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.

Comparing Planck's noise and dust to BICEP2

In case anyone reading this doesn't recall, back in March an experiment known as BICEP2 made a detection of something known as B-mode polarisation in the cosmic microwave background (CMB). This was big news, mostly because this B-mode polarisation signal would be a characteristic signal of primordial gravitational waves. The detection of the effects of primordial gravitational waves would itself be a wonderful discovery, but this potential discovery went even further in the wonderfulness because the likely origin of primordial gravitational waves would be a process known as inflation which is postulated to have occurred in the very, very early universe.

The B-mode polarisation in the CMB as seen by BICEP2. Seen here for the first time in blog format without the arrows. Is it dust, or is it ripples in space-time? Don't let Occam's razor decide!

I said at the time, and would stand by this now, that if BICEP2 has detected the effects of primordial gravitational waves, then this would be the greatest discovery of the 21st century.

However, about a month after BICEP2's big announcement a large crack developed in the hope that they had detected the effects of primordial gravitational waves and obtained strong evidence for inflation. The problem is that light scattering of dust in the Milky Way Galaxy can also produce this B-mode polarisation signal. Of course BICEP2 knew this and had estimated the amplitude of such a signal and found it to be much too small to explain their signal. The crack was that it seemed they had potentially under-estimated this signal. Or, more precisely, it was unclear how big the signal actually is. It might be as big as the BICEP2 signal, or it might be smaller.

Either way, the situation a few months ago was that the argument BICEP2 made for why this dust signal should be small was no longer convincing and more evidence was needed to determine whether the signal was due to dust, or primordial stuff.

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?

A new cosmological coincidence problem?

One of the consequences of the BICEP2 data from last week, should it hold up to scrutiny, and be seen by other experiments (I hope it holds up to scrutiny and is seen by other experiments), is that there is a significant lack of "power" in the temperature anisotropies on large angular scales.

What that sentence means is that when you look at the CMB in very large patches on the sky (about the size of the moon and bigger) its temperature fluctuates from patch to patch less than we would expect.

This was already somewhat the case before the BICEP2 discovery, but BICEP2 made it much more significant. The reason for this will hopefully turn into a post of its own one day, but, essentially, the primordial gravitational waves that BICEP2 has hopefully discovered would themselves have seeded temperature anisotropies on these large angular scales. Previously, we could just assume that the primordial gravitational waves had a really small amplitude and thus didn't affect the temperature much at all. Now, however, it seems like they might be quite large and therefore, this apparent lack of power becomes much more pertinent.

That's all fine and is something that any model of inflation that hopes to explain the origin of these gravitational waves will need to explain, despite what many cosmologists already writing papers on the ArXiv seem to want to believe (links withheld). As a side, ever-so-slightly-frustrated, note, the only papers I've seen that have actually analysed the data, rather than repeating old claims, have confirmed this problem that was clear from, at the latest, the day after the announcement.

But why does it imply a "cosmological coincidence problem"? And why is it a new coincidence problem? What's the old one?

Preliminary: Cosmological impacts of BICEP2 + Planck

If anybody is interested, I'm currently drip-tweeting some of the constraints one can obtain from considering Planck and BICEP2 data together. BICEP2 did do a bit of this in their paper, but they only considered specific scenarios. They were also often a bit coy about the implications of the combined analysis. I'll try not to be ;-).

The results should only be seen as indicative, these aren't published, and never will be in this form (maybe they could be cited if used in a paper though!). They were provided to me by Sussex Uni's resident obtaining-cosmology-from-the-CMB expert Antony Lewis, after a hurried Tuesday adding the BICEP2 data to the Planck cosmology pipeline (i.e. CosmoMC) and may contain mistakes.

Antony has himself also made some of these results public at the Cosmo Coffee website.

Questions here, or on Twitter are most welcome. If you want to see specific cosmologies, I'll do my best to show them (if I have them), or ask Antony very nicely to provide them (no guarantees, of course).

You can find my Twitter account here: @just_shaun. Feel free to share!

