Comment on "Extended Born-Infeld theory and the bouncing magnetic
  universe"
  In a recent paper [Phys. Rev. D{\bf 85}, 023528 (2012)] the authors proposed
a generalized Born-Infeld electrodynamics coupled to general relativity which
produces a nonsingular bouncing universe. For a magnetic universe the resulting
cosmic evolution inevitably interpolates between asymptotic de Sitter states.
Here we shall show that (i) the above theory does not have the standard weak
field Maxwell limit, (ii) a sudden curvature singularity -- not better than the
big bang -- arises, (iii) the speed of sound squared is a negative quantity
signaling instability against small perturbations of the background energy
density, and that (iv) the conclusion about the inevitability of the asymptotic
vacuum regime in a magnetic universe is wrong.
