  It is widely accepted that the primordial universe experienced a brief period
of accelerated expansion called inflation. This scenario provides a plausible
solution to the horizon and flatness problems. However, the particle physics
mechanism responsible for inflation remains speculative with, in particular ,
the assumption of a scalar field called inflaton. Furthermore, the comparison
with the most recent data raises new questions that encourage the consideration
of alternative hypotheses. Here, we propose a completely different scenario
based on a mechanism whose origins lie in the nonlin-earities of the Einstein
field equations. We use the analytical results of weak gravitational wave
turbulence to develop a phenomenological theory of strong gravitational wave
turbulence where the inverse cascade of wave action plays a key role. In this
scenario, the space-time metric excitation triggers an explosive inverse
cascade followed by the formation of a condensate in Fourier space whose growth
is interpreted as an expansion of the universe. Contrary to the idea that
gravitation can only produce a decelerating expansion, our study reveals that
gravitational wave turbulence could be a source of inflation. The fossil
spectrum that emerges from this scenario is shown to be in agreement with the
cosmic microwave background radiation measured by the Planck mission.
