Universe (cosmology)

The totality of space, time, matter, and energy, including observable regions and any regions beyond observation. Its large-scale history and structure are studied through cosmology.

Big Bang: The early hot, dense phase from which the observable universe expanded and evolved. It describes the universe’s earliest well-supported history, not an explosion into preexisting space.
General relativity: Einstein’s theory of gravity, describing gravitation as the curvature of spacetime by matter and energy. It supplies the equations used to model cosmic expansion and spacetime on large scales.
Edwin Hubble: American astronomer whose observations established the extragalactic nature of many nebulae and helped reveal cosmic expansion. His galaxy measurements helped overturn the idea that the Milky Way was the whole universe.
Observable universe: The region from which light or other signals have had time to reach an observer since the universe began. It is the measurable portion of the universe, not necessarily its entire extent.
Big Bang nucleosynthesis: The formation of most primordial helium and small amounts of other light nuclei during the universe’s first minutes. Its predicted abundances provide an independent test of the early universe’s temperature and density.
Metric expansion of space: The increase over time of distances between locations carried with the expanding universe. It accounts for large-scale cosmic expansion without requiring galaxies to move through space from a central point.
Hubble–Lemaître law: The relation between a galaxy’s distance and its recession speed in the expanding universe. Its measured proportionality provides direct evidence for cosmic expansion.
Georges Lemaître: Belgian priest and physicist who developed an expanding-universe model and proposed an early ‘primeval atom.’ His theoretical work linked cosmic expansion to a finite past.
Cosmic horizon: A boundary beyond which events cannot currently send signals to an observer because of cosmic history and expansion. Horizons limit what can be known directly about regions beyond observation.
Recombination (cosmology): The era when electrons and nuclei combined into neutral atoms, allowing light to travel freely through the universe. The released light is observed today as the cosmic microwave background.

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Isolated systemBroader topic: In many physical models, the universe is treated as an isolated system.