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I What is t-mode CMB spectrum?

  1. May 10, 2017 #1
    The Wikipedia article
    DASI first detected the CMB polarization, and CBI provided the first E-mode polarization spectrum with compelling evidence that it is out of phase with the T-mode spectrum.​
    A footnote [48] leads to
    which says
    Ground-based interferometers provided fluctuation measurements with higher accuracy, including the Very Small Array, the Degree Angular Scale Interferometer (DASI) and the Cosmic Background Imager (CBI). DASI first detected the CMB polarization,[46][47] and CBI provided the first E-mode polarization spectrum with compelling evidence that it is out of phase with the T-mode spectrum.[48]
    Another Wikipedia article
    E-modes were first seen in 2002 by the Degree Angular Scale Interferometer (DASI).
    Cosmologists predict two types of B-modes, the first generated during cosmic inflation shortly after the big bang,[57][58][59] and the second generated by gravitational lensing at later times.[60]

    I was able to find some discussions about E-mode, M-mode, and B-mode, but nothing about T-mode.

    I would appreciate any information which would help me understand what T-mode polarization is.
  2. jcsd
  3. May 11, 2017 #2


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    You have apparently been victimized by a typical triki-wiki ploy. T is in reference to the temperature anisotropy of the CMB. This is discussed on page 5 of reference 48 in the wiki article... "A given correlator output sample, or visibility, can be one of the four polarization products RR, RL, LR, or LL. These can be related to the fundamental CMB polarization modes T (temperature), E, and B (polarization) (37)." The wording is admittedly a bit confusing implying there is something called the temperature polarization mode T, but, if such a thing actually exists, it is not commonly referred to as 't-mode polarization'. To see a depiction of the various CMB anisotropy measurements, including T, E and B, see Figure 1 on p3 of https://arxiv.org/pdf/1210.6008.pdf. Hope this helps.
  4. May 13, 2017 #3
    Hi @Chronos:

    Thank you for posting the reference. I have been trying to to gain some understanding from it, but it is hard work. There are two things in particular which I think I am able to understand, but I cannot find it in the cited article.

    1. How is the CMB temperature distribution over the sky represented? Is there some online database of this information that I might be able to access? I am envisioning some table of temperature values, or delta values with respect to the overall average temperature, organized by some sort of longitude and latitude with respect to some chosen equator and prime meridian.

    2. I did not understand how the perturbations are calculated. I am envisioning some kind of 2D Fourier coefficient calculations, or possibly correlation coefficients. Do you know of any reference that describes the perturbation calculation methods used to derive results from the CMB temperature distribution data?

    I am looking forward to whatever help you can give me.

  5. May 14, 2017 #4


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    You are probably looking for the Time Ordered Data set, as discussed here http://space.mit.edu/home/tegmark/cmb/pipeline.html. I'm unsure where this might be available, but, assume some diligent searching could probably turn some of it up. Of course this data has been 'cleaned', a vital step in its compilation. Nobody really wants to see data warped by galactic synchrotron radiation, point source contributions, or Doppler drift due to where the antenna is pointed relative to its direction of motion.
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