Barely a hundred years after the discovery of phosphorus in 1669, Joseph Wright of Derby commemorated the event with a painting whose full title ran long: The Alchymist, in Search of the Philosopher’s Stone, discovers Phosphorus, and prays for the successful conclusion of his operation, as was the custom of the ancient chymical astrologers. The image is lit by the glow of a boiling flask, with the titular “alchymist” kneeling beside it. His workshop is a dark cathedral, but the contents of the flask light his face. Accompanied by two assistants, the man is surrounded by the astrological scribblings that have brought him this far, to the moment of truth. Now his eyes turn upward, and he yields to the mystery.
Despite the dramatic retelling, viewers acquainted with the history of phosphorus may snicker. Hennig Brandt, the ostensible subject of Wright’s painting, followed the alchemical tradition of keeping his recipe a secret, but others soon worked out step-by-step instructions for isolating phosphorus, which started with boiling urine — were the painting’s Smell-O-Vision activated, it might convince viewers that the alchemist’s tears were more than devotional.
Just a century after Brandt, it was possible for Wright to imagine him as an ancient graybeard, a figure whose bearing was less industrial than biblical. This transition was not so unusual. In sciences that make cumulative progress, even the hardest-nosed investigators of the past may come to seem primitive.
But even for his time, Brandt was odd. One of his fellow chemists described him as “a man little known, of low birth, with a bizarre and mysterious nature in all he did.” Compared with Andreas Sigismund Marggraf, who would study phosphorus soon afterward, Brandt seems displaced in time, a figure from an entirely different era. Marggraf was a scrupulous investigator, one of the fathers of analytical chemistry. Historians have credited him with discovering that animals contain and excrete phosphorus because they ingest it with food, not because their bodies spontaneously produce it.
This discovery took three years of Marggraf’s systematic work. But modern readers may be surprised to find that Marggraf also believed that phosphorus was “the unexpected generation of light and fire out of water, fine earth, and phlogiston.” To modern ears this sounds barely better than alchemy. Our estimation of Marggraf shifts abruptly when his speculations emerge in an obsolete framing.
But perhaps we should be less dismissive. The discoveries of Brandt and Marggraf — roughly, discoveries of how to isolate phosphorus from urine and seeds — were no less real because our descriptions of what they discovered might differ from their descriptions. We can’t deny that they found out some things, even if we might disagree with them on exactly what they found out.
This distinction should be kept in mind, now, as we abruptly turn to modern cosmology.
Some sixty years ago, Arno Penzias and Robert Wilson were scraping pigeon droppings from a large radio antenna at Bell Labs in New Jersey. They were radio engineers, and their project was straightforward. They wanted to see if they could eliminate the persistent background noise in radio signals, and they supposed that capturing the pigeons roosting in the horn and cleaning it might help. They realized that it didn’t.
Today, the radio noise they detected has been identified as the cosmic microwave background, a subject of intense scientific scrutiny. The pigeon traps have been displayed as relics by the Smithsonian’s National Air and Space Museum.
P. J. E. Peebles, known to colleagues as Jim, was a young postdoc at the time of this observation, in the early 1960s, just beginning his long career at Princeton University. He was working at the time with Robert H. Dicke, who was a pioneer of experimental relativity and was focused on testing the predictions of Einstein’s theories. Dicke had tasked two of his group members, Peter Roll and David Wilkinson, with measuring the long-wavelength light that theoretical calculations suggested would be left over from a hot “big bang” (as the idea’s skeptics called it). Peebles, fortuitously, was the group’s theorist.
When Dicke heard during a phone call with a colleague that Penzias and Wilson (who by then were asking academic researchers for suggestions) had identified a faint radio signal that was approximately the same in any direction they looked, he immediately suspected that it was exactly what he had asked Roll and Wilkinson to investigate. He returned to his underlings with the quip, “Well, boys, we’ve been scooped.” Penzias and Wilson agreed to publish their observations alongside the Dicke group’s explanation, mainly the work of a young Peebles, in the July 1965 issue of The Astrophysical Journal.
