Feedback processes at cosmic dawn

The word "feedback" is by far one of the most used ones in modern cosmology where it is applied to a vast range of situations and astrophysical objects. However, for the same reason, its meaning in the context is often unclear or fuzzy. Hence a review on feedback should start from setting the definition of feedback on a solid basis. We have found quite useful to this aim to go back to the Oxford Dictionary from where we take the following definition. Feed' back n. 1. (Electr.) Return of fraction of output signal from one stage of circuit, amplifier, etc. to input of same or preceding stage (Positive, negative, tending to increase, decrease the amplification, etc). 2. (Biol., Psych., etc) Modification or control of a process or system by its results or effects, esp. by difference between the desired and actual results. In spite of the broad description, we find this definition quite appropriate in many ways. First, the definition outlines the fact that the concept of feedback invokes a back reaction of a process on itself or on the causes that have produced it. Secondly, the character of feedback can be either negative or positive. Finally, and most importantly, the idea of feedback is intimately linked to the possibility that a system can become self-regulated. Although some types of feedback processes are disruptive, the most important ones in astrophysics are probably those that are able to drive the systems towards a steady state of some sort. To exemplify, think of a galaxy that is witnessing a burst of star formation. The occurrence of the first supernovae will evacuate/heat the gas thus suppressing the star formation activity. Such feedback is then acting back on the energy source (star formation); it is of a negative type, and it could regulate the star formation activity in such a way that only a sustainable amount of stars is formed (regulation). However, feedback can fail to produce such regulation either in small galaxies where the gas can be ejected by the first SNe or in cases when the star formation timescale is too short compared to the feedback one. As we will see there are at least three types of feedback, and even the stellar feedback described in the example above is part of a larger class of feedback phenomena related to the energy deposition of massive stars. We then start by briefly describing the key physical ingredients of feedback processes in cosmology.