Predicted Constraints on Cosmic String Tension from Planck and Future CMB Polarization Measurements

Abstract

We perform a Fisher matrix calculation of the predicted uncertainties on estimates of the cosmic string tension Gmu from upcoming observational data (namely, cosmic microwave background power spectra from the Planck satellite and an idealized future polarization experiment). We employ simulations that are more general than others commonly used in the literature, leaving the mean velocity of strings, correlation length of the string network, and "wiggliness" (which parametrizes smaller-scale structure along the strings) as free parameters that can be observationally measured. In a new code, StringFast, we implement a method for efficient computation of the C_l spectra induced by a network of strings, which is fast enough to be used in Markov Chain Monte Carlo analyses of future data. Performing a calculation with the string parameters left free results in projected constraints on Gmu that are larger than those obtained by fixing their values a priori, typically by a factor of ~2-7. We also find that if Gmu is equal to the current observational maximum, Planck will be able to make a confident detection of strings. However, if Gmu is two orders of magnitude smaller, even a perfect, lensing-free measurement of polarization power spectra will not be able to detect a nonzero string tension at better than 2 sigma confidence.

8 pages, 3 figures; references added and typos fixed to match published version; StringFast code available at http://www.astro.ubc.ca/people/scott/stringfast.html

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• Planck 2013 results. XXV. Searches for cosmic strings and other topological defects
• Constraints on cosmic string tension imposed by the limit on the stochastic gravitational wave background from the European Pulsar Timing Array
• Cosmic String constraints from WMAP and the South Pole Telescope
• Cosmic string parameter constraints and model analysis using small scale Cosmic Microwave Background data
• Forecast constraints on cosmic strings from future CMB, pulsar timing and gravitational wave direct detection experiments
• The information content of cosmic microwave background anisotropies
• Probing cosmic string spacetime through parameter estimation
• Peak-peak correlations in the cosmic background radiation from cosmic strings
• Evading the pulsar constraints on the cosmic string tension in supergravity inflation