The map of Cosmic Microwave Background Radiation. Credits: ESA/Planck

The conundrum of the cold-spot in the relic of Big Bang: Cosmic Microwave Background Radiation

A few weeks back, when my cable operator shut-down my channel subscriptions, I could only see nothing but the “Radiation of Big Bang” sarcastically and philosophically; I could see nothing but everything at the same time. CMB is one of the very discussed, researched, and debated topics among cosmologists all over the world; why not? It’s one of the proofs of the big bang, and many regard it as “Light of Beginning of the Creation itself.” 

 Last year, I had the privilege to learn and meet one of the most active and vigilant cosmologists in Pakistan, Faisal Ur Rahman (Currently a Ph.D. Candidate, Researcher & Visiting Member at the University of Karachi), who also happens to be a part of an International Group of Cosmologists; “Investigating the Cosmic Microwave Background Cold-Spot.” I walked by him a few miles and asked him a number of questions regarding his work on the “Cold Spot,” which happens to fall into a long series of explanations about the phenomenon, which I’ll be discussing throughout this textual endeavor. 

His-ideas are narrated in my words: Our standard model of the big-bang tells us that our Universe was a hotter, extremely-dense plasma type, made up of elementary particles such as nuclei, electrons, and protons only for the first few hundred thousand years. And just after 380,000 years old, our space-time in the Universe had expanded and cooled to below 3000K, allowing the birth of atoms and the first atoms of the first element “Hydrogen” formation. As the free electrons and protons decreased to a very small fraction, the photons were finally able to move around the Universe and till the existence of today’s CMB. And that’s why to regard it as an evidentiary account of the Birth of the Universe. Further, as we know, the Universe is expanding today; thanks to the discovery by Astronomer Hubble, the current CMB wavelength is stretched and has a temperature of just about 2.7K. 

Faisal Ur Rahman giving a seminar on “Big Bang: The Birth of Universe” on the invitation of the Royal Aeronautical Society Pakistan. 

When analyzed at around 10-5K, it’s all isotropic, and the entire CMB has a fine blackbody spectrum (fully uniform temperature). But, as we change it to the microkelvin scale, we start seeing the variations in the graph of Temperature in terms of different temperatures, some cold, some hot, and some hotter. It is important to note that matter wasn’t evenly distributed after the big bang due to the tiny quantum fluctuations in densities. Instead, we can always see some areas have more matter or galaxies, and some don’t, eventually giving rise to the cosmic web we see in the models of the Universe. The CMB photons traveling from the surface of the last scattering moving through a denser region have to overcome a deeper gravitational pull which results in an apparent redshift, and the photons passing through a less dense region will appear blue-shifted.

The increase or decrease in the wavelength of the CMB photons is caused by the changes in μK scale or parameters. More vividly, there’s a relation between the large-scale universe and temperature anisotropies, different temperatures at larger scales, referring to different properties. There’s a Sachs-Wolfe effect, which is a gravitational effect on the large-scale anisotropy of the CMB, which has two kinds. First is the “Ordinary Sach-Wolfe effect,” the non-integrated type, which is related to the early Universe only and tells us about the redshift of light from the last scattering surface. Then the other is the “Integrated Sach-Wolfe (ISW) effect,” which depends on the changes in gravity as the matter-density in the Universe evolves over time, eventually a change in CMB photons.

The evolution of the Universe, with main events occurring at different intervals of time. Credits: Phys.libretexts.org

The primitive fluctuations in the CMB have the strong support of evidence for cosmic inflation, and cosmologists believe that the Universe had the uttermost rapid expansion when it was merely seconds old. Researchers estimate the relative abundance or amount of Dark Matter and Dark Energy in the Universe by the CMB fluctuations, along with Gravitational Lensing as predicted by Einstein’s theory of General Relativity and the Sunyaev-Zel’dovic Effect: In which the CMB photons are distorted due to the high level of energetic electrons in the clusters of galaxies they come through. 

Penzias and Wilson stand at the 15 meter Holmdel Horn Antenna that brought their most notable discovery. Credits: ESA
Penzias and Wilson stand at the 15 meter Holmdel Horn Antenna that brought their most notable discovery. Credits: ESA

Since the discovery of CMB in 1964 by Physicist Arno Penzias and Radio-Astronomer Robert Wilson (For which they won the Nobel Prize of 1978), the Cosmic Microwave Background has given scientists oceans of information and curiosity. Although, it has raised some of the most substantial and complex questions and anomalies, as the strange incongruity of the CMB, as seen from both opposite hemispheres of the sky. 

