An ignoble approach to large field inflation

Abstract

We study an inflationary model developed by Kaloper and Sorbo, in which the inflaton is an axion with a sub-Planckian decay constant, whose potential is generated by mixing with a topological 4-form field strength. This gives a 4d construction of "axion monodromy inflation": the axion winds many times over the course of inflation and draws energy from the 4-form. The classical theory is equivalent to chaotic inflation with a quadratic inflaton potential. Such models can produce "high scale" inflation driven by energy densities of the order of (10 16 GeV ) 4 , which produces primordial gravitational waves potentially accessible to CMB polarization experiments. We analyze the possible corrections to this scenario from the standpoint of 4d effective field theory, identifying the physics which potentially suppresses dangerous corrections to the slow-roll potential. This yields a constraint relation between the axion decay constant, the inflaton mass, and the 4-form charge. We show how these models can evade the fundamental constraints which typically make high-scale inflation difficult to realize. Specifically, the moduli coupling to the axion-four-form sector must have masses higher than the inflationary Hubble scale ( < ∼ 10 14 GeV ). There are also constraints from states that become light due to multiple windings of the axion, as happens in explicit string theory constructions of this scenario. Further, such models generally have a quantum-mechanical "tunneling mode" in which the axion jumps between windings, which must be suppressed. Finally, we outline possible observational signatures.

Poly-instanton inflation

“…The progress in moduli stabilisation also opened up the possibility of realising inflation in the moduli sector. String inflationary models based on single-field slow-roll can be broadly classified, based on the origin of the inflaton field [9], into open string models [10][11][12] and closed string models [13][14][15][16][17][18][19][20].…”

Tunneling in axion monodromy

“…Axion monodromy, however, is susceptible to a new kind of problem: membrane nucleation [14]. More precisely, it is possible for a membrane to nucleate during inflation and mediate a spontaneous decay to a lower energy state.…”

“…In the recent literature [5,14,[16][17][18][19] this tunneling rate has been estimated using results derived by Coleman (and de Luccia in the case of a gravitational theory) [20,21]. However, there are reasons to doubt the validity of these results when applied to axion monodromy.…”

“…For the theory to be consistent, the natural periodicity of the axion φ → φ + 2πf must be matched by k shifts in the flux F 4 [14,27] leading to the condition 2πf = ke µ (2.9) JHEP10(2016)025 Figure 2. Potential for vacuum decay as studied by Coleman. Physically, this means that k of the membranes described above will separate two regions of space which lie in the same position in the axion's periodic domain.…”

“…This tunneling rate was estimated in [5,14,[17][18][19] using formulas derived in field theory by Coleman [20] and in a gravitational theory by Coleman and de Luccia (CdL) [21].…”