## Monday, September 2, 2013

### A long summer

Indeed it has been a long summer, though the good weather appears to be drawing to a close. Over the last few months, I have attended three cosmology conferences or workshops and also been on a two-week holiday in the Dolomites, where I occupied my time by doing things like this:

 La Guglia Edmondo de Amicis, near the Misurina lake.
and enjoying views like this:

 Cima Piccola di Lavaredo, from the Dibona route on Cima Grande.
This explains the lack of activity here in recent times.

Returning home a couple of weeks ago, I was full of ideas for several exciting blog posts, including a summary of all the hottest topics in cosmology that were discussed at the conferences I attended, and perhaps an account of my argument stimulating discussion with Uros Seljak. However, it has come to my attention that there are other physicists in other parts of the world who happen to be working on the exact same topic that my collaborators and I have been investigating for the last few months. The rule in the research world is of course "publish or perish" (though some wit has suggested that "publish and perish" is more accurate) – so most of my time now will be spent on avoiding being scooped, and the current hiatus on this blog will continue for a short period. Looking on the bright side, once normal service resumes, I hope to have some interesting science results to describe!

In the meantime, I can only direct you to other blogs for your entertainment and enlightenment. Those of you who like physics discussions and have not already read Sean Carroll's blog (a vanishingly small number perhaps?) might enjoy this post about Boltzmann brains. I personally also enjoyed this argument against philosopher Tom Nagel.

For people interested in climbing news, I can report that my friends on the Oxford Greenland Expedition that I mentioned once here have returned safely after a successful series of very impressive climbs. I found their regular reports of their activities in the expedition diary well-written and rather thrilling – not just the climbing, but also the account of the journey to Greenland by sea in the face of seemingly never-ending gales! Well worth a read, as is this.

## Thursday, July 11, 2013

[A little note: This post, like many others on this blog, contains a few mathematical symbols which are displayed using MathJax. If you are reading this using an RSS reader such as Feedly and you see a lot of $signs floating around, you may need to click through to the blog to see the proper symbols.] People following the reporting of physics in the popular press might remember having come across a paper earlier this year that claimed to have detected the "largest structure in the Universe" in the distribution of quasars, that "challenged the Cosmological Principle". This was work done by Roger Clowes of the University of Central Lancashire and collaborators, and their paper was published in the Monthly Notices of the Royal Astronomical Society back in March (though it was available online from late last year). The reason I suspect people might have come across it is that it was accompanied by a pretty extraordinary amount of publicity, starting from this press release on the Royal Astronomical Society website. This was then taken up by Reuters, and featured on various popular science websites and news outlets, including New ScientistThe Atlantic, National Geographic, Space.com, The Daily Galaxy, Phys.orgGizmodo, and many more. The structure they claimed to have found even has its own Wikipedia entry.  Obligatory artist's impression of a quasar. One thing that you notice in a lot of these reports is the statement that the discovery of this structure violates Einstein's theory of gravity, which is nonsense. This is sloppy reporting, sure, but the RAS press release is also partly to blame here, since it includes a somewhat gratuitous mention of Einstein, and this is exactly the kind of thing that non-expert journalists are likely to pick up on. Mentioning Einstein probably helps generate more traffic after all, which is why I've put him in the title as well. But aside from the name-dropping, what about the main point about the violation of the cosmological principle? As a quick reminder, the cosmological principle is sometimes taken to be the assumption that, on large scales, the Universe is well-described as homogeneous and isotropic. The question of what constitutes "large scales" is sometimes not very well-defined: we know that on the scale of the Solar System the matter distribution is very definitely not homogeneous, and we believe that on the scale of size of the observable Universe it is. Generally speaking, people assume that on scales larger than about$100$Megaparsecs, homogeneity is a fair assumption. A paper by Yadav, Bagla and Khandai from 2010 showed that if the standard$\Lambda$CDM cosmological model is correct, the scale of homogeneity must be less than at most$370$Mpc. On the other hand, this