The effective energy density of relic gravitons

The energy density, the pressure and the anisotropic stress of the relic gravitons do not have a unique gauge-invariant and frame-invariant expression since the equivalence principle ultimately forbids the localization of the energy-momentum of the gravitational field. All the strategies proposed so far suggest compatible descriptions inside the effective horizon but they lead to sharply different answers when the wavelengths of the gravitons exceed the Hubble radius by remaining shorter than the typical extension of causally connected patches. We present a coherent discussion of the energy-momentum pseudotensors of the relic gravitons in the different kinematical regimes with the aim of scrutinizing the mutual consistency of the competing suggestions. The various proposals are systematically compared by deducing and analyzing the explicit form of the observables in realistic physical situations. General lessons are drawn on the most plausible parametrization of the energy-momentum pseudotensor of the relic gravitons.