Relic gravitational waves: latest revisions and preparations for new data

TL;DR

This paper revises the evidence for relic gravitational waves using updated WMAP7 data with CosmoMC, discusses assumptions in CMB analysis, and assesses future detection prospects with Planck data.

Contribution

It critically re-evaluates relic gravitational wave indications with CosmoMC and outlines strategies to improve future CMB data analysis for detecting these waves.

Findings

Indications of relic gravitational waves reduced from ~2σ to ~1σ.

Assumptions in CMB data analysis critically affect gravitational wave detection.

Detection prospects with Planck are promising but require better foreground mitigation.

Abstract

The forthcoming release of data from the Planck mission, and possibly from the next round of Wilkinson Microwave Anisotropy Probe (WMAP) observations, make it necessary to revise the evaluations of relic gravitational waves in the existing data and, at the same time, to refine the assumptions and data analysis techniques in preparation for the arrival of new data. We reconsider with the help of the commonly used CosmoMC numerical package the previously found indications of relic gravitational waves in the 7-year (WMAP7) data. The CosmoMC approach reduces the confidence of these indications from approximately 2$\sigma$ level to approximately 1$\sigma$ level, but the indications do not disappear altogether. We critically analyze the assumptions that are currently used in the Cosmic Microwave Background (CMB) data analyzes and outline the strategy that should help avoid the oversight of relic gravitational waves in the future CMB data. In particular, it is important to keep away from the unwarranted assumptions about density perturbations. The prospects of confident detection of relic gravitational waves by the Planck satellite have worsened, but they are still good. It appears that more effort will be required in order to mitigate the foreground contamination.