The mystery of the ‘cold spot’ in the CMB

An international collaboration of researchers may have found the solution to the mystery of the so-called Cold Spot visible in the map of the cosmic background radiation. The observations suggest that it is a huge region of the sky where the density of galaxies is much lower than that typical of the space observable. The results were published in Monthly Notices of the Royal Astronomical Society.


Credit: ESA/Planck collaboration

A group of researchers led by Istvan Szapudi the Institute of Astronomy at the University of Hawaii may have found an explanation of the so-called Cold Spot, ie the large cold spot in the map of the cosmic microwave background that, according to the same Szapudi, would the single largest cosmic structure that has ever been identified. The results are published in Monthly Notices of the Royal Astronomical Society.
In 2004 astronomers identified in the map of the cosmic microwave background (Cosmic Microwave Background, CMB) wide region of the sky, about the size of 150 Mpc, or 500 million light years, where the temperature was much lower than the mean value : it was given the term or Cold Spot which literally means “cold spot”. On average, the typical fluctuation of the temperature of the cosmic background radiation is of the order of 10-5, while the “cold spot” is 70 μK cooler than the average temperature of the CMB (about 2.7 K).
The standard cosmological model of the Big Bang predicts the existence of regions of various sizes, which can be colder or warmer than the average value of the temperature of the space, but a region of this size was really a surprise. If this particular region of the sky has originated from the Big Bang, it could be a sign of a more exotic physics that the standard cosmology can not explain. If, instead, the “cold spot” was caused by an underlying structure located between us and the CMB, then it could be an indication that there would be a very rare cosmic structure on a large scale that is distinguished by the distribution of matter in the media Universe.
Thanks to the latest data obtained with the telescope Pan-STARRS1 (PS1) located at Haleakala, Hawaii, and with satellite WISE NASA, the team was able to identify a huge Szapudi “supervuoto cosmic”, a vast region of the sky that stretches for 1.8 billion years-light and in which the density of galaxies is much lower than that typical of the space observable. This region of the Universe was found by combining the observations obtained from PS1 in visible light with the WISE infrared during a study that aimed to estimate the distance and location of each galaxy in the sky area.
Previous studies, always conducted in Hawaii, they observed a much smaller area in the direction of the Cold Spot, without noticing the presence of any structure in that portion of the sky. In an almost paradoxical, one could say that it is much easier to identify the distant cosmic structures than the local Universe since we need to map areas of the sky to see larger structures closer. For this study, were used huge three-dimensional maps made with PS1 and WISE by Andras Kovacs of Eötvös Loránd University, Budapest, Hungary. The supervuoto is just 3 billion light-years, a cosmic distance relatively short compared with the typical scale at which they are localized these giant structures.
Cross a supervuoto can take millions of years, even traveling at the speed of light, and such a large structure can affect the cosmic background radiation by an observable effect, they speak of the Integrated Sachs-Wolfe (ISW) Effect that could provide scientists the first explanation of one of the biggest anomalies that were found before the WMAP satellite and then from the Planck satellite, the map of the cosmic background radiation.


Credit: IfA/University of Hawaii

To understand what astronomers have observed, we imagine that there is a huge cosmic void where there is little matter between the observer and the cosmic background radiation. Now think of this structure as if it were a kind of “hill”. When light enters into the void, it must overcome the slope of the hill. If the expansion of the Universe was not speeding, then the cosmic void not evolve significantly and the light would fall along the slope of the hill again gaining its energy, which lost initially, when out of the vacuum. But with the accelerated expansion, the hill is deformed as the light passes through it. Now, from the moment when the light descends the slope, the hill becomes flatter so the light can not gain all the energy that is lost when entry in the cosmic void. In other words, the light comes out from the vacuum with less energy and therefore to a longer wavelength, which corresponds therefore to a lower temperature.
While the existence of supervuoto and its effect on the CMB does not fully explain the large “cold spot”, it is unlikely that the supervuoto and Cold Spot represent a spatial coincidence in the same region of the sky. Researchers now will continue to work on this property using the new observations of the telescope PS1 and Dark Energy Survey (DES) to study the Cold Spot and supervuoto, as well as another great cosmic void located near the constellation Draco.

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