Combined constraints from BICEP2, Keck, Planck and WMAP on primordial gravitational waves

This week, the joint analysis of BICEP2 (+ BICEP2's successor Keck) and Planck has finally arrived. The result is more or less what was expected, which is that what BICEP2 saw last year in the B-mode polarisation signal of the CMB was not actually primordial gravitational waves (as had originally been hoped and claimed), but was unfortunately actually due to dust in the Milky Way. Such is life. Though we did of course have the best part of a year to come to grips with this reality.

Combined constraint on \(r\) from polarisation and temperature measurements (in blue). Freshly digitised in the spirit of modern cosmology. Gives \(r\lesssim 0.09\) at \(95\%\) confidence.

As a result of subtracting the dust component in BICEP2/Keck's signal (obtained by comparing the measurements from BICEP2/Keck and Planck), the final constraint on the "tensor to scalar ratio" (or \(r\)) from the BICEP2/Keck measurement is that \(r<0.12\) at \(95\%\) confidence. This \(r\) parameter essentially measures the amplitude of a primordial gravitational wave signal, so the net result is that the subtraction of dust takes BICEP2's high significance measurement of non-zero \(r\) and converts it into simply an upper bound.

I've seen some comments on blogs, in media, on Twitter, etc that there is still evidence of some sort of excess signal in BICEP2/Keck over and above the dust, but I can't see any evidence of that in any of their published results. The final likelihood for \(r\) (shown above in black) shows a plot consistent with \(r=0\) at less than \(1-\sigma\) (i.e. \(r=0\) is less than one standard deviation away from the maximum likelihood value). In fact, it would seem that the measurement of the dust that has been obtained by comparing BICEP2/Keck's measurements with Planck's measurements has been so good that the B-mode constraint on \(r\) from BICEP2/Keck is now competitive with (or even slightly better than) the constraint arising from temperature measurements of the CMB. This was always going to happen at some point in the future and it seems that this future has now arrived.

The Cold Spot is not particularly cold

(and it probably isn't explained by a supervoid; although it is still anomalous)

In the cosmic microwave background (CMB) there is a thing that cosmologists call "The Cold Spot". However, I'm going to try to argue that its name is perhaps a little, well, wrong. This is because it isn't actually very cold. Although, it is definitely notably spotty.

That's the cold spot. It even has its own Wikipedia page (which really does need updated).

Why care about a cold spot?

This spot has become a thing to cosmologists because it appears to be somewhat anomalous. What this means is that a spot just like this has a very low probability of occurring in a universe where the standard cosmological model is correct. Just how anomalous it is and how interesting we should find it is a subject for debate and not something I'll go into much today. There are a number of anomalies in the CMB, but there is also a lot of statistical information in the CMB, so freak events are expected to occur if you look at the data in enough different ways. This means that the anomalies could be honest-to-God signs of wonderful new physical effects, or they could just be statistical flukes. Determining which is true is very difficult because of how hard it is to quantify how many ways in which the entire cosmology community have examined their data.

However, if the anomalies are signs of new physics, then we should expect two things to happen. Firstly, some candidate for the new physics should come up, which can create the observed effect and produce all of the much greater number of other measurements that fit the standard cosmological model well. If this happens, then we would look for additional ways in which the universe described by this new model differs from the standard one, and look for those effects. Secondly, as we take more data, we would expect the unlikeliness of the anomaly to increase. that is, it should become more and more anomalous.

In this entry, I'm not going to be making any judgement on whether the cold spot is a statistical fluke or evidence of new physics. What I want to do is explain why, although it still is anomalous, and is definitely a spot, the cold spot isn't very cold. Then, briefly, I'll explain why, if it is evidence of new physics, that new physics isn't a supervoid.

So, what is the cold spot, and why is it anomalous?

The universe as seen by Planck - Days Three and Four I

Sorry for the delay on this. I was pretty tired on Friday, travelling home on Saturday and doing physics on Sunday. I figured it would be better to write something with a little more care today.

Those who were following last week will know that on March 21 ESA finally released some cosmological results from the measurements they were taking with the Planck satellite. And, last week, they had their first scientific conference. I decided to blog about this. I had the initial ambition of one post for each day, but the conference dinner on Thursday beat me and all I got out was a brief teaser post. This post now will be comprised of a summary of what I found interesting on both Thursday and Friday, along with a summary of the whole conference at the end.

