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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More results from the Planck collaboration are coming in, this time from conference in Italy. There’s been a lot of excitement building up to this new release, particularly given some of the unresolved issues in the standard model of cosmology.

One of these issues is the tension between the earlier WMAP data and the Planck data regarding the average temperature of the universe. Early indications were that they differed by about 1.5%, which isn’t huge, but was concerning given the precision of both data sets. The new analysis finds they agree within 0.3%, which is within the uncertainty range of Planck. So, nothing new or interesting there.

Another result puts further constraints on parameters for dark matter. The new results are precise enough to eliminate some dark matter models. Back in April of this year there was a big announcement that the AMS detector on the space station had detected a possible dark matter signal. The bold claim wasn’t substantiated by the data, and as a result it met with quite a bit of criticism. The new Planck data confirms that the AMS claim was wrong. Again, this is what most of us expected.

The fluctuation data matches theory extremely well.
Credit: ESA – Planck collaboration.

The one interesting thing about the new results regards what wasn’t mentioned. What about BICEP2 and the issue of whether its signal was due to interstellar dust or evidence of early cosmic inflation? So far there’s been no announcement. In fact, in the released graphs on the fluctuations of the cosmic background, and the smallest scale fluctuations aren’t shown. That’s where dust becomes an issue. Just how much of an issue remains to be seen.

But at larger scales the Planck data matches theory astoundingly well. Of course WMAP matched it exceptionally well too. Planck just added a bit more precision to what we already knew. So for now it seems rather ho-hum. Nothing really new to see, no sensational headline to be found.

But I just can’t see it that way. Because this latest data confirms that we understand the origin and evolution of the universe really well. A bunch of humans standing on a rock have come to understand the cosmos. Our theory works, and it keeps working. This is an absolutely astounding achievement.

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This month Astronomy & Astrophysics released 31 articles on data gathered by the Planck satellite. This includes nine all-sky surveys at a range of wavelengths from radio to infrared. It represents the most detailed map of the cosmic sky to date, and already there are some interesting results.

What makes the various sky-surveys useful is that each different wavelength gives a different view of the sky. For example, at the 1 cm range the radio emissions of our Milky Way galaxy tend to dominate. At the 300 micron level is the far infrared, which is where the signal of the infamous cosmic dust is strongest. In the middle range (around the 2 millimeter wavelength) is where the cosmic microwave background dominates.

From all these we can better distinguish the foreground effects (all the stars, galaxies, dust, etc.) from the distant cosmic background itself. This not only gives us a clear image of the cosmic background, it also gives us lots of data on all the foreground stuff. This isn’t just noise to be discarded. It actually helps pin down various cosmological parameters.

One of these parameters is the Hubble constant, which determines the rate of cosmic expansion. When we look at the redshift of galaxies, much of it is due to the cosmic expansion, but some is due to the motion of the galaxies through space (which is different). From redshift alone we can’t tell the difference, but the motion of galaxy clusters distorts the cosmic microwave background through what is known as the SZ effect. Basically, low energy photons from the cosmic background collide with fast moving electrons in a galaxy cluster. The photons are then scattered with a great deal of energy, thus changing their wavelength. We can see this as a shift in the wavelength distribution of the cosmic background in the region of the cluster. By measuring this effect we can distinguish between cosmic redshift and Doppler redshift.

Of course the big goal is the detailed mapping of the cosmic background itself, and this new work looks at that in detail. A few of the papers look at the possibility that the background isn’t quite the same in all directions, something that has been hinted at in earlier data, and seems to still be hinted at in this new data. Then of course there is the issue of early cosmic inflation, which entered the news when BICEP2 announced evidence of its existence, then had to tone down some of its claims. These new results put the strongest constraints on cosmic inflation thus far.

There’s a lot here. Far more than can be covered in a single post. As I work through the papers I’ll have much more to say. But that will have to wait for another day.

Papers: A&A special feature: Planck 2013 results. Astronomy & Astrophysics, volume 571, November 2014.

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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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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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Measuring the magnetic fields of our galaxy poses an interesting challenge.  The galactic magnetic field doesn’t emit or absorb light, and of course we can’t directly measure it at various places like we can for Earth’s magnetic field. The galactic magnetic field does, however, interact strongly with things such as ionized gas and electrically charged dust, so we can indirectly measure the field by the way it affects these things.One way that the field has been measured is by looking at the radio bursts of pulsars.  When pulsars emit a burst of radio waves, they are polarized.  That is, the oscillation of the radio waves has a particular orientation, similar to the way you can shake a jump rope up and down, or side to side.  When a radio wave passes through a region of ionized gas, it causes the gas to vibrate slightly.  But if there is a magnetic field in the region of the gas, the gas can move easily along the magnetic field, but not in other directions.  As a result, the polarization of the radio waves rotates, and by measuring this rotation we can determine the strength of the magnetic field between us and the pulsar.  The advantage of this method is that it is relatively easy to do, but the big disadvantage is that we can only measure the magnetic field along the direction of a pulsar.

