Planck, Pointillism and the Axle of Elvis

The reason I was out of the office yesterday was that I was in Cambridge, doing a PhD oral in the Cavendish Laboratory so the first thing to say is congratulations Dr Johnston! It was one of those viva voce examinations that turned out to be less of an examination than an interesting chat about physics. In fact the internal examiner, Prof. Steve Gull, seemed to spend more time asking me questions rather than the candidate!

Afterwards I met up with Anthony Lasenby, the candidate’s supervisor. Not surprisingly the main topic of our brief discussion was today’s impending announcement of results from Planck. Anthony is one of the folks who have been involved with Planck for about twenty years, since it began as a twinkle in the eye of COBRAS/SAMBA. I was looking forward to getting in bright and early this morning to watch the live streaming of the Planck press conference from Paris.

Unfortunately however, I could feel a bit of a lurgy coming on as I travelled to Cambridge yesterday. It got decidedly worse on the way home – it must have been the Cambridge air – and I even ended up passing out on the train from Victoria to Brighton. Fortunately, Brighton was the terminus so someone woke me up when we got there and I got home, coughing and spluttering. I suspect many cosmologists didn’t sleep well last night because of excitement about the Planck results, but in my case it was something else that kept me awake. Anyway, I didn’t make it in this morning so had to follow the announcements via Twitter. Fortunately there’s a lot of press coverage too; see the ESA site and a nice piece by the BBC’s redoubtable Jonathan Amos.

Anyway, without further ado, here’s Planck’s map of the cosmic microwave background:

It’s rather beautiful, in a pointillist kind of way, I think…

It will take me a while in my weakened state to complete a detailed study of the results – and I’m sure to return to them many times in the future, but I will make a couple of points now.

The first is that the papers and data products are all immediately available online. The papers will all appear on the arXiv. Open Access sceptics please take note!

The second is that the most interesting result (as far as I’m concerned) is that at least some of the cosmic anomalies I’ve blogged about in the past, such as the Axle of Elvis Axis of Evil and the famous colder-than-it-should-be cold spot, are still present in the Planck data:

The other results excite me less because, at a quick reading, they all seem to be consistent with the standard cosmological model. Of course, the north-south asymmetry is a small effect on could turn out to be a foreground (e.g. zodiacal emission) or an artefact of the scanning strategy. But if it isn’t a systematic it could be very important. I suspect there’ll be a rush of papers about this before long!

I’m sure to p0st much more about the Planck results in due course, but I think I’ll leave it there for now. Please feel free to post comments and reactions through the box below.

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DEUS

Well, I’m back home from Copenhagen after a very interesting and stimulating workshop called “DEUS” (subtitled “Current and Future Challenges of the Dark and Early Universes”). I just thought I’d post a brief message to thank the organizers for inviting me and for arranging such an interesting and varied programme, and especially for giving so many young researchers the chance to give talks (as well as some old farts like me).

Although I’d originally planned to talk about something else, I evenually decided to do a variation on the theme of cosmic anomalies, a topic I’ve blogged about at various times over the past couple of years. In a nutshell this was a quick overview of various features of the observed universe that seem to suggest departures from the standard “Lambda-CDM”  (or LCDM, for short) cosmological model, including the famous WMAP Cold Spot, the Axis of Evil, and various other statistical hints of anomalous behaviour in present-day observations.

To add a bit of audience participation I gave those attending my talk the chance to vote on what they thought about these – I was genuinely interested to see what this particular audience felt about whether the standard model is threatened or not.  I asked specifically about these in order to exclude other niggling worries people might have about LCDM from other astrophysical arguments, such as galaxy formation. Anyway, I thought it might be fun to repeat the poll here, so feel free to add your vote here:

As for the results of the vote during my presentation, I was somewhat surprised to see a roughly equal division between A and B, but there were even a few in C. I had assumed the vast majority would vote “A”….

Anyway, thanks again to the organizers of a fun meeting. That’s three trips to Copenhagen in as many months. I guess it will be a while before I go back again. 😦

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The Axle of Elvis

An interesting paper on the arXiv yesterday gave me a prod to expand a little on one of the cosmic anomalies I’ve blogged about before.

