Noelia Sanchez Gonzalez

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.

We study the string theory dynamics of the volume scalar rolling down an exponential potential during the period between inflation and reheating, in a background of cosmic superstring loops. In the context of the LVS potential, we demonstrate the existence of a novel string loop attractor tracker solution, in which 75% of the energy density of the universe is in the form of a gas of fundamental cosmic superstring loops (a configuration preferred over the standard radiation tracker). On this tracker, it is the continual reduction in the string tension as the volume scalar evolves that makes the loops stable against decay. For more general non-LVS potentials, mixed radiation-loop trackers can also occur.

We describe a new mechanism, whose ingredients are realized in string compactifications, for the formation of cosmic (super)string networks. Oscillating string loops grow when their tension μ decreases with time. If 2H+μ'/μ<0, where H
is the Hubble parameter and μ' denotes the time derivative of the tension, loops grow faster than the scale factor and an initial population of isolated small loops (for example, produced by nucleation) can grow, percolate, and form a network. This condition is satisfied for fundamental strings in the background of a kinating volume modulus rolling toward the asymptotic large volume region of moduli space. Such long kination epochs are motivated in string cosmology by both the electroweak hierarchy problem and the need to solve the overshoot problem. The tension of such a network today is set by the final vacuum; for phenomenologically appealing large volume scenario vacua, this would lead to a fundamental string network with Gμ∼10^(-10).