.. py:currentmodule:: tissue_forge
The universe is everything according to a model as instructed to Tissue Forge. It is the spacetime of a simulation and contains every model object therein, where objects interact, processes are performed, and events occur. The universe can be thought of as the repository for all representations of physical objects in a simulation.
Very little exists in the universe unless it is asserted to Tissue Forge. The universe does not exist without space and a description of its boundaries, and a time is always assigned to the current configuration of the universe (even if it is empty). Otherwise, the universe according to Tissue Forge is a void unless instructed otherwise.
The universe is automatically created when :ref:`initializing a simulation <running_a_sim>`, and exactly one universe always exists. When objects are created and assigned a position (e.g., when creating :ref:`particles <creating_particles_and_types>`), they are placed at the prescribed position in the universe according to a Cartesian :ref:`coordinate system<coordinate_systems>`. Whatever objects may be in the universe, their dynamics will only occur if the universe is :ref:`integrated in time <running_a_sim>`. What occurs when an object attempts to cross a boundary of the universe is governed by prescribed :ref:`boundary conditions <boundary>`. The universe has no intrinsic units except those implied by the objects and interactions it contains.
For convenience, the universe can always be accessed as a top-level variable :py:attr:`Universe` in Python. The universe contains a number of useful methods that provide information about both the universe and everything in it in its current configuration. For example, the current time and time step can be accessed with the universe properties :attr:`time <Universe.time>` and :attr:`dt <Universe.dt>`, respectively (static methods :meth:`Universe::getTime` and :meth:`Universe::getDt`, respectively, in C++). Likewise, an inventory of all particles and bonds can be :ref:`accessed <accessing>` with the universe methods :meth:`particles <Universe.particles>` and :meth:`bonds <Universe.bonds>`.