Planck First Light

By Sean Carroll | September 17, 2009 12:59 pm

If you haven’t heard that Planck has seen first light, you haven’t been reading the right cosmology blogs: see Andrew Jaffe, Peter Coles, and Planck’s own Twitter feed. Planck is of course the European Space Agency’s microwave background satellite experiment, which was launched back in May. Since then it’s been tumbling in space about once every minute, doing a leisurely scan of the sky. The survey is not nearly completed, but all systems seem to be running smoothly.

Here’s the region it’s looked at so far, superimposed over a visual-light map of the Milky Way:


And here’s a zoom in on one region, as seen in two different wavelengths:


So far the scientists are playing with the data to learn about the instrument, not so much about the microwave background. Andrew predicts a big splash of papers from Planck in August 2012. We’ll be looking for a bunch of things: Are the overall features of the CMB consistent with predictions from inflation? Are there “non-Gaussian” features indicating extra power in some regions? Is the strength of the perturbations equal on all scales, or does it gradually diminish at smaller distances? Did we learn anything surprising from the polarization, such as tensor modes that could come from inflation or an overall rotation that could come from quintessence? Does the universe have a preferred direction?

I’m sure it will be front-page news, whatever that news turns out to be. Stay tuned.

  • Thanny

    It will be interesting comparing the full image, when it’s complete, with COBE and WMAP, to see, visually, how much more detail there is.

  • Ellipsis

    Yes, perhaps the first thing to ask is how consistent is the data from this slice with the WMAP data from that region of sky?

  • Ethan Siegel

    Sean, I’m going to hazard an educated guess at your questions, in order:

    Yes, the CMB is consistent with inflation.

    Yes, there are non-Gaussian features in some regions, but the non-Gaussianity is going to be small and not necessarily compelling.

    The strength of the perturbations will be slightly diminished at smaller distances, but nothing surprising (maybe n_s = 0.97).

    No, we won’t learn anything from polarization.

    And no, there’s no preferred direction, at least not with any statistical significance.

    Do you think the answers will be any different from this?

  • Rory Kent

    On that note, does the Planck use the same statistical method as the WMAP (Spherical Mexican Hat Wavelets)?
    If not, could it be used to confirm/deny the “statistcal error” explanation of the cold spot?

  • Sili

    August, eh? Good timing – it’ll be nice to know the answer to the origin of it all before the end of the world.

    (wink goes here)

  • Sean

    Ethan, those are presumably the most vanilla guesses possible, and therefore likely to be right. If there is good evidence that the perturbations aren’t scale free at high statistical significance, that would certainly be very interesting. And there are various studies indicating that there is a preferred direction in the WMAP data, so if Planck says that it’s not there, that’s interesting in its own right.

  • Jason Dick

    What I’m interested in seeing is whether or not the “axis of evil” reappears in Planck data, as people Eriksen pointed out a while back it is possible that correlated noise might cause such an effect, but if so, then Planck with its very different noise properties should show either a very different “axis of evil”, or perhaps not at all.

  • Eugene

    My predictions for Sean’s series of questions :

    Yes. Yes. Yes. No, No. No.

  • JoAnne

    I’m rooting for a preferred direction in space. That would be cool!

  • MedallionOfFerret

    Hey, I can answer the last question. There is a preferred direction to space: down. I know that may seem a little quarky, but that’s exactly what the data will show. Is that cool enough?

  • Brando

    My question is: will Alan Guth be happy with the data? 😉

  • Mark

    My predictions:

    1. The scale of slow-roll inflation is low

    2. No promordial gravity waves are observed

    3. The CMB is mostly generated by a late decaying modulus field and not the inflaton

  • Ethan Siegel

    Sean, I think n_s = 1.00 is even more vanilla, but overall I agree with you. I’ve just seen so many 2- and 3-sigma results over the years in many different subfields of physics simply get overturned when statistics improve and/or new data comes in. I think that — with the exception of non-Gaussianity — there’s very little that’s going to surprise us about Planck. But we don’t know until we do the experiment, and that’s why we do it.

  • Cartesian


    It does look like something fractal, see :


  • Peter Coles

    If there is non-Gaussianity you might have to rename this blog “Cosmic Skewness”…

  • o1

    I think it won’t be consistent with inflation as I expect CMB to be a result of photon-photon scattering and therefore it should depend on our local Galactic neighborhood.

  • Kaleberg

    This reminds me of a question I’ve been meaning to ask. I’ve heard that when you fall into a black hole, everything you see above is blue shifted and is much brighter than it was when you started your fall. How bright does the CMB get? Do you need CMB-block? (Obviously, you’ve got bigger problems, but I was kind of wondering and what with the LHC not producing any convenient black holes …)

  • Eric M

    @15: Cosmic Kurtosis rolls off the tongue better.

  • Peter H

    When will WMAP respond to Liu and Li? Planck will need a stick in the sand to move on from.


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Cosmic Variance

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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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