We’re in an exciting and somewhat nerve-racking moment now in which the experimentalists have detected something in the sky that may be a cosmic thunderclap from the origin of the universe — or it may be something far more quotidian (my new favorite word!). For background, please refer to my story in March about the report from the BICEP2 team, and then my follow-up story and a blog item suggesting that the scientists may have seen only an effect created by dust in the galactic foreground.
Last night, a new paper popped up on Arxiv, the physics preprint site (the papers aren’t yet peer-reviewed or published in a journal), and it is sure to be read closely by the members of the community of scientists who study the Cosmic Microwave Background (CMB) radiation. (The CMB is the closest thing we have to a fingerprint of the Big Bang. The CMB saturates the universe and is the residue of the moment when the cosmos became transparent to light, roughly 300,000 years after the primordial, hot, dense universe began to expand and cool.)
The lead author of the new paper is cosmologist Raphael Flauger of the Institute for Advanced Study in Princeton. The gist of the paper is that the BICEP2 results reported in March do not rule out a galactic, rather than cosmological, explanation of the polarization of the CMB. The paper concludes:
We conclude that the predicted level of polarized emission from interstellar dust in this field might leave room for a primordial gravitational wave contribution, but could also be high enough to explain the observed excess B-mode power. Thus, no strong cosmological inference can be drawn at this time.
Let’s talk about what this does and doesn’t mean.
1. It doesn’t mean that BICEP2 didn’t see anything. On the contrary, BICEP2 has taken a long, deep look at the universe and measured a strong polarization of the CMB. A scientist tells me by e-mail: “BICEP2 has made the deepest map of a patch of sky at 150 GHz and has detected a very specific form of polarization. This meant having to measure fluctuations in the radiation at the tens of nano-Kelvin level, and the fact that the BICEP team was able to make these measurements is an incredible achievement.” These maps will be useful for cosmologists no matter how this plays out.
2. If subsequent data bolsters the BICEP2 interpretation of the polarization, this will strengthen the case for the theory of cosmic inflation developed by Alan Guth, Andrei Linde, Paul Steinhardt (who later turned against the theory) and others. This is the idea that the universe underwent a brief, turbo-charged (if you will) inflationary spasm before settling into the more stately expansion we see today. [Update: Guth, Linde, Steinhardt and BICEP2 scientist John Kovac are among those on a panel Friday night at the World Science Festival in NYC; I’m told it will be live-streamed here.] And there are other fascinating theoretical implications, as noted in the Flauger et al paper:
The signal would constitute direct evidence for quantum fluctuations in the spacetime metric. It would provide very strong additional support that a period of cosmic inflation occurred in the early Universe. The inferred amplitude of the tensor modes would provide a measurement of the Hubble rate during inflation … and evidence that the inflation underwent a trans-Planckian excursion in field space. In addition, it would have important implications for axion physics, would essentially exclude cosmologically stable moduli with masses below [10 to the 14 GeV], and would motivate serious consideration of the graviton problem. It would also place a direct bound on the graviton mass …
You don’t have to understand all this to grasp that it’s cosmos-shaking stuff. This is like turning the CMB into a lens to look back to the origin of time and space and measure quantum effects directly — something heretofore the province only of theories scrawled on whiteboards. (And fyi I am suddenly remembering a tequila party at the beach circa 1978 when I underwent a trans-Planckian excursion in field space and then someone loaded me onto the tailgate of a pickup. But that’s another story.)
3. If the signal turns out to be from foreground dust, that will not disprove the inflation theory. It’ll mean that astrophysicists will keep looking. There are at least seven experiments looking for these gravitational waves.
4. If inflation theory comes into disfavor, that will not disprove the theory of the Big Bang, which is on solid observational and theoretical ground.
5. If the signal turns out to be cosmological, at least in part, it will be a huge triumph for the BICEP2 team, but the lesson will linger that when making a dramatic announcement, one should always hedge one’s language to take in the possibility of gremlins in the foreground messing up your data. A good rule would be: The bigger the apparent discovery, the more cautious the announcement.
6. As we say of writers: A scientist is someone for whom science is more difficult than for other people. These CMB experiments are excruciatingly delicate, and in this case required the construction of a telescope at the SOUTH POLE. This one’s hard. No one’s going to get this answer by Googling it.
[Update: Nature has an in-depth article on the Flauger et al paper and another recent paper analyzing the BICEP2 report.]