18 - The Physical Universe

Summary

Here, I consider our current view of the universe. I start with the Hubble Ultra-Deep Field, which shows about 10,000 galaxies in a tiny field of view. The whole of the observable universe contains over two trillion galaxies. I discuss two important principles regarding the nature of the universe and our place within it. The cosmological principle holds that the universe is homogeneous provided that we make comparisons at a high enough level of spatial scale. The Copernican principle maintains that our position within the universe is not central. We are certainly not central in the solar system or the galaxy; whether we are central in the universe is a tougher question to grapple with. We are at the centre of our own observable universe, but by definition any other observer is at the centre of theirs. We then turn from seeing galaxies in general to seeing individual events. These include long-known phenomena such as the Crab Nebula, which was produced by a supernova explosion. They also include more recently observed events such as collisions of neutron stars. We end by looking at the relative power of radio signals produced by biological and non-biological sources.