Galileon inflation (1009.2497v5)

Abstract: Galileon inflation is a radiatively stable higher derivative model of inflation. The model is determined by a finite number of relevant operators which are protected by a covariant generalization of the Galileon shift symmetry. We show that the nongaussianity of the primordial density perturbation generated during an epoch of Galileon inflation is a particularly powerful observational probe of these models and that, when the speed of sound is small, fNL can be larger than the usual result fNL ~ 1/c_s^2.

• The paper introduces a Galileon inflation framework that incorporates higher-derivative operators while maintaining stability and avoiding ghost instabilities through symmetry protection.
• The paper demonstrates that the model produces observable non-Gaussian signatures in the bispectrum, with fNL deviating from the typical cₛ⁻² scaling seen in canonical inflation.
• The paper suggests that the Galileon inflation approach extends the effective field theory of inflation, offering robust and testable predictions for future cosmic microwave background and large-scale structure surveys.

Analysis of Galileon Inflation

The paper "Galileon Inflation" investigates a model of inflation based on Galilean-invariant fields, characterized by a covariant generalization of the Galileon shift symmetry. This model is particularly notable for its radiative stability and the ability to include higher-derivative operators without introducing ghost instabilities, a common challenge in theories that go beyond canonical kinetic terms in inflationary scenarios.

Model Overview

The Galileon inflation framework builds from the Galileon models initially studied in the context of modified gravity theories, especially massive gravity and extensions like the DGP (Dvali-Gabadadze-Porrati) model. These models are characterized by their robustness against quantum corrections, ensured by the shift symmetries they possess. This symmetry protection is crucial for maintaining the stability and predictivity of the model.

Key Features of Galileon Inflation

1. Higher Derivatives and Stability: The authors show that Galileon inflation allows for the inclusion of a finite number of higher-derivative operators while maintaining stability and avoiding ghosts, thanks to the enforcing shift symmetry. This is contrasted with other higher-derivative theories, which often suffer from ghost instabilities.
2. Observable Nongaussianities: A significant portion of the paper focuses on the non-Gaussian features of the primordial density perturbation produced during Galileon inflation. The bispectrum's non-Gaussianity parameter, denoted as $f_{NL}$, in the Galileon model offers a distinct signature from both Dirac-Born-Infeld (DBI) and canonical inflation models, particularly when the speed of sound, $c_s$, is small. The findings suggest that $f_{NL}$ can deviate from the expected $c_s^{-2}$ scaling normally associated with non-canonical inflation models.
3. Theoretical Implications: The model aligns with efforts to systematically explore the effective field theory of inflation, suggesting new forms of interactions and potentially different cosmic signatures. This extends the theoretical landscape for constructing viable inflationary models that are structurally protected from quantum instabilities.

Implications for Future Research

The Galileon inflation model opens new pathways for understanding the early universe, offering a framework with distinctive observational signatures that could be probed by precision cosmology. The theoretical robustness against radiative corrections ensures that predictions remain accurate even at higher orders, making this a strong candidate for future observations focused on the non-Gaussian structure of the cosmic microwave background and large-scale structure.

Practical Considerations

The practical implications stem from the model's potential to fit observational data with a distinct non-Gaussian signature. Future observations, especially those from CMB and large-scale structure surveys, could provide tests for the underlying assumptions of the Galileon inflationary model, particularly its prediction of significant non-Gaussianities not aligned with traditional models.

Conclusion

The research on Galileon inflation enriches the theoretical toolkit available for cosmic inflation, providing a robust and predictive framework that accommodates higher-derivative actions without succumbing to ghost instabilities. By predicting unique non-Gaussian signatures, this model stands out as an intriguing candidate for understanding the mechanics of the early universe, warranting further exploration both theoretically and observationally.