Free The Science of Discworld Revised Edition by Terry Pratchett

for each possible wavelength of light – the distance from one electromagnetic peak to the next. For red light, this distance is 2.8 hundred thousandths of an inch (0.7 millionths of a metre).
    Hubble noticed that something funny was happening to the light emitted by stars: the colours were shifting in the red direction. The further away a star was, the bigger the shift. He interpreted this ‘red shift’ as a sign that the stars are moving away from us, because there is a similar shift for sound, known as the ‘Doppler effect’, and it’s caused by the source of the sound moving. So the further away the stars are, the faster they’re travelling. This means that the stars aren’t just moving away from
us
– they’re moving away from each other, like a flock of birds dispersing in all directions.
    The universe, said Hubble, is expanding.
    Not expanding
into
anything, of course. It’s just that the space inside the universe is
growing
. 3 That made the physicists’ ears prick up, because it fitted exactly one of their three scenarios for changes in the size of the universe: stay the same, grow, collapse. They ‘knew’ it had to be one of the three, but which? Now they knew that, too. If we accept that the universe is growing we can work out where it came from by running time backwards, and this time-reversed universe collapses back to a single point. Putting time the right way round again, it must all have grown
from
a single point – the Big Bang. By estimating the rate of expansion of the universe we can work out that the Big Bang happened about 15 billion years ago.
    There is further evidence in the Big Bang’s favour: it left ‘echoes’. The Big Bang produces vast amounts of radiation, which spreads through the universe. Over billions of years, the remnants of the Big Bang’s radiation smeared out into the ‘cosmic background’, a kind of low-level simmering of radiant energy across the sky, the light analogue of a reverberating echo of sound. It is as if God shouted ‘Hello!’ at the instant of creation and we can still hear a faint ‘elloelloelloelloellox …’ from the distant mountains. On Discworld this is
exactly
the case, and the Listening Monks in their remote temples spend their whole lives straining to pick out from the sounds of the universe the faint echoes of the Words that set it in motion.
    According to the details of the Big Bang, the cosmic background radiation should have a ‘temperature’ (the analogue of loudness) of about 3° Kelvin (0° Kelvin is the coldest anything can get – equivalent to about -273° Celsius). Astronomers can measure the temperature of the cosmic background radiation, and they do indeed get 3° Kelvin. The Big Bang isn’t just a wild speculation. Not so long ago, most scientists didn’t want to believe it, and they only changed their minds because of Hubble’s evidence for the expansion of the universe, and that impressively accurate figure of 3° Kelvin for the temperature of the cosmic background radiation.
    It was, indeed, a very loud, and hot, bang.
    We are ambivalent, then, about beginnings – their ‘creation myth’ aspect appeals to our sense of narrative imperative, but we sometimes find the ‘first it wasn’t, then it was’ lie-to-children unpalatable. We have even more trouble with becomings. Our minds attach labels to things in the surrounding world, and we interpret those labels as discontinuities. If things have different labels, then we expect there to be a clear line of demarcation between them. The universe, however, runs on processes rather than things, and a process starts as one thing and
becomes
another without ever crossing a clear boundary. Worse, if there is some apparent boundary, we are likely to point to it and shout ‘that’s
it
!’ just because we can’t see anything else worth getting agitated about.
    How many times have you been in a discussion in which somebody says ‘We have to decide where to draw the line’?