Back at the end of August, Chris Lee described a discovery that was a bit of a stumper. The WMAP probe, which has been refining our picture of the cosmic microwave background, found what appears to be a cosmic cold spot: an exceptionally large area of space that’s apparently more or less empty. The probabilities of this occurring by chance are vanishingly small, and it has left cosmologists scratching their heads.
A number of them apparently stopped scratching and started calculating, and the fruits of some of that labor are being released by Science today. The authors of the paper have done calculations that suggest the cold spot is a remnant of an exotic form of phase transition that took place early in the history of the universe. This is something that has been kicked around by the theorists for decades, but hasn’t been observed as of yet.
The idea behind the phenomenon is that, at very high energies, some particles become “symmetric”—they are functionally interchangeable, and can’t be distinguished. As the universe expanded after the big bang, it gradually cooled, diluting the average energy density. This cooling should sporadically drop the energy below the point where some forms of symmetry are maintained and, because of the irregularities in the universe’s cooling observed by WMAP, the breaking of symmetry should happen unevenly.
The net result is that neighboring areas of the universe will be at states above and below the symmetry point. This creates a situation vaguely analogous to the phase change between liquid water and ice, where there is a fundamental discontinuity between the two regions. In the cosmic version, the phase change boundary should cause significant changes in the wavelength of light that crosses it, potentially creating a relatively “cold,” or red-shifted, patch in the microwave background (hot patches should also occur).
How likely is this exotic explanation? Compared to the obvious alternative explanation—a combination of instrument error and random fluctuations—pretty likely, according to the authors. They did a Bayesian analysis of the probabilities of both, and found that the phase shift was a more probable solution. In addition, the work suggested that phase changes should cause only one cold spot large enough to detect at the resolution that we’ve currently achieved in our measurements of the cosmic background radiation.
The phase change explanation also provides two testable predictions, namely that we’ll find more of these when we get better resolution maps, and that a certain amount of polarization should be detectable in the area at the edges of the cold spot. The size of the cold spot also places an eye popping number on the energy at which the phase change took place: 8.7 × 1015 GeV. The Nobel Intent staff was unable to think of a reasonable analogy to describe how energetic that is.