Gowdy spacetimes (or universes) are simple expanding model universes. In contrast with the better known Friedmann-Lemaître-Robertson-Walker universes (the basis for the big bang models), Gowdy universes are not homogeneous. Instead, they are filled with a regular pattern of gravitational waves. A Gowdy T3 universe is the simplest kind of Gowdy universe, in which space has the shape of a three-dimensional torus.
More information about Gowdy universes can be found in the spotlight text Of gravitational waves and spherical chickens.