Detectability of nGauss primordial magnetic fields in the B-mode polarization of CMB

Abstract: The origin of large-scale magnetic fields in the Universe is still unknown. Observations suggest the presence of nGauss primordial magnetic fields at the last scattering surface. The presence of such fields will affect the evolution of cosmological perturbations and potentially leave an imprint on the CMB anisotropies. Here, we show that the B-mode power spectrum carries a clear signature of the stochastic primordial magnetic fields up to a few nGauss. Specifically, the presence of nGauss primordial magnetic fields changes the BB power spectrum at all scales. At large scales, the tensor modes contribute to the B-mode spectrum, while non-vanishing vector modes contribute at small scales. We show that the B-mode of CMB carry a distinct signature of the primordial magnetic fields. To validate our prediction, we use BICEP2 and POLARBEAR data and find that B-mode observations from the experiments are consistent with non-zero primordial fields. We also use the BICEP2 observations to constrain the primordial magnetic field. We provide a detailed analysis of the B-mode polarization of the CMB with primordial magnetic fields and investigate the implications for the upcoming CMB missions.

• The paper demonstrates that nGauss primordial magnetic fields (PMF) significantly enhance the Cosmic Microwave Background (CMB) BB power spectrum, providing a primary means for their detection, while minimally affecting other spectra.
• The study validates theoretical predictions for PMF signatures by comparing simulated CMB power spectra against observations from BICEP2 and POLARBEAR experiments, deriving constraints on PMF parameters.
• Findings highlight the critical role of high-sensitivity B-mode measurements at higher multipoles from future CMB missions in potentially confirming or constraining the presence of nGauss PMF.

Detectability of nGauss Primordial Magnetic Fields in the B-mode Polarization of CMB

The study of primordial magnetic fields (PMF) is essential for understanding the magnetic phenomena observed across the cosmos. This paper investigates the detectability of nano-Gauss (nGauss) PMF within the Cosmic Microwave Background (CMB) B-mode polarization, leveraging data from BICEP2 and POLARBEAR experiments. It demonstrates that, while nGauss PMF may not significantly alter the TT, EE, and TE power spectra, its presence markedly affects the BB power spectrum.

Recent observations hint at PMF at the last scattering surface, which influences cosmological perturbations and imprints on CMB anisotropies. The paper argues that the B-mode power spectrum contains distinct signatures of stochastic PMF up to a few nGauss, affecting the BB power spectrum universally. Specifically, it is shown that tensor modes primarily affect the B-mode spectrum on large scales, whereas vector modes contribute at small scales.

Key Findings and Methodology

1. Impact on Power Spectra: The paper begins by noting PMF’s negligible effects on the TT, EE, and TE power spectra but highlights a pronounced increase in the BB spectrum due to the presence of PMF. Utilizing CAMB simulations, the authors establish that PMF enables non-vanishing vector modes, thereby augmenting the BB spectrum at large multipoles. The study asserts that the observed BB-spectrum significantly rises with increased PMF strength, demonstrating discernible deviations from scenarios with no PMF.
2. Validation with Current Data: To substantiate theoretical predictions, the analysis compares simulated CMB signatures against BICEP2 and POLARBEAR observations. The authors confirm that power spectra, accounting for non-zero PMF, align compatibly with these datasets, indicating potential presence of PMF. Constraints are derived on the PMF strength and magnetic spectral index using BICEP2 data, further delineating parameter space consistent with observed CMB characteristics.
3. Implications for Future Observations: The results illuminate the importance of B-mode studies at higher multipoles, postulating that upcoming missions, which aim to enhance the sensitivity of B-mode measurements, could more stringently constrain PMF properties or potentially confirm nGauss-scale PMF presence in the Universe. Such findings would bear profound implications for theoretical models of early Universe magnetogenesis.

Implications and Directions

The implications of this research are manifold. Firstly, its findings establish a substantial theoretical basis for the presence of primordial magnetic fields detectable through the B-mode spectrum of the CMB, challenging researchers to refine their predictions and observational strategies. Secondly, with future CMB missions like those under the LiteBird, CORE, and PICO ideation, the heightened precision in measuring B-modes promises heightened sensitivity to PMF-related signatures. Should these missions validate the paper’s findings, it could necessitate revising prevailing paradigms within cosmology and astrophysics regarding magnetic field evolution and origin.

In conclusion, the paper illustrates a clear path towards detecting and constraining primordial magnetic fields using the B-mode polarization of the CMB. It substantiates its claims with rigorous theoretical underpinnings and aligns them with present observational data, setting the stage for future observations to potentially confirm these findings. This delineates a constructive interplay between theoretical predictions and observational capabilities, underscoring the vitality of nuanced empirical measurements in advancing the understanding of cosmological magnetic phenomena.