Simple Nature - Light and Matter

2.3.6 ⋆ Evidence for repulsive gravityUntil recently, physicists thought they understood gravity fairlywell. Einstein had modified Newton’s theory, but certain characteristricsof gravitational forces were firmly established. For one thing,they were always attractive. If gravity always attracts, then it islogical to ask why the universe doesn’t collapse. Newton had answeredthis question by saying that if the universe was infinite inall directions, then it would have no geometric center toward whichit would collapse; the forces on any particular star or planet exertedby distant parts of the universe would tend to cancel out bysymmetry. More careful calculations, however, show that Newton’suniverse would have a tendency to collapse on smaller scales: anypart of the universe that happened to be slightly more dense thanaverage would contract further, and this contraction would resultin stronger gravitational forces, which would cause even more rapidcontraction, and so on.When Einstein overhauled gravity, the same problem reared itsugly head. Like Newton, Einstein was predisposed to believe in auniverse that was static, so he added a special repulsive term to hisequations, intended to prevent a collapse. This term was not associatedwith any interaction of mass with mass, but represented merelyan overall tendency for space itself to expand unless restrained bythe matter that inhabited it. It turns out that Einstein’s solution,like Newton’s, is unstable. Furthermore, it was soon discoveredobservationally that the universe was expanding, and this was interpretedby creating the Big Bang model, in which the universe’scurrent expansion is the aftermath of a fantastically hot explosion. 10An expanding universe, unlike a static one, was capable of being explainedwith Einstein’s equations, without any repulsion term. Theuniverse’s expansion would simply slow down over time due to theattractive gravitational forces. After these developments, Einsteinsaid woefully that adding the repulsive term, known as the cosmologicalconstant, had been the greatest blunder of his life.m / The WMAP probe’s mapof the cosmic microwave backgroundis like a “baby picture” ofthe universe.10 Section subsection 6.1.5 presents some of the evidence for the Big Bang.108 Chapter 2 Conservation of Energy