: The concept of a "particle" becomes local and observer-dependent. Different observers (e.g., one inertial and one accelerating) may disagree on whether a state contains particles or is a vacuum.
Quantum Field Theory in Curved Spacetime: Quantized Fields and Semiclassical Gravity Quantum Field Theory in Curved Spacetime: Quant...
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 : The concept of a "particle" becomes local
In flat (Minkowski) spacetime, Poincaré invariance provides a unique vacuum state and a global definition of "particles". In curved spacetime, these "crutches" disappear: In curved spacetime, these "crutches" disappear: : General
: General curved backgrounds lack global Poincaré invariance and time-translation symmetry, making it impossible to define a unique, preferred vacuum state.
is a linear combination of both the old annihilation and creation operators:
: In the early, rapidly expanding universe, time-varying gravitational fields can "excite" the vacuum, creating elementary particles that seed the large-scale structure of the universe. Robert Wald - Quantum Field Theory in Curved Spacetime