Phenomenology of a Pseudo-Scalar Inflaton: Naturally Large Nongaussianity (1102.4333v1)

Abstract: Many controlled realizations of chaotic inflation employ pseudo-scalar axions. Pseudo-scalars \phi are naturally coupled to gauge fields through c \phi F \tilde{F}. In the presence of this coupling, gauge field quanta are copiously produced by the rolling inflaton. The produced gauge quanta, in turn, source inflaton fluctuations via inverse decay. These new cosmological perturbations add incoherently with the "vacuum" perturbations, and are highly nongaussian. This provides a natural mechanism to generate large nongaussianity in single or multi field slow-roll inflation. The resulting phenomenological signatures are highly distinctive: large nongaussianity of (nearly) equilateral shape, in addition to detectably large values of both the scalar spectral tilt and tensor-to-scalar ratio (both being typical of large field inflation). The WMAP bound on nongaussianity implies that the coupling, c, of the pseudo-scalar inflaton to any gauge field must be smaller than about 10^{2} M_p^{-1}.

• The paper demonstrates that pseudo-scalar inflatons interacting with gauge fields produce large, nearly equilateral nongaussianity.
• It shows that inverse decay processes amplify scalar perturbations when the coupling exceeds 10⁻² Mₚ⁻¹.
• The analysis bridges particle physics and cosmology by quantifying constraints consistent with CMB anisotropy observations.

Phenomenology of a Pseudo-Scalar Inflaton: Naturally Large Nongaussianity

The paper conducted by Barnaby, Namba, and Peloso presents an investigation into the phenomenology of axion inflation characterized by the presence of pseudo-scalar inflatons. The paper focuses on the interactions between these pseudo-scalars and gauge fields, which are described by the interaction term $c\phi F \tilde{F}$, and their implications for the generation of cosmological perturbations.

In many realizations of chaotic inflation, pseudo-scalar axions are naturally coupled to gauge fields. This coupling results in the production of gauge quanta due to the rolling inflaton during inflation, which in turn affects scalar field fluctuations through a process termed "inverse decay." The decay manifests in the form of highly non-gaussian perturbations. Notably, these induced perturbations are significant enough to dominate over standard vacuum fluctuations under specific conditions, particularly when the coupling $c$ of the pseudo-scalar inflaton to the gauge field exceeds $10^{-2} M_p^{-1}$.

The distinctive feature of the pseudo-scalar inflaton model is the naturally large nongaussianity it generates, which has a nearly equilateral shape. This finding provides a nontrivial pattern of cosmological fluctuations that diverge from those predicted by simple single-field inflaton models.

In addressing the strong numerical results, the paper quantifies the nongaussianity using the bispectrum shape function and discusses the correlation with the well-known equilateral and orthogonal templates extensively utilized in the analysis of the CMB. The analysis concludes with a vital constraint placing the pseudo-scalar coupling $c$ below $10^{2} M_p^{-1}$ based on WMAP bounds.

The implications of these findings offer a bridge between particle physics and cosmological observations, pushing for a detailed assessment of axion-like interactions in inflation models. The model not only expands the theoretical framework for inflationary dynamics by incorporating these interactions but also paves the way for future experimental probes into nongaussianity through precise CMB measurements and other cosmological data.

On a theoretical level, the paper challenges the perception that large nongaussianity is a signal of non-standard dynamics, positing instead that it could naturally arise in well-motivated particle physics models of inflation like axion inflation, which embeds string-theoretical concepts. Practically, the findings suggest that different particle physics models could lead to varied predictions on the observed anisotropies of the CMB, thereby opening a window for more tests and falsification of theoretical predictions.

Future developments in this area are likely to focus on further refining measurements of nongaussianity and tensor-to-scalar ratios, which can act as discerning features of the various inflationary models under consideration. Enhanced observational facilities will provide robust datasets to test the claims and constraints posited by this paper, potentially leading to concrete evidence of axion-like fields influencing early universe dynamics. In sum, the research extends our understanding of how fundamental axion interactions can naturally lead to significant cosmological signatures, thereby enriching the theoretical landscape and providing fertile ground for observational pursuits in cosmology.