The CMB cold spot – explained?

The cosmic microwave background (CMB) is the afterglow of the Big Bang, cooled to the microwave region of the electromagnetic spectrum by the expansion of the Universe for ~14 billion years. Across the entire sky radiation at 2.7 K is evident once local sources are removed. It is not completely uniform in temperature however, with deviations to 1 part in 100, 000.

According to standard cosmology, the CMB gives a snapshot of the Universe around 380,000 years after the Big Bang. The temperature of the expanding Universe cooled until it reached about 3000 K, at which point electrons and protons combined to form hydrogen atoms. Radiation was scattering in the free electrons and protons of the younger, hotter Universe, but once hydrogen formed radiation could travel unimpeded.  The Universe therefore became ‘transparent’ to radiation, which we see as the CMB. This time is generally known as the ‘time of last scattering’ or the epoch of recombination.

Cosmologists have realised that measuring the angular variations, or anisotropy, in this radiation would provide important clues to why matter (like galaxies and clusters) is distributed the way it is today. The regions that are slightly more dense than others gravitationally attract photons, causing them to lose some energy in transition. These regions thus appear to be at a lower CMB temperature. Regions that are less dense suffer less from this effect and the radiation appears to be at a relatively higher CMB temperature.

In 2004 while examining a map of the CMB obtained by the WMAP satellite, astronomers discovered the Cold Spot, a larger-than-expected unusually cold area of the sky. The physics surrounding the Big Bang predicts warmer and cooler spots of various sizes in the infant Universe, but a spot this large and this cold was unexpected.

An all-sky picture of the cosmic microwave background (CMB) created from nine years of WMAP data. The image reveals temperature fluctuations, shown as colour differences, that correspond to the ‘seeds’ that eventually became galaxies. This image shows a temperature range of ± 200 µK . The dark blue Cold Spot is at bottom right of the image. (Credit: NASA / WMAP Science Team)

The Planck satellite also detected the CMB Cold Spot (as well as other suggested anomalies initially seen in the WMAP data.). The Cold Spot deviates by about -70 µK from the average CMB temperature and is centred on (l, b) ~ (209°, -57°) in Galactic coordinates. The Cold Spot is perhaps the most significant among the ‘anomalies’ in the CMB that are potential departures from isotropic or Gaussian statistics.

Explanations of the Cold Spot range from a statistical fluke through to unknown physics, an imprint of a parallel universe, to the Integrated Sachs-Wolfe (ISW) effect from a ~200 h−1 Mpc supervoid, which is an area lacking in galaxies, centred on the Cold Spot.

In the ISW effect photons from the CMB can be gravitationally redshifted or blueshifted due to intervening gravitational fields. If photons have to travel through a dense cluster of galaxies they gain energy by “falling” into the cluster’s gravitational potential. As the photons “climb” out of the cluster potential they lose energy, but not as much as they have gained since the Universe expanded in that intervening time. The net effect is that these photons are slightly warmer. For photons travelling through a void the effect is the opposite. The photons exit the void with less energy and therefore at a longer wavelength, which corresponds to a colder temperature.

Hence the ISW effect is driven by the intervening galaxy distribution and can slightly alter the temperature of the CMB photons that we detect.  If correct, the void that could cause a Cold Spot in the CMB would be readily detectable in large-scale structure surveys of galaxies.

An all sky map of the CMB separated into the two hemispheres of the sky, indicated by the curved white line. The Cold Spot that extends over a patch of sky that is much larger than expected is circled in the bottom left of the map. (Credit: ESA and the Planck Collaboration)

So what is the origin of this CMB Cold Spot? A recent study by István Szapudi of the University of Hawaii and collaborators of galaxy properties in and around the Cold Slot has recently been published in the Monthly Notices of the Royal Astronomical Society. This new work uses the extensive WISE-2MASS all-sky infrared databases and Pan-STARRS1 (PS1) data set of galaxies that have a mean z ~ 0.14.

The Cold Spot area resides in the constellation Eridanus in the southern galactic hemisphere. The insets show the environment of this anomalous patch of the sky as mapped using PS1 and WISE data and as observed in the CMB temperature data taken by the Planck satellite. The angular diameter of the vast supervoid aligned with the Cold Spot, which exceeds 30 degrees, is marked by the white circles. (Credit: graphic by Gergő Kránicz, image by ESA Planck Collaboration)

They find a very large void – a large under dense region of galaxies – in the constellation of Eridanus with a radius Rvoid ~ 220 Mpc centred at z = 0.22. This equates to a void that is 1.8 billion light-years across, in which the density of galaxies is much lower than usual in the known Universe. This corresponds to a 3.3σ fluctuation in a CMB expected for a Λ Cold Dark Matter model for the Universe.

Overall the detection of a very large void in the galaxy distribution is a viable cause for the CMB Cold Spot. However the current estimates of the ISW effect are smaller than the observed -70 µK deviation. Could the void in Eridanus actually be much larger than ~220 Mpc? This is possible. The team that studied the Cold Spot in Eridanus is now taking a look at another CMB cold spot near the constellation of Draco. These further studies should give us better insight into the physical properties of the CMB.

For more information, see

[Glen Mackie]

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