  Infrared gravitons are continually produced during inflation. Like all
particles, their contribution to the vacuum energy comes not only from their
bare kinetic energy but also from the interactions they have with other
gravitons. These interactions can be substantial -- despite the particles being
highly infrared -- because they occur over the enormous spatial volume of the
universe. Furthermore, the interactions grow with time evolution because more
and more such gravitons come into causal contact with one another. Since
gravity is universally attractive, these interactions can act to slow and
eventually stop accelerated expansion.
