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Cold spot

The , also known as the WMAP Cold Spot, is an anomalously large and cold region in the (CMB) radiation, the relic glow from the that permeates the at an average temperature of 2.725 kelvins. It spans about 5 degrees across the sky—roughly the width of 10 full moons—and is located in the within the constellation . First detected in 2004 by NASA's (WMAP) and later confirmed by the European Space Agency's Planck satellite, the spot's core temperatures reach up to 140–150 microkelvins below the CMB mean, far exceeding the typical fluctuation amplitude of around 18 microkelvins. This feature stands out as a statistical , with less than a 1% probability of arising from random Gaussian fluctuations in the standard cosmological model. Its unusual size and depth have prompted extensive study, as they challenge expectations for the uniformity of the early . The leading explanation attributes the cold spot to the integrated Sachs-Wolfe (ISW) effect caused by the supervoid, a vast underdensity in the cosmic web approximately 1.8 billion light-years across and located about 2–3 billion light-years from . In this process, photons passing through the void lose energy due to as the void expands under dark energy's influence, resulting in observed cooling of approximately 15–30 microkelvins. A 2022 analysis using Dark Energy Survey data confirmed the supervoid as a significant underdensity with an extent of approximately 20 degrees, the largest known cosmic structure, providing evidence that it partially explains the cold spot via the ISW effect, though not the full depth. While alternative hypotheses—such as a statistical fluke or even exotic ideas like a collision with another —have been proposed, the supervoid model aligns best with gravitational lensing and distribution data, though debates continue on whether it fully accounts for the . Ongoing research with missions like the upcoming Simons Observatory continues to probe this enigma, which may offer insights into and the large-scale structure of the .

Background

Cosmic Microwave Background

The (CMB) is the uniform that permeates the , originating as from the hot, dense early . It fills all of space with a nearly perfect at a of 2.725 ± 0.001 . This radiation is observed as microwaves today due to the , which has redshifted the original photons from higher energies. The CMB's uniformity underscores the large-scale homogeneity of the , serving as a key pillar of the model..pdf) The formed during the epoch of recombination, approximately 380,000 years after the , when the had cooled sufficiently (to about 3,000 ) for electrons and protons to combine into neutral atoms. Prior to this, the was a hot opaque to photons due to frequent ; recombination made it transparent, allowing these photons to decouple and propagate freely ever since. Traveling for nearly 13.8 billion years, the thus captures the state of the at this pivotal transition from opacity to transparency, often termed the surface of last . Key properties of the CMB include its near-perfect isotropy, with temperature variations across the sky amounting to only about one part in 100,000 (or fluctuations of order $10^{-5} K), and a spectral shape that deviates from a blackbody by less than 0.03% of the peak intensity. These characteristics align precisely with theoretical predictions for relic radiation from a hot Big Bang. The CMB was first detected in 1965 by Arno Penzias and Robert Wilson at Bell Laboratories, who observed an unexplained excess antenna temperature of roughly 3.5 K in all directions while calibrating a radio telescope—serendipitously confirming earlier theoretical expectations..pdf) Subsequent observations by the Cosmic Background Explorer (COBE) in 1992 provided definitive confirmation of the 's blackbody spectrum and revealed its tiny temperature anisotropies, validating the inflationary paradigm. In cosmology, the acts as a fossil record of the early , encoding primordial density perturbations that served as seeds for the leading to the formation of galaxies, clusters, and the cosmic web of large-scale structure. These perturbations, imprinted as acoustic oscillations in the before recombination, allow precise measurements of fundamental parameters like the 's composition and expansion history.

