Forecast constraints on cosmic string parameters from gravitational wave direct detection experiments

Gravitational waves (GWs) are one of the key signatures of cosmic strings. If GWs from cosmic strings are detected in future experiments, not only their existence can be confirmed, but also their properties might be probed. In this paper, we study the determination of cosmic string parameters through direct detection of GW signatures in future ground-based GW experiments. We consider two types of GWs, bursts and the stochastic GW background, which provide us with different information about cosmic string properties. Performing the Fisher matrix calculation on the cosmic string parameters, such as parameters governing the string tension $G\mu$ and initial loop size $\alpha$ and the reconnection probability $p$, we find that the two different types of GWs can break degeneracies in some of these parameters and provide better constraints than those from each measurement.

Figure 1

The values of $\gamma$ plotted as a function of $p$ for the radiation- and matter-dominated era.

Figure 2

The rate of GW burst $dR/d\ln h$ as a function of $fh$, for $G\mu ={10}^{-7}$, $\alpha ={10}^{-16}$, $p=1$, and $f=220\mathrm{Hz}$, represented by the black thick line. Bursts in the left region (orange) form the GW background and those in the right region (blue) are observed as an isolated burst. The red, green, and purple dotted lines represent the contribution from $1\le z\le 10$, $0.1\le z\le 1$, and ${10}^{-2}\le z\le 0.1$, respectively.

Figure 3

The burst rate $dR/d\ln h$ in terms of $fh$ for various parameter sets.

Figure 4

The background spectrum ${\Omega }_{\mathrm{GW}}$ versus present frequency $f$ for various parameter sets.

Figure 5

The parameter regions excluded by current experiments and cosmological constraints, as well as regions which can be probed by Advanced LIGO, for different values of $p$. The colored regions are excluded by current experiments. Advanced LIGO can probe the region above the yellow solid line with GW background search and that above the yellow dotted line with burst search.

Figure 6

Marginalized $2\sigma$ constraints on cosmic string parameters in $p-\alpha$, $\alpha -G\mu$, and $p-G\mu$ planes, respectively. The fiducial parameters are taken to be $G\mu ={10}^{-7}$, $\alpha ={10}^{-16}$, $p=1$. The solid black line represents the constraints from the burst detection alone, and the red line represents the combined constraints from the burst detection and the GW background measurement.