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Home > Journal of Cosmology and Astroparticle Physics (IOPP/SISSA) > Dissipative axial inflation |

Notari, Alessio (Departament de Física Fondamental i Institut de Ciències del Cosmos, Universitat de Barcelona, Martí i Franquès 1, Barcelona, 08028 Spain) ; Tywoniuk, Konrad (Theoretical Physics Department, CERN, Geneva, Switzerland)

22 December 2016

**Abstract: **We analyze in detail the background cosmological evolution of a scalar field coupled to a massless abelian gauge field through an axial term ϕ/fγ F ̃F, such as in the case of an axion. Gauge fields in this case are known to experience tachyonic growth and therefore can backreact on the background as an effective dissipation into radiation energy density ρR, which can lead to inflation without the need of a flat potential. We analyze the system, for momenta k smaller than the cutoff fγ, including the backreaction numerically. We consider the evolution from a given static initial condition and explicitly show that, if fγ is smaller than the field excursion ϕ0 by about a factor of at least 𝒪 (20), there is a friction effect which turns on before the field can fall down and which can then lead to a very long stage of inflation with a generic potential. In addition we find superimposed oscillations, which would get imprinted on any kind of perturbations, scalars and tensors. Such oscillations have a period of 4–5 efolds and an amplitude which is typically less than a few percent and decreases linearly with fγ. We also stress that the curvature perturbation on uniform density slices should be sensitive to slow-roll parameters related to ρR rather than ϕ̇2/2 and we discuss the existence of friction terms acting on the perturbations, although we postpone a calculation of the power spectrum and of non-gaussianity to future work and we simply define and compute suitable slow roll parameters. Finally we stress that this scenario may be realized in the axion case, if the coupling 1/fγ to U(1) (photons) is much larger than the coupling 1/fG to non-abelian gauge fields (gluons), since the latter sets the range of the potential and therefore the maximal allowed ϕ0∼ fG.

**Published in: ****JCAP 1612 (2016) 038**
**Published by: **Institute of Physics Publishing/SISSA

**DOI: **10.1088/1475-7516/2016/12/038

**arXiv: **1608.06223

**License: **CC-BY-3.0