quasar structure that Clowes et al. found is absolutely enormous: over 4 billion light years, or more than 1000 Mpc, long. Does the existence of such a large structure mean that the Universe is not homogeneous, the cosmological principle is not true, and the foundation on which all of modern cosmology is based is shaky? Well actually, no. Unfortunately Clowes' paper is wrong, on several counts. In fact, I have recently published a paper myself (journal version here, free arXiv version here) which points out that it is wrong. And, on the principle that if I don't talk about my own work, no one else will, I'm going to try explaining some of the ideas involved here. The first reason it is wrong is something that a lot of people who should know better don't seem to realise: there is no reason that structures should not exist which are larger than the homogeneity scale of$\Lambda$CDM. You may think that this doesn't make sense, because homogeneity precludes the existence of structures, so no structure can be larger than the homogeneity scale. Nevertheless, it does and they can. Let me explain a little more. The point here is that the Universe is not homogeneous, at any scale. What is homogeneous and isotropic is simply the background model we use the describe its behaviour. In the real Universe, there are always fluctuations away from homogeneity at all scales – in fact the theory of inflation basically guarantees this, since the power spectrum of potential fluctuations is close to scale-invariant. The assumption that all cosmological theory really rests on is that these fluctuations can be treated as perturbations about a homogeneous background – so that a perturbation theory approach to cosmology is valid. Given this knowledge that the Universe is never exactly homogeneous, the question of what the "homogeneity scale" actually means, and how to define it, takes on a different light. (Before you ask, yes it is still a useful concept!) One possible way to define it is as that scale above which density fluctuations$\delta$generally become small compared to the homogeneous background density. In technical terms, this means the scale at which the two-point correlation function for the fluctuations,$\xi(r)$, (of which the power spectrum$P(k)$is the Fourier transform) becomes less than$1$. Based on this definition, the homogeneity scale would be around$10$Mpc. It turns out that this definition, and the direct measurement of$\xi(r)$itself, is not very good for determining whether or not the Universe is a fractal, which is a question that several researchers decided was an important one to answer a few years ago. This question can instead be answered by a different analysis, which I explained once before here: essentially, given a catalogue with the positions of many galaxies (or quasars, or whatever), draw a sphere of radius$R$around each galaxy, and count how many other galaxies lie within this sphere, and how this number changes with$R$. The scale above which the average of this number for all galaxies starts scaling as the cube of the radius, $$N(<R)\propto R^3,$$ (within measurement error) is then the homogeneity scale (if it starts scaling as some other constant power of$R$, the Universe has a fractal nature). This is the definition of the homogeneity scale used by Yadav et al. and it is related to an integral of$\xi(r)$; typically measurements of the homogeneity scale using this definition come up with values of around$100-150$Mpc.  The figure that proves that the distribution of quasars is in fact homogeneous on the expected scales. For details, see arXiv:1306.1700. To get back to the original point, neither of these definitions of the homogeneity scale makes any claim about the existence of structures that are larger than that. In fact, in the$\Lambda$CDM model, the correlation function for matter density fluctuations is expected to be small but positive out to scales larger than either of the two homogeneity scales defined above (though not as large as Yadav et al.'s generous upper limit). The correlation function that can actually be measured using any given population of galaxies or quasars will extend out even further. So we already expect correlations to exist beyond the homogeneity scale – this means that, for some definitions of what constitutes a "structure", we expect to see large "structures" on these scales too. The second reason that the claim by Clowes et al. is wrong is however less subtle. Given the particular definition of a "structure" they use, one would expect to find very large structures even if density correlations were exactly zero on all scales. Yes, you read that right. It's worth going over how they define a "structure", just to make this absolutely clear. About the position of each quasar in the catalogue they draw a sphere of radius$L$. If any other quasars at all happen to lie within this sphere, they are classified as part of the same "structure", which can now be extended in other directions by repeating the