The existence of early cosmic inflation is one of the big questions in cosmology. It seems to be necessary to explain the uniformity of the cosmic microwave background, but confirming it observationally has been a challenge. Last March, BICEP2 released announced that they had detected evidence of primordial gravitational waves in the polarization of the CMB, which is one prediction of early inflation. This started a firestorm of debate over whether it was an actual detection of inflation, or whether it was simply an effect of light scattering off interstellar dust. Yesterday a new paper was released combining results of BICEP2, Planck, and Keck studies, and it’s being presented in the press as the killer of cosmic inflation. But in reality things aren’t so cut and dry.

BICEP2 and Keck observation surveys (outlines) are where dust effects should be low.
Credit: Caltech Observatory

It should be noted that this new paper hasn’t been peer reviewed, and is even designated as a draft on the paper itself, so it shouldn’t be taken as authoritative just yet. What the paper does is combine the data from these three independent studies to see what the combined results say about cosmic inflation and the question of dust. This is useful because BICEP2 and the Keck Array (both located in Antarctica) look at specific patches of sky where dust contamination is expected to be low. Planck, in contrast, made an all-sky survey including both dusty and non-dusty regions. Combining these very different approaches is a good way to test the validity of the results.

This new paper makes several conclusions. First, it compares the B-mode polarization results of BICEP2 and Keck with those of Planck, and finds that they strongly match. The agreement is at a 7-sigma level, which means the chance of the data not being an actual result is about 1 in 390 trillion. In short, we are absolutely detecting B-mode polarization in the cosmic microwave background. This is great, because it means there’s no debate about whether the data is valid.

There’s a hint of cosmic inflation in the data. Credit: BICEP/Keck

The next question is whether this B-mode polarization is due to primordial gravitational waves from early inflation, light scattering off interstellar dust, or some combination of the two. (There’s actually a third effect due to gravitational lensing, but we understand that effect pretty well.) The primordial gravitational waves are measured in terms of what is known as an r factor, where a larger r means stronger gravitational waves and therefore stronger inflation. The original results from BICEP2 found that r is between 0.15 and 0.27, with the best result being r = 0.2 at a 5-sigma level. Taken by itself, this would seem to be a home-run discovery of cosmic inflation. But BICEP2 didn’t have great data on the distribution of interstellar dust, which Planck has.

In this new combined-data result, the teams find that r is 0.12. So the new results do show evidence of cosmic inflation. However, the confidence of this result is only 2-sigma, or about 95%. This means there’s about a 5% chance of it being a false positive. Given the significance of such a discovery, this isn’t remotely strong enough to declare it valid. What this means is that the BICEP2 results didn’t “discover” cosmic inflation. As it stands, there is evidence of cosmic inflation, but not strong evidence. The data hints at early inflation, but we don’t yet have a smoking gun.

In short, this new paper confirms the validity of the BICEP2 data, but disproves the conclusion drawn from that data. Early inflation has not been disproven, but it hasn’t been proven either. So we’ll have to dust ourselves off and keep looking for an answer one way or the other.

Paper: A Joint Analysis of BICEP2/Keck Array and Planck Data. new.bicepkeck.org/BKP_paper_20150130.pdf

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One of the recent sagas in cosmology began with the BICEP2 press conference announcing evidence of early cosmic inflation. There was some controversy since the press release was held before the paper was peer reviewed. The results were eventually published in Physical Review Letters, though with a more cautious conclusion than the original press release. Now the Planck team has released more of their data. This new work hasn’t yet been peer reviewed, but it doesn’t look good for BICEP2.

As you might recall, BICEP2 analyzed light from the cosmic microwave background (CMB) looking for a type of pattern known as B-mode polarization. This is a pattern of polarized light that (theoretically) is caused by gravitational waves produced by early cosmic inflation. There’s absolutely no doubt that BICEP2 detected B-mode polarization, but that’s only half the challenge. The other half is proving that the B-mode polarization they saw was due to cosmic inflation, and not due to some other process, mainly dust. And therein lies the problem. Dust is fairly common in the Milky Way, and it can also create B-mode polarization. Because the dust is between us and the CMB, it can contaminate its B-mode signal. This is sometimes referred to as the foreground problem. To really prove you have evidence of B-mode polarization in the CMB, you must ensure that you’ve eliminated any foreground effects from your data.

When the BICEP2 results were first announced, the question of dust was immediately raised. Some researchers noted that dust particles caught in magnetic fields could produce stronger B-mode effects than originally thought. Others pointed out that part of the data BICEP2 used to distinguish foreground dust wasn’t very accurate. This is part of the reason the final results went from “We found inflation!” to “We think we’ve found inflation! (But we can’t be certain.)”

