High-frequency Gravitational Waves from Superstring Phases in the Early Universe

Abstract: When moduli roll in the early universe, all physical scales - including string tensions - simultaneously evolve. The dynamics of cosmic string loops with time-varying tension can produce cosmic string loop trackers in which most of the energy density of the universe lies in the form of string loops. This solution can exist as an attractor until the rolling modulus reaches its minimum, when the loops ultimately decay through gravitational wave emission. We explore the spectrum of gravitational waves produced by such string loop trackers. The resulting spectrum is high-frequency and peaks in the GHz regime today. The amplitude of the signal is diluted by any subsequent matter-dominated epochs, and thus the potential observability of the signal crucially depends on the duration of the moduli-dominated epoch that follows once the moduli settle down and oscillate about their minimum.