Proc. Natl. Acad. Sci. USAVol. 90, pp. 4878-4881, June 1993Colloquium Paper

This paper was presented at a coUoquium entited "Physical Cosmology," organized by a committee chaired by David N. Schramm,held March 27 and 28, 1992, at the National Academy of Scinces, Irvine, CA.

An ancient revisits cosmologyJESSE L. GREENSTEINDepartment of Astronomy and Palomar Observatory, California Institute of Technology, 105-24, Pasadena, CA 91125

ABSTRACT In this after-dinner speech, a somewhat light-hearted attempt is made to view the observational side ofphysical cosmology as a subdiscipline of astrophysics, still in anearly stage of sophistication and in need of more theoreticalunderstanding. The theoretical side of cosmology, in contrast,has its deep base in general relativity. A major result ofobservational cosmology is that an expansion of the Universearose from a singularity some 15 billion years ago. This has hadan enormous impact on the public's view of both astronomyand theology. It places on cosmologists an extra responsibilityfor clear thinking and interpretation. Recently, gravitationalphysics caused another crisis from an unexpected observationalresult that nonbaryonic matter appears to dominate. Willobtaining information about this massive nonbaryonic compo-nent require that astronomers cease to rely on measurement ofphotons? But 40 years ago after radio astronomical techniquesuncovered the high-energy universe, we happily introducednew subfields, with techniques from physics and engineeringstill tied to photon detection. Another historical example showshow a subfield of cosmology, big bang nucleosynthesis, grew incomplexity from its spectroscopic astrophysics beginning 40years ago. Determination of primordial abundances of lighternuclei does illuminate conditions in the Big Bang, but theobservational results faced and overcame many hurdles on theway.

Some History, Metaphors for the Beginning, Reductionism

To justify my speaking here, I must turn time backwards andinvoke authority. I have worked on and been mostly inter-ested in how stars evolve. I am fascinated by atomic physics,the spectra of atoms, how many atoms there are of eachelement, and, why based on low-energy nuclear physics,there are that many. I recognize and admire some stars asindividuals and enjoy the strange things they do. My lastcontribution to cosmology was to participate in recognizingthe redshift of quasars (then called QSRS) the day Schmidtbroke that logjam. Especially so since I had already devel-oped an exotic but incorrect hypothesis, which claimed thatthey were stripped supernova cores. Their spectra were ofsupposedly unrecognizable, highly ionized extinct radioac-tivities. This was an incorrect but fashionable idea. Schmidtand I wrote what I still think is a good paper (1) in 1964. Itprovided a first model of the broad-line emitting regions ofquasars; we confessed to having no explanation for theenormous energy released within so small a volume. Stars arestill my passion. But at this meeting I cannot help beingentranced by the growth in the kinds and amounts of dataabout the larger universe and by its increasing complexity. Iam sure that many of you begin to realize the extent of yourtask; it is exciting for me to be near discoverers of the mostly

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unexpected. A wide gap between observation and theorymust always exist. That is not a discouraging fact, andastronomers have long benefited from the contradictions theyuncovered. An ideal recent example is the Cosmic Back-ground Explorer, results from which assert that smoothnessdominates at large redshift, z. But we obviously live amongclumpy galaxies and clusters. Important theoretical ques-tions arise from this contrast, which may lead to a deepinsight into particle physics, as discussed extensively at thismeeting.

In spite of new knowledge, one must still worry about thelarge gaps between "fact," "interpretation," and "mean-ing." I may be telling you what you already know, but,viewed from the outside, the present situation in cosmologysomewhat resembles that in stellar spectroscopic astrophys-ics 30-40 years ago. Enormous increases have since occurredin the (i) available data, its quality, and wavelength range; (ii)laboratory astrophysics and computation ofatomic structure;and (iii) stellar atmosphere theory, mainly from the increasedpower of computers. As a result, the once negative conno-tation of the phrase "astrophysical accuracy" has, I hope,disappeared. Some believe we know the helium/hydrogenratio to 1% of itself. In observational cosmology, however,there is much further still to go; the expansion rate (Hubbleconstant), Ho, is a subject ofargument at the 50% level. A staris essentially an individual, may lose or accrete matter, haveits individual peculiar history, but long remain a stable,nearly closed system. Meaningful physical parameters can beassigned to it, the luminosity predicted as a function of mass,composition, and age. Stellar astronomy smells like physics.My hope is that observational cosmology may soon acquirea similar depth of physical understanding.

