Lessening Anomalies

An interesting paper caught my eye on today’s ArXiv and I thought I’d post something here because it relates to an ongoing theme on this blog about the possibility that there might be anomalies in the observed pattern of temperature fluctuations in the cosmic microwave background (CMB). See my other posts here, here, here, here and here for related discussions.

One of the authors of the new paper, John Peacock, is an occasional commenter on this blog. He was also the Chief Inquisitor at my PhD (or rather DPhil) examination, which took place 21 years ago. The four-and-a-half hours of grilling I went through that afternoon reduced me to a gibbering wreck but the examiners obviously felt sorry for me and let me pass anyway. I’m not one to hold a grudge so I’ll resist the temptation to be churlish towards my erstwhile tormentor.

The most recent paper is about the possible  contribution of  the integrated Sachs-Wolfe (ISW) effect to these anomalies. The ISW mechanism generates temperature variations in the CMB because photons travel along a line of sight through a time-varying gravitational potential between the last-scattering surface and the observer. The integrated effect is zero if the potential does not evolve because the energy shift falling into a well exactly balances that involved in climbing out of one. If in transit the well gets a bit deeper, however, there is a net contribution.

The specific thing about the ISW effect that makes it measurable is that the temperature variations it induces should correlate with the pattern of structure in the galaxy distribution, as it is these that generate the potential fluctuations through which CMB photons travel. Francis & Peacock try to assess the ISW contribution using data from the 2MASS all-sky survey of galaxies. This in itself contains important cosmological clues but in the context of this particular question it is a nuisance, like any other foreground contamination, so they subtract it off the maps obtained from the Wilkinson Microwave Anisotropy Probe (WMAP) in an attempt to get a cleaner map of the primordial CMB sky.

The results are shown in the picture below which presents  the lowest order spherical harmonic modes, the quadrupole (left) and octopole (right) for the  ISW component (top) , WMAP data (middle) and at the bottom we have the cleaned CMB sky (i.e. the middle minus the top). The ISW subtraction doesn’t make a huge difference to the visual appearance of the CMB maps but it is enough to substantially reduce to the statistical significance of at least some of the reported anomalies I mentioned above. This reinforces how careful we have to be in analysing the data before jumping to cosmological conclusions.


There should also be a further contribution from fluctuations beyond the depth of the 2MASS survey (about 0.3 in redshift).  The actual ISW effect could therefore  be significantly larger than this estimate.

2 Responses to “Lessening Anomalies”

  1. Chris Crowe Says:

    I spotted this paper too, it is indeed interesting. Will the mitigation of the anomalies in the CMB by the integrated ISW will continue into higher redshifts?

  2. Without having the data one can’t tell. The reason this does reduce the anomaly, however, seems to be that the ISW component is aligned in roughly orthogonal fashion to the WMAP data. The ISW quadrupole in particular is not far from zonal while the WMAP quadrupole is predominantly sectoral…

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