Oscillations in the CMB from Axion Monodromy Inflation

TL;DR

The study analyzes oscillatory features in the CMB predicted by axion monodromy inflation, deriving an analytic modulated scalar power spectrum and exploring resonance-driven particle production. It confronts the model with five-year WMAP data and then embeds the scenario in explicit string-theory realizations, deriving axion decay constants, backreaction effects, and higher-derivative constraints. By combining observational limits with microphysical consistency requirements, the authors map out regions of parameter space where detectable oscillations in the power spectrum and possibly in the bispectrum could arise in the near future, alongside a robust tensor signal around r≈0.07. The work argues that resonant signatures are a characteristic, testable hallmark of axion monodromy inflation in well-controlled string constructions and identifies concrete avenues for future investigation and model-building refinement.

Abstract

We study the CMB observables in axion monodromy inflation. These well-motivated scenarios for inflation in string theory have monomial potentials over super-Planckian field ranges, with superimposed sinusoidal modulations from instanton effects. Such periodic modulations of the potential can drive resonant enhancements of the correlation functions of cosmological perturbations, with characteristic modulations of the amplitude as a function of wavenumber. We give an analytical result for the scalar power spectrum in this class of models, and we determine the limits that present data places on the amplitude and frequency of modulations. Then, incorporating an improved understanding of the realization of axion monodromy inflation in string theory, we perform a careful study of microphysical constraints in this scenario. We find that detectable modulations of the scalar power spectrum are commonplace in well-controlled examples, while resonant contributions to the bispectrum are undetectable in some classes of examples and detectable in others. We conclude that resonant contributions to the spectrum and bispectrum are a characteristic signature of axion monodromy inflation that, in favorable cases, could be detected in near-future experiments.

Figures (10)

• Figure 1: The solid line is the analytical result for $\delta n_s$ as a function of $f$, for $b=0.08$, while the dots are the numerical result obtained from an adaptation of the code used in inflation.
• Figure 2: This plot shows the 68% and 95% likelihood contours in the $\delta n_s$-$f$, and $bf$-$\log_{10}f$ plane, respectively, from the five-year WMAP data on the temperature angular power spectrum.
• Figure 3: The left plot shows the angular power spectrum for the best fit point $f=6.67\times 10^{-4}$, and $\delta n_s=0. 17$. The right plot shows the angular power spectrum for the best fit point together with the unbinned WMAP five-year data.
• Figure 4: These plots show the 68% and 95% likelihood contours for the five-year WMAP data on the temperature angular power spectrum in the $\delta n_s$ - $\Omega_Bh^2$ plane and $\delta n_s$ - $\Delta\phi$ plane for an axion decay constant of $f=3\times 10^{-2}$ and $f=1.5\times 10^{- 2}$, respectively.
• Figure 5: This figure shows a triangle plot for some of the parameters that were sampled in a Markov chain Monte Carlo for an axion decay constant of $f=10^{-2}$. The contours again represent 68% and 95% confidence levels.