Before explaining what this is all about, let me just briefly introduce a bit of lingo. The pattern of variations fluctuations in the temperature of the cosmic microwave background (CMB) across the sky, such as is revealed by the Wilkinson Microwave Anisotropy Probe (WMAP), is usually presented in terms of the behaviour of its spherical harmonic components. The temperature as a function of position is represented as a superposition of spherical harmonic modes labelled by two numbers, the degree l and the order m. The degree basically sets the characteristic angular scale of the mode (large  scales have low l, and small scales have high l). For example the dipole mode has l=1 and it corresponds to variation across the sky on a scale of 180 degrees; the quadrupole (l=2) has a scale of 90 degrees, and so on. For a fixed l the order m runs from -l to +l and each order represents a particular pattern with that given scale.

The spherical harmonic coefficients that tell you how much of each mode is present in the signal are generally  complex numbers having real and imaginary parts or, equivalently, an amplitude and a phase.  The exception to this are the modes with m=0, the zonal modes, which have no azimuthal variation: they vary only with latitude, not longitude. These have no imaginary part so don’t really have a phase. For the other modes, the phase controls the variation with azimuthal angle around the axis of the chosen coordinate system, which in the case of the CMB is usually taken to be the Galactic one.

In the simplest versions of cosmic inflation, each of the spherical harmonic modes should be statistically independent and randomly distributed in both amplitude and phase. What this really means is that the harmonic modes are in a state of maximum statistical disorder or entropy. This property also guarantees that the temperature fluctuations over the sky should be described by  a Gaussian distribution.

That was perhaps a bit technical but the key idea is that if you decompose the overall pattern of fluctuations into its spherical harmonic components the individual mode patterns should look completely different. The quadrupole and octopole, for example, shouldn’t line up in any particular way.

Evidence that this wasn’t the case started to emerge when WMAP released its first set of data in 2003 with indications of an alignment between the modes of low degree. In their  analysis, Kate Land and Joao Magueijo dubbed this feature The Axis of Evil; the name has stuck.They concluded that there was a statistically significant alignment (at 99.9% confidence) between the multipoles of low degree (l=2 and 3), meaning that the measured alignment is only expected to arise by chance in one in a thousand simulated skies. More recently, further investigation of this effect using subsequent releases of data from the WMAP experiment and a more detailed treatment of the analysis (including its stability with respect to Galactic cuts) suggested that the result is not quite as robust as had originally been claimed. .

Here are the low-l modes of the WMAP data so you see what we’re talking about. The top row of the picture contains the modes for l=2 (quadrupole) and l=3 (octopole) and the bottom shows l=4 and l=5.

 

The two small red blobs mark the two ends of the preferred axis of each mode. The orientation of this axis is consistent across all the modes shown but the statistical significance is much stronger for the ones with lower l.

It’s probably worth mentioning a couple of neglected aspects of this phenomenon. One is that the observed quadrupole and octopole appear not only to be aligned with each other but also appear to be dominated by sectoral orders, i.e those with m=l. These are the modes which are, in a sense, opposite to the zonal modes in that they vary only with longitude and not with latitude. Here’s what the sectoral mode of the quadrupole looks like:

Changing the phase of this mode would result in the pattern moving to the left or right, i.e. changing its origin, but wouldn’t change the orientation. Which brings me to the other remarkable thing, namely that the two lowest modes also have  correlated phases. The blue patch to the right of Galactic centre is in the same place for both these modes. You can see the same feature in the full-resolution map (which involves modes up to l~700 or so):

I don’t know whether there is really anything anomalous about the low degree multipoles, but I hope this is a question that Planck (with its extra sensitivity, better frequency coverage and different experimental strategy) will hopefully shed some light on. It could be some sort of artifact of the measurement process or it could be an indication of something beyond the standard cosmology. It could also just be a fluke. Or even the result of an over-active imagination, like seeing Elvis in your local Tesco.

On its own I don’t think this is going to overthrow the standard model of cosmology. Introducing extra parameters to a model in order to explain a result with a likelihood that is only marginally low in a simpler model does not make sense, at least not to a proper Bayesian who knows about model selection…

However, it is worth mentioning that the Axis of Evil isn’t the only cosmic anomaly to have been reported. If an explanation is found with relatively few parameters that can account for all of these curiosities in one fell swoop then it would stand a good chance of convincing us all that there is more to the Universe than we thought. And that would be fun.