Short answers, pulled from the story.
Erwin Schrödinger delivered his lecture on superposition in Dublin in the year 1952. He warned the audience that his discussion of multiple histories happening simultaneously might seem lunatic. This event marked a key moment in the history of quantum mechanics and the multiverse concept.
The American philosopher and psychologist William James first used the term multiverse in 1895. He used the term in a different context than the modern scientific definition used today. This historical usage predates the modern scientific debate that began in 1895 between Boltzmann and Zermelo.
Dr. Ranga-Ram Chary found a signal 4,500 times brighter than expected in the cosmic radiation spectrum in 2015. This signal arose from the formation of atoms during the era of recombination and suggests a universe with 65 times more matter particles than our own. There is a 30% chance that this signal is noise rather than evidence of a parallel universe.
Max Tegmark outlined four levels of multiverses in his classification scheme. Level IV, known as the Ultimate Ensemble, considers all universes to be equally real and described by different mathematical structures. This level subsumes all other ensembles and brings closure to the hierarchy of multiverses.
The Many-Worlds Interpretation of quantum mechanics proposed by Hugh Everett III suggests that every quantum event creates a branching of the universe into multiple realities. In this view, all possible outcomes of a quantum event correspond to different worlds that are as real as our own. This interpretation implies that Schrödinger's cat thought experiment results in both outcomes being real in at least one world.