Dylan R. Sutton, Chang Feng, Christian L. Reichardt

We present new limits on the amplitude of potential primordial magnetic fields (PMFs) using temperature and polarization measurements of the cosmic microwave background (CMB) from Planck, BICEP2/Keck Array, POLARBEAR, and SPT-pol. We reduce twofold the 95% CL upper limit on the CMB anisotropy power due to PMFs, from $A_{pmf}$ < 0.39 for Planck alone to $A_{pmf}$ < 0.22 for the combined dataset. We also forecast the expected limits from soon-to-deploy CMB experiments (like SPT-3G, Adv. ACTpol, or the Simons Array) and the proposed CMB-S4 experiment. Future CMB experiments should dramatically reduce the current uncertainties, by one order of magnitude for the near-term experiments and two orders of magnitude for the CMB-S4 experiment. The constraints from CMB-S4 have the potential to rule out much of the parameter space for PMFs.

S. Vilchinskii, O. Sobol, E. Gorbar, I. Rudenok

By assuming the kinetic coupling $f^{2}(\phi)FF$ of the effective inflaton field $\phi$ with the electromagnetic field, we explore magnetogenesis during the inflation and preheating stages in the Starobinsky model. We consider the case of the exponential coupling function $f(\phi)=\exp(\alpha\phi/M_{p})$ and show that for $\alpha \sim 12-15$ it is possible to generate the large scale magnetic fields with strength $\gtrsim 10^{-15}$ Gauss at the present epoch. The spectrum of generated magnetic fields is blue with the spectral index $n=1+s$, $s>0$. We have found that for the relevant values of the coupling parameter, $\alpha=12-15$, model avoids the back-reaction problem for all relevant modes.

sxk1031

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys4021.html

Özenç Güngör, Bryan W. Lynn, Glenn D. Starkman

The weak-scale $U(1)_Y$ Abelian Higgs Model (AHM) is the spontaneous-symmetry-breaking (SSB) gauge theory of a complex scalar $\phi$ and a vector $A^\mu$. In Lorenz gauge, the SSB AHM has: a massless pseudo-scalar $\pi$; a conserved (up to gauge-fixing terms) $U(1)_Y$ global physical current; no conserved $U(1)_Y$ charge; and a Goldstone Theorem, where $\tilde \pi$ in the Kibble representation becomes a true massless Nambu-Goldstone boson (NGB). Two towers of "1-soft-pion" SSB global WTI govern the $\phi$-sector. The 1$^{st}$ tower gives relations among connected off-shell Green's functions, and powerful constraints on the all-loop-orders $\phi$-sector effective AHM Lagrangian. The 2$^{nd}$ tower, governing on-shell connected T-matrix elements, further severely constrains the physics. It guarantees IR finiteness for zero NGB mass; includes the Lee-Stora-Symanzik (LSS) Theorem, forcing the on-shell T-matrix element with only two external pseudoscalar legs to vanish; and is the source of an additional NGB global shift symmetry. Using the Green's functions and the LSS Theorem of the theory, we construct the manifestly symmetric quantum effective potential of the scalar sector of the Goldstone mode of the AHM, in the Lorenz gauge, to all orders in quantum loops. To do so, we solve the Ward Takahashi identities of the theory recursively, and express arbitrary $\phi$-sector Green's functions in terms of those with no external scalar legs and an even number of pseudoscalar legs. The resulting expression for the effective potential is much easier to evaluate at a given order in quantum loops, since many fewer diagrams need be computed for a given number of external $\phi$ legs. In the Kibble representation, the so-constructed effective scalar potential is gauge-independent.