Quantum mechanics and the Planck-spectrum

[The following is a guest post from Bjoern Malte Schaefer. Bjoern is one of the curators of the Cosmology Question of the Week blog, which is worth checking out. This post is a historical look at some of the early parts in the history of quantum mechanics, in particular, the black-body spectrum. Questions are welcome and I'll make sure he sees any of them. Image captions (and hyper-links, in this case) are, as usual, by me, because guest posters don't ever seem to provide their own.]

Two unusual systems

Quantum mechanics surprises with the statement that the microscopic world works very differently from the macroscopic world. Therefore, it took a while until quantum mechanics was formally established as the theory of the microworld. In particular, despite the fact that two of the natural systems on which theories of quantum mechanics could initially be tested were very simple, even from the point of view of the physicists of the time, one needed to introduce a number of novel concepts for their description. These two physical systems were the hydrogen atom and the spectrum of a thermal radiation source. The hydrogen atom was the lightest of all atoms with the most simply structured spectrum. It exhibited many regularities involving rational numbers relating its discrete energy levels. It could only be ionised once implying that it had only a single electron and from these reasons it was the obvious test case for any theory of mechanics in the quantum regime. Werner Heisenberg was the first to be successful in solving this quantum mechanical analogue of the Kepler-problem, i.e. the equation of motion of a charge moving in a Coulomb-potential, paving the way for a systematic understanding of atomic spectra, their fine structure, the theory of chemical bonds, interactions of atoms with fields and ultimately quantum electrodynamics.

The Planck-spectrum was equally puzzling: It is the distribution of photon energies emitted from a body at thermal equilibrium and does not, in particular, require any further specification of the body apart that it should be black, meaning ideally emitting and absorbing radiation irrespective of wave length: From this point of view it is really the simplest macroscopic body one could imagine because its internal structure does not matter. In contrast to the hydrogen atom it is described with a continuous spectrum. In fact, there are at least two beautiful examples of Planck-spectra in Nature: the thermal spectrum of the Sun and the cosmic microwave background. The solution to the Planck-spectrum involves quantum mechanics, quantum statistics and relativity, and unites three of the four the great constants of Nature: the Planck-quantum h, the Boltzmann-constant \(k_B\) and the speed of light c.

The spectrum (basically intensity against wavelength or frequency) of the light from the sun (in yellow) and a blackbody with the same temperature (grey). I'm actually surprised by how similar they are.

Limits of the Planck-spectrum

Although criticised at the time by many physicists as phenomenological, the high energy part of the Planck-spectrum is relatively straightforward to understand, as had been realised by Wilhelm Wien: Starting with the result that photons as relativistic particles carry energies proportional to their frequency as well as momenta inversely proportional to their wave length (the constant of proportionality in both cases being the Planck-constant h), imposing isotropy of the photon momenta and assuming a thermal distribution of energies according to Boltzmann leads directly to Wien's result which is an excellent fit at high photon energies but shows discrepancies at low photon energies, implying that at low temperatures the system exhibits quantum behaviour of some type.

How does one measure the mass of a neutrino, using cosmology?

I'm going to tell you how, soon, humanity might measure the masses of neutrinos just by observing past events in the universe. I like this topic because it is one of the few situations in fundamental physics where a measurement of the greater universe might detect something about fundamental particles and/or their interactions, before we manage to measure it in a lab. Another example is the existence of dark matter; however the mass of dark matter will almost certainly be first measured in a lab. Perhaps with neutrinos it will go in the other direction?

What is a neutrino?

I guess that before telling you how to measure a neutrino's mass, it might be pertinent to tell you what a neutrino is and how we can know it has mass before we've measured that mass. Well...

When an atomic nucleus decays, the decay products we see are other nuclei, electrons and/or positrons. These visible products always carry less energy and momentum than the amount that the initial nucleus had. This suggests strongly that some unknown other particle is also being created in the decay and that we just can't see it. This hypothetical particle was dubbed the neutrino and when theories were developed for the force responsible for nuclear decays, the neutrino became an important part of them. And, eventually, neutrinos were detected directly. It took a while because neutrinos interact incredibly weakly, which means you need either a lot of neutrinos or a lot of transparent stuff for the neutrino to interact with (or both) before you will see them.