The map of Cosmic Microwave Background (CMB) temperature as data collected by the ESA’s Planck satellite, and confirmed fluctuations, the circled) cold spot, which extends over the large area of the sky and has a too lower temperature. Credits: ESA/Planck/Physics World

In the CMB, there lies a substantial question, a huge cold spot, which is around a billion light-years across. This cold spot was first seen by WMAP in 2004 and more strongly discovered by ESA’s Planck Satellite. We call it “CMB Cold Spot,” it’s around 70µK cold on average than the entire CMB temperature of the Universe and located in the southern celestial hemisphere.

 In other ways, it may be a possibility that the cold-spot was born the first of the big bang density fluctuations or irregularities that bought the entire CMB temperature anisotropies into existence because those anisotropies have a Gaussian distribution, which only permits small variations (at the scale of around 16µK) but not such large. A few found places the temperature of the spot is around 150μK colder than the average CMB, more than of which is permitted from a Gaussian distribution. The radius of the cosmic cold spot extends about an angle of 5°, but apparently, on the other hand, the largest fluctuation found in the primordial CMB temperatures occurs at 1°, making it peculiar and uncanny. 

If we go through the literature review, the researchers have postulated many different hypotheses of this mysterious cold spot in our Universe, and a lot of debate is going on to reveal its mystery and reason of occurrence. Few explanations suggest that the spot is nothing but forefront contamination of the matter (as a kind of dust or synchrotron radiation) within our galaxy or some kind of celestial object. But, profoundly, the observations of NRAO Very Large Array Sky Survey (NVSS), 2 Micron All-Sky Survey (2MASS), Sloan Digital Survey, with proper detailed optical images of Hubble, show us that ‘no object’ found at those coordinates. 

Void and null

One suggested simple but questionable explanation for it is that there might be a “super huge void” between us and the cold spot, but our universe structure on a larger scale, the galaxies have already been found in voids, their density can be much lesser in some cases (1/10th) than other densely populated areas in the Universe. On another side, the Universe also has superclusters – regions where there are more galaxies or quantities of matter in easy terms than normal cases.  

Veiled void? One promising explanation for the phenomenon is that there exists a vast cosmic “supervoid” between us and the cold spot, thanks to the large-scale structure of the Universe. While galaxies still exist within voids, their density of matter is much less (one-tenth of the average) than in other regions of the Universe. On the flip side, the Universe also contains superclusters – huge regions with many more galaxies than normal. 

Faisal Ur Rahman (Physics World, 2019) says that according to analysis, “Both supervoids and superclusters have a very impactful effect on the photons of the CMB as the Integrated Sachs-Wolfe effect varies due to the physical properties of it, it can also be termed as late-time ISW effect. As the CMB photons pass through the superclusters (passing through huge gravitational effects), they gain potential energy as they pass through the superclusters and have a thermal energy increase. Everything is fine; the increase in energy within the photon should be dispersed once the photon comes out of the gravitational valley of the supercluster. But, there’s another factor, Dark Energy, that comes into action, which is responsible for the overall expansion of our Universe. 

During the time when a photon travels across a supercluster, dark energy expands the valley, stretching it in a way that the photon leaving does not have the extra energy to climb back out of the valley. That’s why the CMB photons have some heat. In the same manner, the super-voids work as gravitational hills, and the entering photon of the CMB loses more energy while getting into a void and getting out with regain in energy as they are leaving the void (as now the void is big due to the dark energy). This phenomenon colds the photons. The ISW has a possibility in this phenomenon for the CMB cold spot, and there must be a supervoid existing in the place of the cold-spot then. 

Lambda-CDM Model of the Universe.

Hide and seek

Discussing another study in 2007 at the University of Minnesota by Rudnick and colleagues claimed the presence of supervoid in that place, but a prominent dip in extragalactic brightness and numbers of radio sources that were present there, as seen through the NVSS radio catalog. But another study done by University of Cambridge researchers under Kendrick Smith and collaborators of the University of Michigan in 2010 found that there’s no data evidence for the presence of dips in NVSS maps, which dismissed the presence of a supervoid. Further later in 2014, Seshadri Nadathur of the University of Helsinki concluded that the ISW influence on the CMB light from the believed supervoid is not a possible explanation of the cold-spot. They said that for the possibility of the necessary photon temperature decrease via ISW, the supervoid must be extremely massive and without matter that our standard model of the cosmology will be violated and changed, known as ΛCDM.