Temperature Anisotropies

Temperature anisotropies in the () refer to small spatial variations in the radiation's temperature, deviating from the uniform mean value of about 2.725 by relative amounts typically on the order of \Delta T / T \sim 10^{-5}. These fluctuations provide a snapshot of the early universe at the epoch of recombination, around z \approx 1100, when the and photons last scattered. In the standard cosmological framework, such anisotropies arise from quantum fluctuations amplified during cosmic , which seeded density perturbations that evolved into the observed patterns. The primary sources of these anisotropies originate at the last scattering surface and include generated by , the from the motion of the emitting , and the Sachs-Wolfe effect due to gravitational potentials at recombination. from produce nearly scale-invariant adiabatic perturbations that dominate the large-scale (low-multipole) signal. The contributes through the radial peculiar velocity of the baryon-photon fluid, imprinting oscillations on smaller angular scales. The Sachs-Wolfe effect, described by \Delta T / T \approx \phi / 3 where \phi is the , causes photons to when climbing out of potential wells, primarily affecting large angular scales greater than a few degrees. Secondary anisotropies, generated after recombination as CMB photons propagate through the evolving , include the integrated Sachs-Wolfe (ISW) effect and the Sunyaev-Zel'dovich () effect. The ISW effect arises from the time variation of gravitational potentials during the late-time expansion, particularly in low-density universes dominated by , leading to a net blueshift or of photons and contributing on scales of about 10 degrees. The SZ effect occurs due to inverse of CMB photons by hot electrons in : the SZ distorts the spectrum with a decrement at low frequencies, while the kinetic SZ induces a Doppler-like shift from cluster bulk motions, both acting as localized secondary imprints. Statistically, CMB anisotropies are characterized by the angular power spectrum C_\ell, which quantifies the variance of temperature fluctuations as a function of multipole moment \ell, corresponding to angular scales of \sim 180^\circ / \ell. In the standard \LambdaCDM model, this spectrum features a series of acoustic peaks, with the first prominent peak at \ell \approx 220 reflecting the sound horizon at recombination, modulated by (BAO) in the pre-recombination photon-baryon fluid. These oscillations, driven by pressure waves, leave imprints where even (odd) peaks correspond to compression (rarefaction) phases, with amplitudes sensitive to density \Omega_b. The \LambdaCDM model predicts that the overall distribution of these fluctuations is nearly Gaussian, consistent with random quantum origins from , with any non-Gaussianity constrained to be minimal.

Discovery and Observation

Initial Detection with WMAP

The (WMAP), launched by on June 30, 2001, and operational until its decommissioning in 2010, provided the first high-resolution full-sky maps of the () radiation across five frequency bands from 23 to 94 GHz, achieving an of approximately 1°. The initial detection of the occurred in analyses of WMAP's first-year data released in 2003. Using the spherical Mexican hat wavelet (SMHW) method to probe for non-Gaussian features in the temperature fluctuations, researchers identified a significant manifesting as an unusually cold region in the southern galactic hemisphere. This feature, centered at galactic coordinates (l, b) = (209°, -57°) in the constellation , exhibited excess at scales around 4°–5°, with a detection probability of about 0.1%–0.4% compared to Gaussian simulations of the standard cosmological model. Subsequent detailed examination confirmed the cold spot as a ~10°-diameter region approximately 70–80 μK colder than its surroundings after filtering, with a minimum excursion of around -78 μK at the center. The appeared in the combined QVW channel map (merging data from 41, 61, and 94 GHz bands to minimize foreground contamination), where only ~0.2% of 10,000 simulated Gaussian maps showed a comparable cold feature, underscoring its non-random nature. Early visualizations from the WMAP data, such as coefficient maps and filtered temperature skies, highlighted the spot's extent as a prominent circular depression amid typical fluctuations of ±200 μK, prompting further scrutiny of its statistical rarity.