Initially, it was assumed that neutrinos are massless. They don't need to be massless, but for a long time there was no evidence that they did have mass, so the simplest assumption was that they didn't. There are three types of neutrinos: those emitted in interactions with electrons, those emitted in interactions with muons and those emitted in interactions with tau particles. If neutrinos were massless, then a neutrino emitted as an electron neutrino would always remain an electron neutrino. Similarly, a muon neutrino would always remain a muon neutrino. However, if neutrinos do have mass, then a neutrino emitted in an interaction with an electron will actually travel as a superposition of an electron neutrino, muon neutrino and tau neutrino. The net result being that this neutrino could be detected as a different type of neutrino. Therefore, a smoking gun thing to look for when determining whether neutrinos have mass is this characteristic signal whereby one type of neutrino appears to oscillate into another type of neutrino.

This effect was then seen and seen and seen again. Neutrinos appear to have mass. From the perspective of particle physics this is a bit weird. Neutrinos must have really small masses and it is unclear why these masses are so small. Unfortunately, this mechanism of neutrino oscillations doesn't directly give the masses of the neutrinos. Although, it can be used to measure the differences of the masses of the neutrinos, thus setting lower bounds on the possible masses of the neutrinos.

What has this got to do with cosmology?

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

[Continued from yesterday...]

In the first piece of this post I covered the implications of Planck for the paradigm of inflation. This piece covers the rest.

The anomalies

This is what the CMB would look like in an unphysical Bianchi universe. A worry for our physicality is that this unphysical Bianchi universe seems to fit the data better than a physical \(\Lambda\)CDM universe.

It would be impossible to provide an overview of this conference without mentioning the features and anomalies that Planck has chosen to draw significant attention to. I have a bunch of notes that I've written down that I might one day turn into a new blog post, but I'm not going to delve into them now.

These features and anomalies are clearly going to become a contentious issue in cosmology for the next few years. In fact, the words believer, atheist and agnostic were even being used by speakers during talks regarding whether the anomalies are real or statistical effects. Each time someone declared themselves an anomaly atheist or anomaly agnostic, someone in the audience inevitably spoke up and passionately defended the significance of the questioned anomaly.

The list of potential anomalies is long. There is the cold spot, the anomalously low quadrupole, the hemispherical asymmetry, the statistical difference between the odd and even multipoles at large scales, there is the dipole modulation, there is the general lack of power at large scales, there is the feature in the temperature power spectrum at small scales, the fact that the universe seems to be in an unphysical Bianchi model and there is the "axis of evil" (to name a few).

Pick your side. Atheist, believer or agnostic. The great anomaly wars of cosmology are about to begin (another inevitable consequence of an observational, rather than experimental science, I suppose - i.e. there is only a finite quantity of information available to us, so for some observables we can't just do the experiment again to check who is right).

What should we make of Planck vs SPT and Planck vs the local universe?

The universe as seen by Planck - Day Three (two rumours)

The conference dinner here is about to start (has already started), so I don't have time for a proper post. However, there were some very interesting rumours/revelations today so I'll write them down super-quickly. In increasing order of potential interest (note this post might be a bit technical, I'll explain all of this before the end of the weekend):

The feature at l=1700

A senior Planck figure gave a talk today on the features in the Planck angular power spectrum. Much of his talk was devoted to the apparent feature at \(l\simeq 1700\). In the 15 months worth of data that Planck has used to generate the cosmological results shown in their released papers, the statistical significance of this feature (when any feature is looked for) was \(\sim 3\sigma\). This was with a look elsewhere effect that took into account the possibility of the feature occurring at another \(l\) value.

What he let slip was that, when they analyse this same feature with the full temperature data set, the significance of the feature drops to \(\sim 2\sigma\).

Of course, not too much should be read into this because the additional data isn't quite as well understood as that first 15 months; however, its the same telescope looking at the same sky and foregrounds, so there shouldn't be too many complications. Note that this feature is out of the resolution range of Planck's polarisation capabilities, so the new temperature data is the only additional data we will get in the next data release.

