  We study strong types and Galois groups in model theory from a topological
and descriptive-set-theoretical point of view, leaning heavily on topological
dynamical tools. More precisely, we give an abstract (not model theoretic)
treatment of problems related to cardinality and Borel cardinality of strong
types, quotients of definable groups and related objecets, generalising (and
often improving) essentially all hitherto known results in this area. In
particular, we show that under reasonable assumptions, strong type spaces are
"locally" quotients of compact Polish groups. It follows that they are smooth
if and only if they are type-definable, and that a quotient of a type-definable
group by an analytic subgroup is either finite or of cardinality at least
continuum.