Confirmation with Planck and Other Surveys

The European Space Agency's Planck satellite, launched in 2009, achieved higher angular resolution (5–10 arcminutes) and greater sensitivity than the (WMAP), enabling detailed full-sky maps released in 2013 and refined in 2015. These observations independently confirmed the existence of the , first identified in WMAP data, demonstrating its persistence across multiple component-separation methods without evidence of instrumental artifacts. In the Planck results, the cold spot appeared prominently in frequency channels from 70 to 217 GHz, with a central temperature decrement of approximately -70 μK relative to the surrounding , and no significant frequency dependence that would suggest foreground contamination from sources like or emission. Statistical analyses, including transforms and peak statistics, yielded a detection significance of about 3σ, consistent with but more robust than WMAP findings due to Planck's improved foreground subtraction. The 2015 Planck data release further strengthened this confirmation through multi-filter approaches, such as the Mexican Hat Wavelet (SMHW) and needlet analyses, which highlighted non-Gaussian in the cold spot region with probabilities below 2% under Gaussian assumptions, indicating a low likelihood of random occurrence. The profile showed a central drop ranging from 70 to 150 μK, surrounded by a hot ring, with the anomaly robust to masking and consistent across the , NILC, SEVEM, and SMICA maps. These results refined the cold spot's edges and ruled out foreground residuals, attributing the to cosmological origins rather than Galactic . Reanalyses of WMAP's 3-year data in 2007 had already bolstered the initial 2004 detection by confirming non-Gaussianity at the 2–3σ level using improved multi-frequency cleaning, setting the stage for Planck's higher-precision verification. Overall, these post-WMAP surveys established the cold spot as a reliable feature of the , with around 2.5σ in combined analyses, emphasizing its deviation from expected Gaussian fluctuations.

Physical Characteristics

Size and Temperature Profile

The is characterized by an of approximately 5° to 10°, rendering it the largest identified anomaly in the () temperature distribution. This extent aligns with low-multipole modes, particularly ℓ ≈ 10–20, which probe large-scale structures in the early . The temperature profile features a central depression of roughly 70 μK below the average CMB temperature of 2.725 K, encompassing the broader region, while an inner cold core exhibits a deeper decrement of up to 140 μK. This core is encircled by a hotter ring, approximately 10–15° in radius, resulting from the convolution effects of or Gaussian filtering applied during data processing. Relative to typical CMB anisotropies, which display a root-mean-square variation of about 18 μK on comparable angular scales, the Cold Spot's amplitude surpasses 3–4 standard deviations, highlighting its exceptional nature. Radial temperature profiles derived from WMAP and Planck observations depict this structure vividly, with smoothed maps revealing the pronounced central dip flanked by elevated temperatures in the surrounding annulus. These visualizations, often generated using expansions or transforms, emphasize the spot's coherence across multiple datasets and scales.

Location in the Sky

The CMB Cold Spot is positioned at equatorial coordinates of right ascension 03h 15m 05s and declination −19° 35′ (J2000 epoch), which correspond to galactic coordinates of longitude l = 209° and latitude b = −57°. These coordinates place the anomaly in a relatively clean region of the sky for microwave observations. This location situates the Cold Spot within the constellation Eridanus in the southern celestial hemisphere, far from the galactic plane at a latitude of approximately −57°, thereby reducing potential interference from galactic dust and synchrotron emission that could mimic temperature anisotropies. The southern declination further complicates ground-based observations from northern hemisphere facilities, as the region rises low on the horizon or remains inaccessible for much of the year, underscoring the importance of space-based telescopes like the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck for high-fidelity mapping. In terms of alignment with large-scale structures, the Cold Spot overlaps the line of sight toward the Eridanus supervoid, a vast underdensity spanning hundreds of megaparsecs and with an angular extent over 30 degrees as mapped in 2025 observations, as revealed by galaxy redshift surveys such as those from the Dark Energy Survey (DES) and earlier radio continuum data. However, there is no evident direct superposition with prominent galaxy clusters along this path, consistent with the supervoid's characterization as a region of low matter density rather than clustered overdensities.

