2D universe

A 2D universe is a theoretical object in physics and a type of dimensional analogy.

Properties
A 2D universe usually describes a theoretical universe where all constituent matter has two spatial dimensions and one temporal dimension. This is more specifically known as a 2 + 1 universe; two spatial dimension and one temporal dimension. Universes with 1 + 1 dimensions, which could also be described as a "2D universe," are largely unexplored in physics and mathematics. The term 2D universe is also occasionally used to discuss the possibility of a Holographic universe.

It has been speculated by some physicists that not only could a 2D universe be theoretically possible, but that life could exist there. For example, physicist James Scargill explored the possibility of life in a 2D universe in his paper Existence of Life in 2 + 1 Dimensions.

Allegory
In the early 20th century, writers and artists became aware of the possibility of an extra spatial dimension. This lead to a boom of media that featured allegories for higher dimensions. Flatland: A Romance of Many Dimensions features both a 2 + 1 and a 1 + 1 universe, exploring alternate numbers of spatial dimensions as an allegory for how a 4D space interacts with a 3D space.