Alternate universes, often referred to as "parallel" universes, are a legitimate part of theoretical physics and cosmological speculations.  (See cosmology.)  Which doesn't make them real but does make speculation about them at least somewhat respectable.
    Below is an entirely fictional presentation of how alternate universes are understood to work, according to the Xiox, an entirely fictional, deep-spacefaring extraterrestrial culture.  It might even crudely resemble, in one aspect or another, how such things operate — if there actually are alternate universes.  In cooking it up, of course, I fitted it to the story needs of the novel it was written for (and the prequel thereto).  Thus my "scientific basis" is that arcane study known as Balonium.
    You may find it entertaining; possibly even thought provoking.


    Parallel universes do not result from randomly or regularly generated alternatives.  They result when a sophont decides, knowingly or unknowingly, between two or more alternative actions of sufficiently effective differences.  
    Like a stone thrown into a pond, the results of choice propagate outward in what can be thought of as a ripple effect.  But unless the initiating decision is quite potent, the difference will not maintain itself against the tendency toward the conservation of established universes.  The separation does not perpetuate, and only one of the two alternatives continues.  
    But if the changes are potent enough, "parallel" universes result, or "divergent" universes, if you prefer.  None of them having any material trace of the other or others.  However, within the reality matrixp, the primal matrix, the causal complex persists for a considerable period as shadow events.  And by focusing on selected fields, adepts can discern critical events imprinted by their primal causality.  And with sufficient knowledge of pre-event conditions, they can then be given context and identity.

    Thus, in year 1983 of the Terran Common Era, in what we can call the stem universe, a sequence of extreme political events and military posturing led to an American naval task force holding exercises in the vicinity of Korea.  Within weeks, the government of the Soviet Union replied with a large-scale demonstration of naval power within five hundred miles of the Hawaiian Islands.  Given the experience of 1941, the American Pacific Fleet was sent out to confront it, a response sufficiently threatening, it was misread as an impending attack.  
    Ordered by Soviet Pacific Fleet Command, the Soviet force commander ordered a single tactical nuclear missile launched to destroy the American flagship.  However, the order was incorrectly transmitted, and all of his missile ships fired.
    The Soviet admiral, appalled by the error, immediately notified Moscow.  At the same time, an American satellite monitoring the confrontation reported this multi-missile launch, and the U.S. responded immediately with a launch not only against the Soviet task force, but against naval shore targets in the Soviet Far East.
    The chief Soviet executive, Yuri V. Andropov, had responded instantly.  Assuming the Americans would launch a wider-ranging nuclear response than they actually did, he ordered an ICBM attack on numerous strategic American targets.  This massive launch was reported promptly, and the Americans raised the ante "while they still could."
    The critical mis-transmission of the Soviet firing order resulted from a choice made by the admiral's signalman -- to covertly drink ethanol on watch.  The result was a space-time bifurcation, and two parallel universes.  The first drink initiated one of them.  The other was the continuation of the stem universe, in which the deadly choice was not made.  In each of those universes, choices made during the next few minutes created a veritable spray of incipient new universes.  This often happens at critical junctures in the violent decline of sapient life forms.
    In the universe of interest here, which we will call Universe Terra One, hundreds of fusion warheads exploded in the atmosphere and on the surface.  The risk of such a war had been foreseen.  Scientists had predicted not only extensive shock wave and radiation damage, but extensive urban, forest and grass fires; a major increase in albedo that would take several years to decline to pre-war normal; the effective destruction of planetary technical infrastructures, including food production and transportation; enormous direct and indirect human fatalities; and the collapse of law and order.
    Their predictions, however, were not met.  Instead, an unforeseen effect resulted which still is not understood: a major imbalance in the local sector of the underlying reality matrix.  Which was promptly adjusted by the  erasure of the still localized universe of the cataclysm, and its replacement by another, one in which Terra differed from its precursor in some but by no means all respects.  In it, many effects of then-recent Terran history virtually disappeared, remaining only as more or less vague memories in the surviving, ethnically redistributed sophonts.  Sophonts confused not only by the new and unexplainable world actually experienced, but by vague images of horrors in a stillborn universe, horrors that never quite happened.  
    In the Xiox sphere of interest, this was the first, and so far only known event of a reality matrix rebalancing itself in that manner.  Its study gave rise to whole new areas of research, and in time to the cosmology that today we take for granted.