# How does the expanding universe affect quantum fields?

The universe is expanding. It then seems logical to say that the QM fields are expanding as the universe expands. My question is how does this happen? When I consider an expanding field it forces me to consider the actual properties of the field. Are the QM fields becoming less dense or are these fields only mathematical constructs that enable predicts to be made. If the fields are not only mathematical then does their expansion require the creation of new field “stuff” to fill in the new space being created. QM mathematics is way out of my skill level so an answer that respects my curiosity and understands my limitations is appreciated.

• There's a good discussion of this in Birrell & Davies, in which a plane-wave solution evolves in a universe with $a^2=A+B\tanh\rho\eta$. – J.G. Jan 21 '18 at 21:07