Searching for Standard Clocks in the Primordial Universe

TL;DR

This paper searches for primordial standard clock signals in the CMB power spectrum, finds no significant evidence in WMAP7 data, and forecasts the potential for future experiments like Planck to detect such features, which could reveal the universe's expansion history.

Contribution

It introduces a method to detect primordial standard clock signals in CMB data and assesses the sensitivity of future experiments to these features.

Findings

No statistically significant clock signals found in WMAP7 data.

Forecasts show Planck-like experiments can detect features with at least 20% modulation.

Sensitivity zone identified for wavenumbers 0.01 to 0.1 Mpc^(-1).

Abstract

Classically oscillating massive fields can be used as "standard clocks" in the primordial universe. They generate features in primordial density perturbations that directly record the scale factor evolution a(t). Detecting and measuring these "fingerprint" signals is challenging but would provide a direct evidence for a specific primordial universe paradigm. In this paper, such a search is performed for the power spectrum of the Cosmic Microwave Background (CMB) anisotropies using the WMAP7 data. Although a good fit to the data privileges a scale around k=0.01 Mpc^(-1), we do not find statistical significance for, neither against, the presence of any feature. We then forecast the expected constraints a Planck-like CMB experiment can impose on the fingerprint parameters by using Markov-Chain-Monte-Carlo (MCMC) methods on mock data. We exhibit a high sensitivity zone for wavenumbers ranging from 0.01 Mpc^(-1) to 0.1 Mpc^(-1) in which fingerprints show up first on the posterior probability distribution of the wavenumber at which they occur, and then on the modulation frequency. Within the sensitivity zone, we show that the inflationary paradigm can be inferred from a single feature generating at least a 20% modulation of the primordial power spectrum. This minimal value sensitively depends on the modulation frequency.