Transparency Temperature in Big Bang

Transparency Temperature

透明温度

At temperatures higher than about 3000 K where the average kinetic energy of particles is about 0.26 electron volts, the formation of stable atoms is hindered. Above that temperature, matter exists in a plasma state of ionized atoms, which strongly absorbs electromagnetic radiation of all wavelengths, i.e., the plasma is opaque.

When the plasma cools below about 3000K, it is cool enough for hydrogen and helium nuclei to collect electrons and become stable atoms. The electrons in stable atoms exist in specific quantum energy states that are characteristic of those atoms. They will absorb only radiation of the specific frequencies (specific photon energies) that will cause an electron to jump to a higher quantum state in that atom, unless the photon energy of the incoming radiation is high enough to take the electron all the way out of the atom (ionize the atom). This means that the cooling gas cloud of the expanding universe has a point at which it forms stable atoms and becomes transparent to almost all wavelengths, at least for photons with quantum energy less than the ionization energy of the atoms.

当等离子体冷却至约3000K以下时，温度足够低，使氢和氦原子核能够捕获电子并形成稳定的原子。稳定原子中的电子存在于特定的量子能级中，这些能级特征于这些原子。它们只会吸收特定频率（特定光子能量）的辐射，使电子跃迁到更高的量子态，除非入射辐射的光子能量足够高，能够将电子完全从原子中移除（电离原子）。这意味着膨胀宇宙中的冷却气体云有一个点，在该点形成稳定的原子并变得对几乎所有波长透明，至少对于光子能量低于原子电离能的光子而言是如此。

Hydrogen

Helium

At this stage of the universe, almost all the ordinary matter was hydrogen and helium (>99%).

在宇宙的这一阶段，几乎所有的普通物质都是氢和氦（>99%）

氢氦

To illustrate the idea of transparency, consider a range of the hydrogen emission spectrum and the helium emission spectrum in the visible wavelengths. The three frequencies of light emitted in the top image represent transitions between the quantum levels of the hydrogen atom. The emitted wavelengths from hot hydrogen gas are also the frequencies that could be absorbed by cool hydrogen, so if you illuminated the hydrogen gas with all of the wavelengths in this range, it would be transparent to most of them - only those three narrow energy bands would be absorbed. So the stable hydrogen gas is transparent to most wavelengths. The helium would have more absorbable frequencies, but for both hydrogen and helium, most wavelengths would pass through without absorption.

为了说明透明的概念，考虑氢的发射光谱和氦的发射光谱在可见光波段的范围。上图中三个频率的光代表氢原子量子水平之间的跃迁。热氢气体发出的波长也是冷氢气可以吸收的频率，因此如果用该波段内的所有波长照射氢气，它对大多数波长都是透明的——只有这三个狭窄的能带会被吸收。因此，稳定的氢气对大多数波长都是透明的。氦气会有更多的可吸收频率，但对氢和氦来说，大多数波长都能通过而不被吸收。

This transparency point is a crucial concept in the modeling of the expanding universe and in the modeling of star formation. Key information about it is provided by the 3 K background radiation.

这个透明点是宇宙膨胀模型和恒星形成模型中的关键概念。有关它的关键信息由3 K背景辐射提供。

在约3000 K以上的温度下，粒子的平均动能约为0.26电子伏特，此时稳定原子的形成受到阻碍。在此温度以上，物质处于离子化的等离子体状态，其中原子强烈吸收所有波长的电磁辐射，即等离子体是不透明的。

Radiation Pressure

辐射压力

Stars can maintain fairly stable sizes because of the radiation pressure exerted by the radiation coming from the hot core. This radiation pressure comes into play in a major way at point during the stellar evolution where the collapsing gas cloud becomes opaque to electromagnetic radiation. Striking this opaque ionized region, the radiation is said to "scatter" off the ions, exerting a net outward pressure which halts the gravitational collapse.

恒星能够维持相对稳定的大小，是因为来自炽热核心的辐射所施加的辐射压力。这种辐射压力在恒星演化的某个关键阶段起着重要作用，此时坍缩的气体云对电磁辐射变得不透明。当辐射撞击到这个不透明的离子化区域时，辐射会被说成‘散射’在离子上，产生一个净的向外压力，从而阻止了引力坍缩。

There is a strong connection between the transparency point and radiation pressure. Trefil makes the analogy to the air in a tire - the pressure exists because the molecules bounce back from the tire "the tire remains inflated because the rubber walls are very efficient at scattering air molecules." Before the transparency point of the "big bang", the ions and electrons of the plasma were efficient scatterers of light, but after they condense into atoms, they are very inefficient scatterers of light - you can easily see 100 miles through air on a clear day.

透明点与辐射压力之间存在强关联。Trefil将这一现象类比为轮胎内的空气——压力的存在是因为分子反弹回轮胎，“轮胎保持充气是因为橡胶墙壁非常高效地散射空气分子。”在“大爆炸”透明点之前，等离子体中的离子和电子是光的高效散射体，但一旦它们结合形成原子，它们就成为光的非常低效散射体——你可以在晴朗的日子里轻松看到十英里远。

Arthur Eddington is credited with calculating a radiation pressure of some 25 million atmosperes for a model star and with calculating that for a star of more than about a hundred solar masses the radiation pressure alone would tear it apart.

阿瑟·爱丁顿被认定为计算了一个恒星模型中的辐射压力约为2500万大气压，并计算出对于超过约百倍太阳质量的恒星，仅靠辐射压力就会将其撕裂。