A lot of pop science articles (for example, this Space.com article) discuss the cold spots on the WMAP data of Cosmic Microwave Background (CMB) radiation, but have any studies been done on the large mass of hot spots on the right of the image below?

    CMB data from WMAP

Is this a side-effect of our observation methods or does it represent some actual feature of the early history of the Universe?


1 Answer 1


I don't remember reading any paper title specifically about the hot spots you refer to, so I guess that they are not as "strange" as the cold spot.

Assuming that the temperature fluctuations are well described by a Gaussian distribution with mean 2.72548 and variance 0.00057 source, the probability of having the cold spot of the observed size and temperature is very low. According to a recent Planck paper this probability is of order of 1% or smaller. The probability of the hot spots is larger (1.5 to 5%), making them more "normal".

This probably explains while the cold spot got more attention that the hot ones.

Is this a side-effect of our observation methods or does it represent some actual feature of the early history of the Universe?

This is tricky. A few scenarios:

  1. The CMB map in the question shows fluctuations of order 10^-4 K. But extracting this signal from observations is very complicated by the fact that on top of the CMB radiation is "polluted" by foreground objects (mostly our own Galaxy, but also extragalactic point and extended sources) that create spurious fluctuations orders of magnitude larger than the CMB ones. This video is great to have an idea of this. If any of the above foreground components is not well understood or some is not accounted for, we could be left with some residuals that we wrongly interpret as CMB anomalies.

  2. The physics we know is the correct one and the cold spot is simply there. But we physicists don't like that in the Universe there are special places

  3. The cold spot is a signal of new/unknown physics or that our standard model is not correct. For instance could be that the assumption that the CMB is Gaussian could be just wrong and we have to re-think our theories about the early universe

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    $\begingroup$ For now I will accept this as it is a very thorough answer and likely the best one I'm going to get for now. Good research effort, good summary of results! +1 $\endgroup$
    – called2voyage
    Oct 21, 2013 at 12:41
  • $\begingroup$ Thanks! It took me a lot of time and thinking to write it. I'm not used to do outreach and it's very hard to try explaining things that you more or less know is a easier but complete way $\endgroup$ Oct 21, 2013 at 13:14

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