Finding Evidence for Inflation and the Origin of Galactic Magnetic Fields with CMB Surveys

Kavli Affiliate: Toshiya Namikawa

| First 5 Authors: Sayan Mandal, Neelima Sehgal, Toshiya Namikawa, ,

| Summary:

The origin of the $mumathrm{G}$ magnetic fields observed in galaxies is
unknown. One promising scenario is that magnetic fields generated during
inflation, larger than 0.1 $mathrm{nG}$ on Mpc scales, were adiabatically
compressed to $mumathrm{G}$ strengths in galaxies during structure formation.
Thus, detecting a scale-invariant primordial magnetic field (PMF) above
$0.1,mathrm{nG}$ on Mpc scales just after recombination would indicate an
inflationary origin of galactic magnetic fields. This would also provide
compelling evidence that inflation occurred since only an inflationary
mechanism could generate such a strong, scale-invariant magnetic field on Mpc
scales. In contrast, constraining the scale-invariant PMF strength to be below
$0.1,mathrm{nG}$ would imply an 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, and, in particular, that CMB-HD would lower the upper bound to
$0.072,mathrm{nG}$ at the $95%$ CL, which is below the critical
$0.1,mathrm{nG}$ threshold for ruling out a purely inflationary origin. If
inflationary PMFs exist, we find that a CMB-HD survey would be able to detect
them with about $3sigma$ significance or higher, providing evidence for
inflation itself.

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