Planck's data analysed on the SPT sky

One of the curiosities of the Planck release was that it seems to give cosmological results that are slightly discrepant with what the South Pole Telescope was giving. If Planck disagrees with BAO or supernovae, or galaxy clusters this is all interesting, but potentially the result of Planck and/or one of those other analyses getting it wrong. However, SPT is another CMB experiment, the fact that Planck and SPT are a bit discrepant is very confusing.

Perhaps SPT made a mistake and the CMB they measured is not the correct CMB?

The obvious way to test this is to analyse the Planck data on the same part of the sky that SPT measured. I overheard a conversation between lead figures in Planck, WMAP and SPT and it seems this is exactly what SPT have done (in unpublished work).

The result is striking.

They found a cosmology that agrees with SPT.

If true, this means that it isn't just Planck and SPT that are slightly discrepant, but different regions of Planck's sky.

What this means cosmologically is unsure. I'll speculate a bit tomorrow.

Power asymmetry

There was quite a bit of excitement over a plot that showed power asymmetry in different directions of the sky. I was going to write about it, but upon reflection, the excitement seems confusing. I'll try to explain the excitement and background before the end of the week.

[The final summary is now available here]

The universe as seen by Planck - Day Two

The cosmic microwave background (CMB) is the best probe we've yet found to study the early universe. The CMB's temperature is very nearly uniform. However this temperature does have very small anisotropies that can be used to study sound waves that existed in the primordial universe. The Planck satellite (an ESA funded experiment) has mapped these temperature anisotropies over the entire sky with the best resolution to date. Last month, Planck released its data and it immediately became the new benchmark for the testing of cosmological models and the measurement of cosmological parameters.

This week ESA is hosting the first conference since Planck released its data. The conference is at ESTEC in the Dutch town of Noordwijk. I am attending this conference and will be doing my best to write updates about what was discussed during the week.You can read my introductory post where I give my motivation for doing this, here.

The CMB is not just useful for studying the primordial universe. As soon as the CMB forms, everywhere in the universe, it travels freely, in every direction, at the speed of light. This means that, in every direction, the CMB we measure here on Earth today has travelled to us from a point billions of light years away. In principle, this makes the CMB not just a really good probe of the state of the universe where and when it was emitted, but also of everything it passed on its way to us.

This secondary use for the CMB turns out to be very useful and many of the highlights from Planck relate to the way in which the CMB interacts on its way to us. The existence of matter in the universe affects the CMB gravitationally. This causes the CMB to bend towards regions of over-density and away from regions of under-density. It also causes the CMB's temperature to shift as it falls into and out of over and under-dense regions. This first effect is known as lensing and one of Planck's most impressive results is a map of the locations of matter in the universe through this lensing effect. The second effect is known as the Sachs-Wolfe effect, something I've written about in some detail.

There is a third way that the CMB is significantly affected by the intervening universe. Within clusters of galaxies there is a lot of hot gas. If the CMB passes through a cluster it can scatter off electrons in this hot gas. The effect of this scattering on the CMB is known as the Sunyaev-Zeldovich (SZ) effect. Therefore, we should be able to use the CMB to detect the lines of sight along which the most massive clusters lie.

We can. And Planck has.

The universe as seen by Planck (conference)

The 47th ESLAB symposium. All the cool kids will either be there, or watching it live on the webcast. Are you one of the cool kids?

This week I will be at a scientific conference, organised by ESA. In ESA's words, this conference is "An international conference dedicated to an in-depth look at the initial scientific results from the Planck mission". The conference is taking place in the small Dutch down of Noordwijk. At this conference there will be many people from within the Planck collaboration, who I'm sure will be delighted to finally be able to talk about their work and many people like myself who have spent the last few years eagerly anticipating the Planck collaboration's results.

The conference will have a live webcast here, you should watch some of it.

I will also be blogging during the conference. My goal is to try to write a new post here each day summarising the most interesting talks and discussions from the conference that day.

Why am I doing this?

An absolutely wonderful image showing how the various all sky images of the CMB anisotropies have improved each decade.

