Simple Nature - Light and Matter

a variant on the Penrose singularity theorem that applied to BigBang singularities. By the Hawking singularity theorem, the level ofuniformity we see in the present-day universe is more than sufficientto prove that a Big Bang singularity must have existed.The cosmic censorship hypothesisIt might not be too much of a philosophical jolt to imagine thatinformation was spontaneously created in the Big Bang. Settingup the initial conditions of the entire universe is traditionally theprerogative of God, not the laws of physics. But there is nothingfundamental in general relativity that forbids the existence of othersingularities that act like the Big Bang, being information producersrather than information consumers. As John Earman of theUniversity of Pittsburgh puts it, anything could pop out of sucha singularity, including green slime or your lost socks. This wouldeliminate any hope of finding a universal set of laws of physics thatwould be able to make a prediction given any initial situation.That would be such a devastating defeat for the enterprise ofphysics that in 1969 Penrose proposed an alternative, humorouslynamed the “cosmic censorship hypothesis,” which states that everysingularity in our universe, other than the Big Bang, is hiddenbehind an event horizon. Therefore if green slime spontaneouslypops out of one, there is limited impact on the predictive abilityof physics, since the slime can never have any causal effect on theoutside world. A singularity that is not modestly cloaked behind anevent horizon is referred to as a naked singularity. Nobody has yetbeen able to prove the cosmic censorship hypothesis.p / An expanding universewith positive spatial curvaturecan be imagined as a balloonbeing blown up. Every galaxy’sdistance from every other galaxyincreases, but no galaxy is thecenter of the expansion.The advent of high-precision cosmologyWe expect that if there is matter in the universe, it should havegravitational fields, and in the rubber-sheet analogy this should berepresented as a curvature of the sheet. Instead of a flat sheet, wecan have a spherical balloon, so that cosmological expansion is likeinflating it with more and more air. It is also possible to have negativecurvature, as in figure e on p. 425. All three of these are valid,possible cosmologies according to relativity. The positive-curvaturetype happens if the average density of matter in the universe is abovea certain critical level, the negative-curvature one if the density isbelow that value.To find out which type of universe we inhabit, we could tryto take a survey of the matter in the universe and determine itsaverage density. Historically, it has been very difficult to do this,even to within an order of magnitude. Most of the matter in theuniverse probably doesn’t emit light, making it difficult to detect.Astronomical distance scales are also very poorly calibrated againstabsolute units such as the SI.Instead, we measure the universe’s curvature, and infer the den-434 Chapter 7 Relativity