The Extravagant Universe
by Robert P. Kirshner
282 pages - Princeton University Press, 2002
Reviewed in American Journal of Physics by Mark P. Silverman.
On the occasions when I have asked research colleagues, former students, and acquaintances at conferences what induced them to become physicists, the reasons given have almost invariably fallen into one of two categories: In their childhood or adolescent years, respondents were intrigued either by the weirdness of quantum mechanics or the origin and fate of the universe. Not, by any means, did most of the representatives of my small sample become quantum physicists or cosmologists, but it was the strange attraction of the physics of the nanoscopically small or the astronomically large that stimulated interest in the first place. I suspect that many other physicists were probably attracted to physics in the same way before going on to do more practical things in their careers than worry about the implications of entangled states or big bangs.
Basic quantum physics, however, for all the sensationalism that science popularizers generate unnecessarily, is reasonably well-understood; the behavior of ordinary matter, however contrary to common sense schooled in Newtonian physics, follows predictably from the Schrödinger equation, Dirac equation, or some other quantum equation of motion. But cosmology, the science of the universe, is still an open book - a book whose chapters are being written daily (in the New York Times?) by research teams studying such disparate things as the explosions of stars, the abundance of the elements, and the temperature of the cosmos.
Author Robert Kirshner is part of one such group that has helped establish the use of Type Ia supernovae as standard candles throughout the universe, and, in so doing, has arrived at a conclusion as bizarre as any that has emerged from physics during the past hundred years: namely, that the expansion of space is accelerating, possibly as a result of an unknown form of all-pervasive energy. The conclusion is as disconcerting as it was unexpected, for one would have thought initially that gravity, being a long-range attractive force, would retard the recession of all sources of matter from one another and lead to a cosmic deceleration. But this does not seem to be the case in the present epoch. Furthermore, the nature of the matter in the universe is itself a puzzle. Observations of the orbital speed of outlying stars or gas clouds about the center of their galaxy (for virtually any galaxy), or the relative speeds of galaxies bound in clusters, suggest the presence of far more mass (at least 10 times more) than can be discerned from the luminosity of these structures at all wavelengths across the electromagnetic spectrum.
The conundrum of this so-called "dark matter" and "dark energy" may be summed up succinctly in the composition of a parameter called Omega (Ω), the ratio of the density of mass-energy in the Universe to the critical density below which the universe would inevitably contract to a big crunch if there were no dark energy present. Separate lines of evidence from the supernova research discussed by the author and from independent investigations of the cosmic background radiation consistently point to a universe right on the "edge", Ω = 1, with dark energy contributing about 70% and all matter contributing about 30%. Of this 30%, known nuclear physics and the cosmic abundance of light elements (principally deuterium and helium) make it highly implausible that ordinary matter as we know it (protons, neutrons, electrons, etc.) constitutes more than a few per cent, much of which is not visible. Physics may have come a long way from cannon balls dropped from the Tower of Pisa (something Galileo almost certainly never did) to quantum chromodynamics, but if current astronomical observations and their interpretations are correct, the content of the universe consists mostly of unknown forms of matter and energy.
The title of Kirshner's book reflects the perplexing complexity of this composition. "A neater universe crafted by Occam's razor might have just one form of dark matter," Kirshner eloquently wrote,
"but our extravagant universe apparently must have at least three: some dark baryons, a pinch of neutrino mass, but mostly something else. Instead of a minimalist universe, we seem to live in a rococo one: we have everything you can think of and more than you can think of. Perhaps we should not be so quick to...reject wild ideas: we need even wilder ones to interpret these startling results."
It is good to hear this from an astronomer, for I proposed one such imaginative idea (I prefer "imaginative" to "wild") a few years ago. The idea is that - in contrast to very massive bosons (WIMPs) which would have clumped too much at the centers of galaxies and which, in any event, can not be found despite lengthy searches - perhaps dark matter consists instead of very light bosons. Above a certain calculable temperature, light bosons behave like hot dark matter (neutrinos). Below that temperature, however, the particles form a Bose-Einstein condensate (BEC) and behave like cold dark matter. It is, I believe, a beautiful idea; it led to a characteristic length scale for the size of dark matter halos, a universal galactic rotation curve, and the prediction of superfluid-like vortices of dark matter within galaxies.