Cosmology has grown enormously as a science since then. Partly because of Peebles’s work in the decades that followed, notions of “dark matter” and “dark energy” have become commonplace. His importance to cosmology has won Peebles many awards, including the 2019 Nobel Prize in Physics.
Recognitions aside, Peebles never became a celebrity scientist, and his understatements in print have verged on unintentional comedy. His first book released after winning the Nobel — Cosmology’s Century: An Inside History of Our Modern Understanding of the Universe (2020) — was dense with equations and, despite friendly claims to the contrary, seemed written solely for his peers. Personal asides were limited to little flourishes, like when in the discussion of a previously unpublished calculation he adds the parenthetical, “I don’t remember making it, but it’s in our file, marked October 1968, so who else?”
But now Peebles has released a follow-up book, The Whole Truth: A Cosmologist’s Reflections on the Search for Objective Reality, that at last acknowledges that non-scientists might be interested in his work. The book functions as a humblebrag, arguing that those ideas Peebles has done the most to advance — the big bang, dark matter, dark energy — are likely to be permanent contributions to science, while also insisting that these discoveries were just waiting to be found. Even with his extensive achievements, Peebles supposes that if he were never born, the development of cosmology would hardly be different at all.
As a newly public figure, Peebles is an oddity. In one recent profile, for Princeton Alumni Weekly, a colleague compared him to the wizard Dumbledore, “in every way brilliant and benevolent.” His path, it seems, has been quite cozy.
The graduate of a tiny high school near Winnipeg, Peebles claims never to have thought about a career in science before enrolling at the University of Manitoba, where he started as an engineering student but found he had a taste for physics. A professor instructed him to go to Princeton for graduate school, so he went, and stayed there for the rest of his career. Each of the books he wrote without co-authors has been dedicated to his wife, Alison, whom he married before leaving Winnipeg. At the beginning of Principles of Physical Cosmology, Peebles also greets his three daughters.
His plain-spoken and self-effacing style can make one forget that Peebles is a genuine giant in his field, the man who first introduced the world to the theory that has become cosmology’s standard model. This is the ΛCDM model, pronounced by saying the letters: “lambda cee dee em.” The “Λ” refers to dark energy in the form of Einstein’s cosmological constant, and the “CDM” refers to “cold dark matter,” which is “cold” insofar as it moves with the galaxies we can see and is not speedy enough to leave them behind.
Peebles wears his prestige lightly, and in The Whole Truth he reflects on what cosmology shares with the broader quest for knowledge. “The theme of this book,” he writes in the preface, “is that the empirical results from natural science, presented in the worked example of physical cosmology, add up to a persuasive case for observer-independent reality.” What emerges in this “worked example” is a case that, with or without his help, cosmologists would eventually have settled on something like the ΛCDM model.
And if cosmologists would have arrived at the model sooner or later anyway, it must be pointing to the truth itself. This conclusion, for Peebles, is intimately tied up with what it means for cosmology to point toward an “observer-independent reality.” But is this reasoning enough? Peebles argues that the intersecting and reinforcing lines of evidence from the past decades make it difficult to imagine a successor to his model that doesn’t inherit many of its features.
This argument is fair, but it pushes the question down another level. That level is the question of what successor theories must inherit from their parent theories — and what might conceivably be thrown out in the process. Once chemistry got rid of phlogiston, was the science essentially unchanged? (We can still isolate phosphorus.) Did some residue of the old explanation remain?
Now, some readers might be wary of any physics book with the word “reality” in its title, having been burned enough times already. But The Whole Truth is less popular physics than amateur philosophy. In Cosmology’s Century, Peebles apologized for his limitations as a historian, sociologist, and philosopher, and The Whole Truth is heavily influenced by the reading he has done to shore up these weaknesses.
Of course, this influence is likely to annoy a different type of reader. Very few scientists (and even very few humanists) doubt that there is an objective layer to the world. Peebles also knows this, but roughly the first half of The Whole Truth functions as an olive branch to the humanists, valorizing their work to identify the social processes in science, before discounting their importance in the book’s second half.
