Wednesday, July 3, 2013

Planck's summer: CMB observations without CMB photons

CQW presents "Planck's summer", a series of posts composed by recent graduate Youness!

Imagine a planet with an atmosphere that is, for some reason, completely opaque for low-energy photons such that a direct ground-based detection of the CMB is impossible. Furthermore, the inhabitants of the planet do not yet possess the technology to build spacecraft that leaves the atmosphere.  However, they are very good experimentalists on the ground. How could they test whether there is a CMB confirming a basic prediction of big bang cosmology?


  1. Make use of the Lorentz boost to find out that there is a (minute) heat source which "circles" their planet at the same period as their parent star, but at 90 degrees offset in direction?

    Well, they probably would mistake it as a very slight asymmetry in the emission of their parent star. But they would wonder why this asymmetry should rotate on the star with exactly the angular speed of their own planet.

    Of course they would need to be EXTREMELY good experimenters. Their job would be a bit easier if they were circling a white dwarf rather than a star.

    Greetings from a Gaia meeting at Leiden, Holland,
    Uli Bastian

  2. I would guess they could detect the presence of the GZK cutoff in ultra-high energy cosmic rays. A high-energy proton experiences the CMB boosted to their frame, i.e. the head-on are blueshifted. If that blueshift is stronger than a certain threshold, the photons can be absorbed by the proton, which then emits pions via the Delta resonance, and loses energy until it drops under that energy threshold (whopping 5 x 10^19 eV for protons).
    Thus, cosmic rays from sufficiently far away will be significantly sparser above that cutoff energy than below, and that should be something that this civilization should be able to find, and from that to deduce the presence of a (homogeneous) photon fluid at a certain temperature that effectively brakes the protons. Given that these high-energy events are rare anyway, it would need a lot of time and luck to establish homogeneity ...
    Robert Schuhmann

  3. CQW hint: think of the boomerang nebula!

  4. I had in fact considered looking for the minimum local temperature in the universe via line observations at other wavelengths (other than typical CMB, I mean). But then the Boomerang nebula is the best example that such an idea won't work!
    Thus I am perfectly puzzled by the hint, and even more curious what ideas might come up.
    Uli Bastian

  5. CQW is sorry for providing a confusing hint! In fact we wanted to aim at the fact that it's possible to measure temperatures with optical spectroscopy: already in 1940 A. McKellar measured the temperature of CN-molecules and found first hints of the CMB. of course the method relies on the assumption that there's no source of energy (for instance, gravitational binding energy released in the formation of the molecular cloud). the boomerang nebula is a very unusual object, optically thick and adiabatically expanding, so that is has a temperature lower than the CMB. in conclusion we emphasise that there might a number of ways, but it'd be very difficult.

  6. Hi guys,

    when I proposed the question, I had McKellar's famous determination of the "temperature of deep space" (see spirou's post) and the Greisen-Zatsepin-Kuzmin limit (pointed out by Robert) in mind.

    It would be interesting to know whether there are additional options though.