The stochastic gravitational wave background from cosmic superstrings

We study the stochastic gravitational wave background sourced by a network of cosmic superstrings and demonstrate that incorporating higher-mass string species, beyond the fundamental string, is crucial for accurately modeling the resulting gravitational wave spectrum across frequencies ranging from nanohertz to kilohertz. Using the multi-tension velocity-dependent one-scale model to evolve the cosmic superstring network, we perform several fits to the NANOGrav 15-year dataset and obtain expectation values for the fundamental string tension, string coupling and effective size of compact extra dimensions. We find that the cosmic superstring best-fits are comparable in likelihood to Supermassive Black Hole models, thought by many to be the leading candidate explanation of the signal. The implications of the best-fit spectra are discussed within the context of future gravitational wave experiments. We obtain expectation values for the fundamental string tension of log 10 (G μ 1 ) = -11.4 +0.3 -0.2 (-11.5 +0.3 -0.2 ) for gravitational waves originating from large cuspy (kinky) cosmic superstring loops and log 10 (G μ 1 )=-9.7 +0.7 -0.7 (-9.9 +1.0 -0.5 ) for small cuspy (kinky) loops. We also place 2σ upper bounds on the string coupling, finding g s < 0.7 in all cases, and comment on the implication of our results for the effective size of the compact extra dimensions.