In the very first chapter, “On Science and Reality,” Peebles describes the arguments over realism that took place among the American pragmatists at the turn of the twentieth century. Peebles is especially fond of Charles Sanders Peirce and quotes an 1878 Popular Science essay of his at length. Peirce wrote,
Different minds may set out with the most antagonistic views, but the progress of investigation carries them by a force outside of themselves to one and the same conclusion…. The opinion which is fated to be ultimately agreed to by all who investigate, is what we mean by the truth, and the object represented in this opinion is the real. That is the way I would explain reality.
This notion that “the truth” is “the opinion which is fated to be ultimately agreed to by all who investigate” defines the logic of Peebles’s considerations about ΛCDM. Peirce used the speed of light as a scientific fact whose consistent value could be reached through multiple different avenues. Peebles presents dark matter and dark energy in a similar vein, as two more conclusions reachable by many inroads.
Though he discusses some of their disagreements, Peebles is more likely to concede points to philosophers than to argue with them. For instance, Peebles briefly addresses Thomas Kuhn, the influential philosopher of science who proposed that after revolutionary “paradigm shifts,” scientists often misread their predecessors, as these shifts alter what counts as “normal science.” Peebles suggests that science more typically grows through “paradigm additions” than paradigm shifts, but allows that Kuhn’s influence must be acknowledged, since “the mutual influence of science and society seems obvious now.”
In the second chapter, when The Whole Truth turns to “The Social Nature of Physics,” Peebles lays the groundwork for a broader argument, introducing, in among the potted literature reviews, two ideas that recur throughout the book. One is the existence of multiples in scientific discovery — that discoveries are typically arrived at by multiple independent investigators. The other idea is the no miracles argument for scientific realism — the argument that it would be a “miracle” for science to be so successful if it didn’t realistically represent the world. It will be worth our time to linger over these concepts.
Singletons and Multiples in Scientific Discovery: A Chapter in the Sociology of Science” is a 1961 article by the American sociologist Robert K. Merton. In it, Merton discusses how “singletons,” those isolated geniuses who make unique discoveries, are very rare in the history of science. Much more common are “multiples,” those co-discoverers, or co-co-discoverers, who happen upon similar ideas at approximately the same time. This leads Merton to speculate about the inevitability of scientific progress. Merton’s view of scientific progress can be understood by picturing a game where you roll a ball downhill. A scientific genius, in Merton’s view, is not someone who changes the game being played. The genius merely runs fast enough to kick the ball off its plateau before others reach it.
This near-inevitability of scientific discovery drives scientists into competition, for they fear — like Robert Dicke — that they’ll get scooped. Peebles notes that “when the material culture allows a discovery, more than one group can take advantage of the opportunity and produce a multiple.”
As he surveys cosmology, Peebles sees these multiples everywhere. And so what? For him, they seem to function as proof that the internal logic of science has pointed in a particular direction — toward “the opinion which is fated,” as it were.
Of course, a skeptic might view it differently, and it would be hard to tell the difference. We might suppose that scientists across the globe just share a similar intellectual culture. When new observations come to light, why shouldn’t they reach for similar explanations?
Were Peebles forced to judge these ideas about how scientists work — as independent rational observers, or as uncoordinated group-thinkers — I suspect he’d grant that some of each is going on. Besides, he might demur, once we have the models, don’t they take on a life of their own?
This is where the no miracles argument comes in. Whether we frame scientists as the inventors of science or merely its midwives, scientific models are eventually tested on their own merits, and the best ones can be used to make impressive predictions about what happens in the natural world. And why is that? The least adventurous reason one might forward is that the models accurately describe real things in the world. As the philosopher Hilary Putnam wrote, “The positive argument for realism is that it is the only philosophy that doesn’t make the success of science a miracle.” Throughout the book, Peebles suggests that accurate predictions show that models indeed depict reality.
Naturally, a skeptical view is possible here, too. Future theories might salvage the empirical content and predictive acumen from our best theories today without retaining all their metaphysical commitments. Copernicus and Kepler and Newton kicked away Ptolemy’s old system of deferents and epicycles, but the old astronomers’ calendars were still able to predict the motions of the planets as well as ever.
