And the CMB spoke unto WMAP
On Thursday afternoon, the WMAP team released its latest data about the origin and fate of the universe. For readers with social lives, WMAP is the Wilkinson Microwave Anisotropy Probe, which was launched in 2001 and cost $150 million. While that’s less than a third the cost of a single Space Shuttle launch, keep in mind that WMAP has taught us next to nothing about the effects of weightlessness on snails, toads, or even fish. Its sole mission is to study nerdy, technical things like what the universe is made of and whether it’s finite or infinite.
I was at Perimeter Institute on Thurday morning, and people there were awaiting the data as if (har, har) the fate of the universe depended on it. I especially enjoyed chatting with Justin Khoury, a cosmologist who studies the “ekpyrotic scenario.” What is the ekpyrotic scenario? Well, three things I know about it are that
- it posits that our universe is a 4-dimensional “brane” embedded in a 5-dimensional manifold, and that the Big Bang was caused by a different brane slamming into ours 13.6 billion years ago,
- it doesn’t say where the branes or the manifold came from originally, and
- it was co-invented by the father of my former MathCamp roommate.
Like its chief rival — Alan Guth’s inflationary cosmology — the ekpyrotic scenario predicts the fluctuations in the cosmic microwave background that WMAP (as well as its predecessor COBE) observed. But inflation also predicts long-wavelength gravity waves, while the ekpyrotic scenario doesn’t. There was a tiny chance that Thursday’s WMAP release would show evidence of such waves — in which case the ekpyrotic scenario would be killed (or in technical terms, “braned”).
As it turns out, though, the latest results mostly confirm what we already thought, albeit with better precision. The observable universe looks to be 4% “normal stuff” (mostly intergalactic baryons, but also free AOL trial CD’s), 22% cold dark matter, and 74% dark energy. There’s no doubt at all that the dark energy is there, and that it will continue pulling the universe apart (so if you want to visit a different galactic supercluster, leave now). The “scalar spectral index” seems to be slightly less than 1, which is apparently is what you’d expect if inflation were true. Also, space continues to look pretty flat — but then again, the Earth also looks pretty flat, even from the window of a commercial airliner. At least we can say that, if space has a nontrivial curvature, then the radius is a lot bigger than the 14 billion light-years we can see.
(Note that it’s logically possible for space to be finite — that is, to “loop back on itself” — despite having zero curvature. In that case, the universe would be like one of those arcade games where, when your spaceship goes off the edge of the screen, it reappears on the other edge. The questions of the geometry and topology of space are related but different.)
What general conclusions can we draw from all this?
First, that we theoretical computer scientists really ought to get ourselves one of these space probes — one that can peer directly into the face of God and report back to us on whether P=BPP, whether BQP is in AM, and so on. What the physicists do feels like cheating to me, like peeking at the answers in the back of the book. (When I griped about this to Lee Smolin, he offered the following consolation: “At least when you guys answer a question, it stays answered.”)
Second, that space is where the excitement is in fundamental physics these days. If you don’t believe me, look at these awesome slides by John Baez (as well as this from Baez and this from Lee Smolin). Baez points out that, of the three big discoveries of the past 25 years — dark matter, dark energy, and neutrino mass — all three came from astronomy (not from particle accelerators), and not one was predicted by theorists (who’ve been busily trying to explain them post hoc). From my outsider perspective, it seems clear that the astrophysicists have some sort of unfair advantage here, and that the only way to rectify the situation is to cut NASA’s space science budget. Fortunately, that’s exactly what W. has done.
The third conclusion is that it’s time for a new religion: one that would celebrate the release of new CMB data as an event roughly analogous to Moses descending from Sinai with new tablets in hand, and that would regard the Space Shuttle as a blasphemy, an orbiting golden calf. Seriously — am I the only person who sees measuring the CMB fluctuations as a religious obligation?
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Comment #1 March 18th, 2006 at 4:32 pm
— am I the only person who sees measuring the CMB fluctuations as a religious obligation?
No
Comment #2 March 18th, 2006 at 5:50 pm
I see it as far more important than a religious obligation.
Comment #3 March 18th, 2006 at 9:26 pm
and not one was predicted by theorists
I object!
What do you mean by “predicted”? Certainly I know and we knew decades ago many ways to “predict” a neutrino mass, to “predict” a cosmological constant, and I’m full of ideas of candidates for dark matter. I mean, realy, Einstein “predicted” dark energy years ago. (and then, of course, he “unpredicted” it, BTW!)
