Abstract
Abstract
We compute the evolution of the entanglement entropy for a massless field within a
spherical region throughout the inflationary period and the subsequent era of radiation
domination, starting from the Bunch-Davies vacuum. In order to focus on the entanglement of modes
that are directly accessible to observations, we impose an ultraviolet cutoff set by the
wavelength of the last mode that exited the horizon at the end of inflation. The transition of
each mode towards a squeezed state upon horizon exit during inflation and the additional squeezing
when radiation domination sets in enhance the entanglement entropy. Shortly after the transition
to the radiation-dominated era, a volume term develops and becomes the leading contribution to the
entropy at late times, as is common for systems lying in squeezed states. We estimate the
magnitude of the entropy and discuss its interpretation in the light of the quantum to classical
transition for modes exiting the horizon during inflation. Our results raise the possibility that
the quantum nature of weakly interacting fields, such as gravitational waves resulting from tensor
modes during inflation, may be detectable in today's universe. On the other hand, an observer with
no knowledge of the degrees of freedom beyond the horizon would interpret the entropy as
thermal. From this point of view, the reheating after inflation would be a result of
quantum entanglement.