Your January column “This Month in Physics History”
gave what I thought a fair and balanced account of the discovery of the accelerating universe, including the contributions of both research groups, so I was surprised at Robert Kirshner’s letter
responding to it in the February issue. Having reviewed his fine book, The Extravagant Universe
, in the New York Times Book Review
and edited a SLAC Beam Line
article on the research written by Gerson Goldhaber of Lawrence Berkeley Laboratory, I feel in a good position to comment further.
The column might indeed have delved more deeply into the experimental techniques involved—and Kirshner cited a few of the specific contributions of his High-Z Supernova Search team. Omitted from both accounts, however, was the central role of Saul Perlmutter of the LBL Supernova Cosmology Project in pioneering the core technique used by the two groups. This method involves taking successive photographs with a CCD camera of the same patches of night sky about four weeks apart during the new moon; by comparing individual pixels in this wide field of view, researchers can identify candidate supernovae for further observation during the next few months on dedicated telescopes. By following a supernova’s light output over this period, they can obtain the correction factor Kirshner mentions and thereby establish the supernova as a valid “standard candle.”
The LBL team, composed mostly of experimental particle physicists familiar with manipulating vast quantities of data, felt equal to this daunting task. But many astronomers and astrophysicists figured that the technique would never work. Thus the High-Z group found itself playing catch-up in the mid-1990s after the LBL team showed that it did
I vividly recall sitting in the front row at a UC Santa Cruz physics colloquium on 8 December 1997, when Perlmutter gave the first public (beyond Berkeley) presentation of the results that attracted so much attention a month later. Having just edited Goldhaber’s article and been primed on the significance of this research by my UCSC colleague Joel Primack, I was sitting on the edge of my seat, waiting for the numbers, which came only in the last few minutes of the talk. Based on 38 Type Ia supernovae analyzed until then, Perlmutter said, they could conclude that the universe was open: it had only about 30 percent of the critical mass density needed to slow the Hubble expansion to zero. I don’t recall him making any further conclusions, but Primack was not so reluctant. In the ensuing discussion period, he stood up and pointed out that these results implied the previously unthinkable: the need for a cosmological constant.
To be fair to Kirshner and the High-Z team, these preliminary LBL data could not yet rule out dimming of the supernovae light due to absorption by intervening dust—which his group could eliminate by making observations at three different wavelengths. But in early 1998, when these astonishing results began to surface in press accounts, the High-Z team had a statistically weak sample of only about 10 supernovae, while the LBL group by then had accumulated over 40. Nobody—at least not in the particle physics community—would have accepted the momentous conclusion of an accelerating universe as valid based on such a single small sample had there not been another, independent result with significantly better statistics. In the final historical analysis, it was the joint results of both teams, each covering weaknesses in the other’s analysis, that convinced the wider scientific community so rapidly about such an unexpected, revolutionary result.
Historians of science find these priority disputes rather tiresome, but then we don’t have any Nobel prizes at stake! The current dispute about the discovery of the accelerating universe reminds me of my favorite adage: “One of the most difficult things to divide is success.” Michael Riordan Santa Cruz, CA