  In the Hamiltonian language we provide a study of flat-space cosmology in
bigravity and massive gravity constructed mostly with de Rham, Gabadadze,
Tolley (dRGT) potential. It is demonstrated that the Hamiltonian methods are
powerful not only in proving the absence of the Boulware-Deser ghost, but also
in solving other problems. The purpose of this work is to give an introduction
both to the Hamiltonian formalism and to the cosmology of bigravity. We sketch
three roads to the Hamiltonian of bigravity with the dRGT potential: the
metric, the tetrad and the minisuperspace approaches.
