Multiverse in Buddhism?

Can anyone expand on this, or simplify it?

Sure. I can simplify it into one word - 'superstition'.

In original Buddhism, the term "loka" ("world") refers to a state of mind, such as:

For it is in this fathom-long carcass with its perception and mind that I describe the world, its origin, its cessation and the practice that leads to its cessation.

Api cāhaṃ, āvuso, imasmiṃyeva byāmamatte kaḷevare sasaññimhi samanake lokañca paññāpemi lokasamudayañca lokanirodhañca lokanirodhagāminiñca paṭipadanti.

AN 4.45

For those that misunderstand the above, again:

With the fading away of rapture, they enter and remain in the third absorption.

pītiyā ca virāgā … pe … tatiyaṃ jhānaṃ upasampajja viharati—

This is the grounded path for realizing a world of perfect happiness.

ayaṃ kho sā, udāyi, ākāravatī paṭipadā ekantasukhassa lokassa sacchikiriyāyā”ti.

MN 79

For example, if we read SN 11.5 about the 'deva' and 'asura' gods, the impression is they are debating about how to conduct affairs with people in earth. Thus, a 'deva' appears to be a benevolent ruler; where as an 'asura' appears to be a violent ruler.

I think this entire world system with it's thousandfold systems can be thought of as a single system of conceivable points of reference or observers as in special relativity.

The Blessed One said: "And what is the origination of the world? Dependent on the eye & forms there arises eye-consciousness. The meeting of the three is contact. From contact as a requisite condition comes feeling. From feeling as a requisite condition comes craving. From craving as a requisite condition comes clinging/sustenance. From clinging/sustenance as a requisite condition comes becoming. From becoming as a requisite condition comes birth. From birth as a requisite condition, then aging & death, sorrow, lamentation, pain, distress, & despair come into play. This is the origination of the world. https://www.accesstoinsight.org/tipitaka/sn/sn12/sn12.044.than.html

That in the world by which one is a perceiver of the world, a conceiver of the world—this is called the world in the Noble One’s Discipline. And what, friends, is that in the world by which one is a perceiver of the world, a conceiver of the world? The eye is that in the world by which one is a perceiver of the world, a conceiver of the world . The ear … The nose … The tongue … The body … The mind is that in the world by which one is a perceiver of the world, a conceiver of the world. That in the world by which one is a perceiver of the world, a conceiver of the world—this is called the world in the Noble One’s Discipline. https://suttacentral.net/sn35.116/en/bodhi

Therefore you get many conceived & perceived worlds in a conceived & perceived world.

One of Guatama Buddhas appellations is 'Teacher of the Devas'. I quote from that Access To Insight article (which is the Therevada perspective, & refers to the Pali canon):

The Buddha has directly seen the origins of Maha Brahma and understands what it requires to be reborn in his world. In the Brahmajāla sutra (DN 1) the Buddha describes how a supposed Creator God came to believe himself omnipotent and how others came to rely on his sovereignty. His description was based, not on speculation or hearsay, but on his own direct knowledge. The Buddha explains that when our world system disintegrates, as it regularly does after extremely long periods of time, the lower sixteen planes are all destroyed. Beings disappear from all planes below the seventeenth, the plane of the Abhassara gods. Whatever beings cannot be born on the seventeenth or a higher brahma plane then must take birth on the lower planes in other remote world systems.

Eventually the world starts to re-form. Then a solitary being passes away from the Abhassara plane and takes rebirth on the plane of Maha Brahma. A palace created by his kamma awaits him there: "There he dwells, mind-made, feeding on rapture, self-luminous, moving through the air, abiding in glory. And he continues thus for a long, long time." After ages pass, he becomes lonely and longs for other beings to join him. It just so happens that shortly after the brahma starts craving for company, other beings from the Abhassara plane, who have exhausted their lifespans there, pass away and are reborn in the palace of Brahma, in companionship with him.

Because these beings seemed to arise in accordance with the first brahma's wish, he becomes convinced that he is the almighty God: "I am the Great Brahma, the Vanquisher... the Lord, the Maker and Creator, the Supreme Being." The other brahmas, seeing that he was already present when they took birth in his world, accept his claim and revere him as their creator.

Eventually this misconception of a Creator God spreads to the human plane. One of the other brahmas passes away and is reborn here. He develops concentration and learns to recollect his previous life with Maha Brahma, but none of his lives before that. Recollecting that existence he recalls that Maha Brahma was considered the "father of all that are and are to be... permanent, stable, eternal." As he is unable to remember further back, he believes this to be absolute truth and propounds a theistic doctrine of an omnipotent Creator God

I am honestly surprised more Buddhists don't know about this account of Buddha teaching Maha Brahma.

