Unbelievable; yet again, for the fourth or fifth time since July, something related to high-energy physics hits the front pages. This time it’s not a shocking claim of a novel phenomenon, but a thoroughly expected Nobel prize awarded for the field’s greatest discovery of the 1990s: that not only is the universe expanding (as Edwin Hubble discovered in 1929), it is expanding more and more rapidly. The teams in which the prize-winners participated used Type Ia supernovas to make the first direct observations indicating that the universe is dominated by something like a “cosmological constant”, often now called “dark energy”. [What these terms really mean deserves a longer article.]
It is interesting, given the historical juxtaposition of this award with the chaos and media circus of ten days ago, to compare this result with the OPERA experiment’s claim that neutrinos arrive earlier than expected. I am very skeptical of the OPERA result, as are most particle physicists that I talk to; I and most of my particle physics colleagues were also initially very skeptical of today’s Nobel Prize winning result. Are there any differences?Yes, interesting ones worth keeping in mind, though I’m not predicting the future in making these comparisons.
First, three years before the Type Ia supernova measurements were announced in 1998, I had already heard or read detailed presentations by several famous theoretical astrophysicists and cosmologists (including at least Jeremiah Ostriker and Paul Steinhardt, and separately Lawrence Krauss and Michael Turner) pointing out that various other types of measurements were indirectly pointing to the existence of a cosmological constant or something like it. [Essentially, if I remember right, the density of the universe was appearing to have the critical value that makes the geometry of the universe flat (so that parallel lines crossing the universe remain never cross or diverge), but the amount of matter in the universe, including both ordinary and dark matter, appeared to be only 1/4 of that value — implying that 3/4 of the universe must be something else, with a cosmological constant being the choice most consistent with the data.] So the supernova result was not really a bolt from the blue; other data was already (somewhat weakly) pointing toward it.
Second, while one can identify measurements related to Einstein’s special relativity that are on their face (though not strictly) in some conflict with the OPERA result, forcing one to do some theoretical gymnastics to make all the experiments consistent, this was not true of the supernova-based result. What the claim of an accelerating universe really ran counter to was the expectations of particle physicists, and some astrophysicists, but not so much prior data. In fact it was, according to the papers I mentioned above, in rough accord with a lot of prior data, confirming an already-noticed trend.
Third, these now Nobel laureates set out specifically to measure the history of the universe, and designed their research programs to carry out their observations as well as could be done. OPERA, by contrast, was designed to measure an effect of neutrino oscillations [an article on this is coming shortly], not to measure neutrino speed. Indeed, if you set out specifically to measure neutrino speed, OPERA is definitely not the experiment you would have designed — the measurement technique has a number of significant challenges, including the difficulty of calibrating the timing and distance measurements, and the fact that (as discussed here) the pulses of neutrinos used in the measurement last 10,000 nanoseconds, yet need to be used to measure a time shift of 60 nanoseconds.
Finally, the supernova result was obtained nearly simultaneously, after many years of work, by two separate and competing teams of researchers, Perlmutter’s Supernova Cosmology Project, and Schmidt and Reiss’s High-z Supernova Search Team. OPERA currently stands alone, perhaps boldly, perhaps lonely… but certainly unconfirmed.
Now here’s a question: how did particle physicists go from extremely skeptical to largely accepting of the supernova results? Already it was impressive that two similar measurements gave a similar striking result, and that there were prior suggestions that other observations were pointing in this direction. But perhaps the most important cause was that it did not take long for other, independent measurements — observing completely unrelated objects and using entirely different methods — to show independent evidence consistent with an accelerating universe and a similar-sized cosmological constant (or something like it). So it was only a matter of a few short years before the field largely concluded their measurement was correct. Thereafter it was pretty obvious that these two groups would get the credit for being first, and their leading scientists would soon win a Nobel prize for it.
The question for OPERA is whether the future will take a similar path, or instead take the path most often followed by history: a sequence of contradicting experiments, followed by gradual oblivion.
Meanwhile, what of the supernova observation itself? The astrophysics and cosmology of the measurement will be best described by others (such as my friends over at Cosmic Variance). But in brief, it involved using Type Ia supernovas (such as the one that has gone off recently [you can read my post on the subject here] in the Pinwheel galaxy 21,000,000 light years away, and not to be confused with Type II supernovas, such as the 1987 supernova that exploded in the Large Magellanic Cloud, about 160,000 light years away) to estimate distances to far-away galaxies. It’s not easy to measure distances in the universe — you can’t just put down a tape-measure to something millions of light years away, or even bounce a laser beam off it. Instead you have to play the following game.
Suppose you saw a distant light bulb, and you wanted to know how far away it was. Well, if you knew it was a 60 Watt bulb, then you could figure that out, since apparent brightness falls off with distance in a predictable way. But if you didn’t know if it was a 60 Watt, a 40 Watt, or a 2000 Watt bulb, then you wouldn’t be able to do that; it might be dim because it’s intrinsically a dim bulb, or it might just appear dim because it is far away. Well, most stars and galaxies and other identifiable things in the universe don’t come with their wattage marked on the box. But Type Ia supernovas do. They are very regular, in that the way they behave over time is closely related to their maximum brightness. So if you measure the change of a Type Ia’s brightness over time, and look at its maximum brightness in your telescope, you can figure out how bright it must have been intrinsically — a trick for determining the wattage of the bulb.
And then by looking at many galaxies with a supernova in them, and figuring out how far away they are [and remembering that any supernova in the distant universe that you can see today blew up long, long ago, due to the finite speed of light], you can work out the shape and size of the universe over time. So these teams of researchers measured a couple of dozen supernovas each, and came to the conclusion that the universe’s expansion was not decelerating, as most astronomers and almost all particle physicists had expected, but had been accelerating for some time.
Why this discovery shocked particle physicists is a long but interesting and important story, to be told soon; stay tuned.