Abstract
Axion inflation coupled to Abelian gauge fields via a Chern-Simons-like term of the form \( \phi F\overset{\sim }{F} \) represents an attractive inflationary model with a rich phenomenology, including the production of magnetic fields, black holes, gravitational waves, and the matter-antimatter asymmetry. In this work, we focus on a particular regime of axion inflation, the so-called Anber-Sorbo (AS) solution, in which the energy loss in the gauge-field production provides the dominant source of friction for the inflaton motion. We revisit the AS solution and confirm that it is unstable. Contrary to earlier numerical works that attempted to reach the AS solution starting from a regime of weak backreaction, we perform, for the first time, a numerical evolution starting directly from the regime of strong backreaction. Our analysis strongly suggests that, at least as long as one neglects spatial inhomogeneities in the inflaton field, the AS solution has no basin of attraction, not even a very small one that might have been missed in previous numerical studies. Our analysis employs an arsenal of analytical and numerical techniques, some established and some newly introduced, including (1) linear perturbation theory along the lines of ref. [1], (2) the gradient expansion formalism (GEF) developed in ref. [2], (3) a new linearized version of the GEF, and (4) the standard mode-by-mode approach in momentum space in combination with input from the GEF. All these methods yield consistent results confirming the instability of the AS solution, which renders the dynamics of axion inflation in the strong-backreaction regime even more interesting than previously believed.
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Acknowledgments
M. P. and K. S. would like to thank the organizers of the Bethe Forum on Axions at the Bethe Center for Theoretical Physics in October 2022, where this project was initiated. The authors are grateful to Valerie Domcke, Yohei Ema, Kyohei Mukaida, and Stefan Sandner for fruitful discussions. The work of R. v. E. and K. S. is supported by the Deutsche Forschungsgemeinschaft (DFG) through the Research Training Group, GRK 2149: Strong and Weak Interactions — from Hadrons to Dark Matter. M. P. is supported by Istituto Nazionale di Fisica Nucleare (INFN) through the Theoretical Astroparticle Physics (TAsP) project. O. S. is supported by a Philipp Schwartz fellowship of the University of Münster and by the National Research Foundation of Ukraine Project No.2020.02/0062. The work of L. S. is partially supported by the US-NSF grant PHY-2112800.
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von Eckardstein, R., Peloso, M., Schmitz, K. et al. Axion inflation in the strong-backreaction regime: decay of the Anber-Sorbo solution. J. High Energ. Phys. 2023, 183 (2023). https://doi.org/10.1007/JHEP11(2023)183
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DOI: https://doi.org/10.1007/JHEP11(2023)183