= Universe
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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