Kirshner has written a lively, witty, humorous, irreverent, and informative account of his participation in this once-per-lifetime adventure of discovering something truly significant about the universe. This book is one of the clearest popular expositions I have read of the intricacies of attempting to answer the questions "Where did the universe come from and where is it going?" One route to those answers depends on knowing how far away distant galaxies are from our own, a problem that can be solved if there exist throughout the universe bright objects (standard candles) whose intrinsic luminosity is known and whose apparent luminosity is measurable. Like the unfolding of a Sherlock Holmes mystery, the author unravels for us the observations and reasoning leading to the recognition of Type Ia supernovae as the ideal standard candle, despite the fact that not all members of this class have the same intrinsic luminosity.
A Type Ia supernova is a thermonuclear explosion of a white dwarf star, in contrast to a Type II supernova which entails the gravitational core collapse of a giant star many times the mass of the Sun. The latter event may lead to formation of neutron stars (pulsars) or black holes, but no stellar object appears to remain in the aftermath of a Type Ia explosion1. Not every white dwarf explodes. According to current belief, a white dwarf in binary association with another star accretes gas from its companion until its own mass exceeds the so-called Chandrasekhar limit (of about 1.4 solar masses), whereupon the catastrophic nuclear burning of carbon and oxygen take place leading to total disruption of the star. Because these stellar nuclear bombs ignite at approximately the same mass and can be seen more than halfway across the universe, their distances, and therefore the distances of the galaxies in which they reside, can be determined. From the redshifts of identifiable spectral lines the line-of-sight velocity of recession of the embedding galaxy can also be determined. Together, the variation of distance with redshift of a significant number of Type Ia supernovae comprises a Hubble diagram whose geometric shape and regression parameters can reveal the past and future evolution of the Universe.
In the poem, "When I Heard the Learned Astronomer", Walt Whitman decries the obsessive concern of science with quantity as stifling the beauty of nature, writing how the astronomical "charts and diagrams" made him "tired and sick" until he wandered off into the "mystical moist night-air" and looked up "in perfect silence at the stars." Whatever its literary merits, I have always thought this was a rather awful poem. Whitman is looking at the stars not only in perfect silence but in total ignorance, missing the beauty intrinsic to understanding how they got there, why they shine as they do, what eventually happens to them, and what they can tell us about the universe. When I lecture on cosmology to nonspecialists, I read Whitman's poem to them, and then show them the composite Hubble diagram of Type Ia supernovae out to a redshift z ≈ 1 (when the universe was about half its present size), compiled independently by the two principal supernova search groups: the High-Z Supernova Search Team to which the author contributes and the Supernova Cosmology Project (SCP). I tell them that, if they see but one astronomical diagram in their entire lives, this is the diagram they should look at.
A supernova is a relatively rare event, occurring in a galaxy about once per century. Nevertheless, there are hundreds of billions of galaxies whose light can reach us, and therefore stars are exploding somewhere all the time. To find them, however, is a prodigious undertaking - and one of the most interesting parts of Kirshner's book to me is a fast-paced narrative, in terms of the specific team members who perform each task, of the orchestrated procedure by which supernovae are found, captured (on film or CCDs), classified, and (if Type Ia) followed up. It may take up to two years between the time a supernova is discovered and the final measurement of the background sky after the event has faded from view.
The author's writing style is entertaining although at times his colorful use of simile and metaphor is blue-shifted well beyond purple prose into the far ultraviolet. For example, it is difficult for us to see the flattened disk of a galaxy in which we are embedded, "just as a slice of pepperoni sizzling amid the mozzarella has a hard time seeing the whole pizza." I never thought of the Galaxy quite like that before - and now, thanks to the author, pictures of the Milky Way make me hungry.
In some ways the author's sharp, uncomplimentary characterizations of various associates reminded me of James Watson's Double Helix. For example, I laughed out loud at a recreation of conversations between the author and the inimitably crusty but ingenious Fritz Zwicky - and the photograph of Zwicky demonstrating a "spherical bastard" (a bastard any way you looked at it) is hilarious. Of Allan Sandage, the author wrote: "Sandage took a personal view of the Hubble constant - if you disagreed with him, you must be wrong, and possibly malicious. And if you changed from agreement to disagreement, you must be treacherous or stupid or both." From this, the reader may deduce that Kirshner had a disagreement with Sandage. I do not know the author personally, having never met or corresponded with him, but I infer from his narrative that he had quite a few disagreements with other people as well.
