Until recently, physicists thought they understood gravity fairly |
well. Einstein had modi ed Newton’s theory, but certain charac- |
teristrics of gravitational forces were rmly established. For one thing, they were always attractive. If gravity always attracts, then it is logical to ask why the universe doesn’t collapse. Newton had answered this question by saying that if the universe was in nite in all directions, then it would have no geometric center toward which it would collapse; the forces on any particular star or planet ex- erted by distant parts of the universe would tend to cancel out by symmetry. More careful calculations, however, show that Newton’s universe would have a tendency to collapse on smaller scales: any part of the universe that happened to be slightly more dense than average would contract further, and this contraction would result in stronger gravitational forces, which would cause even more rapid contraction, and so on. |
When Einstein overhauled gravity, the same problem reared its |
ugly head. Like Newton, Einstein was predisposed to believe in a universe that was static, so he added a special repulsive term to his equations, intended to prevent a collapse. This term was not asso- ciated with any attraction of mass for mass, but represented merely an overall tendency for space itself to expand unless restrained by the matter that inhabited it. It turns out that Einstein’s solution, like Newton’s, is unstable. Furthermore, it was soon discovered observationally that the universe was expanding, and this was in- terpreted by creating the Big Bang model, in which the universe’s current expansion is the aftermath of a fantastically hot explosion.1 |
An expanding universe, unlike a static one, was capable of being ex- |
plained with Einstein’s equations, without any repulsion term. The universe’s expansion would simply slow down over time due to the attractive gravitational forces. After these developments, Einstein said woefully that adding the repulsive term, known as the cosmo- logical constant, had been the greatest blunder of his life. |
1 Book 3, section 3.5, presents some of the evidence for the Big Bang. |
 |
This was the state of things until 1999, when evidence began to |
turn up that the universe’s expansion has been speeding up rather than slowing down! The rst evidence came from using a telescope as a sort of time machine: light from a distant galaxy may have taken billions of years to reach us, so we are seeing it as it was far in the past. Looking back in time, astronomers saw the universe expanding at speeds that ware lower, rather than higher. At rst they were morti ed, since this was exactly the opposite of what had been expected. The statistical quality of the data was also not good enough to constute ironclad proof, and there were worries about sys- tematic errors. The case for an accelerating expansion has however been nailed down by high-precision mapping of the dim, sky-wide afterglow of the Big Bang, known as the cosmic microwave back- ground. Some theorists have proposed reviving Einstein’s cosmo- logical constant to account for the acceleration, while others believe it is evidence for a mysterious form of matter which exhibits gravi- tational repulsion. The generic term for this unknown stu is “dark energy.” |
As of 2008, most of the remaining doubt about the repulsive ef- |
fect has been dispelled. During the past decade or so, astronomers consider themselves to have entered a new era of high-precision cos- mology. The cosmic microwave background measurements, for ex- ample, have measured the age of the universe to be 13.7 ± 0.2 billion years, a gure that could previously be stated only as a fuzzy range from 10 to 20 billion. We know that only 4% of the universe is atoms, with another 23% consisting of unknown subatomic parti- cles, and 73% of dark energy. It’s more than a little ironic to know about so many things with such high precision, and yet to know virtually nothing about their nature. For instance, we know that precisely 96% of the universe is something other than atoms, but we know precisely nothing about what that something is. |