Big Bang

The Big Bang is the early hot, dense state and subsequent expansion that describe the observable universe’s history. It is a model of cosmic evolution, not an explosion into preexisting space.

Metric expansion of space: The increase over time of distances between widely separated points in the universe. It describes cosmic expansion without requiring galaxies to fly from a central point through space.
General relativity: Albert Einstein’s theory describing gravity as the curvature of spacetime caused by matter and energy. Its equations provide the framework for modeling the universe’s expansion on large scales.
Georges Lemaître: Belgian priest and physicist who proposed an expanding universe and an early, compact state of cosmic matter. His theoretical work anticipated the modern picture before the strongest observational evidence accumulated.
Steady-state cosmology: The family of cosmological models asserting that the universe has no changing large-scale appearance over time. Its timeless picture conflicts with the Big Bang’s evolving density, temperature, and cosmic history.
Dark matter: Matter inferred from its gravitational effects that does not emit or absorb detectable light. Its abundance shaped structure formation, but its physical identity remains unknown.
Cosmic microwave background: The relic radiation released when the early universe became transparent, now observed as a nearly uniform microwave glow. Its spectrum and small temperature variations preserve evidence from about 380,000 years after the beginning of expansion.
Hubble–Lemaître law: The relation between a galaxy’s distance and its recession speed in the expanding universe. The relation connects observed galaxy redshifts to the expansion history described by the Big Bang.
Edwin Hubble: American astronomer whose observations established that galaxies lie beyond the Milky Way and relate distance to recession speed. His galaxy observations helped turn cosmic expansion into a compelling empirical result.
Eternal inflation: A possibility in which inflation continues indefinitely in some regions while ending in others. It extends inflation beyond the observable universe, where its implications are difficult to test.
Dark energy: The name for whatever drives the observed acceleration of cosmic expansion. Its nature determines how the universe’s expansion may develop in the far future.

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