Planck and the CMB

In May of 2009, the European Space Agency launched the Planck Satellite into orbit. On paper, Planck sounds very similar to the WMAP satellite launched 8 years earlier, or the COBE Satellite launched in 1989: measure and map the perturbations in the Cosmic Microwave Background (CMB) radiation.

The CMB radiation is a very nearly uniform remnant from the early universe, roughly 380,000 years after the big bang when the universe was approximately 1000 times smaller, and thus 1000 times hotter than it is now. These days, the CMB is only 2.73K and barely noticeable, unless you get very lucky, or happen to be looking for it.

The CMB is also very nearly uniform in every direction, with the temperature only varying by about 1 part in 100,000 from place to place. Those differences, however, mean a lot. The blue spots in the map above correspond to directions slightly cooler than average, while the red spots are slightly hotter. Those hot and cool patches ultimately give rise to the mass perturbations in the universe that we see today, including all of the large scale structure (galaxies, clusters of galaxies, etc.) that we see around us.

The Results

The Planck satellite has a number of advantages over its predecessors. For one thing, it has 9 different frequency channels, giving it increased sensitivity, and allowing the Planck team to effectively remove various types of foreground sources. Planck also measures structure to a much smaller scale. To give you an idea, you can take all of the hot and cold spots and make a spectrum of how much structure there is on various scales:

Credit: Planck Results 16: Cosmological Parameters, Figure 1

If you’ve never seen one of these plots before, large scale (corresponding to the whole sky) is to the left while small scale (corresponding to something like 4 minutes of arc) is to the right. The red curve is a purely theoretical model, while the blue dots (with error bars) show the average of the data. This is astounding precision science!

Earlier this morning, ESA made a public announcement about the data and results from 15 months of observations. Essentially, they adjusted the various cosmological parameters until the red curve above matched the blue dots as closely as possible. In addition, they adjusted to include information from the WMAP satellite and other cosmological measurements. The thirty (!) technical papers will be officially released on the arxiv tomorrow, but there were a few big surprises.

Notably, they found a little more dark matter and a little less dark energy than previously supposed.

Credit: Planck and ESA

In addition, they found a somewhat lower Hubble Constant than previously supposed. Instead of the best-fit 71 km/s/Mpc concluded from WMAP, Planck found a slightly slower expansion, only 67 km/s/Mpc. The age of the universe, when all of these numbers are plugged in, is still about 13.8 Billion Years.

If you are an expert and want the money table, here it is (click to zoom in):

Should you freak out about these differences?


While it’s always interesting when the best-fit cosmological parameters get tweaked, it’s important to remember that these are estimates. There are error bars involved, and in this case, the differences between these results and the WMAP 9 year results are something on the order of 2 sigma or so. Interesting, and likely to bounce around a bit, bit nothing earth-shattering.

In their overview paper, the Planck team did announce a few things that I thought were kind of noteworthy (with a little editorializing on my part):

  • “An exploration of parameter space beyond the basic set leads to: (a) firmly establishing the effective number of relativistic
    species (neutrinos) at 3;”

    We expected this, but if there is another species of neutrino, it’s fairly massive.

  • “(b) constraining the flatness of space-time to a level of 0.1%;”

    Great constraint! And unsurprising given the standard model of inflation.

  • “we find no evidence at the current level of analysis for tensor modes, nor for a dynamical form of dark energy, nor for time variations of the fine structure constant”

    That is inflation seems to be fairly generic, dark energy seems even more likely to be a cosmological constant, and despite some claims to the contrary, physical parameters seem to be fixed.

  • “we do find evidence for deviations from isotropy at low l’s.In particular, we find a coherent deficit of power with respect to our best-fit CDM model at l’s between 20 and 30.”

    This is quite interesting. The large scale structure — small l — doesn’t quite fit. This sort of problem showed up in WMAP as well. In announcements, they’re not making a huge deal about this, however, and personally, I don’t think it presents much of a problem to the newly tweaked standard cosmological model.

  • “the first detection at high significance (42sigma) of the cross-correlation between CMB lensing and the cosmic infrared background, which allows us to constrain the star formation rate at high redshifts.”

    This, and some of their other results, are putting very tight constraints on the epochs of the first stars.

  • “the first robust (2.5-sigma) detection of the Integrated Sachs-Wolfe eff ect via its cross-correlation with Planck-detected lensing, providing independent evidence for Omega_Lambdasimeq 0.7.”

    This is a very big deal. The Integrated Sachs-Wolfe Effect basically measures the difference between the blue shifting of microwave background photons as they fall into forming clusters and the redshifting as they climb out. They measure, in effect, the overall increase in structure over a finite amount of time. Since the growth of structure is directly predicted by cosmology it is a very robust way of putting an additional constraint on the models.

Expect lots of news and discussion over the next few days. While the Planck results don’t overturn the picture of our universe, they are some pretty exciting and astoundingly precise results!


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