Embedded Structure in Quantum Theory, Functional Operators and the Multiverse

Abstract We explore a wider theoretical framework that has quantum field theory built in, taking the fact that quantum mechanics is reconstructed from quantum field theory as a hint. We formulate a quantum theory with an embedded structure by introducing functional operators, and we find that it could describe the level II multiverse. Topics related to the beginning of the universe such as inflation, the third quantization, and the landscape are discussed in our formulation.