For his part, Peebles recognizes this as a logical possibility, but he isn’t holding his breath. He notes,
We have no measure of the probability that there is some other theory that passes other tests that produce an equally satisfactory picture of the world around us. In natural science we simply rely on the intuition that this seems exceedingly unlikely. It is the best science can do.
If they’re not careful, celebrated scientists may find themselves splitting history into just two parts — before and after they arrived. Peebles is alert to this danger, but the attitude sometimes creeps into The Whole Truth. Before his arrival, he is happy to notice the “corruption of physical science by social pressure and over-enthusiastic promotion of theories,” but after his arrival, cosmology seems to have simply followed the data where it led. Given his central place in the story, I can sympathize with Peebles, and, to his credit, he attempts caution, often stopping to distinguish theoretical claims from matters of fact.
But Peebles also appreciates that what sociologists have identified as “social construction” of scientific theories is not just “corruption” in any usual sense. He refers many times to Laboratory Life: The Social Construction of Scientific Facts, the 1979 ethnography of scientists by Bruno Latour and Steve Woolgar, who described how scientific conclusions emerge as scientists navigate social, experimental, fiscal, and other assorted pressures. Peebles agrees with their observation that scientific models slide along a continuum of constructed-ness, from those that are built solely on group opinion to those that sit on a bedrock of fact.
As he explores in the third chapter, general relativity, Einstein’s theory of gravity published in 1915, had very little experimental support until the 1960s, but it was a standard part of Peebles’s education nonetheless. In the years since, he argues, its construction has become empirical, as it continues to pass ever more rigorous tests.
This case seems ambiguous. Was the physics community right to embrace general relativity despite its uncertain empirical status? Is this just an example of scientific survivorship bias?
The question grows even more uncertain in the case of cosmology, where big assumptions are hard to test. Einstein’s cosmological principle, the subject of the fourth chapter, is a case in point. Einstein had originally hoped to use his newfound theory of gravity to describe a big, old, uniform universe — approximately the same in all spatial directions, and approximately unchanging in time. This was why Einstein added his cosmological constant, Λ (lambda). “If this constant has just the right value,” Peebles writes, “then its repulsion balances the attraction of gravity, allowing a static universe.”
Did our universe in fact follow Einstein’s cosmological principle? And was his static universe the real one? Each question suggested empirical tests, not to be settled by philosophical preference alone.
Peebles insists that, as with general relativity, Einstein’s cosmological principle eventually got an epistemological upgrade. “The cosmological principle became an empirical construction by 1980, when we could add the evidence from the isotropy of the seas of X-rays and microwaves, and the test of scaling of the galaxy correlation functions with the depths of the galaxy catalogs.” In other words, to put the cosmological principle crudely, the fact that all these types of stuff — X-rays, microwaves, galaxies — look basically the same, on average, in all different directions, is evidence that the universe is basically the same wherever you might go.
On the other hand, the universe is not static. By the 1930s, Edwin Hubble established that light from more distant galaxies tends to be redshifted — that is, that equivalent wavelengths of light tend to be elongated. This redshift could be interpreted as evidence that faraway galaxies were receding from us more quickly than nearby ones. This result came to be known as “Hubble’s law.”
When Peebles was a graduate student, cosmologists still argued over the meaning of this law. In those days, the fact that galaxies were streaming outward seemed like a clear indication that Einstein had been on the wrong track with his cosmological constant. But what were the other possibilities?
Steady-state theorists, in the spirit of Einstein, proposed that matter might be bubbling up out of the void, keeping the universe at a constant density even as the galaxies recede. But proponents of various “big bang” scenarios contended that Hubble’s law implied that the universe began at some time in the past and was expanding.[*]
Which brings us back to Penzias and Wilson and pigeon droppings.
