The universe is the total physical system described by cosmology, including spacetime, matter and radiation. A cosmological model specifies their large-scale geometry and evolution, for example through a Friedmann-Lemaître-Robertson-Walker metric and the matter content entering the Friedmann equations.
The observable region is limited by the history of light propagation. A finite particle horizon bounds what could have sent signals to an observer since the model's early evolution; it does not imply a physical wall or establish that the entire universe is finite. Measurements of expansion, the cosmic microwave background and large-scale structure constrain the model through different physical processes.
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The universe refers to the vast, all-encompassing expanse that includes everything that exists, such as matter, energy, planets, stars, galaxies, and even the cosmic laws and forces governing them. It is often described in terms of the following key points: 1. **Scope**: The universe contains all celestial bodies and cosmic phenomena, including the Milky Way galaxy, other galaxies, nebulae, black holes, and dark matter.