Dark flow

Dark flow is a reported coherent motion of galaxy clusters toward a common direction, inferred from their cosmic microwave background radiation. Its existence and scale remain disputed.

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Cosmic microwave background: Relic radiation released when the early universe became transparent, now observed across the sky as microwaves. Dark-flow measurements seek a directional imprint on this radiation.
Kashlinsky dark-flow studies: A series of analyses by Alexander Kashlinsky and collaborators reporting coherent cluster motion from cosmic microwave background data. These studies introduced and developed the dark-flow claim.
Lambda-CDM model: The standard cosmological model, combining cold dark matter with a cosmological constant in an expanding universe. Its expected large-scale flows provide a benchmark for evaluating dark flow.
Observable universe: The region of the universe from which light has had time to reach an observer since the Big Bang. Claims that dark flow extends beyond this boundary cannot be tested by direct observation.
Sunyaev–Zel'dovich effect: Changes to cosmic microwave background radiation caused by its scattering from hot electrons in galaxy clusters. Its thermal component can obscure the much smaller motion-dependent signal.
Wilkinson Microwave Anisotropy Probe: A NASA spacecraft that mapped the cosmic microwave background from 2001 to 2010. Its maps supplied key data for early dark-flow analyses.
Cosmic variance: The irreducible uncertainty caused by observing only one realization of a finite universe. It limits how confidently unusually large motions can be distinguished from chance.
Cosmic horizon: A boundary set by the finite speed of light and the universe's expansion, beyond which events cannot currently be observed. It defines the observational limit invoked in discussions of dark flow's possible reach.
Galaxy cluster: A gravitationally bound structure containing hundreds to thousands of galaxies, hot gas, and dark matter. Clusters are the moving objects whose radiation was analyzed.
Planck (spacecraft): A European Space Agency spacecraft that mapped the cosmic microwave background from 2009 to 2013. Planck data enabled later tests of the proposed signal.