Cosmic String Production Towards the End of Brane Inflation

TL;DR

This work analyzes brane inflation in a superstring framework and argues that cosmic strings are copiously produced toward the end of inflation due to tachyon condensation during brane collision. The string tension is estimated from CMB anisotropy to be $G \mu \simeq 10^{-7}$, indicating the primordial fluctuations arise mainly from inflation with a ~10% contribution from cosmic strings. The initial string density is derived from tachyon dynamics and the Kibble mechanism, predicting a scaling cosmic string network that persists and leaves observable imprints. Observational consequences include a measurable string-induced component in the CMB and a stochastic gravitational-wave background detectable by LIGO/VIRGO and LISA, with pulsar timing offering additional constraints; crucially, the predicted signatures depend on the string scale $M_s$, making cosmological data a potential probe of the underlying brane-world string theory model.

Abstract

Towards the end of the brane inflationary epoch in the brane world, cosmic strings (but not texture, domain walls or monopoles) are copiously produced during brane collision. These cosmic strings are D$p$-branes with $(p-1)$ dimensions compactified. We elaborate on this prediction of the superstring theory description of the brane world. Using the data on the temperature anisotropy in the cosmic microwave background, we estimate the cosmic string tension $μ$ to be around $G μ\simeq 10^{-7}$. This in turn implies that the anisotropy in the cosmic microwave background comes mostly from inflation, but with a component (of order 10%) from cosmic strings. This cosmic string effect should also be observable in gravitational wave detectors and maybe even pulsar timing measurements. Keywords : Inflation, Brane World, Superstring Theory, Cosmic String, Cosmology

Figures (2)

• Figure 1: Collision of Branes at angle $\theta$. In the left figures, the two branes wrap around different 1-cycles of a torus with sides $\ell_\parallel$ and $u\ell_\parallel$. For small $u$, $\theta \simeq 2u$. They are separated in compactified directions orthogonal to the torus. When that separation approaches zero, they collide, resulting in two branes. The resulting brane tensions are cancelled by orientifold planes in the model. In the right figures, when the angle $\theta$ is close to $\pi$, annihilation takes place.
• Figure 2: A schematic picture for the Kibble Mechanism. The two large directions represent the uncompactified dimensions while the vertical direction represents a compactified direction that the $p$-branes wrap around. The arrows indicate the phase value of $T$ for a non-trivial vacuum configuration.