I hope you enjoy it (and thanks for the feedback during the week).

Highlights

• What has Planck told us about inflation?
• What should we make of Planck vs SPT and Planck vs the local universe?
• What is next for CMB science?

• Some final thoughts

What has Planck told us about inflation?

Slava Mukhanov. Cosmology can do what it wants, but Mukhanov's  predictions for inflation will remain unchanged. Somehow cosmology always seems to come back to him in the end. Will that last missing piece show up? Will primordial gravitational waves one day be detected? It's starting to look like a "no", but Mukhanov's heard that talk before. Time will tell...

The first talk on Thursday was about inflation, by another one of the scientists who helped found it. This was by Slava Mukhanov, another old-school Russian physicist. Mukhanov was one of the first to realise that inflation wouldn't just cause the universe to expand dramatically and to make it more homogeneous, it would also seed new fluctuations with a very small amplitude. These new, small, fluctuations arise from the stretching (and eventual amplification) of quantum fluctuations in the field driving inflation. This type of realisation was what took inflation from an interesting concept to a testable paradigm.

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?

The “ISW mystery” deepens considerably (II)

[... continued from yesterday]

[Note added April 29: Through correspondence with some of the authors from what I label as the "French group" below I have learned that the density threshold was actually applied in their work as well. This makes things rather confusing as it means that their methods and the methods of "the DHB" are much more similar. However, they have also updated their paper to reflect new knowledge about the void catalogues and see a slightly more significant signal, similar to what Planck find (see note below). Everything is rather confusing right now. Again, once the dust has settled, I will write a post clearing everything up.]

[Note added March 21: Wow, sometimes science moves quickly. Today Planck released its data. They appear to confirm the anomalous spots in the original "Granett" (Hawaiian) result. They also appear to confirm the new anomalous result that was present in the paper that is now retracted (see the note below from March 19), albeit with a slightly reduced significance. It is unclear exactly what is going on, but it is clear that it is something interesting. I will keep you informed as things progress.]

[Noted added March 19: The paper described in the second half of this post (I called its authors the DHB) has been withdrawn from the journal it was submitted to (see the new abstract at this link: http://arxiv.org/abs/1301.6136). It is unclear whether the problems that the authors found in their analysis will affect their conclusions. However, I suggest you are cautious regarding how you interpret the conclusions I have drawn below based on this paper. I will keep you informed as/when things progress.]

A really neat figure from arXiv:1301.5849 showing the locations and sizes of the various catalogues of voids being examined. A larger redshift means the void is further away from us and one Megaparsec (Mpc) corresponds to three million light years. The purple "Granett et al." box is the original catalogue used by the Hawaiian group back in 2008. 

Isn't this just "a posteriori" statistics?

There is another possible explanation for the mystery. The probability of ZOBOV picking out these lines of sight at random is exceedingly small (less than 0.003), but it isn't zero. Might this have just been a crazy fluke?

Suppose 100 different groups of physicists look for unexpected, but interesting, signals in cosmological data. Then, even if each group is very careful you still expect one of them to find something that would seem to them to be unlikely. Unfortunately, they would be the only ones to publish their results. So we wouldn't see one “detection” paper and ninety-nine papers consistent with no detection. We would just see the one “detection” paper.

The best way to determine whether this is what happened is to look for the signal in other surveys.  If the original measurement was a fluke, it won't show up anywhere else. But, if it does show up again, then the chances that it was a fluke will significantly diminish.

The Friday before last a paper appeared that did exactly this. A French group took two catalogues of voids (so no over-densities), which have been produced by applying ZOBOV to a new catalogue of galaxies (these ones are closer to us). The French group then did more or less the same thing as the Hawaiians did. They examined images of the CMB along the lines of sight of these voids, averaged the temperature in all the images and checked whether the resulting signal could have happened at random.

They found no significant result.

This was quite sobering to read on the day. The paper did verify the significance of the original measurement, but not finding it in the new catalogues was highly suggestive that the story I painted above of a sort of community wide “look elsewhere effect” was true.