Dust effects seen by Planck (shaded region) compared with inflation results of BICEP2 (solid line).
Credit: Planck Collaboration

The new results from Planck chip at that claim even further.  Whereas BICEP2 looked at a specific region of the sky, Planck has been gathering data across the entire sky. This means lots more data that can be used to distinguish foreground dust from a CMB signal. This new paper presented a map of the foreground dust, and a good summary can be seen in the figure. The shaded areas represents the B-mode levels due to dust at different scales. The solid line represents the B-mode distribution due to inflation as seen by BICEP2.  As you can see, it matches the dust signal really well.

The simple conclusion is that the results of BICEP2 have been shown to be dust, but that isn’t quite accurate. It is possible that BICEP2 has found a mixture of dust and inflation signals, and with a better removal of foreground effects there may still be a real result. It is also possible that it’s all dust.

While this seems like bad news, it actually answers a mystery in the BICEP2 results. The level of inflation claimed by BICEP2 was actually quite large. Much larger than expected than many popular models. The fact that a good chuck of the B-mode polarization is due to dust means that inflation can’t be that large. So small inflation models are back in favor. It should also be emphasized that even if the BICEP2 results are shown to be entirely due to dust, that doesn’t mean inflation doesn’t exist. It would simply mean we have no evidence either way.

It’s tempting to look at all this with a bit of schadenfreude. Har, har, the scientists got it wrong again. But a more accurate view would be of two rival sports teams playing an excellent game. BICEP2 almost scored, but Planck rallied an excellent defense. Both teams want to be the first to score, but the other team won’t let them cheat to win. And we get to watch it happen.

Anyone who says science is boring hasn’t been paying attention.

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The BICEP2 paper has officially been accepted in Physical Review Letters. Having survived peer review, does that mean we can now declare that inflation has now been officially observed?  Not necessarily.

You might remember the ongoing saga of the BICEP2 results, from the first press release announcing the detection of B-mode polarization within the cosmic microwave background. There are several sources of this kind of polarization, but one of them is inflation in the early universe. The announcement made headlines everywhere, but also stirred a great deal of controversy. It didn’t take long for “peer review” to start pushing back on the results. First was evidence that interstellar dust could create B-mode polarization in addition to other sources. Then results from the Planck satellite were released with some dust polarization results. Then came news that the BICEP2 team had used some “reverse engineered” data from a Planck result, that some argued invalidated the results.

As I’ve pointed out in earlier posts, these kinds of hard hitting attacks are part of what peer review is all about. The fact that it occurs publicly has more do to with the publicity of science rather than some failure in the process.  But behind all the public drama over the results, there has been the real academic process of peer review.  The BICEP2 team submitted their paper to Physical Review, and the paper has been accepted with some changes.  It has officially survived the process that I often tout as the gold standard of science.

Does that mean the BICEP2 team has won? In a way, they have. Surviving peer review is no small feat, particularly for a controversial result such as early inflation. But the revision of their paper is also significant. In particular, the paper admits that their result might not be a real signal of inflation after all. From the abstract, they state:

However, these models are not sufficiently constrained by external public data to exclude the possibility of dust emission bright enough to explain the entire excess signal.

So they have a real observation of B-mode polarization, and it looks like evidence of cosmic inflation, but they can’t rule out the possibility that their signal is just due to interstellar dust. They also state that more data will be needed to answer that question.

This is what peer review gives us. The BICEP2 team made bold initial claims, and they were taken to task. Much of their initial claims have survived the process, but their bold claim has given way to a more cautious result. They got a paper out of the work, but they also had to eat a bit of crow, and that’s never easy.

But what we get in return is a better understanding of the universe.

Paper: P. A. R. Ade, et al. Detection of B-Mode Polarization at Degree Angular Scales by BICEP2. PRL 112, 241101 (2014).

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Back in March, a project known as BICEP2 held a press conference where they announced the discovery of inflation in the early universe. This created quite a stir in the press. When the announcement was made, the results had just been made public, and their paper had not been peer reviewed.  As everyone started analyzing the work, what initially looked like a pretty strong result started to look less strong.  Then there started to be murmurings that perhaps the announcement had been premature.

The key aspect of the BICEP2 results is the analysis of polarization within the cosmic microwave background (CMB). Specifically there is a type of polarization known as B-mode polarization.  This type of polarization can be caused by gravitational lensing of the CMB by galaxies and the like, but it can also be caused by primordial gravitational waves formed during the earliest moments of the universe.  Distinguishing between these two types of B-modes requires a detailed analysis of the CMB data, which is very tricky to do.  The BICEP2 team ran their analysis, and found a polarization signature too strong to be caused by gravitational lensing alone, thus pointing to inflation as a cause.

