Topological Defects in Contracting Universes

We study the behavior and consequences of cosmic string networks in contracting universes. They approximately behave during the collapse phase as radiation fluids. Scaling solutions describing this are derived and tested against high-resolution numerical simulations. A string network in a contracting universe, together with the gravitational radiation it generates, can affect the dynamics of the universe both locally and globally and be an important source of radiation, entropy, and inhomogeneity. We discuss possible implications for bouncing and cyclic models.

Figure 1

Cosmic string network properties in the expansion and contraction phases, as a function of conformal time (such that maximal expansion occurs at $\eta =1$ and ${\eta }_{\mathrm{c}\mathrm{r}\mathrm{u}\mathrm{n}\mathrm{c}\mathrm{h}}=2$). The solid line is a simulation in the matter era; the dashed line is radiation. Plotted are the rescaled correlation length $L/t$ (top left), the scaling law of the correlation length $L\propto a^{\beta }$ (top right), the velocity of the long string network (middle left), the ratio of the loop and long string velocities (middle right), the fraction of the string energy in the form of long strings (bottom left), and the total number of loops (bottom right).