Another way is to look at the light emitted by ionized gas as it moves through the magnetic fields. The charged particles of the plasma spiral along the magnetic field causing them emit radio waves through what is known as synchrotron radiation. These are typically in the form of radio waves, and are also polarized. With this method we can determine the magnetic field wherever there is ionized gas (which is pretty much everywhere), but it is a much fainter signal than that of pulsars.  This emission of polarized radio waves by ionized gas also poses a challenge for detecting evidence of inflation in the early universe. You might recall an earlier post talking about how such radio loops due to galactic magnetic fields look very similar to the B-mode polarization observed by BICEP2. Discerning the difference between polarization due to cosmic inflation, and polarization due to magnetic fields is very difficult.

Now new data from the Planck satellite should make that a bit easier. The Planck team has recently released a map of the galactic magnetic field obtained from emitted polarized light.

In addition to ionized gas, there is also a significant amount of dust throughout the galaxy. This dust is typically cold and doesn’t emit much light, but does emit a small amount in radio and microwaves.  Since most dust particles are not spherical, most of the emitted light is polarized along the length of the grain.  If the grains were simply floating randomly, then all that polarization would wash out and we would just see a faint glow from the dust without any orientation. But in the presents of a magnetic field, the grains will tend to align with the magnetic field. As a result, the light from the dust is polarized along the direction of the magnetic field. By measuring this orientation we can determine the direction of the magnetic field. The stronger the magnetic field, the more strongly the dust will align with the field, so the strength of the polarization also tells us the strength of the magnetic field.

Planck can measure the light emitted by these dust grains very precisely, and the result can be seen in the image. It is the most precise map of our galaxy’s magnetic field ever obtained.  This is a significant achievement in itself, but this map will also allow astronomers to take into account the effect of radio loops on B-mode polarization. With this map we can better filter out galactic effects when looking at the cosmic microwave background. So later if Planck observes B-mode polarization similar to BICEP2, we can be sure that it really is due to inflation.

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Almost fourteen billion years ago, when the universe was only ten human heartbeats old, light appeared in the universe.   The young universe was dominated by dense matter, so at first the light was constantly scattered and absorbed.  The universe was so dense with charged particles that the light could never travel far.  After about 380,000 years the universe cooled to the point that the charged particles (by then mostly nuclei and electrons) united to form atoms of hydrogen and helium, the first atoms of the universe.  With this union light was no longer heavily scattered and absorbed, and so this earliest light of the universe began a journey across the cosmos.

Galaxies formed.  The first stars were born.  Many burned brightly for a while and then exploded as violent supernovae.  New stars and planets rose from the ashes of dead stars.  On at least one world life appeared, and after a time looked up into the vast sea of stars.  Beyond the sea of stars the first light still glows dimly.  Once as bright as the surface of a sun, it has reddened and dimmed due to the expansion of the universe so that it is now just a faint glow of microwaves.  And yet that faint glow still carries the imprint of the first cosmic spark from which it rose.

Humans first observed this cosmic microwave background (CMB) in the 1960s.  Early observations demonstrated the CMB had a temperature of just 3 Kelvin, which demonstrated the origin of the universe from a “big bang”.  By the 1990s the COBE satellite observes that the CMB is not uniform, but has small temperature fluctuations.  Thus we learn that the early universe had structure.  In 2001 a more sophisticated probe known as WMAP begins a 9 year survey of the CMB.  From its data we could confirm a universe about 13.8 billion years old.  An expanding universe consisting of about 5% regular matter,  24% dark matter, and 71% dark energy (as I’ve written about before).  Then in 2009 a new probe named Planck was launched.  This probe examines the CMB at a much higher resolution than WMAP did.  You can see the difference in the image below.

Comparison of WMAP and Planck results. Credit: WMAP/Planck

Last year the first results from Planck were announced.

1. The universe is 13.82 billion years old (a little bit older than we thought).
2. For the mass/energy of the universe, 4.9% is regular matter (stars, planets, and us), 26.9% is dark matter, and 68.3% is dark energy (a bit more regular and dark matter, and a bit less dark energy than we thought).
3. The universe is expanding, with a Hubble constant of 67.3 km/s/Mpc (a bit more slowly than we thought).

For generations humans looked up at the night sky and had only questions.  Now we can look up at the night sky and know.  Not just what stars and planets are, but the whole sky, the universe in its entirety.   And yet there are still more questions to be asked.  Within this new data is evidence of a “lopsided” nature to the universe.  The CMB has a subtle asymmetry to it.  There’s a new kind of structure to the universe that we don’t entirely understand.

We now know so much about our universe, and yet we still look at the sky and wonder.

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