Modern Cosmology: Science or Folktale?
Current cosmological theory rests on a disturbingly small number of independent observations
It appears that everybody is interested in cosmology. In one anthropological study, every one of the more than 60 separate cultures examined was found to have several common characteristics, including "faith healing, luck superstitions, propitiation of supernatural beings, … and a cosmology." Apparently, to be human is to care how the physical world came to be, whether it has boundaries and what is to become of it. Modern cosmology is a highly sophisticated subject funded by governments with hundreds of millions of dollars a year. It is unquestionably interesting, but is it, even in its modern guise, convincing?
The current Big Bang paradigm has it that the cosmos is expanding out of an initially dense state and that by looking outward into space, one can, thanks to the finite speed of light, look back to much earlier epochs. This understanding owes much to two accidents: astronomers' discovery of redshifts in the spectra of distant nebulae and the fortuitous detection of an omnipresent background of microwave noise, which is believed to be the remnant of radiation from a hot and distant past. Set in the theoretical framework of Einstein's general theory of relativity, such observations lead to a model that makes predictions and can thus be tested.
Of late, there has been much excitement over precision measurements of the cosmic background radiation and the discovery of very distant galaxies of great antiquity. There is even talk of a "concordance model" in which all of the observations come together to paint a coherent picture of how the universe must be constructed.
It is true that the modern study of cosmology has taken a turn for the better, if only because astronomers can now build relevant instruments rather than waiting for serendipitous evidence to turn up. On the other hand, to explain some surprising observations, theoreticians have had to create heroic and yet insubstantial notions such as "dark matter" and "dark energy," which supposedly overwhelm, by a hundred to one, the stuff of the universe we can directly detect. Outsiders are bound to ask whether they should be more impressed by the new observations or more dismayed by the theoretical jinnis that have been conjured up to account for them.
My limited aim here is to discuss this dilemma by looking at the development of cosmology over the past century and to compare the growing number of independent relevant observations with the number of (also growing) separate hypotheses or "free parameters" that have had to be introduced to explain them. Without having to understand the complex astrophysics, one can still ask, at an epistemological level, whether the number of relevant independent measurements has overtaken and comfortably surpassed the number of free parameters needed to fit them—as one would expect of a maturing science. This approach should be appealing to nonspecialists, who otherwise would have little option but to believe experts who may be far too committed to supply objective advice. What one finds, in my view, is that modern cosmology has at best very flimsy observational support.
A Short History of Cosmology
Almost a century ago, Einstein's general theory of relativity posited that matter and energy could bend spacetime. This idea was philosophically attractive because it removed the need to worry about cosmic boundaries if the universe closed back on itself.
Unfortunately (or so Einstein then thought), general relativity implied that the universe would have to either collapse or expand. So in 1921 he found room in his theory for a new free parameter, the so-called "cosmological constant," an arbitrary antigravity term that would put a stop to all that. Ironically, the observers who were examining faint nebulae (distant galaxies) at the time discovered that their spectra were dramatically redshifted—hinting that on its largest scale the universe was expanding after all.
In 1965 Arno Penzias and Robert Wilson stumbled accidentally onto the cosmic background radiation, a microwave whisper arriving from all directions of the sky. As cosmologists interpret it now, they were observing optical radiation emitted by the gas of the universe when it was hot (3,000 degrees Celsius), opaque and relatively young (300,000 years old), redshifted through the enormous factor of a thousand by subsequent cosmic expansion. They were looking into the past with a vengeance and seeing the remnants of what astronomer Fred Hoyle dismissively called the "Big Bang." From then on, the expanding universe was accepted, usually without question, as a natural explanation for the microwave background.
At the same time, astrophysicists sought to understand the origin of the elements. It seemed that most had formed from the fusion of pristine hydrogen inside stars and then had been expelled into general circulation when those stars exploded as supernovae. However, some of the lighter elements, in particular helium, deuterium and lithium, would have had to form much earlier, during the first minutes of the Big Bang. The theory of Big Bang nucleosynthesis did a fair job of predicting the relative amounts of most of these substances, lending more support to the notion of an expanding universe.
Robert Dicke meanwhile noticed a worrying paradox in the Big Bang model: Opposite sides of the cosmos look very much the same, even though they had never been sufficiently close to equilibrate—indeed they had never been sufficiently close for any kind of information (which is limited to the speed of light) to travel between them. This difficulty was virtually unadmitted until 1981, when Alan Guth suggested a vague conceptual solution called "inflation": a slow start to expansion, followed by a rapid acceleration. The necessary causal contacts could then have taken place when the universe was young but not yet flying apart too fast. If inflation actually happened, sufficient stretching during that period of rapid acceleration would have lowered the local curvature today so that it would look flat to the observer, even if it wasn't so on a much larger scale (just as the Earth looks flat to someone with a limited horizon).
