Edit: Let me first thank all of you for being patient with me (I’m by no means a physicist) and for answering my question. This is an update to my previous question. I’ve done some research and to the best of my knowledge the idea that multiple universes could exist is a result of cosmic inflation and the principles of uncertainty in quantum mechanics. Basically (as I understand it) the idea is that during cosmic inflation after the Big Bang due to these uncertainties in quantum mechanics different regions of space time began their ‘Hot Big Bang’ period at different intervals. Thus the gap between universes is the result of the continued inflation of the universe after this happens. So my new question is: could we not just travel there via space? Grant it would be very difficult considering space is still inflating making the distance between universes farther and farther as time passes. But let’s say we use a little handwaving and make it so the teleportation of organic matter is possible or we create a wormhole that won’t collapse almost immediately upon formation and won’t kill us. I’m trying to fill the gaps with as much actual science as possible and whatever is still vastly unknown can be filled in with pseudoscience. Thanks again.
It becomes very simple if we are right in our understanding that our universe could be a simulated universe rather than a real one.
Because all manner of simulations are possible it is even possible to have simulations of universes being emulated inside currently running simulations.
(such as when we run gravity simulations etc).
So the chances of being a simulation are much higher than the chances of being in the real thing.
So in this case travel to the multiverse might only involve transfer of data.
Any violations of the laws of physics could also be conveniently explained by the changes data values while the simulation is running.
For instance walking on water could be fixed by running the code
If height < 0 then set height = 0 and the object would appear to be floating or levitating.
Travelling the multiverse is currently magic because our physic and mathematical models are not up to the task of adequately representing fifth or higher order dimensions required to plot an exit point and return safely. So you could do a cheat and "find" an ancient artifact that acts as a portal (tv shows such as Sliders, Twilight Zone or Stargate SG1). Were humans able to build such a device that could breach the veil/barrier it would require a ridiculous amount of energy and would not be something portable in scale. The computations would require the use of a super computer or several working together.
It's almost certainly not possible to provide a plausible scientific explanation
I'm afraid that (especially with the science based tag) the answer will be that there is no plausible explanation for how human beings might travel the multiverse.
I am very sure that (like the Ansible used by Orson Scott Card in "Ender's Game" to communicate instantly across any distance via quantum entanglement) the idea you can hop between universes is nothing more than a literary device based on junk science arising from poor (or unfortunate) choice of words when explaining the physics to laymen.
That it belongs in the realm of fantasy & has nothing to do with any real (or merely "hard") science.
That said, I'd guess this is an idea you're as attached to as I was to quantum communication, so suspect you'll be resistant to my answer & want "proof" (as I did for why the ansible couldn't work).
But it took me a lot of very frustrating research & reading before I fully understood enough about quantum entanglement to know that the idea it could be used for communication was rubbish & I really don't want to put myself through all that again just to provide someone else with proof :)
So I suggest you field a question in the physics stack exchange to get your answers.
But that said how about this
Go with the idea the past is fixed (hot tub time machine played with this idea I think).
So (when someone invents a time machine) every time anyone goes to the "past" they create a new alternative timeline (one where an older version of themselves just appeared out of nowhere) & as long as the "wormhole" stays open while they're there they can step back into their own timeline.
But if the wormhole closes while they're in the past there's absolutely no way to reconnect the two alternate realities, because every time you open one of these time gates it creates a new alternative timeline.
This means nothing they do in the (other) past effects their present.
So they can bring people & things back with them & they won't disappear from history in their own timeline, which means a short wormhole back gets you your doppelgangers with no need to explain how they manage to tune into an appropriately similar alternate world to find one.
Now you just have to work out the equally improbable mechanics of how to build a time machine.
- It is not proper multiverse, but multi simulated universe, with a central control mechanic. Not that it matter.
- The central mechanic control how information pass from one universe to another. It is somewhat similar to "type cast" in programming, where there are similar type in other universe that can "simulate activity" of this information
- The material part of human in one universe do not travel, but they get sensor (eye, touch...) information from that other universe, and their command (movement of body) pass to that other universe. Give feeling similar to how you dream.
- The tech required: information about how to communicate with central mechanic and with other universe. No need for material movement.
As you implied, there is no hard science answer. I can tell you some ideas that I think about sometimes which may have similarities in essence. String theory mixed with modulation. If your character can figure out how to modulate the frequency of string vibrations and create new harmonics the result may cause a disruption in the fabric of the space-time continuum. This technique might be used in conjunction with a device to simultaneously match the characters own string vibrations with another universes string vibrations.