Cosmic microwave anisotropies from BPS semilocal strings

TL;DR

This paper delivers the first CMB anisotropy calculations for semilocal strings using full field-theory simulations in the BPS limit, showing these non-topological defects produce smaller CMB amplitudes than Abelian-Higgs strings and resemble textures in spectral shape. By decomposing defect energy-momentum correlators into a small set of scaling functions and feeding them into a modified Boltzmann code, the authors compare TT, TE, EE, and BB spectra across semilocal strings, Abelian-Higgs strings, and textures, deriving updated constraints on $G\mu$ and the fractional defect contribution $f_{10}$. The results indicate semilocal strings permit higher $G\mu$ bounds than Abelian-Higgs strings and remain compatible with current data, though not decisively favored or excluded; polarization signals, especially the BB spectrum, offer a potential observational discriminator. Overall, the work broadens the landscape of defect-related CMB predictions and informs high-energy inflation model building by showing semilocal strings can loosen tension on symmetry-breaking scales while preserving testable observational signatures.

Abstract

We present the first ever calculation of cosmic microwave background CMB anisotropy power spectra from semilocal cosmic strings, obtained via simulations of a classical field theory. Semilocal strings are a type of non-topological defect arising in some models of inflation motivated by fundamental physics, and are thought to relax the constraints on the symmetry breaking scale as compared to models with (topological) cosmic strings. We derive constraints on the model parameters, including the string tension parameter mu, from fits to cosmological data, and find that in this regard BPS semilocal strings resemble global textures more than topological strings. The observed microwave anisotropy at l = 10 is reproduced if Gmu = 5.3x10^{-6} (G is Newton's constant). However as with other defects the spectral shape does not match observations, and in models with inflationary perturbations plus semilocal strings the 95% confidence level upper bound is Gmu<2.0x10^{-6} when CMB data, Hubble Key Project and Big Bang Nucleosynthesis data are used (c.f. Gmu<0.9x10^{-6} for cosmic strings). We additionally carry out a Bayesian model comparison of several models with and without defects, showing models with defects are neither conclusively favoured nor disfavoured at present.

Figures (13)

• Figure 1: Variation of $r_{\mathrm{min}}$ and the parameters $e$ and $\lambda$ for the $s=0.3$ simulations, which mimic the one used in Ref. Bevis:2006mj. The subscript 0 denotes the value at the end of the simulation.
• Figure 2: Average value of $\xi$, as defined in Eq. (\ref{['defxi']}), for $s=0.0,\,0.3,\,1.0$, in the radiation era. The average is over five different realizations for each value of $s$, and the shaded regions correspond to 1-$\sigma$ and 2-$\sigma$ deviations. The best-fit line for times $\tau>50$ is also shown (dashed line). Note that the best-fit line is an excellent approximation, and that the differences between different values of $s$ are minimal.
• Figure 3: The dependence of the 5 etcs upon $s$ in the radiation era at $\tau_{\rm sim}=128\eta^{-1}$. The shaded regions show the 1-$\sigma$ and 2-$\sigma$ uncertainties for the $s=0.3$ case, while the lines indicate the $s=1.0$ (solid), $s=0.0$ (dot-dash) results, which clearly lie within the statistical uncertainties.
• Figure 4: The equal-time scaling function $\tilde{C}^{\mathrm{S}}_{11}(k\tau,k\tau)$ averaged over five realizations for $s=0.3$ in the radiation era. The different lines correspond to roughly uniformly spaced $\tau$ values in the range $50\eta^{-1}<\tau<128\eta^{-1}$. The shaded regions show the 1-$\sigma$ and 2-$\sigma$ uncertainties in the mean indicated for the latest time (solid). For visualization purposes, the mean offset across the five realizations is used, whereas the actual CMB calculations use independent offsets for each realization.
• Figure 5: CMB temperature power spectrum from semilocal string simulations. The solid black line is the total prediction, which is obtained by adding the contribution of scalar (dash-dot red), vector (thin blue) and tensor (dashed green) modes.