“再电离”时期是“黑暗时代”宇宙的中性气体变成电离的等离子体的时期。模拟表明，原子氢的21-cm光谱跃变将会在标志着“再电离”的中心舞台的红移和尺度上显示一个清楚的波动峰。这一预测是基于以下假设：宇宙气体的加热（发生在“再电离”之前）是由恒星残体（尤其是X-射线双星）造成的，会达到远高于当时的宇宙微波背景的温度(30 开)。然而，Rennan Barkana及同事则报告说，X-射线双星（高能光子的数量多于低能光子）的硬光谱使得这种加热效果不好，导致延迟的、在空间上均匀的加热。在这一新模型中，“再电离”的21-cm特征被修正成具有一个明显最低温度（低于1开）的一个更复杂的信号，它标志着宇宙平均气体温度升至高于微波背景的程度。
Models and simulations of the epoch of reionization predict that spectra of the 21-centimetre transition of atomic hydrogen will show a clear fluctuation peak, at a redshift and scale, respectively, that mark the central stage of reionization and the characteristic size of ionized bubbles. This is based on the assumption that the cosmic gas was heated by stellar remnants—particularly X-ray binaries—to temperatures well above the cosmic microwave background at that time (about 30 kelvin). Here we show instead that the hard spectra (that is, spectra with more high-energy photons than low-energy photons) of X-ray binaries make such heating ineffective, resulting in a delayed and spatially uniform heating that modifies the 21-centimetre signature of reionization. Rather than looking for a simple rise and fall of the large-scale fluctuations (peaking at several millikelvin), we must expect a more complex signal also featuring a distinct minimum (at less than a millikelvin) that marks the rise of the cosmic mean gas temperature above the microwave background. Observing this signal, possibly with radio telescopes in operation today, will demonstrate the presence of a cosmic background of hard X-rays at that early time.