Transparency of hydrogen to different electromagnetic wavelengths in the early universe I can somehow grasp the early universe being compared as a sphere surface filled densly with plasma like a spherical chessboard all filled with chess pieces.So as that surface streched the distances between plasma particles got bigger and hydrogen formed wich was transparent to light but my question is did the early universe had uniform EM energy distribution or there were photons with higher energy and could hydrogen be opaque for certain wavelengths?
 A: Before the hydrogen epoch, there was a plasma of quarks gluons electrons, the photons would continually scatter off the charged particles. When hydrogen dominates photons do not scatter any longer as there are no free charges, i.e. no plasma, nuclei (mostly hydrogen) have formed and tied up the electrons. It is then that the photons of the Cosmic MIcrowave Backround radiation start their journey.


history of universe

Here it describes what happens with hydrogen and photons:

Immediately after the Big Bang, the universe was a hot, dense plasma of photons, leptons, and quarks: the quark epoch. At $10^{−6}$ seconds, the Universe had expanded and cooled sufficiently to allow for the formation of protons: the hadron epoch. This plasma was effectively opaque to electromagnetic radiation due to Thomson scattering by free electrons, as the mean free path each photon could travel before encountering an electron was very short. This is the current state of the interior of the Sun. As the universe expanded, it also cooled. Eventually, the universe cooled to the point that the formation of neutral hydrogen was energetically favored, and the fraction of free electrons and protons as compared to neutral hydrogen decreased to a few parts in 10,000.


Recombination involves electrons binding to protons (hydrogen nuclei) to form neutral hydrogen atoms. Because direct recombinations to the ground state (lowest energy) of hydrogen are very inefficient, these hydrogen atoms generally form with the electrons in a high energy state, and the electrons quickly transition to their low energy state by emitting photons. Two main pathways exist: from the 2p state by emitting a Lyman-a photon - these photons will almost always be reabsorbed by another hydrogen atom in its ground state - or from the 2s state by emitting two photons, which is very slow.


This production of photons is known as decoupling, which leads to recombination sometimes being called photon decoupling, but recombination and photon decoupling are distinct events. Once photons decoupled from matter, they traveled freely through the universe without interacting with matter and constitute what is observed today as cosmic microwave background radiation

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The fact that hydrogen formation neutralizes the plasma allows for the primordial photons to decouple because they no longer interact with high probability with the neutral hydrogen.
It is not a ball of hydrogen but the neutral constituent hydrogen that is allowing the  CMB to exist up to now..
