A review of Axion Inflation in the era of Planck (1305.3557v1)

Abstract: Because the inflationary mechanism is extremely sensitive to UV-physics, the construction of theoretically robust models of inflation provides a unique window on Planck-scale physics. We review efforts to use an axion with a shift symmetry to ensure a prolonged slow-roll background evolution. The symmetry dictates which operators are allowed, and these in turn determine the observational predictions of this class of models, which include observable gravitational waves (potentially chiral), oscillations in all primordial correlators, specific deviations from scale invariance and Gaussianity and primordial black holes. We discuss the constraints on this class of models in light of the recent Planck results and comment on future perspectives. The shift symmetry is very useful in models of large-field inflation, which typically have monomial potentials, but it cannot explain why two or more terms in the potential are fine-tuned against each other, as needed for typical models of small-field inflation. Therefore some additional symmetries or fine-tuning will be needed if forthcoming experiments will constrain the tensor-to-scalar ratio to be r < 0.01.

• The paper reviews axion inflation theory, focusing on how shift symmetry enables prolonged slow-roll and aligns models with CMB observations.
• It examines various implementations like natural inflation, axion monodromy, and multi-axion scenarios while addressing challenges such as super-Planckian decay constants.
• The study discusses key observational signatures including detectable tensor-to-scalar ratios and oscillatory non-Gaussian features relevant to future CMB and gravitational wave experiments.

Axion Inflation in the Era of Planck: A Review

Overview

The paper by Enrico Pajer and Marco Peloso offers a comprehensive review of axion inflation, particularly in light of the data provided by the Planck satellite. It focuses on the theoretical constructs of inflation models, emphasizing those involving axions with a shift symmetry. This symmetry is instrumental in ensuring a prolonged slow-roll phase, significantly impacting observable features such as gravitational waves, oscillatory behavior in primordial correlators, deviations from Gaussianity, and the possible formation of primordial black holes.

Axion Inflation Models

The authors delve into various implementations of axion inflation, including but not limited to natural inflation, axion monodromy, models with multiple axions, and those involving 4-form couplings. Natural inflation, a classical model, employs an axion potential stemming from non-perturbative effects like gauge instantons. However, it requires super-Planckian values for the axion decay constant to fit observational data, which poses theoretical challenges due to the expected breakdown of shift symmetry at such scales.

Axion monodromy models propose a controlled breaking of the shift symmetry, allowing inflationary potentials with monomial forms, such as linear or fractional powers of the inflaton. This broadens the parameter space and provides compatibility with Planck results when considering observable gravitational wave implications.

The authors also examine multiple axion models and N-flation scenarios, where collective excitation of axion fields leads to significant field excursions aligned with observational demands.

Phenomenological Implications

The review highlights various signals and constraints emerging from axion inflation models. A crucial factor in these models is the generation of a tensor-to-scalar ratio, $r$, that remains within observational limits while permitting potentially detectable gravitational waves. The paper provides insights into how axion-driven inflation could lead to distinct features in Cosmic Microwave Background (CMB) data, such as scale dependence and non-Gaussianity characterized by specific oscillatory signatures.

The coupling of axions to gauge fields results in significant phenomena. For instance, the production of gauge quanta during inflation can source non-Gaussian signatures exceeding standard expectations, contributing to rich phenomenological landscapes for future probing.

Implications and Future Prospects

The paper concludes with a discussion on the shift symmetry's role across different inflationary models. While it is essential for large-field models to control quantum corrections across Planck-scale excursions, small-field models still demand additional symmetries or fine-tuning to remain viable as observational constraints become stricter.

This work offers substantial theoretical insights while acknowledging the potential impacts on future CMB polarization studies and gravitational wave detections. These developments will provide empirical tests to assess the robustness of axion inflation models further, potentially revolutionizing our understanding of early universe dynamics.

Conclusion

Pajer and Peloso's review serves as a critical resource for researchers exploring axion inflation and its implications given advancing observational capabilities. It grapples with crucial questions surrounding theoretical constructs and their observational counterparts, effectively bridging the gap between fundamental theory and emerging data. This paper paves the way for continued exploration of axion dynamics in cosmology, fostering progress toward a more profound understanding of inflationary paradigms.