I always thought that cosmology was just a synonym for astronomy...

According to Dictionary.com:

1)The study of the physical universe considered as a totality of phenomena
in time and space.

a)The astrophysical study of the history, structure, and constituent
dynamics of the universe.

b)A specific theory or model of this structure and these dynamics.

Hmm, seems like it could be defined as part of astronomy to me.
Sci.astro.cosmo perhaps?