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?