The Science of Discworld IV Judgement Da - By Terry Pratchett, Ian Stewart Page 0,106
wisdom deals with these problems by bolting on three additional assumptions. The first is inflation, in which the entire universe expanded to a huge size in an extraordinarily short time. The figures are shocking: between 10-36 and 10-32 seconds after the Big Bang the volume of the universe multiplied by a factor of at least 1078. The cause of this rapid growth – an explosion far more impressive than the wimpy Big Bang that started it all – is, we are told, an inflaton field. (Not ‘inflation field’: an inflaton is – well, a quantum field that causes inflation.) This theory fits many observations very well. The main snag is the absence of any direct evidence for the existence of an inflaton field.
To solve the problem of galactic rotation curves, cosmologists propose the existence of dark matter. This is a form of matter that can’t be observed by the radiation it emits, because it doesn’t, not in any quantity that can be observed from here. It’s entirely reasonable that a lot of the matter in the universe might not be observable, but what we can infer indirectly leads to the conclusion that whatever dark matter may be, it’s not made from the fundamental particles that we know about on Earth. It’s a very alien form of matter, which mainly interacts with everything else through the force of gravity. No such particles have ever been observed, but there are several competing suggestions for what they ought to be, the front runner being WIMPs (weakly interacting massive particles). Despite a lot of theorising, the precise nature of dark matter is up for grabs.
The acceleration of the expansion of the universe is attributed to ‘dark energy’, which is little more than a name for ‘stuff that makes the expansion accelerate’ – though, to be fair, supplemented by detailed analyses of what kind of effect this stuff must have, and suggestions for what it might be. One possibility is Einstein’s cosmological constant.
Until recently, these three dei ex machina resolved most significant discrepancies between the naive Big Bang theory and increasingly sophisticated observations. The introduction of these three items of novel physics, all produced out of a hat and without much independent observational support (other than what they were invented to explain), could be justified pragmatically: they worked, and nothing else seemed to. But there is now a growing realisation that the first of those statements no longer holds, but unfortunately the second still does. A growing minority of cosmologists suspect that three dei ex machina is at least two too many for comfort.
It is now realised that if an inflaton field exists, it doesn’t conveniently switch on once and then cease to operate, which is assumed in the usual explanation of the structure of our universe. Instead, the inflaton field can swing into action anywhere, and at any time, repeatedly. This leads to a scenario called eternal inflation, with our region of the universe being just one inflated bubble in a bubble-bath of cosmic foam. A new period of inflation might start in your living room this afternoon, instantaneously blowing up your television set and the cat by a factor of 1078.
Another problem is that almost all inflationary universes fail to match ours, and if you restrict initial conditions to get the ones that do, then a non-inflationary universe that performs just as well is vastly more probable. According to Roger Penrose, suitable initial conditions not requiring inflation outnumber those for an inflationary universe by a factor of one googolplex – ten to the power ten to the power one hundred. So an explanation not involving inflation, although it requires an extraordinarily unlikely initial state, is massively more plausible than an explanation that does involve inflation.
A few mavericks have been devising alternatives to the standard model all along, but now mainstream cosmologists are also having to rethink the theory. There is no shortage of ideas. In some, there is no Big Bang; instead, there is a kind of revival of the steady-state universe, in which a suitably clumpy distribution of matter can survive for hundreds of billions of years, perhaps indefinitely. The redshift is not caused by expansion, but by gravity. Dark matter is not needed to explain rotation curves: instead, relativistic inertial dragging, in which rotating matter carries space along with it, might do the job.
Perhaps more radical is the proposal that either our theory of gravity, or our theory of motion, need to be modified slightly. In 2012 the particle physicist and