Can topological defects mimic the BICEP2 B-mode signal?

Area of Science:

• Cosmology
• Astrophysics
• Cosmic Microwave Background (CMB) Physics

Background:

• The BICEP2 experiment detected a B-mode polarization signal at low multipoles in the cosmic microwave background (CMB).
• Topological defects and primordial gravitational waves (tensor modes) are potential sources for this observed B-mode signal.
• Previous analyses faced challenges reconciling different CMB datasets and theoretical models.

Purpose of the Study:

• To investigate whether the BICEP2 B-mode signal can be fully explained by topological defects.
• To constrain the contribution of cosmic strings to the CMB B-mode polarization.
• To determine the impact of including cosmic strings on cosmological parameter estimations, particularly the tensor-to-scalar ratio (r).

Main Methods:

• Analyzed the BICEP2 low-multipole B-mode polarization data in conjunction with high-multipole B-mode and CMB temperature anisotropy data (e.g., from the Planck satellite).
• Incorporated a model including both tensor modes and cosmic strings.
• Performed statistical analysis to derive constraints on cosmological parameters and the properties of topological defects.

Main Results:

• The observed B-mode polarization signal at low multipoles by BICEP2 cannot be entirely attributed to topological defects.
• This finding is inconsistent with high-multipole B-mode polarization data and existing CMB temperature anisotropy data.
• Including cosmic strings in the model provides a constraint comparable to that from Planck satellite temperature data, and at the derived limit, improves the fit to B-mode data with a tensor-to-scalar ratio of r ≃ 0.15.

Conclusions:

• Topological defects alone are insufficient to explain the full BICEP2 B-mode polarization signal.
• Cosmic strings provide a viable, albeit limited, contribution that aligns with other CMB observations.
• The inclusion of cosmic strings refines the estimation of the tensor-to-scalar ratio, suggesting a value around r ≃ 0.15.