The cosmic microwave background (CMB) is one of the most powerful tools for cosmology. Its polarization could have imprinted the sign of an inflationary background of gravitational waves, which is supposed to have originated at ${10}^{-38}/{10}^{-35}$ seconds after the Big Bang. Detecting this background is extremely difficult because of the weakness of the signal (if any) left on the CMB polarization and because of the need to control the systematic effects. Additionally, the presence of astrophysical foregrounds, the possibility of leakage from curl-free to curl-like components, including gravitational lensing, and the instrumental noise and systematics, require sensitive detectors and smart systematic effect control. We discuss the experimental efforts spent in this field, highlighting the key observational difference and the choice that could lead, in the near future, to the detection of the curl component of the CMB polarization, a clear sign of the inflationary expansion.

中文翻译：

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寻找原始引力波的宇宙微波背景观测现状

宇宙微波背景 (CMB) 是宇宙学最强大的工具之一。它的极化可能留下了引力波暴胀背景的迹象，这应该起源于${10}^{-38}/{10}^{-35}$大爆炸后的几秒钟。由于留在 CMB 极化上的信号（如果有的话）很弱，并且需要控制系统效应，因此检测这种背景非常困难。此外，天体物理前景的存在、从无卷曲到类卷曲组件泄漏的可能性，包括引力透镜、仪器噪声和系统，需要灵敏的探测器和智能的系统效应控制。我们讨论了在该领域所做的实验工作，强调了关键的观察差异和可能导致在不久的将来检测到 CMB 极化的卷曲分量的选择，这是暴胀扩张的明显迹象。