procedure about each of the newly added member quasars. After repeating this procedure over all$18,722$quasars in the catalogue, the largest such group of quasars identified becomes the "largest structure in the Universe". It should be pretty obvious now that the radius$L$chosen for these spheres, while chosen rather arbitrarily, is crucial to the end result. If it is too large, all quasars in the catalogue end up classified as part of the same truly ginormous "structure", but this is not very helpful. This is known as "percolation" and the critical percolation threshold has been thoroughly studied for Poisson point sets – which are by definition random distributions of points with no correlation at all. The value of$L$that Clowes et al. chose to use, for no apparent reason other than that it gave them a dramatic result, was$100$Mpc – far too large to be justified on any theoretical grounds, but slightly lower than the critical percolation threshold would be if the quasar distribution was similar to that of a Poisson set. On the other hand, the "largest structure in the Universe" only consists of$73$quasars out of$18,722$, so it could be entirely explained as a result of the poor definition ... Now I'll spare you all the details of how to test whether, using this definition of a "structure", one would expect to find "structures" extending over more than$1000$Mpc in length or with more than$73$members or whatever, even in a purely random distribution of points, which are by definition homogeneous. Suffice it to say that it turns out one would. This plot shows the maximum extent of such "structures" found in$10,000$simulations of completely uncorrelated distributions of points, compared to the maximum extent of the "structure" found in the real quasar catalogue.  The probability distribution of extents of largest "structures" found in 10,000 random point sets for two different choices of$L$. Vertical lines show the actual values found for "structures" in the quasar catalogue. The actual values are not very unusual. Figure from arXiv:1306.1700. To summarise then: finding a "structure" larger than the homogeneity scale does not violate the cosmological principle, because of correlations; on top of that, the "largest structure in the Universe" is actually not really a "structure" in any meaningful sense. In my professional opinion, Clowes' paper and all the hype surrounding it in the press is nothing more than that – hype. Unfortunately, this is another verification of my maxim that if a paper to do with cosmology is accompanied by a big press release, it is odds-on to turn out to be wrong. Finally, before I leave the topic, I'll make a comment about the presentation of results by Clowes et al. Here, for instance, is an image they presented showing their "structure", which they call the 'Huge-LQG', with a second "structure" called the 'CCLQG' towards the bottom left:  3D representation of the Huge-LQG and CCLQG. From arXiv:1211.6256. Looks impressive! Until you start digging a bit deeper, anyway. Firstly, they've only shown the quasars that form part of the "structure", not all the others around it. Secondly, they've drawn enormous spheres (of radius$33$Mpc) at the position of each quasar to make it look more dramatic. In actual fact the quasars are way smaller than that. The combined effect of these two presentational choices is to make the 'Huge-LQG' look far more plausible as a structure than it really is. Here's a representation of the exact same region of space that I made myself, which rectifies both problems:  Quasar positions around the "structures" claimed by Clowes et al. Do you still see the "structures"? ## Sunday, June 23, 2013 ### Across the Himalayan Axis I had promised to try to write a summary of the workshop on cosmological perturbations post Planck that took place in Helsinki in the first week of June, but although the talks were all interesting, I didn't feel very inspired to write much about them. Plus life has been intervening, so I'll have to leave you to read Shaun's accounts at the Trenches of Discovery instead. I also recently put a new paper on the arXiv; despite promising to write about my own papers when I put them out, I'm going to have to postpone an account of this one until next week. This is because I am spending the next week at a rather unique workshop in the Austrian Alps. (This is one of the perks of being a physicist, I suppose!) Therefore today's post is going to be about mountaineering instead. It is an account I wrote of a trek I did with my father and sister almost exactly seven years ago: we crossed the main Himalayan mountain range from south to north over a mountain pass known as the Kang La (meaning 'pass of ice' in the local Tibetan dialect, I believe), and then crossed back again from north to south over another pass as part of a big loop. In doing so we also crossed from the northern Indian state of Himachal Pradesh into Zanskar, a province of the state of Jammu and Kashmir, and then back again. The