Parallel World

In JoJo's Bizarre Adventure, the existence of Parallel worlds, Alternate universes, realities or dimensions is a recurring concept. It is mainly conceptualized as the premise behind certain Stand abilities.

Cream
Vanilla Ice's Stand, Cream is able to devour itself and its user to become a void that devours everything it comes in contact with. Vanilla describes it as a dimension of darkness, though he is unsure himself where exactly the objects are sent to. While in this dimension, Vanilla and his Stand are invulnerable to attacks.

The Hand
Okuyasu Nijimura's The Hand is similar to Cream, able to erase whatever space its right hand touches. Okuyasu states that not even he knows where the voided objects go, but it seems the objects are eliminated like an extremely sharp blade had sliced clean through them. This ability allows the normally short-range Stand extreme distance in its attacks, as it can simply remove the space between itself and a target to bring them within range.

Made in Heaven
Made in Heaven increases the speed or rate of the flow of time; achieved through its ability to control the gravitational forces of the earth, moon, and seemingly the entire universe (in reference to aspects of the theory of relativity).

As time continues to travel, the universe will hit a "vanishing point", and a new universe will be created, where everything repeats itself, according to "fate". The Stand's user may then alter the characteristics of the universe and the fates of the people in the recreated universe and create the perfect world they desire.

In the story, just after Enrico Pucci's death at the hands of Emporio Alniño and the Stand Weather Report, the universe completes a second cycle, though not with Pucci's intended result. In this renewed world, characters of seemingly very similar identities to Jolyne, Ermes, Anasui and Weather Report fortuitously gather in Emporio's proximity. However, the characters now have different names such as Irene, Annakiss, and Eldis (Anime only name), along with different lives and memories like Alternate Weather Report.

Wired
The hooks and cables stored inside Pork Pie Hat Kid's mouth are said to go through dimensions when Pork Pie Hat Kid sends them into a bowl of water. The hooks then spring from this dimension into the original dimension through the bottom of any object or organism, for example: a bird feather or a beetle.

Dirty Deeds Done Dirt Cheap
President Funny Valentine's Stand, D4C, allows different parallel worlds/dimensions to co-exist at the same time and/or in the same place. It also allows its user to visit these parallel worlds/dimensions and interact with them. Valentine travels by being closed between two objects, shown first by the use of a chair against the floor. This method allows practically anything to travel between dimensions, including other people. Valentine takes advantage of this towards the end of Steel Ball Run to bring forth Diego Brando from Another Universe to aid him. Anything that comes in contact with any of its alternate universe counterparts, be it an object or a person, will be destroyed, which applies for both entities. This even applies to corpses and their alive counterparts, as well. The only exception to this rule is Valentine himself, who can bring other Valentines to this world and interact with them without any harm.

37th Universe
Though most of the novel takes place in the original universe, the other half of JORGE JOESTAR takes place in the 37th universe; after the effects of Made In Heaven resetting the world thirty-six times, it creates a 37th universe. It is where the main protagonist, Joji Joestar—the alternate of the other main protagonist, George Joestar II, goes on his adventure to solve the murder of the detectives in Morioh. The 37th universe has characters from the original universe, like in Part 4, 5, and 6. Most of them have slightly different names, and different Stands. Because of Made In Heaven's effects, thirty-seven universes were created. Only a few are mentioned, like the second universe (also known as the Steel Ball Run universe), the unknown universe where a version of Funny Valentine originates from, and the 37th universe where the other side of the novel is set in with Joji. He is accompanied by a man named Kars from that universe, who leads the other 36 versions of himself in order to return back to earth from his eternal banishment during the time placement in Battle Tendency.

Unknown Universe
During the final chapters of the novel, another Funny Valentine meets Joji and assists him in defeating Dio Brando. His universe of origin is unknown, though it is said he is in a universe between the 2nd and 37th universes.

Cube and Arrow Cross House
In JORGE JOESTAR, Reimi Sugimoto is alive, and is a Stand User. Her Stand,, is a die-shaped facsimile of her childhood home, which functions as a time traveling tesseract allowing Reimi and anyone she entrusts to travel to different universes and time periods. She also has an ability named Cube House, which is built inside of the Arrow Cross House. It only allows one person inside, any more leads to contradictions, which ends up sending one of the persons into an empty universe with no escape in order to make space.

Trivia

 * According to Araki, he conceived the idea of the Steel Ball Run Universe present in Parts 7-9 towards the end of Stone Ocean's serialization, likening the concept of a series returning to its roots to the Italian Renaissance.