CMB polarization power spectra contributions from a network of cosmic strings

TL;DR

This work presents the first polarization CMB calculation from a local cosmic string network by performing Abelian Higgs field simulations to resolve string microphysics and radiative decay. Using unequal-time energy–momentum correlations decomposed into a small set of scaling functions, the authors couple the string network to a Boltzmann solver and compare against inflationary components, finding a shift of polarization power to larger angular scales and a B-mode signal dominated by vector modes. Normalizing to current bounds, they demonstrate that future experiments like CLOVER could either detect strings or tighten the bound to $G\mu<0.12\times10^{-6}$, with BB offering particularly strong constraints. The results also clarify quantitative differences from previous unconnected-segment models and contrast local strings with global textures, highlighting the observational leverage of polarization measurements for probing early-universe physics and string theory scenarios.

Abstract

We present the first calculation of the possible (local) cosmic string contribution to the cosmic microwave background polarization spectra from simulations of a string network (rather than a stochastic collection of unconnected string segments). We use field theory simulations of the Abelian Higgs model to represent local U(1) strings, including their radiative decay and microphysics. Relative to previous estimates, our calculations show a shift in power to larger angular scales, making the chance of a future cosmic string detection from the B-mode polarization slightly greater. We explore a future ground-based polarization detector, taking the CLOVER project as our example. In the null hypothesis (that cosmic strings make a zero contribution) we find that CLOVER should limit the string tension μto Gμ<0.12e-6 (where G is the gravitational constant), above which it is likely that a detection would be possible.

Figures (5)

• Figure 1: A snapshot from an Abelian Higgs simulation in the matter era at a time when the horizon volume approximately fills the simulation box. The lines show the centers of the strings (found using the gauge-invariant phase-winding method of Kajantie:1998bg) while the upper and lower surfaces highlight the additional presence of radiative decay, which must be included in an adhoc manner in Nambu-Goto simulations. The lower surface indicates regions of significant energy density due to the non-vacuum value of the Higgs field while the upper surface shows regions of significant energy from the quasi-magnetic field in the model (see Bevis:2006mj). Note, however, that the strings themselves make the primary contribution to both of these types of energy and the contrast is chosen to highlight the radiation contribution. For example, the circular pattern seen on the left in these slices is due to the recent collapse of a string loop just above the bottom of the simulation (and is seen in both slices due to the periodic boundary conditions).
• Figure 2: The cmb temperature and polarization power spectra contributions from cosmic strings (black), inflationary scalar modes (gray, solid) and inflationary tensor modes (gray, dot-dashed). For the case of the te cross correlation, positive correlations are shown as solid lines and anti-correlations are shown as dashed lines, except that is for the inflationary tensor component for which the sign is not indicated here.
• Figure 3: A comparison of the ee (dashed) and bb (solid) power spectra contributions from local cosmic strings (black) and global textures (gray). Each are calculated at $h=0.72$, $\Omega_{\mathrm{b}}h^{2}=0.0214$, $\Omega_{\Lambda}=0.75$ and $\tau=0.1$, with the normalizations set to yield fractional contributions to the temperature power spectrum of $f_{10}=0.11$ for strings and $f_{10}=0.13$ for textures, which correspond to the $95\%$ upper bounds from cmb data.
• Figure 4: Estimated uncertainties from the future C$\ell$over project under the null hypothesis, that is that the true underlying model contains neither cosmic strings nor primordial tensor modes. The thick solid line indicates the underlying model, while the squares show the $\ell$-binning of the data and the 1$\sigma$ uncertainties estimated. The thin solid line shows the total power spectrum from an $f_{10}=0.0035$ model while the dashed thin shows the string component in that case.
• Figure 5: The effect of $s$ on the results compared to the estimated uncertainties from realization-to-realization variations, with shaded areas showing the 1 and 2-$\sigma$ regions at each multipole. Note that correlations extend over large multipole ranges, that there are statistical uncertainties for each $s$, and that $T$ is the mean CMB temperature.