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I Cosmology Large Angular Scale Surveyor

  1. Mar 21, 2016 #1
    In the spring 2016 issue of the Johns Hopkins Magazine, there is a non-technical article (in graphic novel format) about a new instrument, dubbed CLASS, which is intended to detect "pinwheel patterns" caused by gravitational waves originating in the inflation era acting on the CBR. If these patterns are found, it will be strong observational evidence that the inflation era actually happened.

    The instrument was built by a JHU team of professors and students, and was then shipped and has recently arrived at its site in the Atacama Desert in the Chilean Andes. It is expected to become operational later this year. The leader of the team is Charles Bennett who was also a principal member of the WMAP team.

    I am hopeful that one or more PF participants can answer a few questions for me.

    1. What would the the conjectured pinwheel patterns actually look like? Are they spiral? If so, why?

    2. In reading about the WMAP project's mapping of the CBR,
    I found that the angular resolution of WMAP ranged between
    0.93 deg at 22GHz and < 0.23 deg at 90GHz.​
    Presumably this resolution was not sufficient to detect the conjectured CBR pinwheel patterns.
    What does current theory about inflation and its GWs predict concerning the required angular resolution to detect the pinwheel patterns? At what average angle would the center of pinwheel spirals be from each other?

    Last edited: Mar 21, 2016
  2. jcsd
  3. Mar 21, 2016 #2


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    Sounds like they're talking about what is known as B-mode polarization.

    Polarization of the CMB (or of anything, really) can be broken into two components. One way of breaking them down is to break them into a radial component (where the polarization direction points towards/away from points on the sky), and a circular component (where the polarization points in a circular direction around a point). Mathematically, these are analagous to the electric (E) field (which always points towards/away from electric charges) and the magnetic (B) field (which goes in circles around moving electric charges).

    It's more a matter of sensitivity and the number of frequencies that the instrument observed the sky at.

    There are two main kinds of instruments that are used to detect the CMB: radiometers and bolometers. Radiometers are much like small antennas, and are best for detecting the CMB at low frequencies. They don't need to be particularly cold to operate, but it helps. Bolometers measure the heat from the photons striking them. This means that in order to detect the CMB at all, they need to be at a much lower temperature than the CMB. In order to make WMAP a simple instrument, they did away with any kind of active cooling system. This ruled out bolometers, and prevented any high-frequency measurements. Without high-frequency measurements, it's very difficult to separate between the CMB and foreground sources (such as dust and gas from our own galaxy, or the gas in distant galaxy clusters, among many other sources).

    Furthermore, the fact that WMAP didn't have a cooling system meant that the radiometers themselves weren't as sensitive as they otherwise could have been. The polarization design also wasn't great for systematic errors (the Planck satellite solved these other problems, but still has relatively poor systematic error control for measuring polarization)..

    The difficulty with measuring primordial B-mode polarization is that the signal is extremely faint (if it exists at all). There are lots of things between us and the CMB that are much brighter with this kind of polarization, making it very difficult to separate the CMB from the rest. We really need an instrument with high resolution, high sensitivity, a large number of frequency bands, and that is able to measure polarization accurately.
    Last edited: Mar 21, 2016
  4. Mar 21, 2016 #3
    Hi Chalnoth:

    Thank you much for a very informative answer.

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