## Sunday, March 24, 2013

### Planck: All we need is six numbers to describe the universe

As I'm sure most of the readers of this blog are aware, the Planck data is now out. It turns out I was correct with two out of three of my rumours. I said that the "ISW mystery" was still present, it was. I said that Planck would present ~3$$\sigma$$ evidence for non-zero neutrino masses, they did (though, as I suggested in my rumour, only after including information from galaxy clusters Planck has detected). Finally, I said that there would be 2-3$$\sigma$$ evidence for some type of "non-Gaussianity", there wasn't. I will duly update my should-I-trust-that-rumour? algorithm in the following way: explicit remarks from Planck members, good rumour; wishful thinking from other theorists, bad rumour.

So what were those results? What big news is there?

The answer is that there isn't anything strikingly new or surprising. I've been trained by years as a theoretical physicist to to dread that sentence and, indeed, many of my colleagues have gone into various states of despair. But, for some reason, I spent the second half of last week in a state of excited wonder. Surprisingly, I loved what I saw on Thursday. It was both stunning and beautiful. This post will be me trying to explain why. (For more details of the actual results see Sesh's post and Peter Woit's list of other blog posts).

The model of cosmology that has been gaining traction over the last decade and a bit is called $$\Lambda$$CDM. This stands for $$\Lambda$$ Cold Dark Matter, where the $$\Lambda$$ represents the poorly named "dark energy". This model has a few theoretical issues, but it is incredibly simple. What Planck specifically found is that this model fits the CMB (Cosmic Microwave Background) very well and better than any alternative that they tested.

Why I found what Planck saw to be incredible

As I wrote above, Planck's results last Thursday had me in a state of impressed awe. On the day, I couldn't quite put my finger on why, until I read another cosmologist's tweets marvelling at how everything we were seeing could be described by just six parameters. Then it hit me. For once, cosmology had gotten it right. What Planck measured depends on a significant variety of physical phenomena. If the early universe had more matter, or more radiation than we expected, Planck would have seen it. If the primordial density perturbations had been shaped in a significantly different way to that in which we expected, Planck would have seen it.

Cosmology gets a lot of flak from some directions for the so-called "epicycles" of dark matter and dark energy. I can kind of understand this when people see images like this and are told that we "don't understand" 95% of the energy density of the universe. But what is often missed is that we include the effects of dark matter and dark energy in this $$\Lambda$$CDM model with one, single, parameter each. And once those parameters are fixed, the predictions of all of cosmology are too.

With this firmly in mind, take a look at Planck's most important, headline image below. This (sort of) shows the amplitude of the temperature fluctuations in the CMB as a function of their angular scale. Remember, it takes just six numbers to define what that entire curve should look like. Just six.

## Thursday, March 21, 2013

### Following Planck's results today

For the people (new and old) who follow this blog and are interested in the Planck satellite's results, which are being announced today, here is a run-down of important things to know:

• Richard Easther will be live-blogging the data release at this location. If you can't watch the release yourself you should follow Richard's post.
• The first ESA event is a general-audience press conference, very soon, at 10:00 CET, which you can watch here.
• The second ESA event is a press conference aimed at scientists and science journalists, which will stream at the same location, i.e. here. You should watch that even if you aren't a scientist because it will be when all the interesting bits are revealed. If you're confused, you can follow Richard's live-blogging and/or ask questions of scientists on Twitter with the hashtag #askplanck.
• The release of the scientific papers is scheduled for 12:00 CET at the ESA website (I'm not sure of the precise url, maybe here?).

Although I won't be live-blogging the results, I will write a post later today summarising what we've learned and discussing the fall-out arising from all the new information.

Enjoy the day!

Edit: The papers will appear here at 12:00 CET: http://www.sciops.esa.int/index.php?project=planck&page=Planck_Legacy_Archive

## Tuesday, March 19, 2013

### Planck rumours will soon become Planck results

On Thursday, the Planck satellite will be revealing its first cosmological results. In terms of fundamental physics, this will be the biggest event since the Higgs discovery last year. In the cosmology community it is the biggest event for the best part of a decade (possibly in both directions of time). If you don't follow cosmology too closely, you might wonder why this particular experiment might generate so much excitement. After all, aren't there all sorts of experiments, all of the time?

If so, I hope you've come to the right place.

 The sky as seen by Planck in 2010. Only, they hadn't removed the foregrounds yet. There's a whole Milky Way galaxy in the way. Why must they make us wait so long?

If you're unaware, Planck is a satellite put in space by the European Space Agency to measure the cosmic microwave background (CMB). The CMB is an incredibly useful source of cosmological information. The impending release of Planck's results on Thursday is big news because Planck has measured the CMB with better resolution than any other experiment that can see the whole sky. Planck might have discovered evidence of interesting new physics, such as extra neutrinos or additional types of dark matter. It might even reveal some effects relating to how physics works at energies we could never probe on Earth. But even if it hasn't discovered anything dramatically new, the precision with which Planck has measured the parameters of the standard cosmological model will immediately make it the new benchmark.

There have been surprisingly few rumours leaked to the rest of the cosmology community about what to expect on Thursday. This has resulted in the most pervasive rumour being that they have simply not found anything worth leaking. Whatever the reality, on Thursday rumours will become results.

What has Planck actually done that is so interesting?