But R. M. Wald describes how particles in curved spacetime is not a well defined concept - that fields are defined in curved spacetime but not particles. This seems similar to the Higgs mechanism. Is it the same thing? Matter is not defined when the universe's spacetime was very curved and expanding very fast. Then when spacetime flattens out enough, particles appear, with mass. Could the "undefinedness" of particles in curved space be the "unstable" quantum field in the Higgs mechanism? Both seem to have a field to begin with and then particles appear after some flatness is achieved.

Are these concepts also related to spinfoam theories where there seems to be a QFT defined which includes spacetime in the Feynman type graphs as though it were like any other type of particle? What circumstances give a higher expectation value for this spinfoam theory to produce more spacetime and not particles? Is this correspond to a more highly curved spacetime or faster expanding spacetime?