A Lop-sided Universe?

Over on cosmic variance, I found an old post concerning the issue of whether there might be large-scale anomalies in the cosmic microwave background sky. I blogged about this some time ago, under the title of Is there an Elephant in the Room?, so it’s interesting to see a different take on it. Interest in this issue has been highlighted by a recent paper by Groeneboom & Eriksen that claims to have detected asymmetry in the distribution of fluctuations in the data from the Wilkinson Microwave Anisotropy Probe (WMAP) inconsistent with the predictions of the standard cosmological model. If this feature is truly of primordial origin then it is an extremely important discovery as it will (probably) require the introduction of new physics into our understanding of cosmology, and that will be exciting.

It is the job of theorists to invent new theories, and it is not at all a problem that these bits of evidence have generated a number of speculative ideas. Who knows? One of them may be right. I think it is the job of theoreticians to think as radically as possible about things like this. On the other hand, it is the observational evidence that counts in the end and we should be very conservative in how we treat that. This is what bothers me about this particular issue.

elongatedThe picture on the left shows a processed version of the WMAP fluctuation pattern designed to reveal the asymmetry, with the apparent preferred direction shown in red. This map shows the variation of the across the whole sky, and the claimed result is that the fluctuations are a bit larger around the red dots (which are 180 degrees apart) than in the regions at right angles to them.

It’s a slight effect, but everything in the picture is a slight effect as the CMB is extremely smooth to start with, the fluctuations in temperature being only about one part in a hundred thousand. The statistical analysis looks to me to be reasonably solid, so lets suppose that the claim is correct.scan

The picture on the right (courtesy of NASA/WMAP Science Team) shows the scan strategy followed by the WMAP satellite on the same projection of the sky. The experiment maps the whole sky by spinning its detectors in such a way that they point at all possible positions. The axis of this spin is chosen in a particular way so that it is aligned with the ecliptic poles (out of the plane of the solar system). It is in the nature of this procedure that it visits some places more than others (those at the ecliptic poles are scanned more often than those at the equator), hence the variation in signal-to-noise shown in the map. You can see that effect graphically in the picture: the regions near the North and South ecliptic poles have better signal to noise than the others.

The axis found by Groeneboom & Eriksen is not perfectly aligned with the ecliptic plane but it is pretty close. It seems a reasonable (if conservative) interpretation of this that the detected CMB anomaly could be due to an unknown systematic that has something to do either with the solar system (such as an unknown source of radiation, like cold dust) or the way the satellite scans. The WMAP team have worked immensely hard to isolate any such systematics so if this is such an effect then it must be very subtle to have escaped their powerful scrutiny. They’re all clever people and it’s a fabulous experiment, but that doesn’t mean that it is impossible that they have missed something.

Many of the comments that have been posted on cosmic variance relating to this question the statistical nature of the result. Of course we have only one sky available, so given the “randomness” of the fluctuations it is possible that freakish configurations occur by chance. This misses the essentially probabilistic nature of all science which I tried to describe in my book on probability From Cosmos to Chaos. We are always limited by noise and incompleteness but that doesn’t invalidate the scientific method. In cosmology these problems are writ large because of the nature of the subject, but there is no qualitative difference in the interplay between science and theory in cosmology compared with other sciences. It’s just less easy to get the evidence.

So the issue here, which is addressed only partially by Groeneboom % Eriksen, is whether a lop-sided universe is more probable than an isotropic one given the WMAP measurements. They use a properly consistent Bayesian argument to tackle this issue and form a reasonably strong conclusion that the answer is yes. As far as it goes, I think this is (probably) reasonable.

However, now imagine I don’t believe in anistropic cosmologies but instead have an idea that this is caused by an unknown systematic relating in some way to the ecliptic plane. Following the usual Bayesian logic I think it is clear that, although both can account for the data, my hypothesis must be even more probable than a lop-sided universe. There is no reason why a primordial effect should align so closely with the ecliptic plane, so there is one unexplained coincidence in the lop-sided-universe model, whereas my model neatly accounts for that fact without any freedom to adjust free parameters. Ockham’s razor is on my side.

