Monday, March 25, 2013

Making sense of the Planck results

Without further ado, allow me to present the newest baby picture of the universe.
Image Credit: ESA and the Planck Collaboration
Isn't it adorable! Look at those little anisotropies! Who's a good cosmic microwave background?

Sorry, I got a bit carried away there. Anyway, this is the new image of the anisotropies of the cosmic microwave background. The resolution here is apparently limited only by fundamental physics rather than our ability to observe it, which means this is the best image of the CMB we'll ever get.

Besides making pretty pictures, what did Planck actually find? The first of the results we'll discuss is the new measurement of Hubble's constant, H0. The newly calculated value for Hubble's constant is 67.8±0.77 km/s/Mpc. It is important to note that there are different values reported in the papers released by Planck based on what other data sets the Planck team also used in their analysis to increase their precision. The value I reported above uses the most independent methods together to determine H0. What I found particularly interesting was that this was still within the error bars of the WMAP and HST results, as shown in this figure from one of the many papers released by the Planck science team.

Image credit: Planck Collaboration. Ade et al. 2013 A&A

And now I present the Planck results for the composition of the universe.
68.25% dark energy
31.75% atomic matter + dark matter

Breaking up the matter category yields (roughly)
4.9% atomic matter
26.8% dark matter

If you compare this to the chart I posted last week, you'll note that this is a pretty significant change in what we thought the universe is made of. No, the universe didn't magically lose dark energy between WMAP and Planck. Planck just got better measurements, leading to better data, which gives us a more accurate picture of our universe than we ever had before.

One thing that we non-cosmologists tend to take for granted is that the sum of all of the components of the universe necessarily add up to 100%. As it turns out, there is actually no reason whatsoever for this to be the case. We seem to live in a "critical universe," in which all fractional components sum to 1. We could just as easily have ended up in a universe where this number is either >1 or <1. These are, respectively, called "closed" and "open" universes. The total amount of "stuff" (matter + dark energy) in the universe determines its ultimate fate. In a closed universe, the universe will inevitably collapse back upon itself (unless the universe contains a lot of dark energy) in what we like to call the Big Crunch. An open universe will continue to expand forever until the expansion (completely independent of dark energy, mind you) completely overwhelms all forces in the universe in what is often coined the Big Rip or the Big Freeze. (Man, cosmologists come up with a lot of catchy names for things like this.)

What I discussed above is often called the curvature of the universe, and is parameterized by k. k is positive for a closed universe, and negative for an open universe. While some of the theoretical possibilities you can create by playing around with universes with curvature are rather entertaining, we don't actually need to worry about this. Planck's measurement of the curvature of our universe says that k = 0.0000. That's quite a lot of certainty.

If you recall another different part of my intro to cosmology post, I said that inverting Hubble's constant can give us a rough age for the universe, assuming constant expansion velocity (which we know isn't true, so consider this an upper limit on the age of the universe). With the new calculation of H0, we can determine that the age of the universe is 14.42 billion years. Accounting for the accelerating expansion of dark energy, the newly derived age of the universe is 13.7965 billion years (not a huge change from the final WMAP results, really).

Another particularly important measurement Planck made was looking at the angular sizes of the anisotropies in the CMB. Remember, these tiny fluctuations grew up into the matter distributions we can observe in the universe today through what are called baryon acoustic oscillations (the parts of that Wikipedia article that are purely qualitative seem pretty straightforward, so if you're really interested, give it a read), so those fluctuations are pretty important. What's more is that each peak in the spectrum of the anisotropies contains different information about the universe as a whole, so making these measurements as precisely as possible is very important. Here I present, side-by-side, the measurements of the angular scale of CMB anisotropies as measured by WMAP and Planck, respectively.
Comparison of measurements of the power spectrum of CMB anisotropies from WMAP (left) and Planck (right). Note the change in x-axis length, with Planck's result going out to l = 2500. Image credit: NASA/WMAP Collaboration (Bennett et al. 2013) and ESA/Planck Collaboration (Ade et al. 2013).
Wow. That's quite a difference. While WMAP resolved the first three peaks, (which, in itself was brilliant work), Planck convincingly resolves 5 peaks and (unless I'm fooling myself visually) indicates 6th and 7th peaks as well. Of course, you'd have to find a real cosmologist to tell you what each peak actually means, or you can do what I do, and read the Wikipedia article on the CMB.
"The peaks contain interesting physical signatures. The angular scale of the first peak determines the curvature of the universe (but not the topology of the universe). The next peak - ratio of the odd peaks to the even peaks - determines the reduced baryon density. The third peak can be used to get information about the dark matter density."
 That's pretty damn cool. But now I wonder what all those other peaks tell us about...

Lastly, I'll address a topic that is very near and dear to my heart: neutrinos. For now, all you need to know about neutrinos is that they are very, very light particles, and we observe three different types (which we generally call "flavors" for some reason) in our particle physics experiments today. Turns out that you can actually use CMB data to determine the number of types of neutrinos there are in the universe. I wrote a little paper about this once and I won't bore you with the details. In short, the evolution of the very early universe is surprisingly sensitive to how many different types of this particle with almost negligible mass there are. Better yet, we can even find out the mass of all three neutrino types summed together (not the total mass of neutrinos in the universe, just the mass of one type plus the mass of the other plus the mass of the other).

Later WMAP results (WMAP 7-year and WMAP 9-year) calculated that, apart from the three flavors of neutrinos we already knew about, there was a fourth type flying around out there (uncertainties ruled out three neutrino flavors to two sigma). This was a particularly big deal because the Standard Model of particle physics only has room for three flavors of neutrino. Needless to say, neutrino lovers like myself were pretty excited, and eagerly awaited to see what Planck had to say on the subject. Survey says.... three neutrino flavors. Oh well. But let's not forget the mass constraints! Planck's data says that the summed neutrino masses come to less than 0.23 eV (1 eV = 1.78x10-36 kg), nearly a factor of two lower than WMAP's calculations.

Well, that's all that I can possibly say right now about the Planck results. For those of you expecting to hear something about inflation, I'm sorry to disappoint, but I don't really know enough about inflation to make any meaningful remarks. For the more specific questions, I highly recommend asking a real cosmologist.

No comments:

Post a Comment