Cosmological Magnetic Fields from Inflation and Backreaction
Sugumi Kanno, Jiro Soda, Masa-aki Watanabe
TL;DR
The paper revisits magnetogenesis during inflation by incorporating the backreaction of electromagnetic fields. Using a vector field coupled to the inflaton via $f(\phi) F_{\mu\nu}F^{\mu\nu}$ in an anisotropic background, it shows that backreaction does not terminate inflation but drives it into an anisotropic, attractor regime with an effective coupling $c_{\rm eff}=1$. This backreaction dramatically suppresses the generated magnetic fields, predicting a suppressed amplitude on cosmological scales (e.g., ${\cal O}(10^{-36}\,\text{G})$ on Mpc scales for the flat-spectrum case $c=\tfrac{3}{2}$) and a spectral break at a scale $k_b$. The work clarifies that achieving observationally significant primordial magnetic fields from inflation is more difficult once backreaction is properly accounted, while highlighting potential signatures from the anisotropic expansion. Overall, backreaction alters dynamics and field production but does not end inflation, offering a refined understanding of inflationary magnetogenesis.
Abstract
We study the backreaction problem in a mechanism of magnetogenesis from inflation. In usual analysis, it has been assumed that the backreaction due to electromagnetic fields spoils inflation once it becomes important. However, there exists no justification for this assumption. Hence, we analyze magnetogenesis from inflation by taking into account the backreaction. On the contrary to the naive expectation, we show that inflation still continues even after the backreaction begins to work. Nevertheless, it turns out that creation of primordial magnetic fields is significantly suppressed due to the backreaction.