Especially important are accurate distances and luminos-ities measurable for many stars. But by its nature, a galaxyis a less rigorously closed system; gravitational and thermo-dynamic changes occur; it may even cannibalize neighbors.If so, it enters a stage of rapid star formation and enormousbrightness. Its luminosity as a function oftime is normally thesum over myriads of histories of its stellar (and interstellargas) contents. Galaxy parallaxes cannot be measured, re-flecting the oldest problem of stellar astronomers-the dis-tance scale. If galaxies contained sufficiently luminous indi-vidual stars or groups to serve as standard candles, the galaxydistance scale could be reducible to that of the nearby stars.But at interesting galaxy distances only the most luminoussupernovae are useful individuals. Why should nature assignunique properties to something as violent as gravitationalcollapse and the ultimate explosion producing iron-groupelements? It would be puzzling if it does, in spite of remark-able successes of theory for supernova 1987a. Can a galaxyitself serve as a standard candle? What dictates the brightnessdistribution of its galactic clusters, old or new? Some useful,if unexplained, regularities permit observers to recognizevarious types of galaxies. Regularities exist that provideestimates of luminosity from such an apparently adventitiousproperty as the internal velocity dispersion. If a physical

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theory of galaxy formation becomes available, perhaps someday one might predict a galaxy's luminosity from otherobserved properties. Galaxies do cover an enormous range ofmass, and probably have a wide range of baryonic mass/lightratio. Perhaps external criteria will be found making itpossible to estimate true luminosity. In Hubble's time, it wasreasonable to talk of a mean absolute magnitude with amodest dispersion. But for 70 years even stellar astronomersworried about the Malmquist corrections, which are muchworse in extragalactic studies. Everyone is conscious of thebias in any sample attainable with limited means, but some ofthese are subtle.

In stellar astronomy we begin to hope that theory issoundly based on known physical laws. One can even viewstellar astrophysics as a slightly impoverished, even messy,part of atomic, nuclear, thermodynamic, and plasma physics.It tries to think like physics. But it may well be long beforecurrent concerns about distance scale and statistical bias aresatisfied, as the physics background of observational cos-mology is better understood. There is no reason for discour-agement; the most encouraging change in astronomical so-ciology is how many scientists at this meeting have back-ground training and experimental skills in what was called"physics." Interdisciplinary walls have largely disappeared,and cosmology offers an ultimate challenge. You must andwill succeed.

Complexity exists. For a Californian at a Colloquium heldnear the beaches of the Pacific, images connected with waterare natural. Water has an impressionistic quality. In swim-ming pools, light shimmers on and through water-refractedthrough ripples to the bottom. Hypnotized by myriads ofrunning cusps of light, is it enough to say that they move atrandom? On the Pacific beach, your view of a breaking wavediffers from a surfer's who uses that wave. Adjacent cells ofwater are irrevocably torn apart; some run up the beach,others return to the sea; their neighborhood or topology istorn. Are details predictable or meaningful? I once arguedwith Dick Feynman about whether we should travel to Mars;he asked if I wasn't curious about how the surf looked at theedge of a Martian ocean. I could tell him that Mars had noocean but he had asked the deeper question. How do wavesin another fluid, under a different atmosphere, depend ongravity? Outcomes of simple laws are sensitive to initialconditions and how far away is chaos? How far is it safe topush "reductionism?"We abstract by reducing a full description ofan event to the

measurable part of its significant content. Can we neglect thecolor of a body falling under gravity? If so, how much moreabout it can we omit? Mechanists claim that human con-sciousness is "nothing but" an organized set of chemicalreactions, obeying a set of computing instructions morecomplex than any supercomputer has yet achieved, but notdifferent. Reductionism is quite easy. It works, since it is howpoliticians get elected. Reality forces gross reductionism inlimiting our response to what is or may be observable.