Why was the direction of cosmology altered so decisively when two radio engineers observed the cosmic microwave background? It wasn’t because of just one thing. The “hot big bang” scenario predicted the cosmic microwave background, since matter and light were presumed to be in thermal equilibrium at the beginning. As the universe expanded and cooled, the matter was thought to clump together gravitationally, becoming stars and galaxies, while the light was thought to zip around diffusely, becoming the cosmic microwave background. But the hot big bang also solved other problems. Steady-state theorists posited that hydrogen was being created ex nihilo, while heavier elements could be forged by nuclear fusion in the stars. But their numbers didn’t work out. Steady-state theorists found that helium was more abundant in stars than stellar fusion alone would predict. But in a hot big bang, hydrogen could also be fused into helium at the very beginning.
Thus the hot big bang could explain two observations at once, both the cosmic microwave background and the puzzlingly high abundance of helium. As Peebles points out, “A theory that fits two puzzling and otherwise quite different observations certainly is worth close attention.” In this new way of looking at things, the cosmic microwave background and the overall ratio of hydrogen atoms to helium atoms were fossils, relics to be studied for their secrets.
But as empirical puzzles accumulated, the hot big bang scenario also came to seem incomplete.
Given that dark matter and dark energy continue to be debated in physics today, their introduction might seem like a strange climax for The Whole Truth. But within cosmology, these elements — substances? theoretical placeholders? — were accepted for a similar reason that the hot big bang was accepted. They solved multiple problems at once. To physicists, this has seemed not like some cheap trick to increase the value of junk bonds by bundling them together, but rather like a strategy for the discovery of new truths. After all, it’s difficult enough to build models to match existing evidence. When models accommodate previously unknown facts too, it’s hard not to be impressed.
For instance, readers who keep up with astronomy news will know that stars at the edges of galaxies have been observed to move faster around their galactic centers than one would predict from the gravitational pull of visible matter alone. This was an early reason for introducing “cold dark matter” (the CDM in Peebles’s ΛCDM) as a source for the extra gravity needed to keep fast stars in their orbits. But in the context of the hot big bang, cold dark matter also helped to explain how galaxies had formed in the first place, not to mention that it predicted previously unobserved patterns in the cosmic microwave background.
Likewise, Peebles introduced dark energy — or perhaps reintroduced, since he simply restored Λ, the cosmological constant — into his now-standard ΛCDM model just to satisfy the requirements of general relativity. Extra energy density was needed, beyond the amount provided by the combined contributions of visible and cold dark matter, to explain why our universe at large scales appears “flat” — parallel rays of light stay a fixed distance apart, as in Euclid’s rules — even though general relativity without Λ implied that space should be curved and open — that parallel rays should grow further apart. The fact that ΛCDM could later accommodate observations of the accelerating expansion of the universe was just another sign that it was on the right track.
Peebles discovered the ΛCDM model alone (a “singleton,” as he points out), yet he remains admirably self-effacing and insists that multiple others would have reached it, had he not gotten there first. “I am not being modest; this is my attempt to see how the story might have played out.”
In the last chapter of The Whole Truth, “Lessons from a Scientific Advance,” Peebles summarizes his arguments. There were multiple paths toward his model. It predicted effects beyond its original causes, and satisfied tests that had not been performed when it was proposed.
So then — are dark matter and dark energy real? Peebles thinks so, and claims their acceptance as a fact of history and science alike. As Peebles writes, following a recap of his alternative history,
Was the construction and acceptance of the ΛCDM theory inevitable? The evidence reviewed here makes the case that it would happen about as close to compelling as it can get. And let us note that this is what we expect if our present physical cosmology is a good approximation to objective reality, not a social construction.
Yet at this point, a little more skepticism is in order.
Let’s grant the ΛCDM theory its success. Let’s grant, as a relevant example, that the theory accommodated new patterns in the cosmic microwave background just exactly as they were observed. This was a real scientific discovery, a phenomenon that probably wouldn’t have been sought without the theory. Were Peebles’s claim only that the theory was an effective tool for generating hypotheses that could be tested — and that have been found accurate — then there would be no room for disagreement. But once Peebles mentions reality, a door to criticism opens. The claim does have an intuitive appeal, but the fact that a theory makes reliable predictions does not, unfortunately, imply that the constituents of said theory (the Λ, say, and the CDM) are themselves real.
