The CMB Cold Spot & Eridanus Supervoid
The CMB Cold Spot is a ~5°–10° region in the southern constellation Eridanus (galactic coordinates l ≈ 209°, b ≈ -57°) where Cosmic Microwave Background temperature fluctuations dip to ΔT ≈ -150 μK (roughly 4.5σ below the mean in spherical wavelet filters). While early studies noticed a large foreground underdensity—the Eridanus Supervoid at z ≈ 0.14–0.25—spectroscopic redshift surveys (2dF-VST ATLAS and DES) show that the void's linear Integrated Sachs-Wolfe (ISW) effect accounts for only ΔT_ISW ≈ -20 to -30 μK, indicating the Cold Spot is primarily a rare ~1–2% tail fluctuation of Gaussian primordial curvature perturbations.
Physical Mechanism & Dynamics
When CMB photons traverse an evolving gravitational potential Φ(x, t) in a dark-energy-dominated universe, the potential well of a supervoid decays while the photon is in transit. Photons lose slightly more gravitational blueshift climbing into the void than they regain exiting the shallower potential, producing a secondary cold imprint via the Integrated Sachs-Wolfe (ISW) effect: ΔT/T = (2/c²) ∫ (∂Φ/∂t) dt.
However, in standard flat ΛCDM, even an extreme supervoid of radius R ≈ 150–200 Mpc and central underdensity δ_m ≈ -0.3 at z ≈ 0.2 generates at most ΔT_ISW ≈ -20 to -30 μK. The remaining -120 μK temperature decrement surrounded by a warm ring must therefore arise at the surface of last scattering (z ≈ 1090) unless non-standard dark energy or modified gravity enhances the late-time ISW growth rate.
Quantitatively, this mechanism is governed by Late-Time Integrated Sachs-Wolfe (ISW) Temperature Decrement (ΔT_ISW / T_CMB = (2 / c^3) ∫_0^{χ_*} (∂Φ(χ, η) / ∂η) dχ). Integrates the conformal-time decay rate of the Newtonian gravitational potential Φ along the photon's comoving radial trajectory χ.
Late-Time Integrated Sachs-Wolfe (ISW) Temperature Decrement
ΔT_ISW / T_CMB = (2 / c^3) ∫_0^{χ_*} (∂Φ(χ, η) / ∂η) dχIntegrates the conformal-time decay rate of the Newtonian gravitational potential Φ along the photon's comoving radial trajectory χ.
Quantitative Benchmarks & Parameters
| Parameter / Observable | Measured Value / Scale | Method & Context |
|---|---|---|
| Central CMB temperature decrement | ΔT ≈ -150 μK (mean ~ -70 μK over 5°) | WMAP & Planck SMICA/Commander maps |
| Angular radius & sky location | θ ≈ 5° (ring to ~10°), l = 209°, b = -57° | Southern sky in Eridanus |
| Eridanus Supervoid redshift range | z ≈ 0.14–0.25 (split into sub-voids) | DES & 2dF-VST ATLAS galaxy spectroscopy |
| Predicted ΛCDM ISW contribution | -20 to -30 μK (~15–20% of signal) | Linear & second-order Rees-Sciama modeling |
Observational Evidence & How Scientists Know
Planck 2018 polarization (E-mode) and temperature cross-spectra, combined with Dark Energy Survey (DES) weak gravitational lensing and spectroscopic galaxy counts by Mackenzie et al. (2017), resolved the Eridanus line of sight into three moderate chained underdensities rather than a single monolithic vacuum, ruling out a purely local void explanation within standard GR.
Across reference datasets (Planck 2018 Cosmological Parameters (PR3)), quantitative benchmarks for the cmb cold spot & eridanus supervoid are anchored by: Central CMB temperature decrement (ΔT ≈ -150 μK (mean ~ -70 μK over 5°), via WMAP & Planck SMICA/Commander maps); Angular radius & sky location (θ ≈ 5° (ring to ~10°), l = 209°, b = -57°, via Southern sky in Eridanus); Eridanus Supervoid redshift range (z ≈ 0.14–0.25 (split into sub-voids), via DES & 2dF-VST ATLAS galaxy spectroscopy); Predicted ΛCDM ISW contribution (-20 to -30 μK (~15–20% of signal), via Linear & second-order Rees-Sciama modeling).
Uncertainties, Systematics & Open Questions
Because look-elsewhere corrections across wavelet scales reduce the global p-value of the Cold Spot in Gaussian ΛCDM simulations to p ≈ 0.01–0.02, distinguishing a rare statistical fluke from exotic early-universe physics remains an active benchmark.
Real Astronomical Benchmarks & Cross-Domain Links
Planck CMB Temperature Map
Full-sky submillimeter mission confirming WMAP's detection of the Eridanus Cold Spot
Integrated Sachs-Wolfe Effect
Secondary CMB anisotropy mechanism coupling decaying cosmic potentials to photon energy
Cosmic Microwave Background
Primary 2.7255 K blackbody relic radiation from z ≈ 1090
Guided Learning Path
Related Concepts, Methods & Explorers
Scientific Sources & Provenance
- Planck 2018 Cosmological Parameters (PR3) (PR3 (2020 A&A 641, A6)) — Planck Collaboration, Aghanim, N., et al. 2020, A&A, 641, A6. Archive / Reference ↗
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