Can primordial magnetic fields be the origin of the BICEP2 data?

Abstract

If the B-mode signal in the CMB polarization seen by the BICEP2 experiment is confirmed, it has dramatic implications for models of inflation. The result is also in tension with Planck limits on standard inflationary models. It is therefore important to investigate whether this signal can arise from alternative sources. If so, this could lessen the pressure on inflationary models and the tension with Planck data. We investigate whether vector and tensor modes from primordial magnetic fields can explain the signal. We find that in principle, magnetic fields generated during inflation can indeed produce the required B-mode, for a suitable range of energy scales of inflation. In this case, the primordial gravitational wave amplitude is negligible, so that there is no tension with Planck and no problems posed for current inflationary models. However, the simplest magnetic model is in tension with Planck limits on non-Gaussianity in the trispectrum. It may be possible to fine-tune the magnetogenesis model so that this non-Gaussianity is suppressed. Alternatively, a weaker magnetic field can pass the non-Gaussianity constraints and allow the primordial tensor mode to be reduced to r~0.09, thus removing the tension with Planck data and alleviating the problems with simple inflationary models.

5 pages, 3 figures; v2: POLARBEAR data added, main results unchanged; v3: fig.1 modified, discussion about the magnetic field contribution to the temperature added, main results unchanged, version accepted in PRL

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• Cosmic microwave background bispectrum of tensor passive modes induced from primordial magnetic fields
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