6.3.2 The cosmos
Next time there is a clear dark night, look up at the sky. You will see the stars, of which the Sun is one, that make up our galaxy. Our galaxy, the Milky Way, contains about 100 billion stars and belongs to a cluster of similar galaxies we call the Local Group. The Local Group is an outlying part of a huge cluster of galactic groups called the Virgo Supercluster. On a larger scale still, the universe seems to consist of billions of superclusters strung out through space in huge filaments, with immense voids between them as depicted in Figure 33.
How did the universe get that way? How did all this arise?
One approach to this problem is, again, through simulation. Cosmologists have good evidence that space, time, matter and energy began about 15 billion years ago in a gigantic explosion, the ‘Big Bang’. They also have a good idea of what conditions were like from a few seconds after the Big Bang to the time when the cosmos was about 300,000 years old. Things were quite simple then. The cosmos consisted of:
matter (mostly hydrogen);
‘dark’ matter (no one knows what this is yet – only that it is there);
gravitational and radiation energy.
In the 1990s, the US National Science Foundation's Grand Challenge Cosmology Consortium (GC3) produced a number of simulations of the early universe based on this understanding. All of them followed our familiar broad strategy: division into parts, measuring quantities in each part (sampling) and then mapping to numbers (quantisation). For example, a team led by Michael Norman at the NCSA modelled a cubic section of the universe 340 million light years wide, at about 1 billion years after the Big Bang. They divided the space up into over a hundred million cubes and assigned values for the density of matter, dark matter, the temperature, and so on to points inside each cube. Other models using the same strategy are of earlier eras, going back into the first three minutes of time.
What do these models, of climate or of the cosmos, tell us? Well, nothing if they are just used as a static, simplified picture – inside the boundary – of how things are, or were. But if we set them into motion, they tell us about past, present and future.