Does the Universe Have a Supervoid? Explaining the Cold Spot of the CMB
Studying the cosmic microwave background (CMB) offers one with so many consequences for so many disciplines of science. And as we continue to launch new satellites and get better data on it, we find that our theories are being pushed to a point where they seem likely to break. And on top of that, we encounter new predictions based on the hints that the temperature differentials offer us. One of these is in regards to the cold spot, a troubling irregularity in what should be a homogenous Universe. Why it exists has challenged scientists for years. But could it have an impact on the Universe of today?
In 2007, a team of researchers at the University of Hawaii led by Istvan Szapudi investigated that and developed the supervoid idea in an effort to explain the cold spot. Simply put, a supervoid is a low-density region devoid of matter and may be a result of dark energy, that invisible mysterious force driving the expansion of the Universe. Istvan and others began to wonder how light would act as it transversed such a place. We can look at smaller voids of a similar nature to perhaps gain a grasp on the situation, plus work from the conditions of the early Universe (Szapudi 30).
At that time, quantum fluctuations caused different densities of matter in different locations, and where lots clumped together eventually formed the clusters we see today, while those places lacking matter became voids. And as the Universe grew, whenever matter would fall into a void it would decelerate until it got close to a gravitational source then start accelerating again, therefore spending as little time as possible inside the void. As Istvan describes it, the situation is similar to rolling a ball up a hill, for it slows as it gets towards the top but then again once the top has been peaked (31).
Now, imagine this happening to photons from the cosmic microwave background (CMB), our furthest look into the past of the Universe. Photons have constant speed but their energy levels do change, and as one enters a void its energy level decreases, which we see as a cooling off. And as it accelerates again, energy is gained and we see heat radiating. But will the photon exit the void with the same energy as it entered with? No, for the space it moved through expanded as it travelled, robbing it of energy. And that expansion is speeding up, further reducing the energy. We formally call this process of energy loss the integrated Sachs-Wolfe (ISW) effect, and it can be seen as temperature dips near voids (Ibid).
We expect this ISW to be rather small, around the order of 1/10,000 variations in temperature, “smaller than the average fluctuations” in the CMB. For a sense of scale, if we measured the temperature of something as 3 degrees C, the ISW could cause the temperature to be 2.9999 degrees C. Good luck getting that precision, especially at the cold temperatures of the CMB. But when we look for the ISW in a supervoid, the discrepancy is much easier to find (Ibid).
But what did scientists exactly find? Well, that hunt began in 2007, when Laurence Rudnick (University of Minnesota) and his team looked at the NRAO VLA Sky Survey (NVSS) data on galaxies. The information the NVSS collects is radio waves, admittedly not CMB photons but with similar characteristics. And a void was noticed with radio galaxies. Based off that data, the ISW effect courtesy of a supervoid could be found as far away as 11 billion light years away. The reason for the uncertainty is that the NVSS data is unable to determine distances. But scientists realized that if such a supervoid was that far away, the photons passing through it did so about 8 billion years ago, a point in the Universe where the effects of dark energy would have been way less than now and therefore wouldn’t affect the photons enough for the ISW effect to be seen. But the statistics say that areas of the CMB where warm and cold differentials are high should be present locations of voids (Szapudi 32. Szapudi et al).
And so, the team set the CFHT to look at small places in the cold spot area to get a true gauge of galaxies and see how that matched with models. After looking at several distances, it was announced in 2010 that no signs of the supervoid were seen at distances greater than 3 billion light years. But it must be mentioned that because of the resolution of the data at the time, there was only 75% significance, way too low to be considered a safe scientific finding. Plus, such a small area of sky was looked at, further reducing the result. So, the PS1, the first telescope on the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) was brought in to help augment the data collected up to that time from Planck, WMAP, and WISE (32, 34).
After collecting all of that, it was found that the infrared observations from WISE lined up with the suspected supervoid location. And by using redshift values from WISE, Pan-STARRS, and 2MASS, the distance was indeed about 3 billion light years away, with the required level of statistical significance to be considered a scientific finding (at 6 sigma) with a final size of about 1.8 billion light years. But the size of the void doesn’t match expectations. If it originated form the cold spot then it should be 2-4 times bigger than we see it to be. And on top of that, radiation from other sources can under the right circumstances mimic the ISW effect and on top of that the ISW effect only partially explains the temperature differentials seen, meaning that the supervoid idea has some holes in it (See what I did there?). A follow-up survey using ATLAS looked at 20 regions within the inner 5 degrees of the supervoid to see how the redshift values compared under closer scrutiny, and the results were not good. The ISW effect may only contribute -317 +/- 15.9 microkelvins, and other void-like features were spotted elsewhere on the CMB. So maybe, like all things in science, we need to revise our work and delve deeper to uncover the truth…and new questions (Szapudi 35, Szapudi et. Al, Mackenzie).
Mackenzie, Ruari, et al. “Evidence against a supervoid causing the CMB Cold Spot.” arXiv:1704/03814v1.
Szapudi, Istavan. “The Emptiest Place in Space.” Scientific American Aug. 2016: 30-2, 34-5. Print.
Szapudi, Istavan et al. “Detection of a Supervoid Aligned with the Cold Spot of the Cosmic Microwave Background.” arXiv:1405/1566v2.
© 2018 Leonard Kelley