David Daverio

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 ≃ 0.15.

General relativity and cosmic structure formation

When general relativity is included in large-scale simulations of the cosmic structure of the Universe, relativistic effects turn out to be small but measurable, thus providing tests for models of dark matter and dark energy.

gevolution: a cosmological N-body code based on General Relativity

We present a new N-body code, gevolution, for the evolution of large scale structure in the Universe. Our code is based on a weak field expansion of General Relativity and calculates all six metric degrees of freedom in Poisson gauge. N-body particles are evolved by solving the geodesic equation which we write in terms of a canonical momentum such that it remains valid also for relativistic particles. We validate the code by considering the Schwarzschild solution and, in the Newtonian limit, by comparing with the Newtonian N-body codes Gadget-2 and RAMSES. We then proceed with a simulation of large scale structure in a Universe with massive neutrinos where we study the gravitational slip induced by the neutrino shear stress. The code can be extended to include different kinds of dark energy or modified gravity models and going beyond the usually adopted quasi-static approximation. Our code is publicly available.

Constraining topological defects with temperature and polarization anisotropies

We analyse the possible contribution of topological defects to cosmic microwave anisotropies, both temperature and polarisation. We allow for the presence of both inflationary scalars and tensors, and of polarised dust foregrounds that may contribute to or dominate the B-mode polarisation signal. We confirm and quantify our previous statements that topological defects on their own are a poor fit to the B-mode signal. However, adding topological defects to a models with a tensor component or a dust component improves the fit around

Scaling from gauge and scalar radiation in Abelian-Higgs string networks

\ell=200

New CMB constraints for Abelian Higgs cosmic strings

. Fitting simultaneously to both temperature and polarisation data, we find that textures fit almost as well as tensors (

Energy-momentum correlations for Abelian Higgs cosmic strings

\Delta\chi^2 = 2.0

Fitting BICEP2 with defects, primordial gravitational waves and dust

), while Abelian Higgs strings are ruled out as the sole source of the B-mode signal at low

General Relativistic

\ell

N

. The 95% confidence upper limits on models combining defects and dust are