Large Nongaussianity in Axion Inflation (1011.1500v2)

Abstract: The inflationary paradigm has enjoyed phenomenological success, however, a compelling particle physics realization is still lacking. The key obstruction is that the requirement of a suitably flat scalar potential is sensitive to Ultra-Violet (UV) physics. Axions are among the best-motivated inflaton candidates, since the flatness of their potential is naturally protected by a shift symmetry. We re-consider the cosmological perturbations in axion inflation, consistently accounting for the coupling to gauge fields \phi F \tilde{F}, which is generically present in these models. This coupling leads to production of gauge quanta, which provide a new source of inflaton fluctuations, \delta\phi. For an axion decay constant < 10^{-2} M_p, this effect typically dominates over the standard fluctuations from the vacuum, and saturates the current observational bounds on nongaussianity of the CMB anisotropies. Since sub-Planckian values of the decay constant are typical for concrete realizations that admit a UV completion (such as N-flation and axion monodromy), we conclude that large nongaussianity is easily obtained in very minimal and natural realizations of inflation.

• The paper shows that axion-gauge couplings during inflation produce large nongaussianity by amplifying inflaton fluctuations via inverse decay processes.
• It quantitatively links sub-Planckian axion decay constants to observable signatures in the cosmic microwave background.
• The results offer a testable framework for early universe models and motivate targeted CMB experiments.

Axion Inflation and Nongaussianity: A Detailed Perspective

The paper "Large Nongaussianity in Axion Inflation" by Neil Barnaby and Marco Peloso discusses the potential of axion fields to generate significant nongaussian perturbations in the cosmic microwave background (CMB) during inflation. The paper investigates the mechanisms by which axion inflation, through its coupling to gauge fields, can produce observable levels of nongaussianity, a key parameter in distinguishing different models of the early universe.

In the inflationary framework, conventional models face challenges primarily due to the requirement for a flat scalar potential, which is sensitive to Ultra-Violet (UV) physics. Axion fields, characterized by a shift symmetry, provide a naturally stable potential making them attractive candidates for the inflaton. By re-evaluating cosmological perturbations in axion inflation, the authors discover that the production of gauge quanta can dominate inflaton fluctuations for axion decay constants $f \lesssim 10^{-2}M_p$. This effect is pivotal as it saturates current observational constraints on CMB nongaussianity.

Observational evidence for nongaussianity in the CMB is crucial—it could validate various models of inflation by providing constraints and distinguishing features. Among several axion inflation models, those with sub-Planckian decay constants are inclined towards achieving observable levels of nongaussianity without excessive complexity or fine-tuning.

The production of gauge quanta during inflation, which arises due to the coupling term $\alpha \phi F\tilde{F}$ in the action, is shown to introduce significant contributions to inflaton fluctuations. This interaction catalyzes energy dissipation into the gauge fields, influencing the fluctuation dynamics of both fields. Specifically, the fluctuation $\delta \phi$ is majorly contributed to by inverse decay processes, $\delta A + \delta A \rightarrow \delta \phi$, leading to pronounced nongaussianity, given as $$f_{NL}^{equil} \approx 4.4 \times 10^{-10^3} \xi^9 P^{1/2} e^{6\pi \xi}$$.

The implications of these findings are twofold. Theoretically, they propose a revision to the understanding of primordial field dynamics, suggesting that nongaussianities can indeed emerge naturally from minimalistic axion inflation setups. Practically, these results pinpoint potential signals that forthcoming CMB experiments, such as those involving the Planck satellite, could detect, offering robust tests of the axion inflation hypothesis. Utilizing such nongaussian signals would revolutionize the resolution of inflationary dynamics and the physics of the early universe.

Future research directions may include a deeper investigation into the non-Abelian gauge field extensions, a comprehensive exploration of multi-field inflation scenarios, and detailed numerical simulations of axion gauge field interactions throughout the inflationary period. Furthermore, the exploration of the associated gravitational wave production as an auxiliary prediction could enrich the understanding and observational strategy of axion inflation scenarios. Such advancements could confirm or refute the capability of axions or similar scalar fields in explaining the universe's inflationary epoch.

In conclusion, Barnaby and Peloso's investigation into axion inflation delineates a significant pathway for obtaining large nongaussianity from feasible, UV-complete cosmological models. Their work highlights the importance of axion-gauge couplings in nongaussian generation, laying the groundwork for future explorations and potential discoveries in cosmology and particle physics.