I like clean, simple arguments. I like
inescapable logical conclusions. And when those things appear in a paper
telling us something new about the very early universe – even better.
Today, David Parkinson, a research fellow
at the University of Queensland, came to Melbourne to give a talk on the paper
he just posted to the arxiv today with his colleagues Marina Cortes and Andrew Liddle.
It was incredibly (and not coincidentally) well timed. Just two days ago, I
spend most of the day grappling with the implications of the latest cosmic
microwave background results from the Planck satellite, and now David was going
to come and give a new perspective on it.
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| Marina Cortes |
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| Andrew Liddle |
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| David Parkinson |
But let me back up. In March, researchers
with a telescope at the South Pole – the BICEP2 collaboration –
announced an astonishing new result. They were studying the cosmic microwave background (CMB,
the afterglow of the Big Bang), and they presented what they said was the first
evidence of primordial
gravitational waves – ripples in the fabric of spacetime, from the first
tiny fraction of a second after the Big Bang. The announcement made a HUGE
splash. There were articles in all the major
newspapers
and magazines
hailing the discovery as the start of a new
era in cosmology, and in some cases even as proof of cosmic
inflation. This is a scenario in which the early universe, in the first
billionth of a billionth of a billionth of a billionth of a second after the
moment of creation, expanded extremely rapidly and grew several orders of
magnitude in size. The theory of inflation has been around since the 1980s,
first proposed by Alan Guth, and developed by Andre Linde, Paul Steinhardt, and
others. One of the key predictions of inflation is that it would produce gravitational
waves, and in principle these could be seen as little
swirls in the pattern of polarization of the CMB. The CMB is one of the
strongest pieces of evidence that the Big Bang happened at all, and by studying
its light we can learn a huge amount about what the early universe looked like.
Some of that light is polarized, meaning it is preferentially oriented one way
or another when it reaches the detector. Patterns in the polarization can show
us traces of those early spacetime ripples. Although many experiments had been
looking for these patterns, BICEP2’s signal was unexpectedly strong, and it was
in some tension with previous tensor measurements by the Planck satellite, among others. A viral video went around
showing a flabbergasted Andre Linde receiving the news, and even he, a vocal
supporter of inflation theory, looked shocked.
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| Characteristic swirls in cosmic microwave background polarization, found by BICEP2. Image credit: BICEP2 Collaboration. |
It wasn’t long after BICEP2’s announcement,
though, that problems appeared. Rumors
went around saying that the BICEP2 team had made a mistake in their calculations.
The problem was interstellar stardust. It turns out that dust
can create polarization too, and although the BICEP2 team considered a few
different possibilities for the level of contribution of dust to their signal, several
cosmologists
argued that the estimates were way too low. Two papers came out showing that the
BICEP2 signal – the one that was supposed to be a beautiful picture of
gravitational waves – could have been entirely due to dust in our Galaxy
mimicking the primordial signal.
More articles
appeared, now announcing that the “Big Bang result” has “turned
to dust” (among other clever puns). Paul Steinhardt, who has spent the last
several years developing alternatives to inflation theory, wrote an article
proclaiming that inflation was never a good idea to begin with, and the dust
problems go to show that the hype was all for nothing. Most of the
cosmology community, however, took the attitude that we should probably just wait
and see. There were several other experiments taking data to confirm or
rule out BICEP2’s discovery, and the Planck satellite – the current flagship in
the CMB detection game – would be producing maps of interstellar dust really
soon. That should clear everything up.
Two days ago,
Planck released their dust
polarization results. They specifically addressed the BICEP2 study, and
while they were very measured in their statements (pointing to an upcoming joint
analysis), the upshot of the work was that the dust polarization signal was so
high that it could easily account for everything BICEP2 saw. Maybe the
gravitational waves are there, but if Planck is right about the amount of dust
in the way, there’s really no way to say that BICEP2 actually discovered them.
In physics, a discovery means you’ve shown something to be the case beyond
any reasonable (statistical) doubt. Usually that comes in the form of a
statement of how incredibly unlikely it is that chance or some spurious signal
could have given you the same result. A signal that could just as easily be all
dust is definitely not a discovery.
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| Comparison of original BICEP2 result (left) and Planck dust polarization result (right). The circled region in each shows where the primordial gravitational wave signal is expected to show up for the model supported by BICEP2's result. The colored lines in the BICEP2 figure are their dust models, all well below the signal. The blue boxes in the Planck figure are their estimates for the dust amplitude, and the solid line is where the gravitational wave signal should occur. You can see that the dust amplitude is comparable to the expected gravitational wave signal amplitude, suggesting the two could not be distinguished. Image credits: BICEP2 Collaboration, Planck Collaboration. |
This all brings
us to David’s paper. The details are technical, but David and his colleagues
basically go back to the drawing board to determine how we can analyze data to
get the best, most unbiased estimate of the gravitational wave signal. They
re-analyze the BICEP2 polarization signal, under a couple of different
assumptions, using Planck's previous limits on the gravitational wave contribution as a starting point. First, they assume there was no dust contamination at all. Then
they look at an “optimistic” dust model, where dust contamination is there but
not bad enough to drown out the signal, and a “pessimistic” dust model, where
dust can account for everything. They look at not just the level of primordial
gravitational waves – also known as tensor
modes – but also the “tilt” of the tensor mode spectrum, an important
parameter in inflationary models.
What they find is
striking. In the “optimistic” and dust-free models, they find tensor modes,
just as BICEP2 did, but they also find a tilt that is utterly incompatible with standard models of inflation. Basically,
if BICEP2 and Planck’s previous measurements are correct, and the dust is at a
manageable level, BICEP2 not only doesn’t prove inflation – it just about rules
it out! The only other option is to use the “pessimistic” dust model, in which
case BICEP2 discovered nothing. As it happens, Planck’s new measurements fit
the pessimistic dust model best.
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| Results from Cortes, Liddle & Parkinson paper. The data points are the BICEP2 results, and the circled region can be compared with the regions in the figure above. The black lines are the expected level of the signal for dust plus tensor modes plus the contribution from gravitational lensing. The green line is the tensor contribution -- this can be directly compared to the dashed red line in the BICEP2 figure above. You can see the tilt in the spectrum in the way the green tensor line extends up and to the right in the figure. Image credit: Cortes, Liddle & Parkinson 2014. |
In any case, the
implication is clear, and somewhat unsettling. It presents us with three items
– Planck’s previous tensor limits, BICEP2’s gravitational wave signal, and the inflationary
model – and it says we can pick two.
At least one has to be incorrect.
That’s a bold
statement, and a big deal if it holds up. I love the irony in the suggestion
that keeping the “inflation-proving” result requires disproving inflation. But
it also illustrates the danger of jumping the gun in these kinds of complicated
data analyses. It’s widely believed that BICEP2 made too strong a statement in
their original paper and press-release, both in their optimism about dust
foregrounds and in their statement of confidence in the signal. Now it appears
that their analysis may also have introduced a bias that hid the implications
for the tensor tilt.
To know with any
degree of certainty what the BICEP2 result really means, we’ll have to wait
for a joint analysis being carried out by the BICEP2 and Planck teams in
collaboration, and we’ll have to see what the other experiments find. But it’s
certainly an exciting time, and, as always, it’s fascinating to see the scientific
process in action.
Footnote: My PhD
thesis was partially based on a study with a similar sort of gist – that you
can have two of three theories, but not all of them together. In my case, the
theories were axion dark matter, string theory, and inflation. If you’re really
curious, you can find the paper here.