This won't be an easy task. The conference goes quite late each day and many topics will be covered, but I want to do this anyway. To understand why, first go watch my new favourite video on the internet. Brady Haran makes science videos and if you've never seen them, you should go check them out. I felt like Brady was taking the words out of my mind when I saw that video. One day the utopia that Brady and I envisage will exist and a Planck conference like this will be besieged by legions of fans. One auditorium will be fill of fans of non-Gaussianity and fans of Gaussianity, on opposite side, cheering their preference on. Another auditorium will be filled with fans of dark radiation, cheering their team on. Yet another will be filled with fans of the cosmological constant shouting their favourite chants at their mortal enemies, the quintessence crowd. But that day is not today.

Planck: All we need is six numbers to describe the universe

As I'm sure most of the readers of this blog are aware, the Planck data is now out. It turns out I was correct with two out of three of my rumours. I said that the "ISW mystery" was still present, it was. I said that Planck would present ~3\(\sigma\) evidence for non-zero neutrino masses, they did (though, as I suggested in my rumour, only after including information from galaxy clusters Planck has detected). Finally, I said that there would be 2-3\(\sigma\) evidence for some type of "non-Gaussianity", there wasn't. I will duly update my should-I-trust-that-rumour? algorithm in the following way: explicit remarks from Planck members, good rumour; wishful thinking from other theorists, bad rumour.

So what were those results? What big news is there?

The answer is that there isn't anything strikingly new or surprising. I've been trained by years as a theoretical physicist to to dread that sentence and, indeed, many of my colleagues have gone into various states of despair. But, for some reason, I spent the second half of last week in a state of excited wonder. Surprisingly, I loved what I saw on Thursday. It was both stunning and beautiful. This post will be me trying to explain why. (For more details of the actual results see Sesh's post and Peter Woit's list of other blog posts).

The model of cosmology that has been gaining traction over the last decade and a bit is called \(\Lambda\)CDM. This stands for \(\Lambda\) Cold Dark Matter, where the \(\Lambda\) represents the poorly named "dark energy". This model has a few theoretical issues, but it is incredibly simple. What Planck specifically found is that this model fits the CMB (Cosmic Microwave Background) very well and better than any alternative that they tested.

Why I found what Planck saw to be incredible

As I wrote above, Planck's results last Thursday had me in a state of impressed awe. On the day, I couldn't quite put my finger on why, until I read another cosmologist's tweets marvelling at how everything we were seeing could be described by just six parameters. Then it hit me. For once, cosmology had gotten it right. What Planck measured depends on a significant variety of physical phenomena. If the early universe had more matter, or more radiation than we expected, Planck would have seen it. If the primordial density perturbations had been shaped in a significantly different way to that in which we expected, Planck would have seen it.

Cosmology gets a lot of flak from some directions for the so-called "epicycles" of dark matter and dark energy. I can kind of understand this when people see images like this and are told that we "don't understand" 95% of the energy density of the universe. But what is often missed is that we include the effects of dark matter and dark energy in this \(\Lambda\)CDM model with one, single, parameter each. And once those parameters are fixed, the predictions of all of cosmology are too.

With this firmly in mind, take a look at Planck's most important, headline image below. This (sort of) shows the amplitude of the temperature fluctuations in the CMB as a function of their angular scale. Remember, it takes just six numbers to define what that entire curve should look like. Just six.

Following Planck's results today

For the people (new and old) who follow this blog and are interested in the Planck satellite's results, which are being announced today, here is a run-down of important things to know:

• Richard Easther will be live-blogging the data release at this location. If you can't watch the release yourself you should follow Richard's post.
• The first ESA event is a general-audience press conference, very soon, at 10:00 CET, which you can watch here.
• The second ESA event is a press conference aimed at scientists and science journalists, which will stream at the same location, i.e. here. You should watch that even if you aren't a scientist because it will be when all the interesting bits are revealed. If you're confused, you can follow Richard's live-blogging and/or ask questions of scientists on Twitter with the hashtag #askplanck.

• The release of the scientific papers is scheduled for 12:00 CET at the ESA website (I'm not sure of the precise url, maybe here?).

Although I won't be live-blogging the results, I will write a post later today summarising what we've learned and discussing the fall-out arising from all the new information.

Enjoy the day!

Edit: The papers will appear here at 12:00 CET: http://www.sciops.esa.int/index.php?project=planck&page=Planck_Legacy_Archive

Twitter: @just_shaun

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?

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