Cosmological Inflation Why is the Universe so large, so old, so homogeneous (=the same in all places) and so isotropic (= the same in all directions)? None of these facts is explained by conventional Big Bang cosmology. The explanation proposed by cosmological inflation is that, at some very early stage in its history, the entire visible Universe underwent a brief period of very rapid expansion. This would certainly explain why the Universe is so large, and would also suggest that the overall density of the Universe is very close to the "critical" value required to reverse its expansion, enabling it to live to a very old age. For the same reason, the Universe would be almost homogeneous and isotropic, as suggested by the cosmic microwave radiation, in particular. Energy in empty space? The exponential expansion might have been due to a form of energy in otherwise empty space, that might be related to the Higgs mechanism thought to be responsible for the masses of the particles. This mechanism predicts the existence of one or more Higgs particles, that are being sought actively at particle accelerators. Models of cosmological inflation predict that the Universe should not be exactly homogeneous and isotropic. Rather, small variations in the rate of its expansion, predicted by quantum mechanics, would have created small variations in its density. These may have been seen as the ripples in the Cosmic Background Radiation that are thought to have seeded the formation of galaxies and stars.