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Title: Comment on "extended Born-Infeld theory and the bouncing magnetic universe"
Author: Garcia-Salcedo, R.
Gonzalez, T.
Horta-Rangel, A.
Quiros, I.
Issue Date: 2014
Abstract: In a recent paper [Phys. Rev. D 85, 023528 (2012)PRVDAQ1550-799810.1103/PhysRevD.85.023528] 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 show that (i) the conclusion about the inevitability of the asymptotic vacuum regime in a magnetic universe is wrong, (ii) a sudden curvature singularity - not better than the big bang - arises, but for spaces of positive curvature, (iii) the square sound speed can be a negative quantity, signaling instability against small perturbations of the background energy density, and (iv) the speed of sound can exceed the speed of light, which raises a causality issue. � 2014 American Physical Society.
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