Let us consider the Big Bang. A Greek epigram I rememberpartially is that "In the beginning was the word, and the wordwas made flesh." If that is not an exact quotation, it is acommon Platonic image. First came the logos. We can pastetogether a mystic version from the New Testament, John 1:1and 1:14, to get "In the beginning was the Word, and theWord was with God .... And the Word became flesh." The1961 Alternate Oxford Edition is less elegant but reads"When all things began, the Word already was." For thosewho do not read that far in the Bible, Genesis 1:1 is quiteexplicit "In the beginning God created the heavens and theearth." Apparently, we astronomers had an instrument plat-form already on the first day, and Genesis could justifysupport of planetary missions and submillimeter astronomy.For speculative philosophers and for the public, the numer-

ical values of the Hubble parameters Ho and to are seriouslyinteresting. In current myths about cosmology, as they arepublicized, there is a singularity (or perhaps divine interven-tion) at to. This fascinates the public and produces money-making books in which many of you appear as saints ordevils. Theoretical cosmologists prophesy, saying: It mustbe. Fortunately for an 82-year-old, the Bible provides aptquotations. Ecclesiastes, written by and for old folks, says"Better is the end of a thing than the beginning thereof." Thesingularity must take care of itself, the easier task being todescribe what the universe now contains. Then work back-wards to as close to to as possible. The cosmic backgroundradiation and its interpretation were recent great successes inthis quest. They picture a smooth happening, quite unlike thevisible stars, clusters, galaxies, clusters of galaxies, walls,and voids. Such observed structures are complicated andhave the particularity of any event, not all of which may berationally understandable. We omit what makes the partic-ularity of reality interesting, an unfortunate choice. Most ofthe fun is in the details. A dimly visible universe, in whichobservable baryons represent only a fraction of the totalmass, is a sad fate for a cosmological enterprise that began ona small optimistic scale 70 years ago.Most difficult for me, we may near the end of the historic

dependence of astronomy on the measurement of radiation.If the theorists must have Qi 1.000, if the gravitation (asbeautifully observed in galaxy rotation) requires that darkmatter outweigh luminous matter, our old world of observa-tion appears dark indeed. Velocity dispersions are observedto be larger in galaxy clusters, still larger in the Hubble flow.Good and evil, locked in permanent struggle, are imagina-tively represented as good is light, white (like a cowboy's hat)and evil is dark, black. Added to the peculiar velocities ofgalaxies are gradients in the potential produced by invisiblemass. Galaxy positions and distances are useful merely tolabel coordinates in this potential. But what happens to theunfortunate big-telescope observers? Dark matter, if it exists,becomes the game for experimental and particle physicists.There are not enough brown dwarfs to close the Universe. Itwould be a sad fate for a brilliant science that for 2000 yearstriumphantly extracted information from very few photons,especially so when technology provides such wonderful newgadgets, and computers make thinking easy. Is it time toretire or say: Back to the drawing board? I hope nobody herewould think so-instead let us look for an exotic particlewhose decay will be a photon we could measure. Let me hopethat detection will be by someone calling themself astrono-mer. Astronomers have remained resilient; when earlierchallenges (like cosmic static) occurred, they successfullyacquired the required new techniques, co-opted physicists orengineers with different skills as colleagues. The next newslogan must be: Once more unto the breach, dear friends.Who leads that charge?