account below was first written as a report for the A.C. Irvine Travel Fund, who partly funded this trip, and it has been available via a link on their website for several years. At the time, the Kang La was a very infrequently-used pass, in quite a remote area and only suitable for strong hikers with high-altitude mountain experience. But in the seven years since my trip it has seen quite a rise in popularity — I sometimes flatter myself that my account had something to do with raising the profile of the area! Anyway, the account itself follows after the break. There is also a sketch map of the area I drew myself (it's hard to obtain decent cartographical maps of the area, and illegal to possess them in India due to the proximity to the border), and a few photographs to illustrate the scenery ... ## Tuesday, June 4, 2013 ### CPPP 13 This week I am attending this workshop in Helsinki. The focus of the workshop is on re-evaluating theoretical issues in cosmology in light of the new data from the Planck satellite. Although the data were released in March, so far as I know they have not yet inspired any major theoretical breakthroughs. This is partly because the results were somewhat disappointingly boring, in that there is no smoking-gun indication in the data of failures of our current cosmological model (for more on this, see here), and therefore no clear hints of which extensions of the model we should be looking to explore further. There are still some niggles in the data, to be sure – such as the much advertised "anomalies". But these have not yet led to any major advances either. As a community it seems we cosmologists are still digesting the Planck results. This workshop should aid that process of digestion. There are many scientists from all over the world attending, and I'm looking forward to hearing what they think about what the data mean. The way the workshop has been organised deliberately leaves plenty of time for discussion in between the scheduled talks, which I think is always the best way to go. I'm not giving a talk myself, though some work I did recently with Shaun Hotchkiss and Samuel Flender, who are both based in Helsinki, will feature in the poster session. Samuel gets most of the credit for preparing our poster though! I'm not going to attempt to blog about the workshop in real time. Instead I will try to make a few notes and provide a single post at the end of the week touching on what I thought were the most interesting topics of the week. If you want more detail on each day, you should read Shaun's introduction and day-by-day accounts at The Trenches of Discovery. He did a similar thing before for the official ESA Planck conference, which was very successful. But I'd rather him than me, especially as internet access is rather expensive at my hotel! Meanwhile, the most remarkable fact to note about Helsinki right now is the weather: a thermometer in my hotel said it was 28ºC this morning, which is wonderful outdoors but when combined with quadruple-glazed windows makes nights rather uncomfortable ... ## Monday, May 27, 2013 ### An inconsistent CMB? When the Planck science team announced their results in March, they also put out a great flood of papers. You can find the list here; there are 29 of them, plus an explanatory statement. Except if you look carefully, only 28 of the papers have actually been released. Paper XI, 'Consistency of the data', is still listed as "in preparation". Now, what this paper was supposed to cover was the question of how consistent Planck results were with previous CMB experiments, such as WMAP. We already knew that there were some inconsistencies, both in the derived cosmological parameters such as the dark energy density and the Hubble parameter, and in the overall normalization of the power seen on large scales. We might expect this missing paper to tell us the reason for the inconsistencies, and perhaps to indicate which experiment got it wrong (if any). The problem is that at present there is no indication when we can expect this paper to arrive – when asked, members of the Planck team only say "soon". I presume that the reason for the delay is that they are having some unforeseen difficulty in the analysis. However, if you were paying attention last week, you might have noticed a new submission to the arXiv that provided an interesting little insight into what might be going on. This paper by Anne Mette Frejsel, Martin Hansen and Hao Liu – the authors are at the Niels Bohr Institute in Copenhagen, and in fact all three recently visited Bielefeld for our Kosmologietag workshop – applied a particular consistency check to Planck and WMAP data ... and found WMAP wanting. The test they applied is really pleasingly simple. Suppose you want to measure the CMB temperature anisotropies on the sky using your wonderful satellite – either WMAP or Planck. Unfortunately, there's a great big galaxy (our galaxy) in the way, obscuring quite a large fraction of the sky:  The CMB sky as seen by Planck