Theoretical Explanations

Integrated Sachs-Wolfe Effect and Supervoids

The Integrated Sachs-Wolfe (ISW) effect provides the primary conventional explanation for the Cold Spot, attributing the observed decrement to the interaction of () photons with a large-scale underdensity in the late universe. As the universe's expansion accelerates due to , gravitational potentials associated with overdensities and underdensities begin to , causing photons traversing these regions to a net energy shift. In an underdense region, or void, the potential is shallower and evolves such that photons lose energy, resulting in a cooler observed upon reaching . This late-time ISW effect is negligible during matter domination but becomes prominent at low redshifts (z < 1), where alters the evolution of structure. The ISW contribution to the fractional temperature perturbation is described by the line-of-sight integral \frac{\Delta T}{T} = -2 \int \dot{\Phi} \, d\eta, where \Phi is the gravitational potential, the dot denotes the time derivative, \eta is the conformal time, and the integral is taken along the photon's path from the last scattering surface to the observer. For a supervoid aligned with the Cold Spot's line of sight, the decaying potential (\dot{\Phi} > 0) induces a negative \Delta T, consistent with the observed cooling. This effect contrasts with the primordial Sachs-Wolfe contribution at recombination, which is frozen in the early . The Eridanus supervoid, a vast underdense region implicated in the Cold Spot, spans approximately 1.8 billion light-years in diameter and lies about 3 billion light-years from Earth, corresponding to a redshift of z ≈ 0.15. This supervoid exhibits a matter underdensity of roughly 20-30%, making it one of the largest known cosmic voids and a significant deviation from the average cosmic density. Detected through galaxy surveys and lensing, the void's elongated structure aligns closely with the Cold Spot's position in the Eridanus constellation. Theoretical models of the supervoid predict an ISW-induced cooling of about 50-70 μK at the Cold Spot's center, sufficient to account for much of the observed 70 μK decrement when combined with statistical fluctuations. Simulations incorporating the void's , including its depth and extent, reproduce the Cold Spot's temperature , with the strongest cooling occurring through the void's due to the integrated potential . These models assume a ΛCDM cosmology but highlight that the required ISW amplitude may exceed standard predictions by a factor of 2-5, prompting investigations into enhanced void evolution. Observational evidence supporting this ISW-supervoid link includes lensing measurements that first mapped the underdensity in 2015, revealing a supervoid radius of approximately 220 h⁻¹ Mpc with δ_m ≈ -0.14, consistent with the Cold Spot's location. Subsequent galaxy counts from the Survey in 2021 confirmed the supervoid as a prominent underdensity (signal-to-noise >5) at z < 0.2, with weak lensing signals aligning with the expected mass deficit and providing further validation of the ISW mechanism. While the full Cold Spot anomaly requires additional contributions, the supervoid's ISW imprint remains the most substantiated late-universe explanation.

Primordial Origin and Statistical Significance

The Cold Spot in the cosmic microwave background (CMB) may originate as a direct imprint of quantum fluctuations generated during cosmic inflation, where primordial density perturbations are amplified through gravitational instability on large scales. In standard inflationary models, these quantum fluctuations seed the initial conditions for structure formation, and rare underdense regions could manifest as exceptionally cold patches in the CMB temperature map after recombination. Such a primordial origin posits that the Cold Spot's unusual temperature profile results from an enhanced negative fluctuation at the last scattering surface, without requiring late-time secondary effects. Within the standard ΛCDM cosmology, statistical analyses of the Cold Spot indicate a low probability of occurrence as a natural fluctuation, estimated at approximately 1-2% based on Gaussian random fields. This rarity corresponds to a significance of about 2-3σ, as confirmed in Planck data where the anomaly's presence persists across multiple foreground-cleaned maps. Furthermore, curvature constraints from Planck observations suggest a potential excess in power at low multipoles (ℓ < 30), which could amplify the visibility of large-scale primordial features like the Cold Spot, though this remains within 2σ of ΛCDM predictions. Probes of non-Gaussianity using the Cold Spot's shape and surrounding statistics yield bounds on the local non-Gaussianity parameter fNL that are consistent with Gaussian (fNL ≈ 0 ± 5 at 68% CL from Planck), but the tail of the temperature distribution appears anomalous, hinting at subtle deviations. Monte Carlo simulations of full-sky ΛCDM realizations, incorporating realistic noise and beam effects from Planck, demonstrate that a as extreme as the Cold Spot occurs in roughly 1 in 50 cases, underscoring its status as a statistical . If confirmed as , the Cold Spot would challenge single-field slow-roll models by implying either a rare alignment of fluctuations or slight extensions to the standard paradigm, such as modulated reheating or isocurvature modes, to account for its scale and isolation. This interpretation aligns with the overall consistency of data but highlights tensions in large-scale that ongoing surveys aim to resolve.