So what can we do about this? The answer might be not very much. It is true that, soon, the Planck Surveyor will be launched and it will map the CMB sky againat higher resolution and sensitivity. On the other hand, it will not solve the problem that we only have one sky. The fact that it is a different experiment may yield clues to any residual systematics in the WMAP results, but if it has a similar scan strategy to WMAP, even Planck might not provide definitive answers.

I think this one may run and run!


14 Responses to “A Lop-sided Universe?”

  1. Michael Merrifield Says:

    Have you ever tried cross-correlating the scan pattern with the alleged CMB residuals? Just by eye, I seem to see a wacking great signal, not just in terms of the extrema…

  2. I thought that Planck had a very different scan strategy, which would provide a good test of exactly this sort of worry? At least that’s what E&G seemed to say.

    Otherwise — I think everyone admits that this is much more likely to be a systematic effect of the instrument than something in the primordial sky, absent some sort of confirmation. But the latter could conceivably come from large-scale structure tests.

  3. Anton Garrett Says:

    The WMAP team can only correct for KNOWN effects that correlate with the ecliptic… I’ll go with Ockham and say this is not a coincidence. With the expansion of the universe now known to be *accelerating* by no means known to cosmologists, we now realise that we know less than we thought we did.


  4. telescoper Says:

    Sean, I’m not exactly sure what the Planck scanning strategy is. I’m not an insider. I just assumed it was based on the same idea as WMAP. If I’m wrong about that then there’s better reason to hope Planck will shed light on this. Whether they come from new CMB experiments or alternative observations, such as galaxy clustering, any independent data are very important.

  5. […] Asymmetry: is it all in the Mind? After blogging a few days ago about the possibility that our entire Universe might be asymmetric, I found out […]

  6. […] there might be any hint that the low-level fuzz in the Fermi map might give us about the apparent lopsidedness and other anomalies in the Cosmic Microwave […]

  7. Great post, this is fast becoming one of my favorite blogs. I read this when it was posted and returned only recently for the comments. I find the discussion of a possible connection between the ecliptic and the orientation of the anomaly bewildering. How far off the ecliptic does it have to be before Ockham is left in peace? 45 degrees? 90 degrees? I bet the later would raise more questions than the current, what 20 degrees.

    My point is that the orientation of the anomaly has nothing whatever to do with the ecliptic until some evidence of a connection is discovered. Those who wrongly invoke Ockham to justify what amounts to faith (since it is not accompanied by evidence) are simply wrong minded.

    Any argument that requires belief in “[UN}KNOWN effects that correlate with the ecliptic” seems far more of a WAG than the original paper by Groeneboom and Eriksen.

  8. […] some light on the curiosities we find in the cosmic microwave background that I’ve mentioned here and there, but it seems to peak at too low a frequency to account for much of the overall microwave […]

  9. […] is the issue that I talked about in my seminar in London and have actually blogged about (here and there) previously, which is why the Universe appears to be a bit lop-sided and asymmetrical when […]

  10. […] been written about the possibility of a lop-sided universe that I’ve blogged about here and there, and which is a major topic of current cosmological […]

  11. […] among the list of cosmological anomalies that I’ve blogged about before (for example, here, here and here). I find them interesting but don’t lose sleep worrying that the standard model is […]

  12. […] of temperature fluctuations in the cosmic microwave background (CMB). See my other posts here, here, here, here and here for related […]

  13. […] to is of course the Wilkinson Microwave Anisotropy Probe and I’ve blogged before about the tantalising evidence it suggests of some departures from the standard cosmological theory. These authors do something […]

  14. […] then finally, there is the issue that I have ablogged about (here and there) previously, which is why the Universe appears to be a bit lop-sided and asymmetrical when […]

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