But the real challenge is making sure your data is relatively free of foreground effects.  The CMB is the most distant thing we can observe in the universe, and that means everything else is between the CMB and us.  All of that stuff can distort the CMB, which can give you a false positive result.  This is particularly true in the region around the plane of our galaxy, which is sometimes referred to as the zone of avoidance. These foreground effects are the Achilles heel of any inflation result.

Soon after the BICEP2 announcement, some initial results from the Planck telescope mapped our galaxy’s magnetic field through the polarization of gas and dust within the Milky Way.  This is exactly the type of foreground effect that can distort results. Another paper noted that an effect known as radio loops could produce similar B-mode polarization, and it wasn’t clear whether BICEP2 had taken this into account. Later it was found that the BICEP2 team had used some tentative data from Planck by extracting the data from a PDF slide.

Now a couple of new papers (not yet peer reviewed) argue that there are sufficient foreground effects to wash out the results of BICEP2, thus the BICEP2 result neither confirms nor denies the existence of early inflation.  The BICEP2 team argues that even with foreground corrections the results are still strong enough to be valid.

It is important to keep in mind that the BICEP2 results are still going through peer review.  Whether the result holds up or not is an ongoing question. But this is happening very publicly, and it was initiated with a very public announcement that a great discovery had been made.  If it turns out the results don’t pan out, it can give an impression that we really don’t understand the universe, or that this kind of public point-counterpoint is how peer review works.  If that’s the case, how is that any different from the public debates we see on global warming, or the safety and effectiveness of vaccines?

Of course there is a difference between intense discussion over BICEP2, which is an initial discovery of a new effect, and the endless debate over arguments that have been refuted by the evidence again and again. I have in the past compared peer review to whacking a result like a piñata, but there also comes a point where you stop whacking and come to a conclusion.  Both are a part of peer review.

Much of this public drama began because BICEP2 decided to make a public announcement.  Maybe they shouldn’t have.  Maybe the professionals should keep results to themselves until it passes peer review. Only then should results be announced. It would reduce the drama, and maybe we wouldn’t have such skepticism on controversial topics strongly supported by the evidence.

While I can see the benefit of that approach, I’m not sure that I agree. I think there is value in discussing results that are tentative, and in a public discussion of its strengths and weaknesses.  Perhaps we shouldn’t be so eager to have press conferences, but once the paper was released, it was public.  That’s true of most research these days.  In my field preprints appear on the arxiv long before they are published in a journal.  Trying to keep results private would only make it more difficult to peer review work.  I also think there is a responsibility to ensure science is done publicly.  Most of the work in astrophysics is funded through government support.  The general public pays for it, and should have access to not only the results, but also the data supporting it.

That said, I think there is also a responsibility to be honest and thoughtful about scientific discoveries.  Journalists need to move beyond copy-pasta press releases and point-counterpoint simply for the sake of argument.  When results are tentative that should be made clear, and when the data is conclusive, that should be made clear as well.  As scientists we also need to be willing to communicate ideas and results clearly. We can’t simply disseminate our work among our peers and consider the job done.  As a society we’ve moved beyond that.  And as readers we need to be mindful not to feed the hype machines so prevalent among online media. Science is not simply about data and facts. It is a process that pushes us to be better. To be honest and to think critically.

I’m not sure whether the BICEP2 results will hold up or not.  What I can say is that as it whatever the outcome you’ll hear about it here. Because it’s not about the results, it’s about the process.  In the end, that’s what makes science so cool.

ht to Phillip Buckhaults for prompting this post.

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Recently rumors have been flying that the BICEP2 results regarding the cosmic inflationary period may be invalid. It makes for great headline press, but the reality is not quite so sensational. There may be some issues with the BICEP2 results, but that isn’t what the press is excited about. What they are really excited about is how science groups are airing their dirty laundry, publicly. So what’s really going on?

For those who might not remember, BICEP2 is a project working to detect polarized light within the cosmic microwave background (CMB). Specifically they were looking for a type of polarization known as B-mode polarization. Detection of B-mode polarization is important because one mechanism for it is cosmic inflation in the early universe, which is exactly what BICEP2 claimed to have evidence of.

Part of the reason BICEP2 got so much press is because B-mode polarization is particularly difficult to detect. It is a small signal, and you have to filter through a great deal of observational data to be sure that your result is valid.  But you also have to worry about other sources that look like B-mode polarization, and if you don’t account for them properly, then you could get a “false positive.” That’s where this latest drama arises.

