Essay on "Weak Gravity Strongly Constrains Large-Field Axion Inflation"
The exploration of axion inflationary models is of significant interest in the field of theoretical physics and cosmology, particularly due to their potential role in explaining the observed properties of the universe. The paper "Weak Gravity Strongly Constrains Large-Field Axion Inflation" by Heidenreich, Reece, and Rudelius provides a rigorous examination of the constraints imposed by the Weak Gravity Conjecture (WGC) on large-field inflation models based on axion-like fields. This work extends the understanding of the WGC beyond traditional scenarios, applying it to a variety of axion theories, including multi-field models like N-flation and certain alignment models.
The paper begins by asserting that large-field inflationary models predict detectable primordial gravitational waves and is motivated by the need for a natural explanation of super-Planckian excursions in field space. The authors revisit the Weak Gravity Conjecture, historically applied to theories where a single compact axion derives from an extra dimensional gauge field, and expand its applicability. The WGC suggests that a theory of gravity coupled to electromagnetism must have a state where the charge surpasses the mass in Planck units, effectively ensuring all extremal black holes are unstable. The authors argue that this principle profoundly impacts multi-axion models, proposing constraints on effective field ranges that challenge super-Planckian excursions.
The analysis presented considers the implications of the WGC on models with multiple compact axions, finding that in plausible scenarios, these field ranges cannot exceed the Planck scale. The authors meticulously discuss various alignment models, such as those where axions exhibit kinetic alignment or decay constant alignment. In kinetic alignment, simplicity of both electric charges causing instanton effects and magnetic charges satisfying the WGC in a chosen basis is assumed. In decay constant alignment, the charge matrix linked to instantons and magnetic charges does not display simplicity or small integer forms.
Notably, the paper introduces the Single-EFT Consistency Criterion (SECC), an innovative conjecture suggesting a single effective field theory that covers all moduli space must accommodate modes that do not decouple across this space. This insight, derived from analyzing how mass spectrums change with axion fields, suggests limits on field ranges that prevent super-Planckian excursions, particularly in monodromy models. Alongside this, the Extended Weak Gravity Conjecture (XWGC) is presented, positing that the WGC must apply at every extremum of a field's potential, offering possible bounds on inflationary models based on compact axions.
The theoretical significance of these findings lies in illustrating potential inconsistencies in conventional models of large-field inflation when viewed through the lens of quantum gravity. Practically, this suggests reassessing models of early universe inflation to align with what appears to be a more general principle limiting field ranges associated with axion-like particles.
Moreover, while much of the focus is given to contradictions arising from axion fields originating from 5-form fields reducing to 4-dimensional axion fields, the extensive discourse on implications for specific inflation models is crucial. It pushes the boundary of theoretical possibilities and opens new avenues for exploring large-field inflation in a manner consistent with WGC and its extensions.
Despite the theoretical uncertainties remaining in axiomatic assumptions such as the SECC and XWGC, the paper is a strong advocate for further exploring the constraints of quantum gravity on inflationary models and urges future research to solidify or refute these conjectures. The work is a critical stepping stone in the domain, setting a precedence for examining constraints on inflaton masses and field ranges within axion inflationary cosmological models.