Hold on though!

Things at this date in time did look bad for the anomaly, but there was one important piece missing from the French group's analysis. The Hawaiians only used the most extreme over and under-dense regions in their analysis. ZOBOV found many more than 50 regions for them and if they had used all of them, they also wouldn't have obtained a statistically significant signal. This was always a crucial part of their analysis because we already knew from other observations that the observed ISW effect from most of the universe is as small as the predicted signal.

What would the French group have seen if they had only examined the most extreme voids?

A new observation

Apparently it is a rule of thumb for observers, that the more interesting your observation is, the more boring you are meant to make your title. These guys probably deserve a promotion. The paper is here.

Three days later (last Monday) a mixture of physicists from Durham, Hawaii and Baltimore (the DHB) released a paper. It answered the question posed above. For anybody interested in finding new physics, the answer is very exciting.

The “ISW mystery” deepens considerably

Other than my initials, what secrets does the CMB hide that are waiting to be seen only when the CMB is examined in just the right way?

This time last year I wrote a few posts describing what I called the “ISW mystery” (Part I, II, III and IV). A year has passed, it is time for an update on the mystery.

The very short summary is that things are starting to get more than a little bit exciting. All of the plausible ways in which the calculation of the expected ISW signal could have been wrong have been checked and eliminated as possibilities; if the measured signal is real, it is too large for the standard cosmological model. Much, much more excitingly, the observation that generated the mystery has now been repeated in another region of the universe and a very similar and equally anomalous signal was found; the apparent anomaly was not a statistical fluke.

The preprint of the paper describing this new observation was released just a week ago.

What is the “ISW mystery”?

The image that began the mystery. Why is that spot so hot, and how did it get that cold ring around it?

A quick recap will probably be useful. The integrated Sachs-Wolfe (ISW) effect describes the heating and cooling of light as it passes through gravitational peaks and valleys late in the evolution of the universe. In the standard cosmological model, these peaks and valleys decay with time, so a light ray gains (or loses) more energy entering an over-dense (or under-dense) region of the universe than it loses (or gains) leaving it. The effect is very, very small. Almost every source of light in the universe is not known well enough to be used to detect it. Only the cosmic microwave background (CMB) is uniform enough that these tiny fluctuations could ever be detected.

However, even then, the primary fluctuations in the the temperature of the CMB are bigger than the secondary ones created by the ISW effect. We can measure these fluctuations but we could never know how much is due to the ISW effect and how much is primordial. The only thing we can do is look at the structures in the universe nearby and see if on average the CMB is slightly hotter (colder) along lines of sight where the nearby universe is over-dense (under-dense). The bigger, primordial fluctuations in the CMB should have nothing to do with local structures (the CMB has come from much further away). Therefore, if this signal were to be found in the CMB, the most plausible explanation would be an ISW effect.

A group in Hawaii decided to look for this signal in a slightly unusual way. Firstly, they made a catalogue of significant over and under-dense regions in a particular survey of galaxies. Then, they only examined patches of the CMB that existed along the line of sight of each of these regions. They then found that the patches aligned with over-densities were hotter on average than a randomly selected patch and those aligned with under-densities were colder (with more than “\(4\sigma\)” significance). This is what one would expect from an ISW effect. The “ISW mystery” is that these patches were too hot and too cold. The ISW effect simply shouldn't be that big.

The importance of checking the anomaly from every angle

Highlights from Beijing: COSMO 2012

The obligatory conference photo. The photographer spoke to us in Mandarin. I think what he was trying to say was "more intensity".

Just over a week ago I was at the annual COSMO conference. This year's host was Beijing. I had originally intended to live blog this event, but the Great Wall of China (alternative link) managed to prevent that entirely.

What follows are some reflections on the scientific bits and pieces people presented at the conference that I happened to find interesting. It might be a bit technical, but please ask questions if I use jargon you don't understand. Also, if you're an expert and I write something you want to comment on, please do (especially if something I write is misleading or just plain wrong).

The topics I've chosen below just happen to be what I found memorable. I made no attempt to choose these topics by any sort of theme. I apologise if I've missed anything particularly interesting. Perhaps if you were there and think I missed out something interesting you can either mention it in the comments or write a guest post for us.