In general this challenge is sometimes called the foreground problem.  Basically, the cosmic microwave background is the most distant light we can observe. All the galaxies, dust, interstellar plasma and our own galaxy is between us and the CMB.  So to make sure that the data you gather is really from the CMB, you have to account for all the stuff in the way (the foreground).  We have ways of doing this, but it is difficult. The big challenge is to account for everything.

You might remember a while back I wrote about one foreground effect that BICEP2 didn’t take into account. It involves an effect known as radio loops, where dust particles trapped in interstellar magnetic fields can emit polarized light similar to B-mode polarization. How much of an effect this might have is unclear. Another project being done with the Planck satellite is also looking at this foreground effect, and has released some initial results, but hasn’t yet released the actual data yet.

Now it has come to light that BICEP2 did, in fact, take some of this foreground polarization into account, using results from Planck. But since the raw data hadn’t been released, the team used data taken from a PDF slide of Planck results and basically reverse-engineered the Planck data.  This isn’t ideal, but it works moderately well. Now there is some debate as to whether that slide presented the real foreground polarization or some averaged polarization. If it is the latter, then the BICEP2 results may have underestimated the foreground effect. Does this mean the BICEP2 results are completely invalid? Given what I’ve seen so far, I don’t think it does. Keep in mind that the Planck foreground is one of several foreground effects that BICEP2 did account for. It could be a large error, but it could also be a rather minor one.

Because of all this drama, there are already posts out there claiming that this is evidence of scientists behaving badly, or declaring this kind of thing shows that scientists don’t really know anything. But it is important to keep in mind that the BICEP2 paper is still undergoing peer review.  Critical analysis of the paper is exactly what should happen, and is happening.  This type of dirty laundry used to be confined to the ivory towers, but with social media it now happens in the open.  This is how science is done. BICEP2 has made a bold claim, and now everyone gets to whack at them like a piñata.

Now lets see if their result holds up, or falls apart.

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Last month research project known as BICEP2 announced evidence of inflation within the cosmic microwave background (CMB). Now a new paper argues that a different effect known as a radio loop could produce similar results, which raises the question of whether inflation was detected after all.

The evidence for inflation focused on what is known as B-mode polarization of the CMB.  The B-mode polarization has two causes.  The first is due to gravitational lensing.  The cosmic microwave background we see today has travelled for more than 13 billion years before reaching us.  Along its journey some of it has passed close enough to galaxies and the like to be gravitationally lensed.  This gravitational lensing twists the polarization a bit, giving some of it a B-mode polarization. The second mechanism which is more subtle, and requires more data to analyze. It is due to gravitational waves produced during the inflationary period of the big bang.  What the BICEP2 found was evidence of more B-mode polarization than expected by gravitational lensing alone.  Since there are two ways in which B-mode polarization can occur, they needed to show that this “extra” was not just due to lensing.  Thus, they argued, it must be due to primordial gravitational waves.

Galactic radio loops, with BICEP2 region indicated. Credit: Philipp Mertsch

This new paper looks at another possible source known as radio loops. A radio loop is caused by large magnetic fields that span interstellar space.  When ionized plasma interacts with these magnetic fields, the charged particles spiral along the magnetic field. The spiraling charges then emit radio waves through what is known as synchrotron radiation.  As a result, radio waves are emitted all along the magnetic field loop, hence a “radio loop.”

But the authors point out that these loops can also produce microwave radiation, emitted by charged dust particles within the plasma.  These microwave emissions are in the same wavelength range as the cosmic microwave background.  The emissions are polarized, and their orientation would be similar to the B-mode polarization detected by BICEP2.

Once this “foreground effect” of galactic radio loops is accounted for, there may still be a residual B-mode evidence of inflation.  At this point we can’t be sure because it wasn’t taken into account in the BICEP2 results. What this new paper really points out is a weakness in the BICEP2 results, specifically that it was done at a single wavelength range.

The real test will happen when the Planck team releases their results on B-mode polarization.  The Plank satellite has polarization data at multiple wavelengths.  If these results confirm BICEP2, then we can be sure of an inflationary effect.  If not, then we’ve got more work to do.  And so it goes, loop de loop, back and forth.

This is how science is done.  One team puts forward a result, other teams push back with other ideas, and eventually the best result survives.

Paper: Hao Liu, Philipp Mertsch, and Subir Sarkar. Fingerprints of Galactic Loop I on the Cosmic Microwave Background. arXiv:1404.1899 [astro-ph.CO] (2014).

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