ACDM provides us with an overall understanding of the Universe in a way that dark matter (26.8% of the Universe is mass/energy) and dark energy for 68.3%. The war between the gravity of the dark matter and dark energy’s expansion of the cosmos. Nedathur’s work finds that by anyway, we ought to explain the cold-spot that would be completely anomalous for our knowledge of today. 

More puzzling, another claim rose in 2014, when a team of astronomers under the leadership of István Szapudi of the University of Hawaii found a rare supervoid, with a size of 1.8 billion light-years. And they quoted it as “the largest individual structure ever identified by humanity.” They combined the infrared data from NASA’s Wide-field Survey Explorer (WISE) 2MASS Catalogue and Hawaii’s Pan-STARRS1 (PS1) optical observations for it. They surveyed galaxies near the cold-spot boundaries and found a supervoid. According to the team of Hawaii, the void was 3 Billion light-years away from us, which was not found in previous surveys because searches in the past focused on the early Universe, farther away from us in space and time. Besides this discovery, researchers realized that the large supervoid isn’t enough for the CMB Cold Spot’s temperature down, as the cooling Integrated Sachs-Wolfe effect would be a maximum of 20μK.  

To test the hypothesis of Szapudi, spectroscopic data was used by a team led by Ruari Mackenzie of the University of Durham from the 2dF-VST ATLAS Cold Spot Redshift (2CSz) survey at the Anglo-Australian Telescope in New South Wales, so they could study the redshift in-line of sight to the cold spot. They measured twice from different lines of sight and found three voids out to a distance of three billion light-years and with a possibility of a fourth void as well. Although all the voids were smaller than the void Szapudi predicted in his research, they all had an overall 31μK ISW cooling impact, but simply not most-likely enough to support the cold spot. 

Surprisingly, all the research led to this theory that the voids are not the conclusion here; it’s due to something else, the massive size must have something to with primordial fluctuations. This is so because the standard ΛCDM model will be conflicting if we go through Szapudi’s idea of supervoid. While on the other hand, coded simulations suggest that there’s only a 1 in 50 chance of random, nonGaussian quantum fluctuation in the CMB for the birth of the cosmic cold spot. 

Unusual Structures

Thinking with a different approach, there lies another explanation for the cold spot of the CMB. Maybe the motion of matter or galaxies within that region is unusual, with a super gravitational effect. This could be something similar to the concept of “Greater Attractor,” a kind of abnormality at the mid-point of Laniakea supercluster (the object between our Milky Way) which occurs due to extremity of the presence of mass, and the related “Dipole Repelling,” which works as a gravitational repulsion on the large scale flow of galaxies in our local group, and that might be supposed to exist due to large supervoid. These both have their effect on the CMB dipole, but none such abnormal or unusual structures have been identified within the cold-spot or its surrounding place. Another study from independent sources by Qi Quo and fellows at the Chinese Academy of Science pushed forward the probability of 19 dwarf galaxies that have dark-matter insufficiency, which is very strange because mostly these small galaxies are filled with dark matter normally.

Among these galaxies, 14 are isolated, and they are not the satellite-orbiting type around a large galaxy like ours, which leads to the view that the dark matter deficiency was not the result of any connection with larger or other dwarf galaxies. As per studies, these low-dark-matter dwarf galaxies may change the results of other ISW parameters, all of which are impacted by dark matter. Some cosmologists believe that may be a cold-spot answer lies with the theory of inflation itself? Or maybe after it. Maybe after the whole big-bag, within the infant universe, this patch of the Universe went on for a longer period of infancy, due to which the cold-spot came into being naturally in that part of the Universe. That’s a theory put forward by Yi Wang at the Honk Kong University of Science and Technology and Yin-Zhe Ma of the University of KwaZulu-Natal of South Africa. They proposed a “feature scattering” system of inflation that predicts cold spots (not hot ones). But, still, these substantial questions must be consistent with the validity of other phenomena, including the impact of such anomalies on permutations of density and the overall stars and galaxy evolutions within our Universe.  

Since I’ve discussed so many circulating solutions to uncover this mystery, it’s important to consider each explanation with the larger context of the cosmology, such as Ia supernovae, baryonic acoustic oscillations, and the entire CMB itself for supporting evidence. Some gravitational physicists suggest that a more precise Hubble constant value can be arrived at in the future through mergers of neutron stars or black holes. These observations, with some changes in value for the cosmological parameters, must support the  ΛCDM, and so any solution must be consistent and agreeable with the model.

