Bogomol'nyi equations for Dirac-Born-Infeld cosmic string

TL;DR

This paper demonstrates that Dirac-Born-Infeld cosmic strings can admit BPS solutions with a self-consistent potential, revealing a connection to sine-Gordon models and providing exact first-order equations for these vortices.

Contribution

The authors show that BPS solutions exist for DBI cosmic strings with a specific potential, correcting previous assumptions and deriving exact equations using the BPS Lagrangian method.

Findings

BPS solutions exist for DBI strings with a self-consistent potential.

The BPS potential has a trigonometric form related to sine-Gordon.

BPS tension scales linearly with winding number, with an $oldsymbol{ ext{alpha}}$-dependent deformation.

Abstract

We revisit the question of whether Dirac-Born-Infeld (DBI) cosmic strings can admit Bogomol'nyi-Prasad-Sommerfield (BPS) configurations. Earlier work by Babichev et al. arXiv:0809.2013 concluded that DBI strings with the standard Mexican-hat potential possess no BPS limit, implying an unavoidable nonzero binding energy. In contrast, using the BPS Lagrangian method, we show that DBI strings do admit BPS solutions, provided the potential is chosen self-consistently. Imposing the existence of Bogomol'nyi equations uniquely determines the admissible potential and yields exact first-order BPS equations for DBI vortices. We independently verify the consistency of these equations using the stressless (vanishing-pressure) condition on the energy-momentum tensor. The resulting solutions saturate the Bogomol'nyi bound, exhibit zero binding energy, and smoothly recover the Nielsen-Olesen string in the limit $\alpha \to 0$. Regularity of the gauge-field equation requires $\alpha < \pi^2$. A notable outcome of the construction is that the BPS-compatible potential takes a trigonometric form closely related to the sine-Gordon potential, revealing a natural correspondence between the sine-Gordon string and the BPS DBI string. The BPS tension scales linearly with the winding number $n$ but acquires an $\alpha$-dependent deformation.