## Planck results

March 21, 2013

Big science news today: the Planck experiment has released a huge raft of results based on cosmological observations made during 2009-10. Planck is a satellite-based experiment that has been making precision measurements of the cosmic microwave background (CMB) radiation, the details of which tell us a great deal about the history and structure of the universe. Planck is a truly spectacular project.

I remember that when I was an undergraduate physics student — which was quite a long time ago now — we heard rumours of this satellite, which was then in the planning stages. The hope was that it, and to a lesser extent its predecessor WMAP, would usher in an era of “precision cosmology”, in which cosmologists would have a wealth of high quality measurements against which to judge their theories about cosmic structure and evolution.

Based on the results published today, I would say that those hopes have been triumphantly vindicated. For instance, consider this paper on cosmological parameters; look at Tables 1 and 2. These are amazing results: baryon density is about 2.2%, cold dark matter density about 12%, dark energy density about 68%, Hubble constant about 67, and the age of the universe about 13.8 billion years (with an uncertainty of only about 100 million years!).

There is a lot here for non-specialists to digest — and I certainly count myself in that group. The BBC is on the case.

### 16 Responses to “Planck results”

1. Vince Says:

I’ve read that there are hints of possible lopsidedness in the power spectrum (or whatever it is), but it could be nothing (like how the diphoton excess reported last year by ATLAS for the particular Higgs decay channel has gone down with the extended data set).

2. Vince Says:

I’m a little confused. The total matter density is around 31%, but your numbers add to 14.2%. Do photons and neutrinos make up the rest?

http://www.nytimes.com/2013/03/10/magazine/the-professor-the-bikini-model-and-the-suitcase-full-of-trouble.html?pagewanted=all

3. cburrell Says:

I made a mistake: the density values I quoted were $\Omega h^2$ values, not straight $\Omega$ values. Given the value they measure for the Hubble constant $(h = H_0/100)$, the matter density comes out about right.

That’s quite a story!

4. We got a good summary at CITA yesterday from one of the Planckers. Mostly, Planck nails numbers that had already been converging. Some points of interest:

There are no significant statistical non-Gaussianities on small scales, which means few hints for those interested in handles on non-simple inflation.
However, it appears that there are some larger-scale anomalies, in particular, the “cold spot” in the southern hemisphere that was seen by COBE and WMAP remains; as of yet, there is no compelling physical explanation (unless it is a fluke).
The Hubble parameter is in tension with galaxy and supernovae surveys. Planck measures 67.3 +/- 1.2 km/s/Mpc, while other measurements are in the low seventies; I suspect that there might be systematics in the latter that need to be ironed out.
No evidence for additional neutrinos (some experiments, such as SPT, had been hinting at this), though “sterile” neutrinos or other exotic relativistic species are not necessarily ruled out.
Because of the wide frequency coverage, there is a lot of “ancillary” data (e.g., galactic foregrounds) that will be very useful to non-cosmologists. The presenter speculated that the paper on galactic dust may get more citations than the one on cosmological results.

(Sorry for not taking the time to put the Hubble parameter in LaTeX!)

• cburrell Says:

Thanks, Adam. It is nice to hear from someone who knows this field well. I haven’t had time to look through the papers with anything but the most superficial attention.

The syntax for LaTeX in a WordPress blog is \dollar latex your-latex-here \dollar, where the \dollar should be the actual dollar sign.

5. Hmmm, the HTML unordered list tag doesn’t seem to work for comments . . .

6. Vince Says:

I hope you’re doing well! I bet you’re glad to have one of your own as Pope. I wanted to ask you about something I read about PLANCK. I read that these observations are as good as they’re going to get because we’re not restricted by the instruments, but by astrophysics. Do you know what this means?

Also, do you know when the CMB polarization data is going to come out? Will the CMB fluctuation dataset increase in size (like the dataset for ATLAS and CMS) or are those observations finished?

7. Vince Says:

Oh, and if there will be any primordial gravitational waves detected by PLANCK, will they show up in the polarization data?

8. Stories like this remind me of St. Augustine who wrote that the infinite levels in the bible are for those who need to search out the truths which the simple among us can see without need of delving.

Aristotle grasped the principles long ago, and the Church confirmed them with its understanding of the nature of substantial change. What those sending rockets into space will eventually find is that what they are looking for is right before their eyes and cannot be known by machines because it’s in potency, i.e. prime matter.

9. Vince, the restriction of astrophysics refers to cosmic variance. Because the CMB can be decomposed as random Gaussian variates, at a certain stage one is limited simply by statistics, especially at larger angular scales where there are few enough modes that the $\sqrt{N}$ dominates instrumental noise. Another source of uncertainty is foreground removal. Because of the frequency coverage, they seem to have done an excellent job with the temperature maps. But foregrounds will also be a challenge for polarisation.

I’m not sure when the polarisation results will come out. For the most recent release, they used the WMAP polarisation to help them with the cosmological parameter estimates. Addition of the Planck polarisation will be a boost to this. I don’t think they’ll have much more temperature data as their high-frequency instrument ran out of cryogens last year, and the low-frequency instrument should peter out in a couple of months.

Yes, if there are gravity waves, they will show up in the polarisation; in particular, in the B-modes, i.e., the divergence-free decomposition of polarisation. Given current upper-bounds on the amplitude of gravity waves, however, it is looking less likely that Planck will have a shot at seeing them.

10. Filia Artis Says:

I’m laughing because this has to be THE nerdiest comment stream on a blog post I’ve ever seen. (insert index finger on bridge of glasses as salute to you!)