Institute for Advanced Study/Princeton University Early Universe/Cosmology Lunch Discussion

Magnetic fields of strength ~μG are observed in galaxies today, but their origin is unknown. A promising scenario is that magnetic fields larger than 0.1 nG at Mpc scales generated during inflation were adiabatically compressed during structure formation to give rise to the μG fields in galaxies. Thus, detecting a scale-invariant primordial magnetic field (PMF) above 0.1 nG on Mpc scales just after recombination would indicate an inflationary origin of galactic magnetic fields. Such a detection would also provide compelling evidence that inflation occurred, since such a strong, scale-invariant magnetic field on Mpc scales could only arise from inflation. If the scale-invariant PMF strength is constrained to be below 0.1 nG, it would imply that the inflationary scenario is not the primary origin, since such weak PMFs cannot be amplified enough via adiabatic compression to produce the strength of the galactic fields we observe today. We find that measurements of anisotropic birefringence by future CMB surveys will be able to improve the sensitivity to Mpc-scale inflationary PMFs by an order of magnitude. In particular, the CMB-HD experiment would lower the upper bound to 0.072 nG at the 95% CL, which is below the critical 0.1 nG threshold for ruling out a purely inflationary origin. If inflationary PMFs exist, a CMB-HD survey would be able to detect them with at least 3σ significance, providing evidence for inflation itself.