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Centro de Astrofísica da Universidade do Porto

Physical and invariant models for defect network evolution

C. J. A. P. Martins, M. M. P. V. P. Cabral

We revisit the velocity-dependent one-scale model for topological defect evolution, and present a new alternative formulation in terms of a physical (rather than invariant) characteristic length scale. While the two approaches are equivalent (as we explicitly demonstrate), the new one is particularly relevant when studying the evolution of ultra-relativistic defects. Moreover, a comparison of the two provides further insight on the interpretation of the model's two phenomenological parameters, c related to energy losses and k related to the curvature of the defects. As an illustration of the relevance of the new formulation, we use it to study the evolution of cosmic string and domain wall networks in contracting universes. We show that these networks are ultra-relativistic and conformally contracted, with the physical length scale behaving as Lph∝a and the density as ρ∝a−4 (as in a radiation fluid) in both cases. On the other hand the velocity and invariant length respectively behave as (γv)∝a−n and Linv∝a44−n, where n is the number of dimensions of the defect's worldsheet. Finally we also study an alternative friction-dominated scenario and show that the stretching and Kibble regimes identified in the case of expanding universes can also occur for contracting ones.

Physical Review D
Volume 93
February 2016

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Institute of Astrophysics and Space Sciences

Institute of Astrophysics and Space Sciences (IA) is a new but long anticipated research infrastructure with a national dimension. It embodies a bold but feasible vision for the development of Astronomy, Astrophysics and Space Sciences in Portugal, taking full advantage and fully realizing the potential created by the national membership of the European Space Agency (ESA) and the European Southern Observatory (ESO). IA resulted from the merging the two most prominent research units in the field in Portugal: the Centre for Astrophysics of the University of Porto (CAUP) and the Center for Astronomy and Astrophysics of the University of Lisbon (CAAUL). It currently hosts more than two-thirds of all active researchers working in Space Sciences in Portugal, and is responsible for an even greater fraction of the national productivity in international ISI journals in the area of Space Sciences. This is the scientific area with the highest relative impact factor (1.65 times above the international average) and the field with the highest average number of citations per article for Portugal.

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