This point should be obvious to those who have studied the history of physics, but physicists themselves are often most guilty of such oversights. We often overlook historical examples of theories whose parts, like the Ship of Theseus, have all been replaced without compromising their success. The nineteenth-century French mechanical engineer Sadi Carnot, to take one example, presumed that heat was a fluid, but his model of engine efficiency transitioned smoothly to the mechanical theory of heat. James Clerk Maxwell, to take another, supposed that a mechanical ether undergirded all electromagnetic effects, but his equations still worked without it. If a theory’s mathematical structure persists, its meaning may change while its predictions remain intact.
Of course, ΛCDM may follow a different path. Perhaps one day the existence of dark matter and dark energy will be directly confirmed; maybe they will even be routinely manipulated. Dark matter certainly suffers from no shortage of laboratory proposals, and indirect observations of dark matter have become routine. While direct detection efforts for dark matter haven’t found much, it would be premature to give up hope. Proposals for direct detection of dark energy remain much more speculative, but who knows — someday these, too, could amount to something.
Laboratory tests of ΛCDM aren’t mentioned in The Whole Truth, possibly because they don’t seem relevant. In a section near the end titled “The Future,” Peebles predicts that successful cosmological theories will continue to resemble his theory and that no “final” theory will replace it. “I expect the cycle of search and discovery in physical cosmology to end in exhaustion,” he writes, “rather than the claim of empirical establishment of a final theory.”
Again, could be. But here the short timeline of professional cosmology, just a century of serious work, clashes with the long timeline of human interest in cosmic origins, which is unlikely to end anytime soon.
Throughout both Cosmology’s Century and The Whole Truth, Peebles mentions many possibilities that once seemed promising but were abandoned as more immediately fruitful approaches were found. As the victor, he can afford to be generous toward them. Though he is aware that this looks, from the outside, like the social construction of science, from the inside it looks like simple pragmatism, just doing what works. Given that the cosmology community now has a theory, ΛCDM, that passes so many empirical tests, it’s hard to imagine that anyone simply trying to make progress on outstanding empirical issues would look upon trawling the abandoned possibilities as anything but a waste of time.
But now we might take the long view. We might entertain the worst-case scenario for fundamental physics in the next few decades and suppose that all cosmological measurements can be squared with ΛCDM, but that no direct searches for dark matter or energy find anything. As Peebles appreciates, this scenario could mark the end of cosmology as a vibrant branch of physical science. Should new observations merely serve to confirm the old theory, ambitious scientists will look elsewhere for problems to solve.
But I doubt this would mark the end of cosmological speculation. There will always be some who find the standard account unconvincing, who will continue to suspect that there should be more. If the vast astronomical datasets that have already been collected are not lost, we might imagine one such figure hovering over a glowing screen, like Hennig Brandt over his boiling flask, the datasets whispering like cryptic runes. We might imagine this digital alchemist, this data spelunker, continuing onward in that unending search for the alpha and the omega, the beginning and the end, fueled by the eternal unpragmatic faith that beyond the conclusions of the dogmatists, beyond the exhaustion of doubt, meaningful patterns yet remain to be discovered in the dark cathedral of the stars.
[*] Interested readers may take this as a homework problem. If we assume that today’s version of Hubble’s law held indefinitely far back in the past (cosmologists don’t believe this, but it’s useful as a first approximation), how long ago did the universe begin? Hubble’s law is v = H0r, where v is the average recessional speed of a faraway galaxy, r is its distance from Earth, and H0 is “Hubble’s constant,” a value with units of speed per distance.
The answer will be roughly equivalent to 1/H0. The best modern estimate for the Hubble constant H0 is around 71 km per second per megaparsec. Since 1 megaparsec is around 3.1 × 1019 kilometers, the distances in the numerator and denominator of H0 can be made to cancel, leaving the quantity in terms with units of 1/seconds. If we calculate 1/H0, then, it will have units of time. Since one year is around 3.2 × 107 seconds, the “Hubble time” 1/H0 works out to be around 14 billion years — which, reassuringly, is not far from modern estimates for the age of the universe.
The Big Whimper