The Eridanus Supervoid and the cosmic microwave background (CMB) Cold Spot are linked in one of cosmology’s most intriguing open questions—but they are not the same thing, and neither is evidence that our universe has a neighbour.
The Cold Spot is a large region in maps of the CMB, the faint relic radiation released when the universe became transparent roughly 380,000 years after the Big Bang. In that patch of sky, the CMB temperature is lower than the average by only a tiny amount, but the feature is unusually broad and has a notably steep profile. It first became widely discussed in data from NASA’s WMAP mission and was later seen in ESA’s Planck maps.
What is the CMB Cold Spot?
The CMB is not a photograph of empty space. It is the oldest light we can observe, carrying an image of the early universe. Across the sky it has minute temperature variations, which are expected in the standard picture of cosmology. Most of those variations are ordinary statistical fluctuations.
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The Cold Spot stands out because of its size and temperature pattern. It is an anomaly in the statistical sense: a feature worth investigating, not a confirmed break in physics. An anomaly can be a rare natural fluctuation, an effect caused by structures between us and the CMB, or a sign that a model needs refinement. The data alone do not choose among those explanations.
What is the Eridanus Supervoid?
In roughly the same direction as the Cold Spot lies a broad underdense region of the nearby universe: fewer galaxies and less matter than the cosmic average across a very large volume. It is commonly called the Eridanus Supervoid.
The word “void” can be misleading. A cosmic void is not literally nothing. It still contains galaxies, gas and dark matter; it is simply less dense than the surrounding cosmic web. Modern galaxy surveys and gravitational-lensing maps have confirmed a substantial underdensity in the Cold Spot’s direction.
Could the void make the Cold Spot colder?
Possibly, in part. CMB photons have travelled for more than 13 billion years. During that journey, large-scale structures can slightly alter their energy. In an expanding universe with dark energy, a photon travelling through an underdense region can emerge a little cooler through an effect related to changing gravitational potentials, often discussed as the integrated Sachs–Wolfe effect.
This makes the alignment interesting: a real foreground supervoid is present where the CMB has an unusual cold region. But alignment is not the same as a complete explanation. A 2021 analysis using Dark Energy Survey data confirmed the Eridanus underdensity while concluding that, under the standard cosmological model, its expected contribution accounts for only about 10–20% of the observed CMB temperature depression.
Does it prove a multiverse?
No. The CMB Cold Spot has inspired many imaginative proposals, including the idea of a collision with another “bubble universe.” That remains speculation, not an observational conclusion. The presence of the Eridanus Supervoid gives researchers a concrete nearby structure to test, but it does not turn a multiverse hypothesis into evidence.
NASA’s Astronomy Picture of the Day captured the correct attitude toward the Cold Spot years ago: published ideas have ranged from a supervoid to more exotic possibilities, while a rare statistical fluctuation remains possible. The mystery is real because the feature is real—not because one dramatic explanation has won.
What has recent research changed?
Recent work has made the picture sharper. The Dark Energy Survey confirmed the Eridanus Supervoid as a significant underdensity in the Cold Spot’s direction. A separate 2022 analysis of Planck CMB lensing data found that a single supervoid along the line of sight is unlikely to account for the Cold Spot as a whole.
Those results are complementary. They tell us that there is a genuine large-scale void in the relevant part of the sky, while also cautioning against claiming that the void solves every aspect of the CMB anomaly. The most accurate current summary is therefore simple: the Cold Spot is observed, the Eridanus Supervoid is observed, their relationship is an active research question, and the origin of the full Cold Spot remains unsettled.
Why it matters
Questions like this are how cosmology becomes more precise. Mapping voids helps scientists test how matter is distributed, how cosmic structure grows and how light is affected on its journey across the universe. The Eridanus Supervoid is valuable whether or not it turns out to be the entire answer to the Cold Spot.
Sources
- ESA Planck, “The CMB temperature on large angular scales.”
- ESA, “Cosmic Microwave Background (CMB) radiation.”
- Kovács, A. et al. (2022), “The DES view of the Eridanus supervoid and the CMB cold spot,” Monthly Notices of the Royal Astronomical Society.
- Nadathur, S. et al. (2022), “The CMB cold spot under the lens: ruling out a supervoid interpretation.”
- NASA Astronomy Picture of the Day, “The CMB Cold Spot.”
