The mechanisms of absorption formation in the cosmic microwave background (CMB) spectrum at the frequency of the 21-cm line of the transition between the ground-state hyperfine sublevels of the hydrogen atom are analyzed. We show that a strong nonlinearity at the compression stage of primordial matter density fluctuations can give rise to a significant (in depth) absorption even before the explosions of the first stars. In this case, the main effect is due to the heating of matter in a certain narrow range of temperatures under cloud compression. We consider a steady-state radiative transfer in the 21-cm line in a medium that represents a contracting primordial matter density fluctuation at a given redshift z modeled by a homogeneous spherically symmetric cloud in the state of collapse with an adiabatic change in the gas temperature. For a sequence of cloud states with different degrees of compression we have calculated the frequency profiles of the line in the flux of radiation emerging from the cloud. In the initial state we specify the cloud radius r₀, while the gas density is assumed to be equal to the mean cosmological density for a given redshift. We show that for a separate cloud at z = 20, r₀ = 1 kpc, and a degree of radius compression of 1.9 the absorption depth in the line center can reach 0.9 K. When averaged over an ensemble of clouds, the central frequency of the line and its width are determined by the details of the fluctuation evolution dynamics.

中文翻译：

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非线性压缩阶段原始物质密度波动中21 cm线的吸收

分析了在氢原子基态超细亚能级之间跃迁的21 cm线频率处的宇宙微波背景（CMB）光谱中吸收形成的机理。我们表明，即使在第一批恒星爆炸之前，原始物质密度波动压缩阶段的强烈非线性也会引起显着（深度）吸收。在这种情况下，主要的影响是由于在云的压缩下在一定的狭窄温度范围内物质的加热。我们考虑介质中21 cm线中的稳态辐射传递，该介质表示在给定的红移z下收缩的原始物质密度波动由均匀球形对称云在气体温度绝热变化而崩溃的状态下建立模型。对于具有不同压缩程度的一系列云状态，我们已经计算了从云中发出的辐射通量中线的频率分布。在初始状态下，我们指定云半径r ₀，同时假定气体密度等于给定红移的平均宇宙学密度。我们证明了对于z = 20的独立云，r ₀ = 1 kpc，半径压缩程度为1.9时，线中心的吸收深度可以达到0.9K。当对一团云进行平均时，线的中心频率及其宽度由波动演变的细节确定。动力学。