in the 353 GHz channel. Obviously there's a lot of foreground in the way. (This is not the best frequency for viewing the CMB, by the way. I chose it only because it illustrates the foregrounds quite nicely!) Now, as I've mentioned before, there are clever ways of removing this foreground and getting to the underlying CMB signal. The CMB signal is what is interesting for cosmologists, because that is what gives us the insight into fundamental physics. Foregrounds are about messy stuff to do with the distribution of dust in our galaxy: the details are complicated, but the underlying physics is not that interesting (ok, maybe it is, but to different people). Anyway, using their clever techniques (and measurements of the CMB+foreground at several different frequencies), the guys at Planck or WMAP can come up with the best map they can that they think represents the CMB with foreground removed.  Planck's SMICA map of the CMB. The map above shows the Planck team's effort. Well actually they produced four different such "CMB only" maps, constructed by four different methods of removing the foregrounds. These are known as the SMICA, SEVEM, NILC and Commander-Ruler maps, the names indicating the different foreground-removal algorithms used. For some reason, Commander-Ruler appears not to be recommended for general use. WMAP on the other hand produced only one, known as the Internal Linear Combination or ILC map. (Planck's NILC is meant to be a counterpart to WMAP's ILC.) Now, although the algorithms used to produce these maps are, as I said, very clever, the resultant maps are never going to be completely foreground-free. Let's express this as the equation map = CMB + noise where the "noise" term includes foregrounds as well as instrument noise, systematics and other contaminants. If you have more than one map, they see the same fundamental CMB, but the noise contribution to each is different. So you can subtract one from the other to get a new map consisting of their difference: difference = map1 – map2 = noise1 – noise2. Since most of the residual noise should be due to the galactic foreground, most of the features in the difference map should be around the galactic plane. If the foreground removal has been reasonably successful, these features should also be small. And for the Planck maps, that is in fact what Frejsel, Hansen and Liu find:  NILC–SMICA, NILC–SEVEM and SMICA–SEVEM difference maps. Figure from arXiv:1305.4033. So the various different methods used by Planck seem to give self-consistent answers. The same is not true, however, for WMAP. Of course WMAP only use the one method of removing foreground, but they did provide different maps based on the data they had collected after 7 years of operation and after 9 years. The ILC9–ILC7 difference map looks quite different:  ILC9–ILC7 difference map on the left, and with a galactic mask overlaid on the right. Figure from arXiv:1305.4033. Most of the difference appears well away from the galactic plane, as you can see in the right-hand figure, where the galaxy is masked out. So there is some important source of noise that is not foregrounds – so probably some systematics – that has affected the WMAP ILC map. Even more importantly, it is some kind of systematic effect that has changed between the 7-year and the 9-year WMAP data releases, meaning that the ILC9 and ILC7 maps do not appear to be consistent with each other. Frejsel et al. discuss a method of quantifying this, but I won't go into that here because the impression created by the images is both dramatic enough and entirely in line with the quantitative analysis. As you might have expected, the same method shows that WMAP's ILC9 map is thoroughly inconsistent with the various Planck maps (the picture here is even worse than that between the two ILC maps). But perhaps surprisingly, ILC7 is perfectly consistent with Planck. So it appears that whatever might have affected the WMAP results only affected the final data release. I guess one should be careful not to make too much of a fuss about this. The results from Planck and WMAP are, generally speaking, in pretty good agreement, except for some problems at the very largest scales. It is also true that the WMAP team themselves do not use the ILC map for most of their analysis (except for the low multipoles,$\ell<32$– that is, the very largest scales!). But I'm sure this paper will provoke some head-scratching among the WMAP team as they try to figure out what has happened here. Oh, and if you are cosmologist using the ILC9 map for your own analysis, you should probably check whatever conclusions you draw using some other maps before publishing! All in all, I think I'm rather looking forward to Planck's consistency paper when it does finally come out! ## Tuesday, May 21, 2013 ### One year on It has been a long month-and-a-bit since I last had the time to write a proper post here. Primarily this is because I am not very good at doing more than one thing at a time – at least while attempting