The words you’re looking for are not “predicted” but “precalculated” or something like that. The problem is not the existence of the three, the problem is why they have the values they do. And even then, the standard model has all sorts of things which aren’t “predicted”, they are put in by hand.
But I do agree with you that the most interesting new experimental data has come from astrophysics. Too bad I took all those astro courses at Berkeley and then decided to do quantum computing. I can still remember doing the calculations for the angular spectra of the background radiation and finding: if it bumps this way then this is ruled out, if it bumps that way then this other theory is ruled out. Very very cool stuff.
Also, calling the discovery of neutrino mass, astronomy is a bit of a stretch. Sure a big tub of water is a telescope, but one with a really bad angular resolution.
Comment #4 March 18th, 2006 at 9:37 pm
Dave: You object, but then your actual text seems to support my (and Baez’s) thesis. On the basis of what was known in the 70’s, would one have expected neutrino mass, dark matter, or dark energy? If not, then that’s what I meant. 🙂
BTW, the connection of neutrino mass to astronomy is that the neutrinos come from the Sun, which is in outer space.
Comment #5 March 18th, 2006 at 10:31 pm
As everybody knows, the reason that there were no predictions was the fact that physicists had finally found a Theory of Everything by the end of the 1980s.
(And of course this is the point John Baez is making.)
Comment #6 March 19th, 2006 at 10:55 am
Scott, I have this vague recollection that there was an experiment which had a particle accelerator at one side of the planet shoot a bunch of neutrinos straight at a neutrino detector at the other side of the planet, and this verified observations of neutrinos coming from the sun in a significantly more controlled manner. Doesn’t that give particle physics at least partial credit for the discovery?
Comment #7 March 19th, 2006 at 1:01 pm
The reason is that astrophysicists keep looking at the data, while hep left to look time ago, sure as they are that every useful date was already extracted in the 1970.
For an example, nobody notices that the decay rate of Z0 has a value scaled (with the cube of mass, as any electromagnetic decay) from the decay rate of pion.
(Or that 3/8 is the value of sin weinberg that minimises the decay rate of Z0. Or that the mass quotients of muon and W+ are very close to (g-2)… some of these things are surely random coincidences, some others could be exploited in model building, but they are not, because data is dirty)
Comment #8 March 19th, 2006 at 6:53 pm
For an example, nobody notices that the decay rate of Z0 has a value scaled (with the cube of mass, as any electromagnetic decay) from the decay rate of pion.
(Or that 3/8 is the value of sin weinberg that minimises the decay rate of Z0. Or that the mass quotients of muon and W+ are very close to (g-2)…
Or that there are six flavors of red quark, six of blue, and six of green, which spells out 666, the NUMBER OF THE BEAST…
(You might be onto something, but the way you’ve written it makes it sound like the most abstruse conspiracy theory I’ve ever heard. 🙂 )
Comment #9 March 20th, 2006 at 10:51 am
Okay, I’ll give you your point. But you make it sound, in your article, like physicists had no idea about these things (as for example was the case when Dirac “predicted” the positron.)
Also note that many (all?) neutrino oscillation experiments are done not with that big ball of glowing plasma in the sky, but instead with powerful nuclear reactors. Still astronomy? This is, of course, not to say that neutrino astronomy isn’t in the works (AMANDA for example, has produced a map, but they have seen no sources of continuous emission….yet!) What I’m saying is that I think your definition of astronomy may include my solar powered calculator 😉
Comment #10 March 20th, 2006 at 11:09 am
Scott,
it is a new week for J.Baez and thus your link is outdated.
Comment #11 March 20th, 2006 at 1:11 pm
Thanks, Wolfgang — fixed!
Comment #12 March 20th, 2006 at 1:52 pm
Dave: Even if I begrudge you your neutrinos, I think my general claim about space being where the action is today is still valid.
That could change if the LHC finds supersymmetry. But, based on my deep physical insight into these matters, let me put a prediction on the table: it won’t. 🙂
Comment #13 March 20th, 2006 at 2:12 pm
There seems to be a rather high proportion of heat-to-light being generated in the TOE debates here on good ol’ Planet Earth (read a week or two of Woit and Lubos and see if it doesn’t discourage you from feeling that *any* TOE will bear fruit any time soon). OTOH, it pretty clearly took an advance beyond the then-current Standard Model to explain experiments showing 1/3 the expected number of neutrinos from the Sun. Understandable that “the action is” where advances are clearly acknowledged as such rather than where the value of any purported advance is endlessly (and acrimoniously) debated.
Comment #14 March 20th, 2006 at 7:31 pm
Sorry, professor. New particle accelerators and neutrino mass mensurations have proved the number of the Beast is 616.