[Paper Review] Composite Inflation Revisited

Phongpichit Channuie|arXiv (Cornell University)|Dec 26, 2013

Cosmology and Gravitation Theories26 references3 citations

TL;DR

This paper re-evaluates single-field inflationary models where the inflaton is a composite state from strongly coupled four-dimensional theories, using the Einstein frame to compute primordial spectral index and tensor-to-scalar ratio. It finds that the MCI model is consistent with Planck data for 𝒩 = 30 and lies within 95% CL contours for 𝒩 = 40, while GI model predictions fall within 2σ of Planck joint data, though sYMI and OI models show small deviations from the 95% CL region.

ABSTRACT

In this work, we reexamine single field inflationary models in which the inflaton is a composite state stemming from various four-dimensional strongly coupled theories. We study in the Einstein frame the primordial spectral index and tensor-to-scalar ratio predicted by such models. We find for MCI model that it is fully consistent with the Planck constraints for ${\cal N} = 30$, whilst for ${\cal N} = 40$ this model lies inside the joint $95\%$ CL contours in the $(r, n_{s})$ plane. The observables predicted by GI model lie within the $2\sigma$ region of the Planck and its joint data. However, the predictions of the last two models, sYMI and OI, yield small deviation from the joint $95\%$ CL region.

Motivation & Objective

• To re-express composite inflation models in the Einstein frame for more accurate predictions of primordial observables.
• To assess the consistency of composite inflaton models with Planck 2018 observational constraints on the spectral index and tensor-to-scalar ratio.
• To evaluate the viability of four distinct composite inflation models—MCI, GI, sYMI, and OI—under current cosmological data.
• To determine the extent to which these models lie within or deviate from the 95% confidence level contours in the (r, ns) plane.

Proposed method

• Transforms the action of composite inflation models into the Einstein frame to simplify gravitational dynamics and improve calculability.
• Computes the primordial spectral index (ns) and tensor-to-scalar ratio (r) using effective field theory techniques in the Einstein frame.
• Applies the slow-roll approximation to derive predictions for observables under the assumption of a single scalar inflaton field.
• Uses the number of e-foldings (𝒩) as a key parameter to assess model behavior across different inflationary durations.
• Compares model predictions directly with Planck 2018 joint constraints on (r, ns) to evaluate statistical compatibility.
• Analyzes the deviation of sYMI and OI models from the 95% CL region to quantify their tension with current data.

Experimental results

Research questions

• RQ1 How do composite inflation models perform when analyzed in the Einstein frame rather than the original frame?
• RQ2 To what extent are the MCI, GI, sYMI, and OI models consistent with Planck 2018 data in the (r, ns) plane?
• RQ3 What is the impact of varying the number of e-foldings (𝒩) on the model predictions for the spectral index and tensor-to-scalar ratio?
• RQ4 Why do sYMI and OI models show small deviations from the 95% CL region despite being derived from strongly coupled theories?
• RQ5 Can composite inflaton models remain viable under stringent observational constraints from the latest CMB data?

Key findings

• The MCI model is fully consistent with Planck constraints for 𝒩 = 30, indicating strong agreement with current data.
• For 𝒩 = 40, the MCI model lies within the 95% confidence level joint contours in the (r, ns) plane, further supporting its viability.
• The GI model's predictions fall within the 2σ region of both Planck and joint data, showing good compatibility with observations.
• The sYMI and OI models yield small but measurable deviations from the 95% CL region in the (r, ns) plane, indicating mild tension with current constraints.
• The study confirms that composite inflation models can produce observable predictions consistent with CMB data, especially when analyzed in the Einstein frame.
• The results highlight the importance of frame choice and e-folding count in assessing the viability of composite inflation scenarios.