to do those things properly – and I have been working on some real research papers, which I thought I should try to do properly. As a result science communication, and blogging in general, has had to wait. But I will get back in the swing of things very soon: there are some interesting new results, some interesting rehashes of old arguments about inflation, and one of my own papers that I will write about over the next couple of weeks. One of the many things that I omitted to mention during this little break was the fact that Blank On The Map had its first anniversary a couple of weeks ago. All in all, I would say it has been quite a satisfying year of blogging, and, at least relative to my prior expectations, a reasonably successful one too. Blogger doesn't provide me with many detailed statistics, but it does tell me that despite the low recent rate of new postings, there have been roughly 23,000 clicks over the last twelve months. A sizeable portion of of this traffic came from one link on Peter Woit's blog (boy he must have a huge number of readers!) – though the majority appear to arrive via Google search, either by accident or design. Anyway, to all those who have arrived here at some point over the last year, welcome! I hope you found what you were looking for, and enjoyed what you read. This explains what this blog is about. In case you haven't worked your way through the archives, here is a short selection of some highlights from the last year: I should also use this post to try to get some feedback from (all three) regular readers. Do you think I should post more often? Less often? More posts about cosmology and fewer about mountaineering and other stuff in general? Or the other way round? Let me know through the comments box. Constructive criticism of the writing style or any other aspect of this blog will also be well received, though I can't promise to change ... ## Tuesday, April 9, 2013 ### Celebrating Tom Lehrer This is not a post about physics, but one to mark the birthday today of mathematician, teacher, satirist, lyricist and performer Tom Lehrer. Today he turns 85 – or, since he apparently prefers to measure his age in Centigrade – 29 (I must remember to use that one myself sometime!). To commemorate the occasion, the BBC ran a half-hour long radio feature on his life and work last Saturday. This is available to listen to here for another four days; do try to catch it before then! Even readers who have not heard of Lehrer might have heard of some of his better-known songs, such as The Elements Song. Other pieces of simple comedy gold include Lobachevsky, or New Math. But for me the best of Lehrer's songs are the ones with darkly satirical lyrics juxtaposed with curiously uplifting melodies. (These were probably also part of the reason that he never achieved the mainstream popularity he deserved.) So I want to feature one such example here: Kim Jong-un, I hope you are listening. ## Sunday, April 7, 2013 ### Unnecessary spin A few people have asked me why I have not blogged about the recent announcement about, and publication of, results from the Alpha Magnetic Spectrometer, which were widely touted as a possible breakthrough in the search for dark matter. The reason I have not is simply that there are many other better informed commenters who have already done so. In case you have not yet read these accounts, you could do worse than going to Résonaances, or Ethan Siegel, or Stuart Clark in the Guardian, who provide commentary at different levels of technical detail. The simple short summary would be: AMS has not provided evidence about the nature of dark matter, nor is it likely to do so in the near future. The dramatic claims to the contrary are spin, pure and simple. Siegel in fact goes so far as to say "calling it misleading is generous, because I personally believe it is deceitful" (emphasis his own). So I'm not going to make any more comments about that. However, since this incident brought it up again, I do want to comment on a related piece of annoying spin, which is the habit of physicists in the business of communicating science to the public of making vastly exaggerated claims about the possible practical applications of fundamental physics. The example that caught my attention this week occurred when Maggie Aderin-Pocock – who is apparently a space science research fellow at UCL – appeared on the BBC's Today programme on Thursday to discuss the significance of the AMS findings. At one point in the discussion the interviewer John Humphrys asked a slightly tricky question: I understand that dark matter and dark energy are endlessly fascinating, he said, and that learning about the composition of the universe is very exciting. But what practical benefits might it bring? The answer Aderin-Pocock gave was that if we understood what dark matter and dark energy were, we might be able to use them to supply ourselves with energy – dark matter as a fuel source. I'm sorry, but that is just rubbish. Unfortunately, it's