An Ignoble Approach to Large Field Inflation (1101.0026v1)

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 ($\la\ 10^{14}\ GeV$). 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.

• The paper presents a novel axion monodromy model using a 4-form field to generate a quadratic slow-roll potential near 10^16 GeV.
• It circumvents the η problem by employing a sub-Planckian decay constant and ensuring that modulii masses exceed the inflationary Hubble scale.
• The model predicts detectable primordial gravitational waves and unique CMB signatures, bridging theoretical predictions with observational cosmology.

Overview of "An Ignoble Approach to Large Field Inflation"

The paper "An Ignoble Approach to Large Field Inflation" by Kaloper, Lawrence, and Sorbo proposes a novel inflationary model, axion monodromy inflation, which operates within a four-dimensional framework. The central premise is that the inflaton is an axion associated with a sub-Planckian decay constant. The potential is generated through its interaction with a topological 4-form field strength, suggesting a method to sustain inflation at energy densities on the order of $10^{16}$ GeV. This energy level could produce primordial gravitational waves, possibly detectable through experiments on Cosmic Microwave Background (CMB) polarization.

Model Mechanics and Theoretical Underpinnings

The theoretical foundation relies on the axion's coupling to the topological 4-form, allowing the axion to wind multiple times throughout inflation, with the energy replenished from the 4-form. This setup is akin to chaotic inflation characterized by a quadratic inflaton potential. The model gives rise to "high scale" inflation, a significant departure from typical challenges faced by models requiring super-Planckian field ranges, thus circumventing the notorious "η problem."

The paper delineates the constraints imposed by the model and identifies the roles of the axion decay constant, the inflaton mass, and the charge of the 4-form. A salient feature tackled is the question of fundamental constraints typically impeding high-scale inflation realization. Specifically, modulii coupling is addressed, and arguments are made to ensure these have higher masses than the inflationary Hubble scale, effectively addressing potential corrections that may thwart inflation.

Quantitative and Qualitative Implications

Quantitatively, the paper presents the energy scales and parameters that align with observations, notably from the WMAP. The authors achieve a nontrivial inflationary epoch with a field range feasible under compatible conditions in string theory. The results illustrate that their model avoids dangerous corrections from integrating out additional degrees of freedom, ensuring the slow-roll potential remains intact.

Qualitatively, the implications reach into the observation of primordial gravitational waves, particularly since the high energy scales involved in this scenario often translate to detectable features in the CMB spectrum as tensor modes. This links directly to the broader implications on understanding the universe's inflationary beginnings and helps bridge theoretical physics with empirical data avenues.

Future Developments and Observational Signatures

The derivations suggest potential observational signatures, including non-Gaussianities and specific frequency enhancements in the CMB. These resonant features offer testable predictions and require synthesis with forthcoming observational data to corroborate or refute the model's viability.

The paper does not merely present a mathematical construct but instead forms a theoretical trajectory towards addressing inflation's ambitious energy scales with a feasible mechanism grounded in effective field theory and enriched by cosmological observations. Additionally, the paper alludes to how deeper string theoretic insights or extensions may augment the model, potentially providing an avenue for further explorations in other high-scale phenomena in cosmology.

In summary, this model presents an insightful alternative route to reconcile high-scale inflation with effective field theoretic methods, potentially opening new dialogues between theoretical constructs and observational cosmology. The implications span across theoretical predictions and observable phenomena, reiterating the importance of integrative approaches to understanding inflation in the early universe.