The Science of Discworld IV

difficulty emerges from the observed ‘rotation curves’ of galaxies. Galaxies do not rotate like a rigid object: stars at different distances from the centre move with different speeds. Stars in the galaxy’s central bulge move quite slowly; those further out are faster. However, the stars outside the central bulge all move with much the same speed. This is a puzzle for theorists, because both Newtonian and Einsteinian gravity require the stars to move more slowly in the outer reaches of the galaxy. Virtually all galaxies behave in this unexpected manner, which conflicts with numerous observations.
    The third problem is the 1998 discovery that the expansion of the universe is accelerating, which is consistent with a positive non-zero cosmological constant. This was based on the High-z Supernova Search Team’s observations of the redshift in Type Ia supernovae, and won the Nobel Prize for Physics in 2011.
    The prevailing cosmological 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 10 78 . 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, whichmainly 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 10 78 .
    Another problem is that almost all inflationary universes fail to match ours, and if you restrict initial conditions to get the ones thatdo, 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