Experiment and Theory, Some More History

I have long been connected with use or planning of big optical(and of some radio) telescopes. In the 1960s and 70s, ourcommunity grew to feel that a nearly constitutional rightexisted providing tools for observing fainter objects at avariety of wavelengths. All wavelength ranges are nearlycovered, with planning now of increased collecting area andresolution at each wavelength. A number of experiments inspace (which are very expensive) and numerous large opticaltelescopes (pretty expensive) are planned. They include: twoKeck 10 m under construction; four ESO VLT of 8 m; twoGemini at 8 m; from Japan a most expensive 8 m; the MMTconversion, 6.5 m; the Columbus, Magellan, and otherprojects. This enormous increase of collecting area andcapital costs would never have occurred if it had not been for

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the cosmological problem. Justifying such expenditures is aproblem cosmologists must now face, since they bear theguilt. One or two 8-m telescopes would have kept stellarastronomers busy forever but probably would never havebeen funded.But in cosmological research, do theory and telescope

observation talk about the same universe? Can we honestlysay that the simple data obtainable will be either (i) crucial,(ii) important, or (iii) mostly irrelevant to theory? Here is thepoint where sound theoretical understanding of cosmologicalobservables becomes most critical. We must understand,believe, and be ready to justify our answer; credibility withthe public, federal agencies, and colleagues in other subdis-ciplines is at stake.But worse than scientific squabbles the supercollider

caused is the current political pressure that science concen-trate on improving the competitive status of U.S. industry,provide employment for the uneducated, but maintain re-gional balance in funding. Leaders of government, federalagencies, and college presidents repeat such nonsense, di-sastrous for creativity and planning. To astronomy's greatadvantage, we still retain public interest and support. Thegrandeur and beauty of the contents of our universe docapture the imagination of people. We deserve no credit forthat, but we do invent imaginative names and explanationsthat become new poetic public metaphors. This innocentrespect is deepened by the almost Freudian public emphasison "beginnings" and the temptation to credit cosmology withsubstantial insight into the Beginning. Public images of cos-mology and of the moment of human conception are similar.The price of public enthusiasm is that we must be honestabout what we claim to know or plan to do.

Trite promises and trite phrases about funding becomeritualistic; as a government advisor I often felt ill at such loosetalk. One cannot justify exorbitant expenditure by claimssuch as: (i) we will lose our position of leadership, or (ii) ifwe don't spend this money, it will be wasted on an aircraftcarrier, or (iii) the most important results will be the unpre-dicted ones. Since we still hear these phrases they may in partbe true, but rigorous honesty is preferable. The last claim ismost damning; if theory predicts so little, why test it?We have built on the shoulders of giants, we live in a

fortunate country, and we have been extremely lucky. Prom-ise what is honestly possible; don't insist you deserve it.Our luck consisted in the fact that as the universe was being

unveiled it contained enough regularity to reveal generalpatterns and enough oddity that one could not remain com-fortable generalizing from the little we knew. Observationalcosmology has an American flavor, being semiempirical,relying on large equipment and good climate, confident thatsolutions would arise to explain the novelty that wouldcertainly turn up. The cosmological search, largely Ameri-can, became linked with the public respect for Einstein andhis general relativity. Hubble's first major and serious paper(2) on the observed expansion is fun to read. Its diagramshows a nearly linear velocity-distance relation for 24 neb-ulae reaching out to 2 megaparsecs (Mpc), and to 1000km*sec-l*Mpc-' (old distance scale). In spite of large scatter,the points do show an upward trend. Hubble quotes a MountWilson velocity of 3779 km sec-1 for NGC 7619, for which hisextrapolated relation gave a distance of 7 Mpc. His rate, 513± 60 kmsec-lMpc-1, had enormous consequences. Hisdiscussion is modest and brief. " . . . possibility that thevelocity-distance relation may represent the de Sitter effect,and hence that numerical data may be introduced into dis-cussions of the general curvature of space . .. and a generaltendency of material particles to scatter. The latter involvesan acceleration and hence introduces the element of time

