In the last couple of lectures we shall consider basic **cosmology**. This will require extending our knowledge slightly to a small piece of the *general* theory of relativity, thus allowing us to look at more interesting versions of space-time than the **flat**, feature-less space-time of *special* relativity.

Space-time in general relativity is active, **curved** and allows for much more phenomena to occur and allows us to consider possible evolutions of the universe.

We state the four assumptions that determine the cosmology of our universe, namely that space (not including time) is homogeneous, isotropic, flat and expands over time.

From those assumptions we develop an appropriate coordinate system and associated metric, and in this context derive **Hubble's constant**.