Knowra Observable universe Observable universe The region of the universe from which light or other signals have had time to reach Earth since the universe began. Its boundary is set by cosmic history, not by a physical edge.
Particle horizon : The maximum distance from which signals could have reached an observer since the beginning of the universe. It defines the observable universe’s boundary at the present cosmic time.
Age of the universe : The time elapsed since the universe began expanding from its hot, dense early state, about 13.8 billion years. The finite cosmic age limits the time available for signals to reach Earth.
Universe : The totality of space, time, matter, and energy, including regions beyond what can be observed. The observable universe is only the portion of the universe whose signals have reached Earth.
Cosmic horizon problem : The puzzle of why widely separated regions of the cosmic microwave background have nearly the same temperature. It arises because those regions appear causally disconnected in a non-inflationary early universe.
Speed of light : The speed at which light travels through a vacuum, approximately 299,792 kilometers per second. Its finite value limits how far signals can travel during the universe’s age.
Cosmological principle : The assumption that the universe is homogeneous and isotropic on sufficiently large scales. It lets observations from our location support estimates of the universe’s large-scale structure.
Cosmic event horizon : A boundary beyond which events can never send signals to an observer, even given unlimited future time. Unlike the particle horizon, it concerns signals that may arrive in the future.
Cosmic variance : The unavoidable uncertainty caused by observing only one realization of cosmic structures and fluctuations. A finite observable volume limits how precisely large-scale cosmic properties can be measured.
Cosmic expansion : The increase over time in distances between widely separated, unbound regions of the universe. Expansion makes the observable radius larger than the universe’s age multiplied by light speed.
Hubble–Lemaître law : The relation between a galaxy’s distance and its recession speed caused by cosmic expansion. It connects observed galaxy redshifts to distances across the observable region.
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