Limitations of CMB B -mode template delensing

Measurements of the primordial B modes of the CMB are crucial for testing cosmic inflation. However, gravitational lensing of the CMB photons contaminates these measurements. Here, the authors show, through analytic and numerical work, that the simplest method of removing this contamination, using a template at the gradient level based on lensed E modes, is perhaps unexpectedly, remarkably effective and will be very useful in analyzing CMB data, especially for detecting low values of the tensor to scalar ratio.