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Eridanus Supervoid May Explain Cosmic Microwave Background Anomalous Cold Spot

By: William Brown, Biophysicist at the Resonance Science Foundation

A lot of information about the large-scale nature of the universe can be derived from detailed analysis of its ubiquitous thermal electromagnetic field, called the cosmic microwave background (CMB), for instance analysis of suppressed fluctuations at large wavelengths reveals a closed geometry of the universe— a torus-type geometry as we described in the RSF article A New Signature of a Multiply Connected Universe [1].

Inhomogeneities attributed to quantum fluctuations during the inflationary period are amplified across large-scale universal structure, and when inflation ends, they become density fluctuations and cause the differences in temperature observed in the CMB. Its an intriguing signal to study because it reveals an epoch when atomistic and cosmological structure where one-and-the same, and quantum behaviors that typify nature evolves into large-scale structural features of the universe. (Credit: E. Siegel; ESA/Planck and the DOE/NASA/NSF Interagency Task Force on CMB research)

The hot Big Bang Inflationary standard model of cosmology holds that the universe is homogenous (the same in all locations) and isotropic (the same in all directions), which is why it is exciting to consider the many examples that appear to be in violation of this precept as they reveal properties of the large-scale structure and behavior of the universe:

It has long been presumed that the universe is homogeneous and isotropic... with no identifiable axis or orientation. Indeed, this is known as the cosmological principle. Yet, one of the 20th century’s greatest minds, Kurt Gödel, provided an exact solution of the Einstein field equations that described a rotating universe… In more recent events, there have been several findings that suggest that the universe is indeed not entirely homogenous and isotropic. Such examples come from the so-called axis of evil identified during an analysis of the microwave background radiation, dark flow, Shamir’s report on the Sloan Digital Sky Survey showing that left-twisted galaxies were much more common than right-swirling galaxies; as well as structural mapping such as the BOSS Great Wall and Laniakea. William Brown, The Rotating Universe, 2020 [2]

The CMB is the oldest source of light in the universe, and because of this it allows us to draw many inferences about the extant state of the universe by extrapolating out the structure and density fluctuation signal of the CMB radiation. For the most part, the CMB data agrees with the hot Big Bang Inflationary model and the cosmological principle of homogeneity, however one source of data has remained largely anomalous (unexplained) until recently, and that is the great CMB cold spot—a significantly large volume of the universe that is about 70 microkelvin below the average temperature of the universal background radiation. The coldness at its center is not necessarily the most anomalous feature of the Cold Spot, but rather the combination of a cold interior and a surrounding hot ring, leading to vigorous debate about the nature of the enigmatic inhomogeneity of the CMB.

The Planck CMB temperature map (Planck Collaboration 2018) highlighting the anomalous Cold Spot, unusual not only for its coldness and its extent, but for the fact that it’s surrounded by a hot region on all sides. New research suggests this enigmatic feature may be explained by a nearby supervoid in the constellation of Eridanus. (Credit: A. Kovács et al., 2021, MNRAS)

One intriguing explanation for the CMB Cold Spot is that it is evidence of interaction with a parallel universe—not the kind of parallel universe of the many-worlds interpretation of quantum mechanics, but the kind predicted to occur in the eternal inflation model of the early universe, in which inflation may stop in localized regions, like our universe, but continues indefinitely at larger scales generating a multiverse. Like a boiling pot of water, with the bubbles being parallel universes continually springing into existence—and every now and then two adjacent bubbles might bump into each other. Such an interaction could lead to an anomalous density fluctuation in the CMB data, such as what is observed in the Cold Spot.

Now, a team of researchers who have run a detailed analysis of luminous red-shifted galaxies from the Dark Energy Survey have found a correlation between the CMB Cold Spot location and an approximately 1-billion light year diameter void in the cosmic web, termed the Eridanus supervoid [3]. Because the primordial light of the surface of last scattering can become gravitationally red-shifted as a result of the continued expansion of the universe after the first emission of the CMB— known as the Integrated Sachs-Wolfe effect— underdense regions of the universe can result in an apparent lower temperature to the universal background radiation in locations were Dark Energy dominates, like in supervoids.

A hot spot, a cold spot, or a region of average temperature in the CMB corresponds to an underdense, overdense, or average-density region, respectively, when the first primordial light was emitted following the photon epoch: just 380,000 years after the Big Bang. This is a consequence of the Sachs-Wolfe effect. (Credit: E. Siegel/Beyond the Galaxy)

While the decaying gravitational potential (Φ) of a supervoid will at best produce only a fraction of any observed temperature depression of the CMB via the late-time Integrated Sachs-Wolfe effect, the Eridanus supervoid is possibly the largest underdensity that exists since the Dark Energy dominated epoch, and so the correlation may be significant. However, at this time it remains only an intriguing hypothesis, and cannot be verified until more data is collected—which is expected to come with the ESA’s Euclid mission poised to launch just next year, in 2023, with the stated mission “to map the geometry of the Universe and better understand the mysterious dark matter and dark energy, which make up most of the energy budget of the cosmos. The mission will investigate the distance-redshift relationship and the evolution of cosmic structures by measuring shapes and redshifts of galaxies and clusters of galaxies out to redshifts ~2, or equivalently to a look-back time of 10 billion years. In this way, Euclid will cover the entire period over which dark energy played a significant role in accelerating the expansion of the Universe.”


[1] William Brown, “The Rotating Universe.” (accessed Feb. 08, 2022).

[2] William Brown, “A New Signature of a Multiply Connected Universe.” (accessed Feb. 08, 2022).

[3] A. Kovács et al., “The DES view of the Eridanus supervoid and the CMB Cold Spot,” Monthly Notices of the Royal Astronomical Society, vol. 510, no. 1, pp. 216–229, Dec. 2021, doi: 10.1093/mnras/stab3309.


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