Initial episodes of the chemical evolution of the intergalactic medium

The current state of the problem of the origin and transport of ‘heavy’ ($A>4$) chemical elements in the Universe is discussed. The beginning of stellar nucleosynthesis (SNS) dates apparently to $z\gtrsim 20$ redshift epochs (age of the Universe $t_{\rm U}\lesssim 180$ Myr). Presently, SNS traces are observed in some cases in galaxies at redshift $z\sim 10-15$ ($t_{\rm U}\sim 500-270$ Myr). A massive redistribution of chemical elements from galaxies over the entire Universe became possible, primarily under the action of powerful explosions, in the reionization period at $z\lesssim 6$ {($t_{\rm U}\gtrsim 940$ Myr). A correct interpretation of observational data requires an in-depth understanding of the transport and mixing dynamics of chemical elements in the Universe. Theoretical models predict their extremely nonuniform distribution in a range from the interstellar medium on spatial scales of a few hundred light years to the intergalactic medium spanning tens of millions of light years. This is observed in absorption spectra of quasars up to redshift $z\sim 6$ and results in observational selection. The review focuses on the early stages of the history of the Universe's chemical enrichment as it is currently understood given the observational selection effects. Observational data and theoretical ideas underlying the modern understanding of the complex process of the Universe's chemical evolution are outlined.