You will likely also find the pali Brahmajāla sutra interesting for it's sections '18 beliefs about the past' and '44 beliefs about the future'.

You should read about Buddhist temporal cosmology for the origins & expected end of this particular universe or realm.

Edited to add, in response to comments (I am moving these into the body of the answer because they make it more complete, & it will make this added content searchable)

@Orionixe:

the Buddhist cosmos is very large, and is described over very long eras, see Kalpas in Buddhist thought.

In Hindu thought:

"The life span of the universe is one 'maha kalpa' (311.04 trillion years). This time span is one breath of 'Vishnu', who when he exhales, thousands of universes emerge & one 'Brahma' is born in each universe". From Hindu section of the same Wikipedia article

Buddha sees that Brahmas are reborn from other realms, as detailed in the Pali Brahmanjali sutra. So, there are other realms.

Now, the Many Worlds of quantum mechanics: I interpret the alternate selves, the different outcomes, as not separate, but as part of each of us. The real choices we didn't take, are part of us, along with those we did. I discussed thinking of processes as structures within higher dimensions here: How does biological evolution work in the block universe/b-theory of time?. We make moral decisions by imagining counterfactuals, if I do this = x, if I do that = y, so a valid model of alternatives is important ethically, as it is in quantum mechanics.

I find it interesting to imagine, we get to the end of our lives, and in the bardo some residue of conflicts, contradictions, attachments, makes us wish to restart our lives with some subtly different initial conditions. I feel this can link rebirth, and the multiverse. And I personally find it psychologically useful to imagine this as an exercise, along the lines of Nietzsche's idea of Eternal Return - which I would say is best regarded as a thought-experiment. Rebirth too can be used as a thought-experiment, that is without worrying whether it is a literal picture, but as a mirror or lense to look at our choices in a larger context, and to understand which of our actions will linger, as dissatisfactions, ie attachments.

Modern physics hasn't even decided what it means by multiverse, there is substantial dispute on the details, eg whether there are literally Many Worlds, or if they are meaningfully real when impacting measurements.

I do genuinely think Buddhist thought has useful guidance and insights, having been playing in this 'pool' a lot longer, of a vast ancient self-referencing cosmos.

But I also firmly believe, there will always be mysteries and wonders beyond our capacities - and there will always be frontiers for future people, because increasing our capacities can expand our experiences.

Many different options are on the table: 2 dimensions of time, 10 dimensions of space plus one of time, a fracture plane in the E8 hyperstructure, a closed timelike curve 'spinning out' universes, two universes expanding apart in opposite time directions. Too early to call, at least until we have a theory of quantum-gravity.

Scientific ideas about a multiverse/s were not based/inspired by Buddhist traditions, at all.

It's interesting that the Manhattan Project era physicists were becoming aware of Hindu thought:

"Now I am become Death, the destroyer of worlds" - Oppenheimer, quoting the Bhagavad Gita.

Murray Gell-Mann called his systemising of quarks the Eightfold Way, consciously reflecting the Buddhist phrase. I would link this to moving beyond the narrow geocentric cosmos of Western tradition, and searching for alternative ideas that resonate.

My understanding is knowledge of Buddhist thought has not been deeply or widely absorbed enough philosophically, to have more substantial impact on Western thinkers, yet. Largely because of a lack of texts until modern times, and still a lack of important commentaries (eg from Korean Goryeo era). But Buddhist and Jain contemplatives have been thinking about a vast and ancient cosmos for a long time, as part of their practice, and especially in later developments like The Flower Garland Sutra, there are tools for thinking about intersecting and interpenetrating realms and expanded notions of identity once conventional ideas about time are challenged (by the scope and complexity of the cosmos).

Refer the DN 27. Aggaññasutta which describes how life on earth came to be. (Starting from 3. Candi­masū­ri­yā­di­pātubhāva).

In DN 1. Brahmajalasutta there is a mention about higher planes of existence - 3.1.2. Ekaccasassatavāda and it describes how the world is re-populated after forming. we can also get an idea of the time spans in discussion from section 3. Diṭṭhi.

In, SN 56.11. Dhammacakkappavattanasutta a description of different planes of existence getting the message of Dhamma (Dhamma cakka in motion) is described starting from human plane.

Even though there are discussions of different planes of existence, I haven't come across any direct references to multiverse though. Then again, I have only read a few sutta, so this should not be considered an all encompassing claim.