Astronomy, although not exactly a contact sport, is a highly competitive affair not least because viewing time on large telescopes is, as the author takes pains to describe, hard to get and stringently rationed. There are relatively few scientists in the business of hunting for supernovae, and they are undoubtedly often in a position to judge one another's proposals or referee one another's papers. I am not an astronomer, but, if I were, I would not want to have the fate of my manuscripts decided by the author, all his jokes and bonhomie notwithstanding. The author relates that, as a student, he once won a prize for "useful and polite literature" and that, ever since, he has tried to be useful or polite. This is written, I am sure, with tongue in cheek, for the author's conception of utility invariably turned into a negative assessment of other peoples' proposals, manuscripts, or ideas.
One such episode is described at length in the chapter, "Getting It First", which, to the author's regret, he didn't. It was the competing SCP group under the direction of Saul Perlmutter (whom I also do not know) who apparently got it first - the "it" being a cosmological deduction based on Type Ia supernovae. As the author relates, the first SCP publication fell to him to review and he rejected it and the subsequent revisions as well. Kirshner had his reasons, of course, which he tells the reader. "Maybe, it wasn't possible," Kirshner writes with dubious naïveté, for the authors to deal with his issues in the four-page format of a letter for rapid publication, and "they should consider writing the War and Peace-length version for another journal." Readers who understand the importance and prestige of rapid publication in a letters journal can judge for themselves how helpful this counsel must have appeared to the SCP group. Despite Kirshner's effort to make the episode seem humorous, I did not find it so, and could readily imagine that Perlmutter and his colleagues saw the rejection as coming not from a useful reviewer, but from a self-serving competitor. Utility, like beauty, is in the eye of the beholder.
Kirshner's book is not just a chirpy account of finding supernovae. It is the story, as he sees it, of a race for priority and recognition - science, after all, is a human endeavor - and in that regard The Extravagant Universe raises provocative questions about "the extravagant referee". I have encountered one myself (which scientist has not?) with regard to the BEC dark matter idea mentioned previously. My paper, written with a colleague specializing in general relativity, was sent to a reviewer, who held it as long as he could - before the editor would have had to rip it from his hands personally - and then returned it with a not very helpful brief comment. My colleague and I replied quickly, and the waiting game began anew. At the end of each seemingly interminable procrastination, the reviewer would add a new query, a new suggestion, a new demand, a new reference to find and include. Ultimately, satisfying the progressive escalation of "useful" remarks would have changed our short letter, which reported an exciting new idea, into a Ph.D. thesis - Kirshner's suggestion of a War and Peace-length article - necessitating years of calculation. I wonder whether the SCP group felt the same way. Exasperated, we sent the paper elsewhere. It was subjected to review all over again, and eventually published, although by that time other researchers had reached conclusions similar to ours.
Kirshner, himself, must be aware of these games that referees play, for, when it came time for the High-Z group to publish their own first report (described in the chapter "Getting It Right", in contrast to getting it first), they were mindful to select a journal which they felt was more likely to publish it quickly than to scrutinize it carefully.
1. The fact that there is a Type Ia suggests that there must also be at least a Type Ib. That is correct, but a Type Ib involves gravitational collapse, rather than a thermonuclear explosion, and so a Type Ib is really like a Type II. This is the sort of nomenclature that, as Kirshner points out, "drive[s] physicists crazy." The reason behind the apparent madness is that astronomers classify supernovae on the basis of their spectra, not their mechanism. The spectra of all Type I supernovae show no hydrogen lines.
About the author
Mark P. Silverman is Jarvis Professor of Physics at Trinity College. He wrote of his investigations of light, electrons, nuclei, and atoms in his books Waves and Grains: Reflections on Light and Learning (Princeton, 1998), Probing the Atom (Princeton, 2000), and A Universe of Atoms, An Atom in the Universe (Springer, 2002). His latest book Quantum Superposition (Springer, 2008) elucidates principles underlying the strange, counterintuitive behaviour of quantum systems.