Polarization measurements from Planck superimposed on CMB temperature anisotropies. From the Planckoscope, h/t Bob McNees and Raquel Ribeiro.
The good news is: we understand the current universe pretty darn well! So much so, in fact, that even an amazingly high-precision instrument such as Planck has a hard time discovering truly new and surprising things about cosmology. Hence, the Planck press releases chose to highlight the finding that the earliest stars formed about 0.1 billion years later than had previously been thought. Which is an awesome piece of science, but doesn’t quite rise to the level of excitement that other possible discoveries might have reached.
Power spectrum of CMB temperature fluctuations, from Planck. Now that is some agreement between theory and experiment!
For example, the possibility that we had seen primordial gravitational waves from inflation, as the original announcement of the BICEP2 results suggested back in March. If you’ll remember, the polarization of the CMB can be mathematically decomposed into “E-modes,” which look like gradients and arise naturally from the perturbations in density that we all know and love, and “B-modes,” which look like curls and are not produced (in substantial amounts) from density perturbations. They could be produced by gravitational waves, which in turn could be generated during cosmic inflation — so finding them is a very big deal, indeed.
A big deal that apparently hasn’t happened. As has been suspected for a while now, while BICEP2 did detect B-modes, they seem to have been generated by dust in our galaxy, rather than by gravitational waves during inflation. That is the pretty definitive conclusion from the new Planck/BICEP2/Keck joint analysis.
And therefore, what we had hoped was a detection of primordial gravitational waves now turns into a less-thrilling (but equally scientifically crucial) upper limit. Here’s one way of looking at the situation now. On the horizontal axis we have ns, the “tilt” in the power spectrum of perturbations, i.e. the variation in the amplitude of those perturbations on different distances across space. And on the vertical axis we have r, the ratio of the gravitational waves to the ordinary density perturbations. The original BICEP2 interpretation was that we had discovered r = 0.2; now we see that r is less than 0.15, probably less than 0.10, depending on which pieces of information you combine to get your constraint. No sign that it’s anything other than zero.
Current constraints on the “tilt” of the primordial perturbations (horizontal axis) and the contribution from gravitational waves (vertical axis).
So what have we learned? Here are some take-away messages.
BICEP2 was an amazingly successful experiment, performed by some incredibly talented and dedicated scientists. They measured B-modes in the sky at an unprecedented level of precision! Nothing in the new results overturns this.
What they got wrong was the interpretation. Even there, the original paper they submitted is mostly careful, focusing on the detection and putting the inflationary stuff at the end. And in the revised version they were very conservative indeed. In retrospect they were too stingy in imagining how much contamination there could be from dust, which is certainly a hard problem. For good reasons, they intentionally looked at a part of the sky where there is very little dust, and they incorrectly extrapolated known measurements down to this unknown territory. Which is still, really, not such a big deal; everyone makes mistakes, and in this case the existing data and state of the art were just not that clear.
More problematic was the public announcement of the results. The team placed a heavy emphasis on the inflationary interpretation, even inviting Alan Guth and Andrei Linde to the press conference. In retrospect, this was a misjudgment. If their B-modes really were evidence for inflation, that would have become established over time, and they would have gotten all the credit. The gold standard of scientific caution comes from CMB pioneers Arno Penzias and Robert Wilson, who after discovering the CMB itself wrote a paper titled simply “A Measurement of Excess Antenna Temperature at 4080 Mc/s.” We should all aspire to being so careful! But most of us mere human beings will occasionally get excited, especially when we potentially discovered something new and amazing about the universe.
We’re still looking for those primordial B-modes! The hope is to improve limits (or detections…) to at least r ~ 0.01, and potentially r ~ 0.001, over the next few years. Stay tuned.
Inflation is not ruled out! The BICEP2 figure of r = 0.2 was really at the absolute upper reach of what anyone was predicting from realistic inflationary models. There is plenty of room for inflation to still be right, but the value of r to be so small that we haven’t yet seen it.
But we don’t know whether inflation is correct! The strongest predictions of inflation — spatial flatness, scale-free primordial density perturbations — have an annoyingly generic quality. You might expect them to arise in some non-inflationary scenario for the very early universe, even if we don’t have many good candidates for what such a scenario might be. The small tilt of the spectrum (characterized by the fact that ns in the figure above is less than, rather than equal to, one) is a nice feature that you would expect in inflation, so that adds a bit of support. But the gravitational waves would have been very strong support. In their absence, I think there’s still lots of room for clever people to come up with good alternatives to inflation. As I said on Twitter:
Further in that direction, keep in mind how little we truly understand about the extremely early universe. Had BICEP2’s B-modes actually been primordial, it would have represented a fantastic leap in the energy scales to which we have observational access. The converse is just as important: in the absence of empirical data, we should be extremely cautious in extrapolating our current knowledge (or projecting our favorite theories) into realms that are so far away from the part of the world we’ve actually looked at.
At the end of the day: good science, slight bobble with the publicity, corrected before too long by good science again. I’m certainly disappointed that we haven’t yet obtained any direct empirical information about the first trillionth of a trillionth of a trillionth of a second after the Big Bang, but I’m willing to hang in there and see what the next round of experiments (and theories) will bring.