Alternative Hypotheses

Topological Defects

Topological defects, such as cosmic strings, textures, and domain walls, emerge as remnants of phase transitions in the early universe following , where occurs in scalar fields. These defects form when the universe cools below critical temperatures, leading to the reconfiguration of field configurations that cannot smoothly unwind due to topological constraints. Cosmic strings are one-dimensional, textures are point-like configurations of global scalar fields, and domain walls are two-dimensional sheets separating regions of differing states. In the context of the (), these structures are predicted to arise post-inflation during global or gauge at energy scales around 10^{15}-10^{16} GeV. These defects source local temperature perturbations in the CMB through active gravitational effects, distinct from passive linear evolution in standard cosmology. Unlike primordial Gaussian fluctuations, defects evolve dynamically after the , injecting energy via stress-energy tensor contributions that induce non-Gaussian signatures, including isolated cold spots surrounded by hot rings. For the , cosmic textures are particularly invoked, as collapsing texture knots cause photon energy shifts through gravitational redshifting and lensing, producing localized anomalies without relying on large-scale linear integrated Sachs-Wolfe (ISW) effects. This mechanism yields non-Gaussian profiles that align with the observed ~5° angular scale and central decrement of approximately -70 μK, embedded in a broader ~100 μK feature. Texture models specifically predict CMB features of ~5° in size with amplitudes around 100 μK, arising from the scaling evolution of defects where the correlation length grows linearly with the horizon size. Simulations incorporating these models, such as those employing Bayesian analysis on (WMAP) data, show strong evidence ratios (E_1/E_0 ≈ 160) favoring textures over alternative local explanations like clusters, with posterior probabilities indicating a good match to the cold spot's . These numerical studies demonstrate that a single texture event at z ≈ 6 can replicate the observed non-Gaussianity without excessive parameter adjustment in the defect's energy scale. Planck satellite data provide stringent constraints on topological defect models, limiting the dimensionless string tension to Gμ < 1.5 × 10^{-7} at 95% confidence for Nambu-Goto strings, with similar bounds (Gμ < 10^{-6}) for global textures based on power spectra and bispectra. The cold spot has been proposed as a potential smoking gun for such defects, as it represents a rare, localized signal consistent with texture-induced perturbations, though robust analyses rule out models predicting more than 6 detectable textures across the full sky at 95% confidence. Despite these alignments, defect models require fine-tuning of the symmetry-breaking scale and defect density to isolate a single prominent feature like the cold spot, and they are currently less favored than supervoid explanations due to insufficient corroboration from multi-probe observations such as galaxy surveys or weak lensing.

Multiverse and Bubble Collisions

In the paradigm, our emerges as a within a vast where persists indefinitely, spawning innumerable disconnected bubble universes. Collisions between these expanding bubbles during the early cosmic epoch can perturb the and induce anisotropic temperature variations in the (CMB), manifesting as disk-shaped cold or hot spots with a characteristic radial profile. The has been examined as a for such a collision imprint, given its large extent and pronounced decrement, which deviate from expectations in standard inflationary models. In a seminal Bayesian , Feeney et al. () applied matched-template searches to WMAP 7-year data, identifying the cold spot as one of several potential features consistent with bubble collision geometry; however, the posterior distribution maximized at zero detectable collisions, with an upper limit of fewer than 1.6 such events at 68% confidence, indicating marginal compatibility rather than strong support. Theoretical models predict that bubble collision signatures would display non-Gaussian deviations, a nearly circular boundary with abrupt edges, and an edge-to-interior contrast, contrasting with the smoother, statistically isotropic . Subsequent scrutiny with Planck 2015 data, which offers higher resolution and sensitivity, confirmed the cold spot's at approximately 3σ but ruled out robust non-Gaussian signals indicative of strong bubble collisions, while permitting subtle effects below current detection thresholds; no conclusive evidence for this interpretation has emerged. Verification of a bubble collision origin for the cold spot would constitute empirical support for the framework, implying physical interactions across disconnected spacetimes, yet the inherent unobservability of the broader structure raises profound questions about and empirical validation in .

