In popular culture, it goes back to DC comics, specifically Flash #123 in September of 1961.

https://en.m.wikipedia.org/wiki/Flash_of_Two_Worlds

First, a little comic book history… DC set the standard for Superhero comics in 1938 when they introduced Superman. Following that was the creation of character after character.

This was called “The Golden Age” of comics.

The 1950s rolled around and superhero books kind of faded away, replaced by crime and horror books. This drew the attention of Frederic Wertham, a child psychologist, who sought to blame all of societies ills on comic books.

https://en.m.wikipedia.org/wiki/Seduction_of_the_Innocent

This inspired the creation of the Comics Code Authority which controlled what could and could not be published.

https://en.m.wikipedia.org/wiki/Comics_Code_Authority

2 years after Seduction of the Innocent, DC re-booted superhero comics with Showcase #4, introducing a new version of a Golden Age character called The Flash:

https://en.m.wikipedia.org/wiki/Silver_Age_of_Comic_Books

Shortly after that, just like many characters appearing after the introduction of Superman 20 years earlier, a whole host of new characters appeared in the Silver Age, all approved by the Comics Code.

By 1961, DC was facing questions from fans about “Wait, how can there be two Flashes? Or two Green Lanterns? Where are the original characters?”

To explain this, they invented the concept of “Earth 1” and “Earth 2”.

All the current characters and stories were happening on Earth 1, all the Golden Age characters and stories were on Earth 2.

Two planets separated by a vibrational difference, a difference that the Flashes of both worlds were able to cross.

And so Flash #123 introduced the whole Earth1 / Earth 2 concept in 1961.

Following that, they set up an annual crossover event between the Justice League (Earth 1) and the Justice Society (Earth 2), as a “CRISIS ON EARTH…”

Not an expert, but my understanding is that the multiverse (at least, what we today associate as the multiverse) came about due to the many worlds interpretation of quantum mechanics. Basically, quantum physicists had an observation - particles were moving as though they were being pushed by an invisible wave, and they would pick a random position based on that wave when observed.

The most prevalent explanation for this behavior is the Copenhagen interpretation, which states that the particle is the invisible wave, and the wave collapses into a particle when it is measured. But another common interpretation is the many worlds interpretation, which states that the invisible wave is just a statistical probability of where the particle is. And the reason why the particle seems to pick a random point on the wave when observed is actually because the particle creates branching timelines, and we can only observe what happens in our own timelines. Hence, it seems random to us.

I speculate that the idea of multiple parallel timelines, each slightly different, was probably pretty popular with scifi writers, especially since it’s an easy way to portray “what if” scenarios in their stories, and so the concept became popular because of that

According to a few Google searches it was a physisict Hugh Everett in the late 50s. I am sure there were others in more meta and philosophical concepts in the past but I get the idea he is the known physicist to come up with it.

Disclaimer I didn’t read a lot but his name came up a few times and I assume he had a theory that could relate to physics at the time.

I still like that Flash was probably the first for pop culture, when weird time travel started happening in other shows (after deep into Flash recent series) I started joking it was just Barry messing up crap between the multiverse and time.

Tbf the very concept of the universe as we understand it is a pretty new thing that we don’t actually know as much about as you may think.

We have known for slightly longer than a century that the Andromeda Galaxy was an entire galaxy, IE, we have only known for a hundred years now that there are any galaxies besides our own.

So that’s basically why parallel universes is such a “new” literary concept, even if ideas of there being other planes of existence predate that concept.

In ancient Indian cosmology universes are created and destroyed in an infinite cycle

It was popularised recently (in the past 100 years) because of superhero comic authors not wanting to be bound by preexisting canon

Started it in which time line? There are an infinite number of answers.

Personally, the first time I experienced this.`

I’d guess it pretty closely follows the development of concepts that put words to spacial dimensions - 2 points make a line, 2 lines make a plane, multiple planes establish a volume, etc

If you think of time as a line and follow the same logic as spacial dimensions, you have the ‘line’ that represents reality as you’ve experienced it, but every event that has more than one potential outcome branches out from that point the same way the axes that make up length, width, and depth branch from one point. Instead of a 3-D space, we have… well, the multi-worlds theory.

The issue with ideas is that multiple people might have them at the same time. Also, ideas were shared verbally for most of human history. Written records don’t go back super far, so we can’t always know where an idea came from

There are ancient cultures that developed the idea that inside mirrors are mirrored universes.

Are we sure the Avengers didn’t start all this?