Separation of Gravitational-Wave and Cosmic-Shear Contributions to Cosmic Microwave Background Polarization

TL;DR

The blurring by lensing of small-scale CMB power leads with this reconstruction technique to a minimum detectable GW amplitude corresponding to an inflation energy near 10(15) GeV.

Abstract

Inflationary gravitational waves (GW) contribute to the curl component in the polarization of the CMB. Cosmic shear--gravitational lensing of the CMB-- converts a fraction of the dominant gradient polarization to the curl component. Higher-order correlations can be used to map the cosmic shear and subtract this contribution to the curl. Arcminute resolution will be required to pursue GW amplitudes smaller than those accessible by Planck. The finite cutoff in CMB power at small scales leads to a minimum detectable GW amplitude corresponding to an inflation energy near 10^15 GeV.

Figures (2)

• Figure 1: Minimum inflation potential observable at $1\sigma$ as a function of survey width for a one-year experiment. The left panel shows an experiment with NET $s=25\, \mu{\rm K}~\sqrt{\rm sec}$. The solid curve shows results assuming no CS while the dashed curve shows results including the effects of an unsubtracted CS; we take $\theta_{\rm FWHM}=5'$ in these two cases. The dotted curves assume the CS is subtracted with $\theta_{\rm FWHM}=10'$ (upper curve) and $5'$ (lower curve). Since the dotted curves are close to the dashed curve, it shows that these higher-order correlations will not be significantly useful in reconstructing the primordial curl for an experiment similar to Planck's sensitivity and resolution. The right panel shows results for hypothetical improved experiments. The dotted curves shows results with CS subtracted and assuming $s=1\, \mu{\rm K}~\sqrt{\rm sec}$, $\theta_{\rm FWHM}=5'$, $2'$, and $1'$ (from top to bottom). The solid curve assumes $\theta_{\rm FWHM}=1'$ and $s=1\, \mu{\rm K}~\sqrt{\rm sec}$, and no CS, while the dashed curve treats CS as an additional noise. The long-dash curve assumes CS subtraction with no instrumental noise ($s=0$).
• Figure 2: Contributions to the CMB polarization power spectra. The long-dashed curve shows the dominant polarization signal in the gradient component due to scalar perturbations. The solid line shows the maximum allowed curl polarization signal from the gravitational-wave background, which will be smaller if the inflationary energy scale is smaller than the maximum value allowed by COBE of $3.47 \times 10^{16}$ GeV. The dashed curve shows the power spectrum of the curl component of the polarization due to CS. The dotted curve is the CS contribution to the curl component that comes from structures out to a redshift of 1; this is the level at which low-redshift lensing surveys can be used to separate the CS-induced polarization from the IGW signal. The dot-dashed line is the residual when lensing contribution is separated with a no-noise experiment and 80% sky coverage.