Another way to ask is if I can go to my PhD in Cosmology through Astrophysics. Many people will thought that obviously yes, but notice Cosmology isn't a branch of Astrophysics, but of Theoretical Physics. Now you'll think this is a completely nonsense, so that Astrophysics is very similar to Cosmology. But think for a moment. Almost every Astrophysicist, who isn't also a Cosmologist, does not touch this field in its papers, but every Cosmologist is a Theoretical Physicist, and many aren't Astrophysicists. So, can I go to Cosmology into a PhD un Astrophysics?