Competing claims over cause of cosmic cold spot

Written by Keith Cooper, Physics World

A survey of more than 7000 galaxies has concluded that a mysterious cold spot in the cosmic microwave background (CMB) is not caused by a giant void in space, potentially opening the door for more exotic explanations.

Ruari Mackenzie and Tom Shanks at Durham University in the UK led astronomers in building a 3D map of galaxies in the direction of the cold spot. Although they found several large voids, these were deemed insufficient to explain the cold spot’s comparatively huge drop in temperature.

The CMB is the 13.8 billion-year-old heat from the Big Bang, now cooled by the expansion of space to just 2.7 degrees above absolute zero. The CMB is generally characterized by slight temperature variations of just a millionth of a degree or so, in accordance with a Gaussian distribution where small temperature variations are expected but large variations are not. However, the cold spot is 150 μK below the mean CMB temperature, far in excess of that expected from a Gaussian distribution.

One theory proposes that the cold spot is caused by the presence of a “super-void” that is able to chill CMB photons via a process called the Integrated Sachs-Wolfe (ISW) effect. In 2015 a team led by István Szapudi of the University of Hawaii claimed to have discovered a giant void spanning 1.8 billion light-years in the direction of the spot.

The word “void” is a misnomer – galaxies still exist within voids, but their density is less than in other regions of the universe. If dense areas of matter are considered to be gravitational wells, then less-dense regions are gravitational “hills”. CMB photons lose energy as they enter a void and climb up the hill, but regain the energy as they move down the hill and leave the void. However, the expansion of the universe enlarges the void while the CMB photons are passing through it. The result is the ISW effect, whereby photons depart the void with less energy and consequently that region of the CMB appears cooler than it really is.

Finding the voids

To test this, Mackenzie and Shanks’ team compiled spectroscopic redshifts of galaxies – more accurate than the photometric (colour-based) redshifts that Szapudi initially used – from the 2dF-VST ATLAS Cold Spot Redshift (2CSz) survey at the Anglo-Australian Telescope in New South Wales. They found three definite voids out to a distance of three billion light-years and a possible fourth void beyond that. Although none were as large as Szapudi’s super-void, combined they provided a greater ISW effect than that measured by Szapudi. However, it was still only enough to account for 31 μK of the cold spot – and that’s including the dubious fourth void.

The team also performed the same measurements along a different line of sight that acted as a control, and found a similar density of voids in that direction as in the direction of the cold spot. “[The control] clinched it for us that voids cannot be to blame, since there is no cold spot in the CMB behind our control field,” says Shanks.

Szapudi, who was actually part of the team alongside Mackenzie and Shanks, disagrees with their conclusion – “If I had to bet on the cause of the cold spot, I’d bet on the super-void.” He interprets the individual voids detected in the 2CSz survey as being part of the substructure of the super-void. His confidence comes from a statistical analysis suggesting that the likelihood of the alignment between the void and the cold spot being coincidental is very slim.

Mackenzie isn’t won over by that line of argument, saying that the fact the control field doesn’t have a cold spot “shows fairly robustly that the claim that the alignment is significant doesn’t hold”. Szapudi counters this by pointing out that the control field is qualitatively different, including the presence of a large galaxy cluster that would offset the ISW effect.

Nevertheless, the presence of such a huge void is difficult to explain in the standard cold-dark-matter model of cosmology, plus there is the extra 120 μK that needs to be accounted for. Szapudi says that making his hypothesis work requires tweaking the Standard Model in some way.

Exotic physics

If Shanks and Mackenzie are correct then an alternative explanation for the cold spot must now be found. Simulations have shown that a random, non-Gaussian quantum fluctuation in the CMB has a 1 in 50 chance of creating the cold spot, but other, more exotic possibilities may also come into play. Among them is the idea that the cold spot is where our universe is bumping into another universe created by eternal inflation. This would produce an identifiable polarization signal in the cold spot. Data from the European Space Agency’s Planck spacecraft that might prove or disprove this have yet to be fully analysed. If the polarization signal is there, however, then a collision with another universe would “become the most plausible explanation, believe it or not”, according to Shanks.

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