Karl W. Giberson
The Guy in the Wheelchair
The t = 0 "appearance" of the universe has occasioned much cosmological head-scratching. In the early days of the Big Bang theory, when the evidence was less than compelling, many cosmologists rejected the idea of a beginning. The Belgian cleric/physicist Georges Lemaître, who first proposed it, was accused of smuggling a suspiciously biblical "creation" into science. The enthusiastic agnostic Fred Hoyle developed an alternative "steady state" model, hopefully doing away with what Sir Arthur Eddington had called an "unaesthetically abrupt" beginning to the universe.
The discovery of the predicted background radiation in 1965 dramatically confirmed the Big Bang. Modern cosmology was born. Since then extrapolations have tried to deal with the beginning of the universe, sometimes "explaining" it, other times "explaining it away." Maybe our universe is the daughter of a previous universe or a bubble in a meta-universe or a sibling of many contemporary universes. Or, suggests Hawking, maybe there simply is no beginning.
The most exasperating feature of the Big Bang theory is its increasing vagueness as one approaches the point t = 0 on the cosmic timeline. As a description of today's universe the theory works well; there is ample evidence that the universe is expanding in the way the theory says it should; the radiation left over from the initial "explosion" is spread uniformly throughout space, as we would expect. And, when we look billions of light years "out" into space and see things as they were long ago, they are different in ways that fit with the Big Bang. All of this is comforting, for those who take comfort from such things.
But the picture grows murky as we approach the beginning. On the observational side, we simply cannot look out far enough to see light from 14 billion years ago. We can't even get close, so we are very much in the dark, so to speak, when it comes to observation of this critical point in the history of the universe.
There is, however, a glimmer of light on the theoretical side. Mathematical models of the early universe predict, in a rather straightforward way, astonishingly great densities of matter and very high temperatures. Microcosmic versions of such extreme environments can be created in the laboratory and tested against theoretical models. And the match is excellent for those early stages of the universe that come after the moment of origination.
But what about the actual point t = 0? This cannot be reproduced in the laboratory. Nor does there exist a compelling, generally accepted theory of exactly what this stage would look like.
Absent both observational data and compelling theoretical models, we have an explanatory vacuum—and cosmologists, like nature, abhor vacuums. This particular vacuum is filled with ingenious speculations, including those of Hawking.
The technical version of Hawking's speculation was published in collaboration with James Hartle in the Physical Review, the world's leading physics journal. After a densely mathematical, conceptually opaque presentation of the problem of the temporal and spatial "boundaries" of the universe, they conclude, so cryptically that readers can be forgiven for thinking they understand: "This means that the Universe does not have any boundaries in space or time … . There is thus no problem of boundary conditions."
Hawking repeated the final words of the paper until they became rather famous: "If this were the case, one would have solved the problem of the initial boundary conditions of the Universe: the boundary conditions are that it has no boundary." Note the all-important-but-easily-overlooked conditional, "If this were the case."
Hawking and Hartle's result is remarkable, but it must be placed in context. Interesting ideas in mathematical physics always contain assumptions and simplifications. As remarkable as the fit between the natural world and mathematics might be, the fit can rarely be made without simplifying assumptions; even the simple calculation of the rate at which a body falls to earth must assume that the earth and the body have all of their mass located at their centers of gravity, and that all other gravitational centers are infinitely far away. Such assumptions are necessary to make the "real world" match the "theoretical world," which it often does astonishingly well.