.. ." This time, while much too short (to 2 x 109 yr) wasstill a physically interesting number. As data increased, other

measurable parameters were introduced (e.g., the numbercounts and angular diameter vs. brightness), critical testswere devised for general relativity by R. C. Tolman and laterby H. P. Robertson. These tests had, and still have, a hardtime. But where would astronomy be without the crutch toimagination provided by the Hubble time? How many tele-scopes were and will be built and used to aim at measuringthis slippery number? Granted that the time scale was wrong,the large distances and times captured the attention of allscientists, changed physicists into astronomers, endearedastronomy to the newspapers. We were always good for anew story about the new largest redshift; observationalcosmologists became lords of the universe and possessors ofthe best observing time with large telescopes.Radio telescopes revealed the new high-energy universe,

introducing new areas of physics, and provided the veryluminous radio galaxies as sources of new larger redshifts.The redshift of 3C295 at z = 0.46 more than doubled thelargest z obtained after 30 years of large telescope observa-tion by Hubble, Humason, Minkowski, and Mayall. Clearlyrelativity was one ofthe dominant great ideas ofthe early 20thcentury. But although I admired Humason and Minkowski, Iconfess to relief at their retirement, which made availableobserving time for me at the prime-focus spectrograph of thePalomar 5-m reflector. But what did I, in fact, observe? Whitedwarfs, at a distance of only 30 pc (not Mpc)-less dramaticobjects. But they provided tests for equivalence and specialrelativity, from the gravitational redshift and other laws ofphysics. For example, their maximum mass lies below thetheoretical upper limit, 1.4 Suns; not only does electron massincrease with velocity, but the hydrogen/helium ratio is low;hydrogen has burnt completely. Like the observational cos-mologists, I was seduced by the interplay of relativity, stellarstructure, evolution. Unexpectedly rich details did make mychoice worthwhile; there was slow rotation (caused by loss ofmass and angular momentum), loss of magnetic-field energy,compositions resulting from a-particle burning, and nearlycomplete gravitational separation of elements by diffusion. Itis not cosmology but it is fun and even important.

Nucleosynthesis in the Early Universe

A story ofhow one reacts to a major change in point of view,to a "change in paradigm," may illuminate how observersface new challenges. The abundances ofthe lighter nuclei andtheir isotopes are now useful to Big Bang cosmology. Withmany collaborators, data on stellar compositions were pro-vided, from 1950 to 1970, with eyes on the theory of stellarnucleosynthesis. Involved from the beginning, I give a largelypersonal account of how abundances of the lighter elementswere attacked. In "Gedanken Astrophysics" (read in pre-print), Kurucz (3) discusses the primordial abundances of 1H,2H, 3'4He, 6'7Li as constraints on the early universe. Icondense his words drastically: "The logical way to proceedwould be to make a grid of model universes, varying thebaryon and radiation densities ... until they produce pre-dictions that can be compared directly to observation. Thenmake the necessary observations and test the theories." Hecontinues . . . "That is not the way many astrophysicistsproceed, however, . . . they take observations that can bemade ... and try to determine the primordial abundances,knowing a priori what answer is needed to fit some particularmodel . . ." I must apologize to Kurucz, an importantastrophysicist working on stellar model atmospheres. Itcertainly does not work that way. We observe only what wecan; we are always limited in experiments we can perform,living in a brutally real universe, without the luxury ofchanging it, lacking observational tools to do what we want.Not all experimenter pioneers are also capable of developinga theory ofthe 3 K radiation background; their data forced the

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interpretation; the early prediction by Gamow et al. waswrong.Cosmic abundances (more difficult than those at stellar

