Sean Carroll

Arrgh, I have really wanted to hop back on the blogging bandwagon, but this travel/work reality has made it tough. Next week, though, I plan to be blogging like a banshee. If banshees could blog. And if, when they did blog, they did so frequently and with enthusiasm.

Just got back from the North Carolina Science Festival in Charlotte, where I talked about the Higgs boson. You can find some live-tweeting of the event by searching the hashtag #HiggsTalk. Among all the deep and inspirational points I tried to make, one seemed to create the biggest impression: frogs can see individual photons.

This is an example I got from David Deutsch’s book The Fabric of Reality. It’s an attempt to connect our underlying fundamental description of the world, which is in terms of fields, to what we see when we make a quantum observation, which is in terms of particles — at least if we look closely enough. Deutsch’s point is that human vision is a bit too crude to detect just one photon at a time, but frogs (and presumably other animals) are sensitive enough to see single photons.

Such a fun and quirky fact naturally raised the skeptical instincts of some folks in the room, and to the internet they all went. Is it actually true?

Turns out, to the extent that a few minutes of googling around can reveal, it’s not an easy question to answer. It’s certainly true that the photoreceptors in a frog’s eye are sensitive enough to trigger on individual photons — indeed, researchers are using frog’s eyes to help fashion hybrid light-detector technology. But on the other hand, human photoreceptors are also sensitive enough to trigger on individual photons — and yet, we don’t as a matter of fact actually see photons one at a time. The presumption is that we would be seeing too much noise if our brains actually responded to such low levels of light; in practice, it seems to take several dozen photons before a human will say they see something.

So maybe the same is true for frogs? I wasn’t able to find a definitive-sounding word on the subject, but there is good reason to believe that frogs are at least much more sensitive than we are. The point is that noise we filter out is roughly proportional to body temperature. In a warm-blooded creature, simple thermal motions are constantly jostling the rhodopsin molecules in the eye, which could mimic the act of seeing something. A cold-blooded frog isn’t as susceptible to this problem, so its vision can be usefully much more efficient at low light levels.

Of course none of this matters to the actual point being made in my lecture, which is that light is really a vibration in the electromagnetic field, but careful observations (be they by frogs or artificial photodetectors) reveal individual energy packets call photons. It’s not that the field is “made of” photons, it’s that what we see when we perform measurements in a world governed by quantum mechanics is different from what the world is “actually made of,” to the extent that it’s okay to think about such a concept. Which, with all due respect to my croaky amphibious friends, is more amazing to me than all the eyeballs in the world.

The Planck satellite, a European cosmic microwave background observatory, was launched in 2009 and is finally ready to release its first set of cosmology results. (It has already released findings on galaxies and dust and so forth — what early-universe cosmologists call “foregrounds” and others call “my life’s work.”) They will be showing us the highest-precision all-sky map of the microwave background ever made. The announcement starts at 10 a.m. Paris time, which works out to 2 a.m. Los Angeles time. Don’t expect me to be live-blogging.

So what should we be looking for? Typically an experiment like this isn’t just a fishing expedition; scientists have a pretty clear idea of what questions they would like answered, and what discoveries they might be able to make. Nature is always capable of surprising us, of course. There are some very useful posts on this question by Renee Hlozek and Shaun Hotchkiss. (I hope everyone reading those posts will take a moment to appreciate how wonderful it is that we live in an era where real experts can chime in directly on important scientific questions.)

A CMB map contains an enormous amount of information, especially if you are measuring the polarization as well as the temperature at each point. My understanding is that this edition of the Planck results will not include polarization, but that will be coming some day down the road. (And Max Tegmark’s $100 is safe for another few months.) Nevertheless, a lot of the interesting information boils down to the “power spectrum,” which tells us how strongly the temperature varies on different angular scales. Of course, there are a few observables that go beyond the power spectrum, and those are some of the most interesting ones.

Here are some of the major things cosmologists might want to learn from the CMB temperature anisotropies:

• Did the original perturbations we inherited from the early universe have the same amplitude on all scales, or were the slightly different?
• What are the best fits for cosmological parameters such as the density of dark matter and dark energy, the numbers and masses of neutrinos, and the Hubble constant? Or even spatial curvature?
• Are there persistent “anomalies” that can’t be easily accounted for by a simple theory of primordial perturbations? For example, do the anisotropies somehow define a preferred axis in space?
• Are the perturbations completely random — “Gaussian” — or are there hints of primordial non-Gaussianity, which might help pin down specific models of inflation?

I suspect it would be wise to keep expectations low for Earth-shattering (or universe-shattering) discoveries here, although I’d certainly welcome a surprise. The amplitude of the primordial perturbations has already been nailed down fairly well, by the Atacama Cosmology Telescope as well as by the South Pole Telescope that I blogged about. From Renee’s post, here is a graph of the data from the WMAP satellite as well as ACT and SPT, which as you can see are pretty compatible with each other as well as with the theoretical prediction. We might get a more definite finding that the amplitudes aren’t strictly the same at all scales, which would be good news for proponents of inflation.

We definitely hope to get more precise measurements of cosmological parameters, especially the number of neutrino species and their masses. Evidence from particle physics experiments here on the ground is inconclusive when it comes to the number of neutrino species — very recent results from the MiniBooNE experiment seem to point in the direction of sterile neutrinos that don’t feel the weak interactions. If such neutrinos are produced in the early universe, they could have an effect on the CMB anisotropies. Obviously any definitive statement that there were more than three kinds of neutrinos would be huge news. The other hope for groundbreaking news would be the discovery of nonzero spatial curvature, but nobody really expects that.

As far as anomalies are concerned, Planck has a very different scanning strategy than WMAP had, so it’s possible that it will squelch some people’s favorite anomalies. But there is the problem of cosmic variance (in the original sense) — on very large scales, there is a limited number of modes we can measure, since we only get one universe. If large-scale fluctuations just happen to be statistically anomalous, it might be very difficult to ever decide whether it’s an accident or the sign of new physics.

The search for non-Gaussianities (correlations between fluctuations on different scales) is possibly the most interesting thing we should be looking at in the current release. If inflation is right, you may or may not see deviations from perfectly Gaussian behavior, depending on the kind of inflation we’re talking about. Roughly speaking, we expect perturbations to be Gaussian in simple models of inflation with ordinary dynamics of a single scalar field, but adding bells and whistles to your inflationary model can introduce some non-Gaussianities. So it’s not really evidence for or against inflation, but limits the model space if inflation is the right answer.

Let’s offer early congratulations to the Planck team, who have certainly worked incredibly hard to get to this point.