the kind of rubbish that is increasingly commonly voiced by scientists. You may argue that Aderin-Pocock was simply commenting on something she didn't understand – and if you listen to the whole interview (available here for a few days; skip to the segment between 1h 23m and 1h 26m), including the cringe-worthy suggestion that dark matter and dark energy are the same thing really (because E=mc2, apparently), it's hard to avoid that conclusion. But a few weeks ago I thought I heard Andrew Pontzen and Tom Whyntie suggest something similar about the Higgs boson on BBC Radio 5 (unfortunately this episode is no longer on the iPlayer so I can't check the exact words they used). And here is Jon Butterworth seeming to suggest (in the midst of an otherwise reasonable piece) that the Higgs could be used to power interstellar travel ... Why do people feel the need to do this? It's patently rubbish, and they know better. Do we as a scientific community feel that continued public support of science is so important that we should mislead or deceive the public in order to guarantee our future access to it? Do we feel that there is no convincing honest case to be made instead? Or are we just too lazy to make the honest case, and so rely on the catchy but inaccurate soundbite instead? I think the sensible answer to the question John Humphrys posed would go something like this. Discovering the nature dark matter is a fascinating and exciting adventure. Knowing the answer will almost certainly have no practical applications whatsoever. However, on the journey to the answer we will have to develop new technologies and equipment (made of ordinary matter!) which may serendipitously turn out to have spin-off applications that we cannot yet foresee. More importantly, the very fact that the search is fascinating is part of what draws talented and creative young minds to physics – indeed to science – in the first place, from where they go on to enrich our society in a myriad different ways, none of which may later be connected to dark matter at all. I tried to make this case at greater length here in the early days of this blog. It's a more subtle argument than just throwing empty phrases about "energy source" around, and it might be hard to reduce to a sound-byte. But it is justifiable, and also honest. And since science is after all about careful argumentation, let's have less spin all round please. ## Wednesday, March 27, 2013 ### Explaining Planck by analogy Explaining physics to the public is hard. Most physicists do a lousy job of conveying a summary of what their research really means and why it is important, without the use of jargon and in terms that can be readily understood. So it is not particularly surprising that occasionally non-experts trying to translate these statements for the benefit of other non-experts come up with misleading headlines such as this, or this. Just to be clear: Planck has not mapped the universe as it was in the first tiny fraction of a second. (To be fair, most other reports correctly make this distinction, though they differ widely on when inflation is supposed to have occurred.) I think this is an important thing to get right, and I'm going to try to explain why, and what the CMB actually is. However, I'm going to try to do so with the help of an analogy. This analogy is not my original invention – I heard Simon White use it during the Planck science briefing – but I think it is brilliant, simple to understand and not vastly misleading. So, despite the health warning about analogies above, I'm going to run with it and see how far we get. ## Thursday, March 21, 2013 ### What Planck has seen Update at 16:30 CET: I've now had a chance to listen to the main science briefing, and also to glance at some of the scientific papers released today, albeit very briefly. So here are a few more thoughts, though in actual fact it will take quite some time for cosmologists to fully assess the Planck results. The first thing to say – and it's something easy to forget to say – is just what a wonderful achievement it is to send a satellite carrying two such precise instruments up into space, station it at L2, cool the instruments to a tenth of a degree above absolute zero with a fluctuations of less than one part in a million about that, spin the satellite once per minute, scan the whole sky in 9 different frequency bands, subtract all the messy foreground radiation from our own galaxy and even our solar system, all to obtain this perfect image of the universe as it was nearly 14 billion years ago:  The CMB sky according to Planck. So congratulations and thanks to the Planck team! Now I said all that first up because I don't want to now sound churlish when I say that overall the results are a little disappointing for cosmologists. This is because, as I noted earlier in the day, there isn't much by way of exciting new results to challenge our current model of the universe. And of course physicists are more excited by evidence that what they