But, still exploring alternatives outside our standard model play an important role in studying the anomalies such as itself, the cold-spot. For example, Elenora Di Valentino at the University of Manchester carefully studied the Planck satellite data of 2018, and their findings challenged our understanding of the usual ΛCDM assumption of the Universe being flat. Their results pointed out a “closed universe,” which is contradictory to our present assumptions and challenges the inflation theory. While their research was only 3σ, not conclusive, it needs a parallel study for more investigation as a cross-check. 

The above map shows CMB fluctuations.
The above map shows CMB fluctuations. Credits: ESA
The map shows the amplitude fluctuations polarization. While the temperature map clearly signifies the cold spot, the anomaly. It doesn’t show any statistical signification in anomalies of the polarization map, but doesn’t rule out the potential relevance of seen in the temperature map, but makes it more mysterious as a puzzle to be followed. Credits: ESA

Getting to an exotic route?

As we don’t have any standardized explanation or theory for the cold-spot of the Cosmic Microwave Background Radiation, some thinkers, for instance, philosophical actors, opt for a more weird possibility that the cold spot might be due to a “collision” between our Universe and a parallel universe. This goes with the popular multiverse theory we hear today by physicists all around the world. It says that the Universe collides and interacts over-time. The other Universe must have collided with ours, also thanks to the quantum entanglement between universes before they were separated at first by the cosmic inflation, and the interactions left this footprint on the CMB as a cold-spot. But, still, the “ECREE: Extraordinary claims require extraordinary evidence as said by Dr. Carl Sagan” and must be following the laws of other cosmological observations. If there’s a possibility to the theory of collision between universes, then there would have been an identifiable polarization signal in the coldspot. This has been suggested by Tom Shanks of the Centre of Extragalactic Astronomy at Durham University in 2017. 

The recent data results of the Planck team in 2019 involved further analysis of the polarization found in the CMB (that’s entirely different from its temperature scale) to further dive into the anomalies and mysteries such as the cold spot. Planck’s multi-frequency data are designed to eliminate noises already existing from other microwave sources, including the dust and gas coming in between surveys from our own Milky Way. Even due to this scrupulous and worthy analysis, they failed to find any specific breakthrough traces of anomalies in the polarization maps (shown in maps). Till now, the Planck team has not denied or confirmed any of the nature of such anomalies as cold spots. This leaves an open-minded possibility of testation and explanations that would better our understanding and the implications of laws of physics for parameters at a cosmological scale. 

Giant Magellan Telescope to be finished in 2029 as per gmto.org. Credits: GMTO

Further in the future, we need more data from more sensitive telescopes such as the MeerKAT array and other ventures like Giant Magellan Telescope and Square Kilometre Array. We also need to understand more about the existing forces such as Dark Energy and how it plays a role in the overall evolution of our Universe every passing second to understand the workings of the Integrated Sache-Wolfe effect. We do not reach any steady conclusion about the CMB cold spot with our present knowledge and limitations. So, we direly need better observational data or maybe revisitation in our current universal understandings. On the bright side, we can always use more advanced ground or space-based telescopes to get deeper to find a substantial explanation for the question of this enigmatic scientific occurrence of CMB cold spot.

References:

1. “The Enduring Enigma of the Cosmic Cold Spot.” Physics World, 11 Feb. 2020, https://physicsworld.com/a/the-enduring-enigma-of-the-cosmic-cold-spot/.

2. Dark Matter & Dark Energy | University of Oxford Department of Physics. https://www2.physics.ox.ac.uk/research/dark-matter-dark-energy. Accessed 27 Jan. 2021.

3. “Astronomers Discover Largest Known Structure in the Universe Is … a Big Hole.” The Guardian, 20 Apr. 2015, http://www.theguardian.com/science/2015/apr/20/astronomers-discover-largest-known-structure-in-the-universe-is-a-big-hole.

4. Goss, Heather. “What Created the Universe’s Cold Spot?” Air & Space Magazine, https://www.airspacemag.com/space/what-created-universes-cold-spot-180955343/. Accessed 27 Jan. 2021.

5. Hu W, Dodelson S. Cosmic microwave background anisotropies. Annu Rev Astron Astrophys. (2002) 40:171–216. doi: 10.1146/annurev.astro.40.060401.093926

6. Kolb EW, Turner MS. The early Universe. Front Phys. (1990) 69:1–547.

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