Polarization of the Microwave Background in Defect Models

TL;DR

The paper addresses how CMB polarization signatures distinguish defect-based structure formation from inflation. It employs a two-stage calculation that converts defect field dynamics into a defect stress-energy correlator, decomposes it into coherent sources, and propagates these with a modified Boltzmann solver to obtain T, E, and B spectra. The results show a robust, sizable $B$-polarization signal (≈$1\,\mu\text{K}$) on degree scales driven by vector modes, and a strong decoherence that dampens acoustic features and suppresses $C_l^{TC}$, providing clear observational discriminants. The findings imply that future missions like Planck could decisively test defect models by probing magnetic polarization and cross-correlation patterns, offering a practical path to distinguish between the competing theories of structure formation.

Abstract

We compute the polarization power spectra for global strings, monopoles, textures and nontopological textures, and compare them to inflationary models. We find that topological defect models predict a significant (1 microK) contribution to magnetic type polarization on degree angular scales, which is produced by the large vector component of the defect source. We also investigate the effect of decoherence on polarization. It leads to a smoothing of acoustic oscillations both in temperature and polarization power spectra and strongly suppresses the cross-correlation between temperature and polarization relative to inflationary models. Presence or absence of magnetic polarization or cross-correlation would be a strong discriminator between the two theories of structure formation and will be testable with the next generation of CMB satellites.

Figures (3)

• Figure 1: Power spectra of temperature (T), electric type polarization (E) and magnetic type polarization (B) for global strings, monopoles, textures and nontopological textures. For comparison we also show the corresponding spectra in a standard CDM model with $T/S=1$ (which maximises the B component present). Defect models all predict a much larger component of B polarization on small angular scales.
• Figure 2: The breakdown of the contributions to the total power by the scalar, vector and tensor components for a global string model. Scalars and vectors dominate E and B polarization, respectively. Other defect models give qualitatively similar results.
• Figure 3: The cross-correlation power spectrum (a) and the correlation coefficient (b) for the same defect models as in figure 1, as well as for the standard CDM model. Defect models predict significantly less power in cross-correlation than inflationary models.