The Eridanus Supervoid and the Cold Spot: A Temperature Budget That Does Not Close

The Cold Spot, a cold region of the microwave sky some ten degrees across, sits in the direction of the Eridanus supervoid, and the suggestion that the void explains the spot through the integrated Sachs-Wolfe effect is tested here as an explicit temperature budget. Taking the observed central decrement as -70 microkelvin at the relevant smoothing, the spherical-equivalent model of the supervoid, radius 140 megaparsecs per h at mean contrast minus 0.30, contributes -3.3 microkelvin. A deep variant at contrast minus 0.40 and radius 160 megaparsecs per h contributes -6.0 microkelvin, which is 9 per cent of the observed decrement. Only a variant that incorrectly assigns the central depth of a typical void profile, minus 0.85, to the whole supervoid volume reaches -9.5 microkelvin, and that variant contradicts the catalogue density of the structure. The budget therefore does not close: the supervoid plausibly supplies a minority of the Cold Spot's depth, the remainder being a primary fluctuation that is itself a 2.8 sigma event at the stated smoothing, unusual but not extraordinary. Claims that the void fully explains the spot fail on the arithmetic presented here. All values are computed from stated assumptions, not measured.