Inflaton or Curvaton? Constraints on Bimodal Primordial Spectra from Mixed Perturbations

TL;DR

This paper investigates how current and future cosmic microwave background data can constrain models where primordial fluctuations are a mixture of inflaton and curvaton contributions, focusing on spectral index differences and their detectability.

Contribution

It introduces a bimodal power spectrum model for mixed inflaton and curvaton perturbations and assesses the potential of data sets to constrain their relative contributions.

Findings

Current data favor a pure power-law spectrum.

No significant fit improvement with mixed spectra using existing data.

Planck data can significantly constrain bimodal spectrum parameters.

Abstract

We consider Cosmic Microwave Background constraints on inflation models for which the primordial power spectrum is a mixture of perturbations generated by inflaton fluctuations and fluctuations in a curvaton field. If future experiments do not detect isocurvature modes or large non-Gaussianity, it will not be possible to directly distinguish inflaton and curvaton contributions. We investigate whether current and future data can instead constrain the relative contributions of the two sources. We model the spectrum with a bimodal form consisting of a sum of two independent power laws, with different spectral indices. We quantify the ability of current and upcoming data sets to constrain the difference $\Delta n$ in spectral indices, and relative fraction $f$ of the subdominant power spectrum at a pivot scale of $k = 0.017\ {{\rm Mpc^{-1}}\ h}$. Data sets selected are the WMAP 7-year data, alone and in conjunction with South Pole Telescope data, and a synthetic data set comparable to the upcoming Planck data set. We find that current data show no increase in quality of fit for a mixed inflaton/curvaton power spectrum, and a pure power-law spectrum is favored. The ability to constrain independent parameters such as the tensor/scalar ratio is not substantially affected by the additional parameters in the fit. Planck will be capable of placing significant constraints on the parameter space for a bimodal spectrum.