Recent Developments and Debates

Post-2021 Observations

Following the 2021 results, refinements in 2022 using DES Year 3 data confirmed the presence of the supervoid aligned with the Cold Spot direction, exhibiting an underdensity of approximately 20% (δ ≈ -0.2) in the deepest regions at z ≈ 0.15, based on redMaGiC samples. These analyses indicated that the standard Integrated Sachs-Wolfe (ISW) effect from the void contributes only 10-20% to the observed Cold Spot temperature depression of about -150 μK, falling short of a full . However, observations of excess ISW signals from multiple supervoids (A_ISW ≈ 5.2 ± 1.6 times the ΛCDM expectation across DES and BOSS datasets) suggest a potentially larger contribution. In 2025, the (DESI) Year 3 results, incorporating expansion history maps from and galaxy clustering, provided hints of links between low-redshift voids and CMB temperature anomalies, potentially through modified ISW effects driven by dynamical at z < 0.03. A multi-probe analysis combining galaxy surveys, CMB temperature profiles, and void catalogs reported hotter-than-expected CMB photons in local voids (z < 0.03) at 2.7-3.6σ significance, supporting enhanced late-time structure-CMB interactions. Data from the () DR6 and South Pole Telescope-3G (SPT-3G) 2023-2024 observations, covering high-resolution maps in the southern sky, enhanced angular resolution to arcminute scales. Early 2024 data from weak lensing surveys showed maps consistent with the Eridanus supervoid's projected mass distribution in the Cold Spot direction, though with amplitudes ~30% below ΛCDM predictions, supporting a partial void imprint without requiring exotic primaries.

Ongoing Controversies

A central concerns the adequacy of supervoids in accounting for the Cold Spot's observed properties. Analysis of Planck () lensing data in 2022 indicated that known supervoids along the line of sight can explain only a fraction of the Cold Spot's temperature depth and size, effectively ruling out a single large void as the dominant cause under standard cosmology. Conversely, the 2022 Survey () examination affirmed a strong alignment between the supervoid and the Cold Spot's position, suggesting a partial match via the integrated Sachs-Wolfe (ISW) effect, though the void's depth remains insufficient for a complete explanation within ΛCDM parameters. Another ongoing tension revolves around whether the Cold Spot represents a genuine physical or a statistical artifact. Early assessments pegged its deviation at approximately 3σ from Gaussian expectations in the , but incorporating the —arising from multiple possible sky positions—lowers the significance to around 2σ, rendering it compatible with random fluctuations. This interpretation is complicated by broader discrepancies in the low-ℓ CMB power spectrum, where Planck measurements show a deficit that tensions with baryon acoustic oscillation data from , potentially signaling either enhanced look-elsewhere biases or subtle deviations from ΛCDM. The Cold Spot does not exist in isolation, fueling discussions of correlated anomalies that might point to instrumental systematics or extensions beyond standard cosmology. When considered alongside the hemispherical —a north-south power imbalance in the at large scales—these features collectively challenge assumptions, with joint probabilities under ΛCDM falling below 1% in some analyses, though proponents attribute them to foreground contamination or selection effects. Prospects for resolution lie with next-generation experiments offering enhanced resolution and sensitivity. The Simons Observatory, achieving first light in 2024, will map the at arcminute scales to dissect low-ℓ structures like the Cold Spot, potentially distinguishing ISW imprints from primordial signals. Similarly, the Stage-4 (-S4) project, slated for full operations around 2027, promises deeper measurements to test anomaly origins with unprecedented precision. Despite these advances, no has emerged. The supervoid hypothesis via ISW remains the favored late-universe explanation among many cosmologists, yet or exotic scenarios like bubble collisions persist as viable alternatives, with exclusion probabilities exceeding 0.01 in current datasets.

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