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.

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!

"A major discovery", BICEP2 and B-modes

[Added note (on Monday): Well, wow, the rumours were, if anything, understated. I'm happy to go on record that, unless a mistake has been made, this is the greatest scientific discovery of the 21st century, and may remain so even once the century is over. I (and others) will write many more detailed summaries of what was observed over time, but BICEP2 have announced a discovery of primordial B-modes, which is extremely strong evidence of cosmological inflation (if it turns out to be scale invariant, inflation is as true as most accepted science). Matt Strassler has a good hastily written summary here. As does Liam McAllister at Lubos Motl's blog, here. Of course, this is just one experiment and maybe they've made a mistake, but the results look very robust at the moment.

Congratulations on being alive today readers! We just learned about how particles work at energies \(10^{13}\) times greater than even the LHC can probe, and about what was happening at a time much, much less than a nanosecond after the beginning of the Big Bang.]

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[Added note (on Sunday): It seems highly probable that these rumours are essentially true. Although the precise details of the results aren't yet public, the BICEP2 PI, John Kovac, has sent a widely distributed email with the following information: Data and scientific papers with results from the BICEP2 experiment will go public and be viewable here at 2:45pm GMT on Monday. At the same time a technical webcast will begin at this address.

It's going to be an exciting day!]

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The cosmology rumour mill exploded today. Harvard Astrophysics have issued a press release stating that, on Monday, they will announce a "major discovery".

This is the only hard-evidence of anything interesting on the way and it could be an announcement of anything that fits under the label of "astrophysics". This is important to keep in mind. However, for one reason or another (that is hard to nail down), cosmologists are suggesting that it is going to be about cosmology. The speculation is that it will be about the BICEP2 experiment, which has been measuring the polarisation in the CMB. The speculation is that BICEP2 have seen primordial "B-mode" polarisation.

If this speculation is true, this would be a result immense in its significance.

Primordial B-modes would be a smoking gun signal of primordial gravitational waves. This, alone, makes such a discovery important. Gravitational waves have not yet been observed, but are a prediction from general relativity. Therefore, such a discovery would be on the same level of significance as the discovery of the Higgs particle. We were almost certain it would be there, but it is good to finally see it.

However, the potential significance of such a result goes further because these primordial gravitational waves would need a source. The theory of cosmological inflation would/could be such a source. Inflation is a compelling theory, not without some problems, for how the universe evolved in its very earliest stages. If it occurred when the universe had a large enough temperature, it would generate primordial gravitational waves large enough to tickle the CMB enough to make these B-modes visible in the polarisation. As of yet, inflation has passed quite a few observational tests, but nothing has been seen that could be described as smoking gun evidence. A spectrum of primordial gravitational waves would very nearly be such a smoking gun. If the spectrum was scale invariant (i.e. if the gravitational waves have the same amplitude on all distance scales) that would be a smoking gun for inflation and accolades, Nobel Prizes, etc, etc, would flow accordingly.

All of this is just speculation, but some of it does seem to be coming from reputable sources. And some of my colleagues have been talking about tip-offs from people who wish to remain anonymous, so I figured I'd collect all the speculation I know of here in a post (let me know if I've missed anything):

• Richard Easther, on the rumour and its implications
• Bruce Bassett, on the probability that the rumours are true
• The Guardian (the first major news source to pick up on this) with comments from various prominent cosmologists
• Jester/Resonaances on the context (i.e. earlier constraints on primordial B modes)
• Lubos Motl, amongst other things, explains what B mode polarisation actually is
• Philip Gibbs, at viXra log
• Peter Coles, amongst other things, on why gravitational waves mean there should be B-modes in the CMB polarisation
• Sesh Nadathur on why, amongst other things, the rumoured measurement would appear to be in tension with results from Planck and WMAP

• Sean Carroll has written a very thorough overview of the implications for cosmology (if the rumours are true).

The PI of BICEP2, John Kovac, gave a talk at the annual COSMO conference last year that had some pretty ambitious claims for how sensitive BICEP2 and similar experiments were going to be, so... well... we'll know on Monday. It should also be noted that, although the existence of these gravitational waves is a prediction of inflation, their amplitude is a free parameter and an amplitude this big is potentially a little surprising (for me, lower temperature inflation models just seem more compelling, others might disagree).

Twitter: @just_shaun

[Edit: The video of John Kovac's talk can be found here]