## 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?

Put most simply, Planck has measured the temperature of the CMB along various directions in the sky. The temperature of the CMB is almost uniform, but not quite. There are tiny fluctuations in the temperature of the CMB. These fluctuations are approximately one-millionth the size of the average temperature. To put this in perspective, measuring these fluctuations is like measuring the height of a building that is 100 metres tall, in tenths of a millimetre.

Many other experiments (COBE, BOOMERANG, WMAP, ACT, SPT) have measured these fluctuations. So what makes Planck so special?

 A map of the fluctuations in the CMB as seen by COBE. Pretty poor resolution, but the first ever to see them. The big red band along the middle is residual foreground from our own galaxy

In order to determine the temperature of the Cosmic Microwave Background it is necessary to measure its intensity as a function of its frequency. Unfortunately, the CMB is not the only microwave radiation in the universe. In order to properly determine the CMB's temperature along any given line of sight it is first necessary to determine what parts of this microwave radiation are the CMB and what are from some other source. The wider the range of frequencies at which you measure the microwave radiation, the easier this gets. Compared to comparable experiments, Planck does just this. The effects Planck has been looking for are subtle; therefore, removing this foreground is very important.

Most importantly though, Planck has measured the CMB with better resolution than any other space-based telescope. The Atacama Cosmology Telescope and South Pole Telescope have measured the CMB with a similar resolution; however, as their names suggest, they are Earth based and can only see a small fraction of the sky. Planck, being space based, can obviously see in every direction.

This means that Planck is the first telescope that will be able to tell us about both the smallest scales (i.e. the good resolution) and the largest scales (i.e. the full sky coverage) at the same time. This is Planck's most powerful feature.

But why is the CMB so interesting?

There is lots of stuff in the universe that can give us interesting cosmological information. Moreover, there are many other telescopes making measurements of this other stuff right now and reporting back to us. What is it about the CMB that always gets everyone so much more excited?

The customary reason to give for this is the fact that the CMB is the closest we can get to an image of the Big Bang. The CMB formed only 400,000 years after the Big Bang began and before that time, the universe was opaque. Therefore no light from any event before the formation of the CMB can reach us. This is true, but I think it doesn't quite capture the real reason why the CMB is so interesting.

 The temperature fluctuations in the CMB as seen by WMAP. The resolution is much better than COBE (and someone nicely removed the galaxy). In fact the resolution is so good that you can even make out the initials of a prominent cosmologist. One of the disputes I expect that Planck will help settle is the true identity of this cosmologist.

This "real reason" is that, for most cosmological models, we can predict very accurately what the CMB should look like. We can then compare the measured CMB to the accurate predictions and see which predictions were closest to the reality. The reason we can be so precise is that, at the early time when the CMB formed, the universe was very nearly homogeneous. That is, the temperature and density at every point of the universe was almost the same. You should compare that homogeneous, early, universe to the current universe where we have galaxies, stars, quasars, enormous regions empty of almost all matter and a whole host of other things occupying the universe. When the CMB formed the universe was more like a uniform soup of dark matter, hydrogen and radiation. This smoothness makes the calculations a lot simpler.

The second point that is often made is that, after it was formed, the CMB has only interacted very weakly with anything else in the universe. This is mostly true, although CMB science is now so precise that even these incredibly weak interactions are now useful cosmological probes. For example, Planck will have discovered a number of massive galaxy clusters because of how the CMB scatters of hot electrons within those clusters.

Despite all of this, eventually the CMB will no longer be the optimal cosmological probe. The information contained within the CMB is limited. Studying the CMB can only tell us what the density of the universe was, billions of years ago, on a thin shell, billions of light-years away. It is much more difficult to extract information about the primordial (or even current) density everywhere within that shell, but once we can, the wealth of information provided will be much more significant. These observations will be what comes next in cosmology. That is, very detailed observations of the structures in the universe. The major, expensive, satellite that will be the next generation's Planck (~20 years from now) is called Euclid, which will measure the location of hundreds of billions of galaxies. In the mean time there will be many interesting surveys (e.g. e-Rosita, DES). CMB science may soon be at the point of exhaustion, but observational cosmology will continue for at least the rest of this century.

 The CMB's temperature fluctuations as seen by the South Pole Telescope. Even better resolution than WMAP, but only a small fraction of the sky. Planck will see this level of resolution, but over the whole sky.

Then, after that century has passed, we (or whoever is still around by then) can start real-time cosmology. That is, observing the changes in time of the temperature of the CMB and the locations of structures. These changes will be small, but the longer we wait, the bigger they become. It is conceivable that just at the end of the life-time of the youngest people alive today the first of these measurements will be beginning to occur.

What if nothing new is revealed?

The rumours surrounding Planck and what new things it might discover have had one notable feature to them: they haven't really existed. This has caused one persistent rumour to develop and that is that there is actually nothing interesting to leak.

What would this mean?

Firstly, I won't lie, it would be kind of sad. I'm a young researcher, I'd love to have something new and exciting to contemplate and work on.

But, the situation wouldn't be quite as grim as the corresponding scenario for particle physics will be if all the LHC finds is the Higgs. The reason for this is that the standard model of particle physics is much more strongly established than the corresponding cosmological model. There are also a number of future experiments already taking measurements, or with allocated funding, that will further explore cosmology. If none of them find anything new and I'm writing blog posts like this in twenty years, I will start to use the word grim for cosmology too. The standard cosmological model (SCM) has its problems (such as what dark energy actually is) and, if nothing unexpected shows up, then we can't gain any insight into these problems and future generations might just have to learn to live with them.

But for now, Planck will still be doing a great service. Firstly, and this shouldn't be ignored, it will verify WMAP's measurements. The LHC is great because it has two detectors. If one made a mistake, it is unlikely that the other made exactly the same mistake. So, if both see something, we can be confident it is there. So far, for large enough scales, WMAP is all we've had. Maybe WMAP made a mistake? We wouldn't know. But, on Thursday, we will because a completely different telescope, analysed by different people, will have made the same measurements. Any physics and results based on WMAP will gain that important confirmation step.

Secondly, Planck will narrow the uncertainty on all of the parameters of the SCM (e.g. the density of dark matter, the density of dark energy, and the nature of the primordial density fluctuations). Planck will therefore allow us to make much more precise predictions about what other cosmological events should occur, if the SCM is correct. This will help us decide what types of telescopes and detectors to build and how to analyse the data produced. For very rare events, small changes in these parameters can even make the difference between expecting to see something or nothing. This sounds less interesting than exciting new physics, but it will still advance science significantly.

The closest I can give you to rumours

I have heard a few genuine rumours (I'm not going to identify any sources or make any claims regarding the veracity of these rumours). Here they are:
• The initial "ISW mystery" is still present in Planck's observations.
• When combined with extra data sets (including, I expect, galaxy clusters detected by Planck), there will be some evidence for non-zero neutrino masses, enough to make this become quite a popular field of (cosmological) research.
• With respect to non-Gaussianity, Planck will see evidence for non-Gaussianity at a level comparable to what WMAP has seen (i.e. 2-3 $$\sigma$$).
That's it though, that's all I've heard...

The release