CMB cold spot

The CMB cold spot is an unusually large, low-temperature region in the cosmic microwave background, seen toward the constellation Eridanus. Its origin remains debated.

Integrated Sachs–Wolfe effect: A change in cosmic microwave background photon energy as photons cross evolving gravitational potentials. The effect could contribute to the spot’s temperature through the large-scale structure along its line of sight.
Eridanus supervoid: A reported large underdensity of galaxies in the direction of the CMB cold spot. Its gravitational imprint was proposed as a source of some of the spot’s apparent cooling.
Wilkinson Microwave Anisotropy Probe: A NASA spacecraft that mapped the cosmic microwave background from 2001 to 2010. Its full-sky maps brought the cold spot to broad attention.
Anomaly (cosmology): An observation that appears unusual relative to the predictions of a cosmological model. The cold spot is one of the best-known large-angle CMB anomalies.
Gravitational redshift: A change in light’s frequency caused by its passage through a gravitational field. Photons lose energy crossing a gravitational potential well that evolves while they pass through it.
Cosmic texture: A hypothetical topological defect formed during a symmetry-breaking phase transition in the early universe. A texture collapse has been proposed as a primordial explanation for the spot.
Planck (spacecraft): A European Space Agency observatory that mapped the cosmic microwave background from 2009 to 2013. Its higher-resolution maps enabled detailed checks of the spot’s properties.
Look-elsewhere effect: The increased chance of finding an apparently rare result when many possibilities are examined. Searching across sky positions and scales can make an extreme feature less surprising.
Cosmic microwave background: The relic radiation released when the early universe became transparent to light. The cold spot is an apparent temperature feature within this all-sky radiation field.
Inflation (cosmology): A proposed period of extremely rapid expansion in the universe’s earliest moments. Primordial fluctuations from inflation could produce an unusually cold region without a special local cause.

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Giant VoidCompared with: Its possible association with a supervoid remains uncertain and does not make the two the same phenomenon.