Introduction to Cosmic F- and D-Strings

TL;DR

The work analyzes whether cosmic strings and cosmic superstrings can arise after inflation, persist cosmologically, and be observable. It combines gauge-string reviews with string-theory constructions, showing that warped-brane inflation can produce F- and D-strings (and bound states) with tunable tensions $\mu$ via warp factors, yielding observable gravitational signatures such as cusp bursts in the LIGO/LISA bands. It highlights production mechanisms, stability constraints, and distinctive network properties—especially reconnection probabilities $P$ and $(p,q)$ junctions—that could distinguish superstrings from gauge strings. The results suggest a realistic path for testing string theory through cosmological and gravitational-wave observations, potentially linking microphysics to macroscopic cosmic phenomena.

Abstract

In these lectures I discuss the possibility that superstrings of cosmic length might exist and be observable. I first review the original idea of cosmic strings arising as gauge theory solitons, and discuss in particular their network properties and the observational bounds that rule out cosmic strings as the principal origin of structure in our universe. I then consider cosmic superstrings, including the `fundamental' F-strings and also D-strings and strings arising from wrapped branes. I discuss the conditions under which these will exist and be observable, and ways in which different kinds of string might be distinguished. We will see that each of these issues is model-dependent, but that some of the simplest models of inflation in string theory do lead to cosmic superstrings. Moreover, these could be the first objects seen in gravitational wave astronomy, and might have distinctive network properties. The outline of these lectures follows hep-th/0410082, but the treatment is more detailed and pedagogical.

Figures (3)

• Figure 2: Cosmic string simulation. A side of the cube is around a third of the horizon length. Strings that appear to end are just leaving the simulation volume. From Allen and Shellard Allen:1990tv.
• Figure 3: Instabilities of macroscopic strings: a) Confinement by a domain wall. b) Breakage.
• Figure 4: Gravitational wave cusp signals, taken from Damour and Vilenkin Damour:2001bk. The horizontal axis is $\log_{10} \alpha$ where $\alpha = 50G\mu$. Thus the brane inflation range $10^{-12} \,{\stackrel{<}{{_\sim}}}\, G\mu \,{\stackrel{<}{{_\sim}}}\, 10^{-6}$ becomes $-10.3 < \log_{10} \alpha < -4.3$. The vertical axis is $\log_{10} h$ where $h$ is the gravitational strain in the LIGO frequency band. The upper and lower dashed horizontals are the sensitivities of LIGO I and Advanced LIGO at one event per year. The upper two curves are the cusp signal under optimistic and pessimistic network assumptions; the pessimism is that a large number of kinks may suppress the cusps. The lowest solid curve is the signal from kinks, which form whenever strings reconnect. The dashed curve is the stochastic signal.