Relic gravitational waves from the chiral magnetic effect
  Relic gravitational waves (GWs) can be produced by primordial magnetic
fields. However, not much is known about the resulting GW amplitudes and their
dependence on the details of the generation mechanism. Here we treat magnetic
field generation through the chiral magnetic effect (CME) as a generic
mechanism and explore its dependence on the speed of generation (the product of
magnetic diffusivity and characteristic wavenumber) and the speed
characterizing the maximum magnetic field strength expected from the CME. When
the latter exceeds the former (regime I), the regime applicable to the early
universe, we obtain an inverse cascade with moderate GW energy that scales with
the third power of the magnetic energy. When the generation speed exceeds the
CME limit (regime II), the GW energy continues to increase without a
corresponding increase of magnetic energy. In the early kinematic phase, the GW
energy spectrum (per linear wavenumber interval) has opposite slopes in both
regimes and is characterized by an inertial range spectrum in regime I and a
white noise spectrum in regime II. The occurrence of these two slopes is shown
to be a generic consequence of a nearly monochromatic exponential growth of the
magnetic field. The resulting GW energy is found to be proportional to the
fifth power of the limiting CME speed and the first power of the generation
speed.
