The Multiversal Onion
The universe is a 4th-dimension hypersphere, with reality being the 3d "face" on the sphere. You can travel in any direction infinitely, but if you travel far enough in any direction for long enough, you will eventually loop around back to your original location (like how if you travel in a straight line on the Earth, you will eventually return to your original position). Although, this has never been accomplished due to the size of the universe. Even light can no longer complete this journey, due to the expansion of the universe.
However, infinite alternate universes also exist. All parallel universes are also hyperspheres, with each hypersphere being enveloped by another, like Russian nesting dolls or the layers of an onion. Every alternate universe exists either as a "Higher" universe, enveloping all universes beneath it, or a "Lower" universe, which is further towards the core of the Multiversal Onion. Our universe is just one layer of the Onion, existing in the Goldilocks Zone of the Multiversal Onion (there section of universes where life currently exists).
The higher you ascend through the Onion, each universe you pass through is larger than the last, due to Dark Energy expanding the fabric of spacetime. The highest levels of the Onion have already experienced the Heat Death of the Universe, and are considered Dead Universes, as life is impossible on them and it is impossible to travel to them. Conversely, each Lower Universe is smaller than the one "above" it. The deeper you delve into the Onion, the smaller the universe is until you reach the True Singularity, also known as the Multiversal Core. Eventually, the Lower Universes become so small that they're incredibly dense and hot, like the time just after the Big Bang. Life is impossible in these universes, so they're avoided by multiversal travelers and civilizations. Given enough time, the Lower Universes will eventually expand and become part of the Goldilocks Zone, and the universes that are currently a part of the Goldilocks Zone will eventually expand so much that they experience the Heat Death, and become part of the uninhabitable Higher Universes. At the very center of the Onion/Hypersphere is the Multiversal Core, a singularity with the infinite mass from which all other universes have been birthed. Without it, every universe would eventually become uninhabitable and the Onion would completely die. Some of the multiversal civilizations that still practice religion worship the True Singularity as their god, while others believe that it is the spark of life created by God as an engine of creation. Even scientists that study the structure of the multiverse seem to view it as an entity to be revered, an anomaly that has allowed life to survive even after entire universes die, and is thus worthy of respect. However, it is impossible to travel into the True Singularity, and any attempted journey would definitely be fatal.
Traveling Through the Onion
Multiversal travel is incredibly difficult. Very few objects in the universe can allow for travel between the universes. The main source of multiversal travel are supermassive black holes. Due to the infinite density of black holes, their extreme weight is able to puncture the fabric of reality enter the 4th dimension. Matter that a black hole absorbs travels "down" through the Onion, arriving in a Lower Universe through the wormhole's opposite force, a white hole, which ejects you into your new universe at the speed of light.
Ironically, the safest black holes to travel through are also the largest black holes, those that reside in the center of galaxies. Smaller black holes have a nasty habit of spaghettifying all matter that passes through them. Only supermassive black holes have high enough mass to prevent the travelers from being ripped apart by extreme tidal forces. However, the larger the black hole, the more universes the wormhole travels through, and thus only certain supermassive black holes can be used for safe multiversal travel. Finding the proper black hole that will send you to your desired universe can be very tricky, especially for civilizations new to multiversal travel. Many first expeditions through black holes are suicide missions, unwittingly sending their passengers to the uninhabitable, young universes close to the Multiversal Core.
Once you travel through a black hole, returning to your original universe is even harder than the journey there. You could attempt to return through the white hole, but as you would have to have a vehicle capable of faster-than-light travel, but most universes are not capable of such technology. Again, this is especially true for the civilizations new to multiversal travel. Thus, in this multiverse, Einstein-Rosen Bridges (wormholes) exist, and are fundamental to inter-universal travel, but they tend to be a one-way trip for societies just discovering the multiverse.
Multiversal Societies
Although every universe has a similar origin and follow a similar path at first, they eventually diverge from one another while the universe is fairly young. Thus, there are no "alternate" versions of you in another universe. There is no Alligator Loki equivalent of you on any universe lower or higher than yours. Each universe has its own history, unique to it, that determines how things play out in that universe. Some universes become dominated by an aquatic space-faring species that evolved around the volcanic vents in the depths of their world's oceans. Others have been dominated by technological life, AI programs that eventually became the dominant form of life in their universe and were the first form of life to gain mastery over the universe. There is incredible diversity among the multiverses due to the unique paths they took after their Big Bang.
For a society to gain mastery over multiversal travel, they have to unlock the key to faster-than-light travel in order to return to their original universe through white holes. A few multiversal civilizations have been formed by societies that have traveled through black holes and become stranded in their new universes, later developing faster than light travel, but the majority of multiversal societies (generally the more successful ones) gain the power of FTL first. Due to this Great Filter for multiversal travel, most multiversal societies are extremely old, advanced, intergalactic civilizations that have no reason to interact with "lower" species (either through a sense of superiority to them, or something like Star Trek's Prime Directive). Think civilizations that at least control entire galaxies, can easily and quickly create Dyson Spheres, and have found a way to make themselves immortal (through biology or AI).
The main uses of multiversal travel for societies of this type would be:
- To allow for extremely fast travel across their home universe. Because each lower universe is smaller than each above it, if you travel to a lower universe and fly to a different white hole connected to your original universe, you can return to another location in your original universe without flying as far as you would if you just flew towards your destination in a straight line. If you've played Minecraft, think of a Nether Hub.
- Protection against other multiversal societies. One day, a new, hostile multiversal society might appear and try to wipe out your people. If you are able to control all white holes that lead to the portion of the universe you control, this greatly increases the chance you will be warned before any attacks on your home universe, allowing you to fight back better.
- As a way to escape the Heat Death of your universe (assuming your society survives long enough for that to be a problem). The oldest multiversal societies have survived long enough to have to flee from their home universe to a lower one several times.
Okay, that's my idea. Here's why I would make a multiverse this way.
Firstly, it's unique. I haven't seen any idea like it before in a book, TV show, movie, or video game (if anyone has heard of a cosmology like this before, please point me towards it, I would be very interested in reading/watching it). Most Sci-Fi stories and TTRPGs are humanocentric and take place on Earth, even the ones that focus on the multiverse. This would be a new take on the multiverse very different from the normal "you have alternate versions of yourself in every alternate universe, and travel between them is surprisingly easy".
Secondly, it could house endless stories. One adventure/novel could be about a military spaceship that flew into a supermassive black hole in order to escape an enemy, only to be stranded on a lower universe and have to start over, completely isolated from their galactic civilization. Another could be about a multiversal war between two vastly different, highly developed societies with extremely high stakes. Being capable of housing limitless stories is kind of necessary for a TTRPG setting.
Thirdly (related to the previous point), there is a sad deficiency of sci-fi epics, and I feel this setting would be well-suited for both a sci-fi TTRPG setting and a sci-fi epic. Epic stories about huge wars tend to be more common in fantasy than sci-fi (for every sci-fi epic like Dune, The Expanse, or Mass Effect there are 10 fantasy epics like the Lord of the Rings, Stormlight Archive/Mistborn, the Game of Thrones, Dragon Age, Wheel of Time, Elder Scrolls, etc).
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