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Accueil > A propos du LPP > Communication > Actualités archivées > 2017 > Turbulence of weak gravitational waves in the early Universe
Turbulence of weak gravitational waves in the early Universe
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We are currently experiencing a revolution in astrophysics with the multiplication of direct observations of gravitational waves produced during cataclysmic events such as the merger of black holes or neutron stars. This new astronomy promises us many surprising discoveries in the coming months on the functioning of our near or far Universe.
What about the theoretical study of gravitational waves ? We have just published an article (Galtier & Nazarenko, Phys Rev. Lett 119 (21), 2017) in which we have studied the nonlinear behavior of a stochastic set of weak amplitude gravitational waves. This situation is relevant in extreme cases such as the environment of black holes or the very primordial Universe : indeed, around 10-36s a symmetry breaking of the GUT (Grand-Unified-Theory) is expected which, according to some scenarios, could lead to a first-order phase transition and the generation of vacuum bubbles (see figure) ; the collisions of these bubbles would be a potent source of gravitational waves. In this work, we have obtained - by a rigorous mathematical development - the equations of gravitational wave turbulence as well as their exact solutions. Our calculations are based on the equations of general relativity of A. Einstein who are supposed to be valid beyond the Planck’s time (10-43s).
We show that an initial forcing of the space-time around the wavenumber kF leads to the excitation of fluctuations of the space-time metric at wavenumbers larger and smaller than kF. In the first case, the direct cascade towards small scales is limited by the Planck’s scale under which quantum gravity dominates. We show that in the second case, the inverse cascade is explosive with in principle the possibility to excite fluctuations of the space-time metric from kF to k = 0 (infinite scale) in a finite time. The mechanism however stops at the scale where turbulence becomes strong. This inverse cascade provides an efficient mechanism for homogenizing primordial fluctuations.
At a time where many cosmological questions remain open (the origins of dark energy, dark matter or cosmological inflation), nonlinear physics - highly developed in plasma physics but much less in general relativity - could bring original and surprising answers.
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