Neutrinos and precision cosmology

One of the first images captured by the Dark Energy Survey. The more interesting images it will take will be of very distant galaxies and won't look anywhere near as nice. This one is just for people to put in their blogs.

Jan Hamann gave a talk on the future constraints that cosmology will provide for neutrino physics. I was pleasantly surprised by the power of large scale structure probes, such as Euclid.

We know from particle physics experiments that the difference between the masses of two of the neutrinos is more than 0.06 electron volts. This means that the heaviest neutrino must be heavier than 0.06 electron volts.

The ISW Mystery IV: Where does the evidence lead?

Where does the evidence lead? (Photograph: H Armstrong Roberts/Corbis)

In my last three major posts (I, II and III) I've been talking you through a mystery: the integrated Sachs-Wolfe mystery. This post will be able to be read on its own, but it you will appreciate it much more if you have also read them. In today's post I will be playing detective, examining the evidence, looking for leads and weighing up the various possible solutions to the mystery. Like any good mystery there are hints as to what the resolution might be, but like any good mystery story some of these hints might turn out to be just be red herrings, so we need to be careful.

At the beginning of my most recent post in this series I warned you that it would be my most technical post to date, but encouraged you to stick with it. With this post, the situation is the opposite. That post contained the details of the actual measurement that was made, which is necessarily going to be somewhat dry and technical. This post, however, speculates about what might have caused the effect. As you'll soon see, solving the mystery potentially requires modifications to our understanding of fundamental physics or the initial conditions of the universe. All very exciting stuff, so congratulations for making it to this point.

An overview of the case:

Before embarking on the detective work, let me recap the first three posts in this series. In the first post, I introduced what the integrated Sachs-Wolfe effect is. It is the very subtle heating and cooling of light as it passes through over and under dense regions of the universe. In the second post I explained that this effect is so small that it almost certainly will never be observed directly. The only hope we have to observe it is to look for statistical correlations between the temperature of light on the sky and the density of the matter that the light travelled through to reach us. Only the cosmic microwave background (CMB) is uniform enough that such a statistical correlation could ever be observed. In the last instalment, I told you of a particular measurement that intended to detect this ISW effect by looking at extreme over and under densities in the universe. The measurement appeared to be a success because it did measure a correlation. The only problem, and the source of the mystery, is that the size of the correlation is far too big to be from the ISW effect.

Something in those structures is heating/cooling the CMB, but what?

What does the sound of the Big Bang look like?

Six weeks ago I wrote a post where I tried to explain how we know that the Big Bang definitely happened. There are of course other reasons why we know the Big Bang happened, but I decided to focus on one, relatively easily explained piece of evidence, which is the existence and frequency spectrum of the Cosmic Microwave Background (CMB).

Quickly summarising: The CMB is very cold radiation that permeates the entire universe. It was created when the expanding and cooling universe cooled to a point where it was cold enough for hydrogen atoms to form. Before this point, the electrons and protons in hydrogen had enough energy to be free from each other to form an opaque plasma. Once neutral hydrogen formed the universe became transparent and the CMB was formed and travelled (almost) freely forever after. We have detected and measured this CMB and its intensity as a function of its frequency (effectively, the brightness of each colour) is exactly what the Big Bang predicted. If there was no Big Bang there would be no reason to expect a CMB to exist, let alone for it to have this particular property. For more details please read my previous post and the links within.

When writing that post I had intended to say quite a bit more about the CMB and the Big Bang than I ended up having space for. It is not quite true that the mere existence (and spectrum) of the CMB is enough to conclusively determine that the Big Bang must have happened. However the existence of the CMB did build the metaphorical equivalent of a thousand big, bold and bright neon signs that all pointed aggressively towards the Big Bang being true.

When I began writing that earlier post and claimed that the CMB does conclusively prove that the Big Bang happened I had in my mind what I actually discuss in this post. This is the fact that we can see in the CMB the effects of sound waves that existed in the primordial hydrogen plasma. It is these sound waves and our measurements of them that puts the final nail in the coffin of all things not the Big Bang. They also represent what I claimed in that earlier post to be “jaw-droppingly stunning pieces of detective work”.

The glorious Planck satellite, measurer of all things CMB