Cosmic Ferromagnetism of Magninos
  We study the physical conditions for the occurrence of ferromagnetic
instability in a neutral plasma of fermions. We consider a system of two
species $M$ and $Y$ which are oppositely charged under a local $U(1)_{X}$, with
$M$ much lighter than $Y$. The leading correction to free quasiparticle
behaviour for the lighter species arises from the exchange interaction, while
the heavier species remain spectators. This plasma, which is abelian,
asymmetric and idealised, is shown to be naturally susceptible to the formation
of a completely spin-imbalanced ferromagnetic state for the lighter species
(dubbed a magnino) in large parts of parameter space. It is shown that the
domain structure formed by this ferromagnetic state can mimic Dark Energy,
determining the masses of the two fermion species involved, depending on their
abundance relative to the standard photons. Incomplete cancellation of the
X-magnetic fields among the domains can give rise to residual long range
$X$-magnetic fields. Under the assumption that this $U(1)_{X}$ mixes with
Maxwell electromagnetism, this provides a mechanism for the seed for
cosmic-scale magnetic fields. An extended model with several flavours $M_a$ and
$Y_a$ of the species can incorporate Dark Matter. Thus the scenario shows the
potential for explaining the large scale magnetic fields, and what are arguably
the two most important outstanding puzzles of cosmology: Dark Matter and Dark
Energy.
