Masui Kiyoshi Wesley, Pen Ue-Li, Turok Neil
Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Rd, Vancouver, British Columbia V6T 1Z1, Canada.
Canadian Institute for Theoretical Astrophysics, 60 St. George Street, Toronto, Ontario M5S 3H8, Canada.
Phys Rev Lett. 2017 Jun 2;118(22):221301. doi: 10.1103/PhysRevLett.118.221301. Epub 2017 Jun 1.
We show that three-dimensional information is critical to discerning the effects of parity violation in the primordial gravity-wave background. If present, helical gravity waves induce parity-violating correlations in the cosmic microwave background (CMB) between parity-odd polarization B modes and parity-even temperature anisotropies (T) or polarization E modes. Unfortunately, EB correlations are much weaker than would be naively expected, which we show is due to an approximate symmetry resulting from the two-dimensional nature of the CMB. The detectability of parity-violating correlations is exacerbated by the fact that the handedness of individual modes cannot be discerned in the two-dimensional CMB, leading to a noise contribution from scalar matter perturbations. In contrast, the tidal imprints of primordial gravity waves fossilized into the large-scale structure of the Universe are a three-dimensional probe of parity violation. Using such fossils the handedness of gravity waves may be determined on a mode-by-mode basis, permitting future surveys to probe helicity at the percent level if the amplitude of primordial gravity waves is near current observational upper limits.
我们表明，三维信息对于识别原初引力波背景中宇称破缺的影响至关重要。如果存在，螺旋引力波会在宇宙微波背景（CMB）中诱导出宇称破缺的相关性，这种相关性存在于宇称奇数极化B模式与宇称偶数温度各向异性（T）或极化E模式之间。不幸的是，EB相关性比直观预期的要弱得多，我们表明这是由于CMB二维性质所导致的一种近似对称性。由于在二维CMB中无法辨别单个模式的手征性，从而导致标量物质微扰产生噪声贡献，这使得宇称破缺相关性的可探测性进一步恶化。相比之下，固化在宇宙大尺度结构中的原初引力波的潮汐印记是宇称破缺的三维探测器。利用这些印记，可以逐模式地确定引力波的手征性，如果原初引力波的振幅接近当前观测上限，那么未来的观测有望探测到百分比水平的螺旋度。
