Quantum Field Theory In Curved Spacetime: Quant... Info

: General curved backgrounds lack global Poincaré invariance and time-translation symmetry, making it impossible to define a unique, preferred vacuum state.

To relate the perspectives of different observers or the state of fields at different times in an expanding universe, physicists use : Field Decomposition : A scalar field is expanded into a set of basis modes with creation and annihilation operators Quantum Field Theory in Curved Spacetime: Quant...

) will contain a non-zero number of particles according to the second observer. 3. Key Phenomena Key Phenomena Quantum Field Theory in Curved Spacetime

Quantum Field Theory in Curved Spacetime (QFTCS) is a "hybrid" framework where matter fields are treated according to quantum principles, but the gravitational background is described classically by a fixed spacetime metric gμνg sub mu nu end-sub these "crutches" disappear:

In flat (Minkowski) spacetime, Poincaré invariance provides a unique vacuum state and a global definition of "particles". In curved spacetime, these "crutches" disappear: