Can topological defects mimic the BICEP2 B-mode signal?
  We show that the B-mode polarization signal detected at low multipoles by BICEP2 cannot be entirely due to topological defects. This would be incompatible with the high-multipole B-mode polarization data and also with existing temperature anisotropy data. Adding cosmic strings to a model with tensors, we find that B-modes on their own provide a comparable limit on the defects to that already coming from Planck satellite temperature data. We note that strings at this limit give a modest improvement to the best-fit of the B-mode data, at a somewhat lower tensor-to-scalar ratio of $r \simeq 0.15$. 
Joanes Lizarraga, Jon Urrestilla, David Daverio, Mark Hindmarsh,   Martin Kunz and Andrew R. Liddle
astro-ph.CO hep-ph hep-th