surfaces) present nontrivial problems of interpretation. Whywere primordial abundances of the relevant light nuclei noteasily found? To test the theory of the early universe, weshould know how much that predicted output was processedand altered through stellar interiors. One may feel uncom-fortable if the 2H/H ratio is determined from local interstellarmatter while the H/He ratio comes from gas-starved metal-poor galaxies. The lithium, as a Li/Fe ratio in the oldest stars,remains primordial only over that limited range of surfacetemperatures in which it is unmixed, unburnt. McMillan (4),doing a student experiment in a new accelerator at theKellogg Laboratory in 1933, wrote that the Li isotope ratio inthe Sun is important. Richardson and I (5) followed that upin 1951 but found 6Li/7Li not reliable, even near sunspots.(Later, differing ratios of 6Li/7Li were found in young starswhere a complex of nonprimordial origins exists.) Spallationby cosmic rays occurs in the interstellar cloud from which thestar formed, with products modified by circulation and ther-monuclear burning. The reduction in content from the Earthto the Sun is by a factor of 100. An experiment at Lick foundLi on T Tauri stars, newborn from such gas, led Bonsack andmyself (6) to find they had 100-fold increases. Beryllium (7)is less sensitive to hydrogen burning, which makes it a probeof deeper internal circulation. But what perversity of atomicstructure placed the resonance lines of both neutral andionized Be near the ozone cutoff of stellar spectra, 3300 and3100 A? Although that is a blow, boron is worse-with linesinaccessible at 2500 and 2100 A. When I complained aboutthis in a talk at Princeton in 1954, Henry Norris Russellreassured me that someday I would be able to see the Blines-clearly prophesying current space-based observa-tions. The B lines have now been found in the oldest,metal-poor stars where it may be primordial; ratios of Li/Be/B are now being measured to determine their primordialcomposition.

I could not resist the challenge of the 2H/H ratio; naturewas again unkind, separating 2H(a) from the broad lines ofH(a) by only 2 A and setting their abundance ratio near 10-4.At the Palomar coudd spectrograph I overexposed on emis-sion nebulae for many hours but found that the scattered lightwas too strong. The isotope ratio 3He/4He was also tempting,

with the solar chromosphere providing a brighter source; the3He isotope shifts vary from line to line, giving fine discrim-ination. But I found (8) no evidence for 3He, to a quite lowlimit. Charlie Lauritsen took only 10 minutes to demolish thesignificance of that negative experiment, pointing out howeasily 3He self-destructs, critical in the termination ofthep-preaction and the solar neutrino problem. As an example ofcomplication, Sargent and Jugaku (9) found that 3He is moreabundant than 4He at the surface of a young star, 3 Cen A.If someone here wishes to use potassium as a new indicator,I warn that Nature, with its customary perversity, put KIresonance lines in the heart ofthe atmospheric 02 absorptionband at 7600 A. Similar hard luck stories abound in spec-troscopy and probably in all fields. It would take too long toexplain later ramifications, but I hope to assure you thatprimordial abundances are becoming known and do agreewith standard Big Bang parameters.

In 1950 no theory called for or predicted these light elementabundances; if it had, the experiments demanded might notthen have been possible. Many are still interesting. DaveSchramm tells me that an attempted explanation of abun-dances of the light elements by Fowler, Greenstein, andHoyle (10) (using spallation in the solar system) did slowacceptance of the idea that useful primordial abundances areobtainable.Perhaps we were stupid in the past, perhaps Nature is

ungenerous. The cosmological enterprise will face, but I amsure surpass, similar roadblocks. It must succeed in makingsense of the larger universe, which is, after all, not muchmore complicated than a star. Good luck to your enterprise.

1.2.3.4.5.

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Greenstein, J. L. & Schmidt, M. (1964)Astrophys. J. 140, 1-34.Hubble, E. (1929) Proc. Natl. Acad. Sci. USA 15, 168-173.Kurucz, R. L. (1992) Comments Astrophys. 16, 1-15.McMillan, E. (1933) Phys. Rev. 44, 240.Greenstein, J. L. & Richardson, R. S. (1951) Astrophys. J. 113,536-546.Bonsack, W. K. & Greenstein, J. L. (1960) Astrophys. J. 131,83-98.Greenstein, J. L. & Tandberg-Hansen, E. (1954) Astrophys. J.119, 113-119.Greenstein, J. L. (1951) Astrophys. J. 113, 531-535.Sargent, W. L. W. & Jugaku, J. (1962) Astrophys. J. 134,777-782.Fowler, W. A., Greenstein, J. L. & Hoyle, F. (1962) Geophys.J. R. Astron. Soc. 6, 148-220.

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