have hitherto believed was wrong than by evidence that it continues to appear to be right. There are however still some results that will be of interest, and where I think you can expect to see a fair amount of debate and new research in the near future. Firstly, as I pointed out earlier, Planck sees the same large scale anomalies as WMAP, thus confirming that they are real rather than artifacts of some systematic error or foreground contamination (I believe Planck even account for possible contamination from our own solar system, which WMAP didn't do). These anomalies include not enough power on large angular scales ($\ell\leq30$), an asymmetry between the power in two hemispheres, a colder-than-expected large cold spot, and so on. The problem with these anomalies is that they lie in the grey zone between being not particularly unusual and being definitely something to worry about. Roughly speaking, they're unlikely at around a 1% level. This means that how seriously you take them depends a lot on your personal prejudices priors. One school of thought – let's call it the "North American school" – tends to downplay the importance of anomalies and question the robustness of the statistical methods by which they were analysed. The other – shall we say "European" – school tends instead to play them up a bit: to highlight the differences with theory and to stress the importance of further investigation. Neither approach is wrong, because as I said this is a grey area. But the Planck team, for what it's worth, seem to be in the "European" camp. The second surprise is the change in the best-fit values for the parameters of the simplest$\Lambda$CDM model. In particular the Hubble parameter is lower than WMAP's, which was already getting a bit low compared to distance-ladder measurements from supernovae. This will be a cause for concern for the people working on distance-ladder measurements, and potentially something interesting for inventive theorists. And finally, something close to my own heart. A few days ago I wrote a post about the discrepancy in the integrated Sachs-Wolfe signal seen from very rare structures, and pointed out that this effect had now been confirmed in two independent measurements. Almost immediately I had to change that statement, because one of those independent measurements had been partially retracted. Well, the Planck team have been on the case (here, paper XIX), and have now filled that gap with a second independent measurement (as well as re-confirming the first one). The effect is definitely there to be seen, and it is still discrepant with$\Lambda\$CDM theory (though I'll need to read the paper in more detail before quantifying that statement).

So there's a ray of hope for something exciting.

11:30 am CET: Well, ESA's first press conference to announce the cosmological results from Planck has just concluded. The full scientific papers will be released in about an hour, and there will be a proper technical briefing on the results in the afternoon (this first announcement was aimed primarily at the media). However, here is a very quick summary of what I gathered is going to be presented:
• The standard Lambda Cold Dark Matter Model continues to be a good fit to CMB data
• However, the best fit parameters have changed: in particular, Planck indicates slightly more dark matter and ordinary (baryonic) matter than WMAP did, and slightly less dark energy. (This is possibly not a very fair comparison – my hunch is that the Planck values are obtained from Planck data alone, whereas the "WMAP values" that were quoted were actually the best fit to WMAP plus additional (non-CMB) datasets.)
• The value of the Hubble parameter has decreased a bit, to around 67 km/s/Mpc. Given the error bars this is actually getting a bit far away from the value measured from supernovae, whch is around 74 km/s/Mpc. I think the quoted error bars on the measurement from supernovae are underestimated.
• The Planck value of the spectral tilt is a bit smaller than, but consistent with, what WMAP found.
• There is no evidence for extra neutrino-like species.
• There is no evidence for non-zero neutrino masses.
• There is no evidence for non-Gaussianity.
• There is no evidence for deviations from a simple power-law form of the primordial power spectrum.
• No polarisation data, and therefore no evidence of gravitational waves or their absence, for around another year.
• There is evidence for anomalies in the large-scale power, consistent with what was seen in WMAP. We'll have to wait and see how statistically significant this is – the general response to the anomalies WMAP saw could be summarised as "interesting, but inconclusive"; I don't think Planck is going to do a lot better than this (and the bigging-up of it in the press conference might have had more to do with the lack of other truly exciting discoveries), but I'd love to be surprised!
That's about all I got out of the media briefing. Obviously we are all waiting for more details this afternoon!