Are Therevada's cosmology and the Mahanaya's sutras physically impossible?
Buddhism is a definite, and it is the same Dhamma preached by all Buddhas. Popular science is always changing with new theory replacing old ones. But having said this some similar theories have emerged:


Parallel universe - Multiverse
Multiple worlds - Planetary habitability
Cyclical universe - Cyclic model, One Big Bang, or were there many?, Multiple Big Bang theory could bring 'Copernican revolution' says Astronomer Royal


Also might be of interest:


Loka: The Buddha’s Formulation of the Universe by Richard R. Crutcher
Parallel universes by Piya Tan