A state in quantum field theory that is a local minimum but not the minimum energy state of the system overall (which is called the true vacuum). Tunnelling (see tunnel effect) occurs from the false vacuum to the true vacuum, which can be calculated using the semiclassical approximation. The false vacuum has never been observed but is predicted to exist in several quantum field theories of relevance to elementary particles, including grand unified theories. In particular, it has been suggested that the false vacuum could be responsible for the expansion in the inflationary universe, which could have been important in the early universe.