics, both impervious to the presence of matter, in general relativity spacetime becomes plastic, deformable, responding to the presence of matter in well-determined ways: matter (or, due to special relativity , energy) actually alters the geometry of space and the flow of time. In turn, masses placed in this “bent” spacetime will have motions that deviate from the usual straight motions at constant velocities described by special relativity; they will have accelerated motions. In Einstein’s general relativity, the effects of gravity are understood in terms of motions in a bent spacetime. This intimate relationship between matter and the geometry of spacetime has immense cosmological importance. As Einstein realized soon after he completed his main paper on the general theory late in 1915, if the distribution of matter in the entire Universe could be somehow modeled, then the new theory of gravity should determine the geometry of the Universe as a whole! A new era for cosmology was to begin, the shape of the Universe itself amenable to study through the equations of general relativity. Following Einstein ’s pioneering efforts, new models of the Universe emerged, mathematical universes based as much on different physical assumptions as on personal prejudice. If you mastered the complex mathematics behind the general theory of relativity, you could play around with building universes on a piece of paper. You could play God on a Tuesday afternoon. As in other instances in the history of physics past and present, 244 MODELING THE UNIVERSE what was missing was data, some indication of what was the correct direction cosmology should take. The whole issue could have remained quite academic if not for another revolution, this time in observational cosmology. In a series of remarkable findings during the 1920s, the American astronomer Edwin Hubble not only settled the age-old question as to whether nebulae were other “island universes ” like our own Milky Way (see Chapter 6), but also discovered something even...