Abstract
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 nonlinearities 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 strong 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. Direct numerical simulations can be used to check our predictions and to investigate the question of non-Gaussianity through the measure of intermittency.
Highlights
Understanding the origin of the universe—before or around the Planck time τP ∼ 10−43 s—is currently out of reach because it would require using a quantum theory of gravity that remains to be built
As we show in Appendix A, the rate of such an expansion for the statistically homogeneous and isotropic gravitational wave (GW) fields is sufficient to produce the GW dilution which overpowers the wave–wave interactions if the wave phases are random
The paper is structured as follows: Section 2 is devoted to the Friedmann equations in order to recall the assumptions of the basic model and notations; in Section 3 we briefly present the analytical results of weak GW turbulence published recently and deduce the phenomenological theory of strong GW turbulence; the formation of a condensate and its interpretation in terms of inflation is discussed in Section 4; in Section 5 we show that our prediction is in good agreement with the cosmic microwave background (CMB)
Summary
Understanding the origin of the universe—before or around the Planck time τP ∼ 10−43 s—is currently out of reach because it would require using a quantum theory of gravity that remains to be built. The paper is structured as follows: Section 2 is devoted to the Friedmann equations in order to recall the assumptions of the basic model and notations; in Section 3 we briefly present the analytical results of weak GW turbulence published recently and deduce the phenomenological theory of strong GW turbulence; the formation of a condensate and its interpretation in terms of inflation is discussed in Section 4; in Section 5 we show that our prediction is in good agreement with the CMB measured by the Planck mission; in the last section we conclude with a summary and a discussion. A consideration of the GW dilution in an expanding universe, where the expansion is caused by the GW themselves is presented in Appendix A
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