ABSTRACT
In this chapter I will describe the Universe at the present epoch. My account has to begin with a disclaimer. We don’t know about the whole Universe. Instead, we know only about the part of the Universe that we can see — the observable Universe. The boundary of the observable Universe is a sphere around us, called the horizon.1 This limitation on our knowledge is not as serious as one might think though, because the observable Universe is almost homogeneous. To be precise, the properties of a region containing many galaxies are more or less independent of the location of that region. It is reasonable to expect that the Universe remains homogeneous as we go beyond the horizon, for at least some distance. If that expectation is satisfied, you can take ‘Universe’ to mean, not just the observable Universe but the entire homogeneous patch around us. In this matter, we are in a position similar to a sailor at sea. The sailor may reasonably expect that the sea extends beyond the horizon in all directions, but that could be wrong.
