After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line. This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz. Here we report the detection of a flattened absorption profile in the sky-averaged radio spectrum, which is centred at a frequency of 78 megahertz and has a best-fitting full-width at half-maximum of 19 megahertz and an amplitude of 0.5 kelvin. The profile is largely consistent with expectations for the 21-centimetre signal induced by early stars; however, the best-fitting amplitude of the profile is more than a factor of two greater than the largest predictions. This discrepancy suggests that either the primordial gas was much colder than expected or the background radiation temperature was hotter than expected. Astrophysical phenomena (such as radiation from stars and stellar remnants) are unlikely to account for this discrepancy; of the proposed extensions to the standard model of cosmology and particle physics, only cooling of the gas as a result of interactions between dark matter and baryons seems to explain the observed amplitude. The low-frequency edge of the observed profile indicates that stars existed and had produced a background of Lyman-α photons by 180 million years after the Big Bang. The high-frequency edge indicates that the gas was heated to above the radiation temperature less than 100 million years later.

中文翻译：

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天空平均光谱中以 78 兆赫为中心的吸收曲线

在早期宇宙中形成恒星之后，预计它们的紫外线最终会穿透原始氢气并改变其 21 厘米超细线的激发状态。这种改变会导致气体吸收来自宇宙微波背景的光子，产生光谱失真，今天在低于 200 兆赫的无线电频率下应该可以观察到。在这里，我们报告了在天空平均无线电频谱中检测到平坦的吸收曲线，其中心频率为 78 兆赫，半峰全宽为 19 兆赫，振幅为 0.5 开尔文。该剖面图与早期恒星引起的 21 厘米信号的预期基本一致；然而，轮廓的最佳拟合幅度比最大预测值大两倍以上。这种差异表明，要么原始气体比预期冷得多，要么背景辐射温度比预期高。天体物理现象（例如来自恒星和恒星残骸的辐射）不太可能解释这种差异；在宇宙学和粒子物理学标准模型的拟议扩展中，只有暗物质和重子之间相互作用导致气体冷却似乎可以解释观察到的振幅。观测到的剖面的低频边缘表明，在大爆炸后的 1.8 亿年，恒星存在并产生了莱曼-α 光子的背景。