We discuss whether an unaccounted contribution to the Cosmic Microwave Background polarization B-mode by primordial magnetic fields (PMFs) can bias future constraints on inflationary gravitational waves. As a case-study, we consider a scale-invariant PMF spectrum with amplitude of 1 nG on 1 Mpc scales, compatible with current cosmological bounds. We find a degeneracy in the B-mode spectra between PMFs and inflationary gravitational waves. If PMFs of this amplitude are not accounted for, future CMB experiments could claim a false detection of a tensor-to-scalar ratio r 0.007, close to the predictions of Starobinsky and α-attractor models. The degeneracy can be broken if B-modes are measured also at multipoles ℓ 900: more precisely experiments like CMB-S4 or CORE-M5 would be able to discriminate PMFs from primordial GWs at high statistical significance. Experiments like LiteBIRD or PIXIE will not be able to break the degeneracy and will need complementary bounds coming, for example, from measurements of anisotropies in the Faraday rotation angle of CMB polarization. This reinforces the importance of future experimental constraints on PMFs.
The impact of primordial magnetic fields on future CMB bounds on inflationary gravitational waves / Renzi, Fabrizio. - In: JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS. - ISSN 1475-7516. - 2018:8(2018), pp. 038-038. [10.1088/1475-7516/2018/08/038]
The impact of primordial magnetic fields on future CMB bounds on inflationary gravitational waves
Fabrizio Renzi
2018
Abstract
We discuss whether an unaccounted contribution to the Cosmic Microwave Background polarization B-mode by primordial magnetic fields (PMFs) can bias future constraints on inflationary gravitational waves. As a case-study, we consider a scale-invariant PMF spectrum with amplitude of 1 nG on 1 Mpc scales, compatible with current cosmological bounds. We find a degeneracy in the B-mode spectra between PMFs and inflationary gravitational waves. If PMFs of this amplitude are not accounted for, future CMB experiments could claim a false detection of a tensor-to-scalar ratio r 0.007, close to the predictions of Starobinsky and α-attractor models. The degeneracy can be broken if B-modes are measured also at multipoles ℓ 900: more precisely experiments like CMB-S4 or CORE-M5 would be able to discriminate PMFs from primordial GWs at high statistical significance. Experiments like LiteBIRD or PIXIE will not be able to break the degeneracy and will need complementary bounds coming, for example, from measurements of anisotropies in the Faraday rotation angle of CMB polarization. This reinforces the importance of future experimental constraints on PMFs.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.