Ratio of C^EE to C^BB from synthetic dust polarization at 353 GHz. Top panel: EE and BB power spectra computed from synthetic dust polarization maps with cold gas (T  <  200 K) excluded. The shaded area overlapping the line represents the standard deviation. The gray region denotes the range of C^EE/C^BB ratios measured by Planck. k_inj is the injection wavenumber of turbulence, and k_dis is the numerical dissipation wavenumber. Bottom panel: same as the top panel, but showing spectra computed separately from different gas phases: WNM (T  >  5000 K; red), UNM (200 K <T <  5000 K; green), and CNM (T  <  200 K; blue).