At about this time in Holland, Albert Bosma discovered that spiral galaxies are spinning far too rapidly to be held together by the mutual gravitational tugs of their observable contents. Astronomers concluded that there had to be far more dark than ordinary, visible matter around to keep galaxies (and galaxy clusters) together. Most cosmologists welcomed the possibility of such dark matter, because it might be lumpy enough to get the galaxies formed in the early universe—another serious problem for theorists. The apparent uniformity of the cosmic background radiation had cosmologists struggling to figure out how the present uneven structure of galaxies and clusters evolved out of such a smooth beginning.
They thus posited the existence of primordial "seeds" of unknown origin, which somehow survived the early, hot era when radiation would tear material things apart. Cosmologists argued that these seeds would grow over time, finally collapsing into the galaxies seen today. A type of dark matter that ignored radiation ("cold dark matter") would be the ideal stuff for such seeds. It could condense into lumps, thereafter dragging the much lesser amounts of ordinary matter in afterwards, matter that would eventually light up as stars. By the 1980s the theoreticians' universe was entirely dominated by such invisible material.
Meanwhile, observations of distant supernovae in the late 1990s told an astonishing, almost shocking, story. The results suggested that the expansion, far from being slowed by gravitation, as was expected, had instead accelerated. Moreover, this acceleration had started only in comparatively recent times (7 billion years ago). The physics responsible for this seeming acceleration is entirely unknown and goes under the deliberately inscrutable name "dark energy," which may or may not have something to do with Einstein's cosmological constant.
The Significance of Cosmology
The currently fashionable concordance model of cosmology (also known to the cognoscenti as "Lambda-Cold Dark Matter," or lambda-CDM) has 18 parameters, 17 of which are independent. Thirteen of these parameters are well fitted to the observational data; the other four remain floating. This situation is very far from healthy. Any theory with more free parameters than relevant observations has little to recommend it. Cosmology has always had such a negative significance, in the sense that it has always had fewer observations than free parameters (as is illustrated at left), though cosmologists are strangely reluctant to admit it. While it is true that we presently have no alternative to the Big Bang in sight, that is no reason to accept it. Thus it was that witchcraft took hold.
The three successful predictions of the concordance model (the apparent flatness of space, the abundances of the light elements and the maximum ages of the oldest star clusters) are overwhelmed by at least half a dozen unpredicted surprises, including dark matter and dark energy. Worse still, there is no sign of a systematic improvement in the net significance of cosmological theories over time.
Where Do We Stand Today?
Big Bang cosmology is not a single theory; rather, it is five separate theories constructed on top of one another. The ground floor is a theory, historically but not fundamentally rooted in general relativity, to explain the redshifts—this is Expansion, which happily also accounts for the cosmic background radiation. The second floor is Inflation—needed to solve the horizon and "flatness" problems of the Big Bang. The third floor is the Dark Matter hypothesis required to explain the existence of contemporary visible structures, such as galaxies and clusters, which otherwise would never condense within the expanding fireball. The fourth floor is some kind of description for the "seeds" from which such structure is to grow. And the fifth and topmost floor is the mysterious Dark Energy, needed to allow for the recent acceleration of cosmic expansion indicated by the supernova observations. Thus Dark Energy could crumble, leaving the rest of the building intact. But if the Expansion floor collapsed, the entire edifice above it would come crashing down. Expansion is a moderately well-supported hypothesis, consistent with the cosmic background radiation, with the helium abundance and with the ages inferred for the oldest stars and star clusters in our neighborhood. However, finding more direct evidence for Expansion must be of paramount importance.
In the 1930s, Richard Tolman proposed such a test, really good data for which are only now becoming available. Tolman calculated that the surface brightness (the apparent brightness per unit area) of receding galaxies should fall off in a particularly dramatic way with redshift—indeed, so dramatically that those of us building the first cameras for the Hubble Space Telescope in the 1980s were told by cosmologists not to worry about distant galaxies, because we simply wouldn't see them. Imagine our surprise therefore when every deep Hubble image turned out to have hundreds of apparently distant galaxies scattered all over it (as seen in the first image in this piece). Contemporary cosmologists mutter about "galaxy evolution," but the omens do not necessarily look good for the Tolman test of Expansion at high redshift.
In its original form, an expanding Einstein model had an attractive, economic elegance. Alas, it has since run into serious difficulties, which have been cured only by sticking on some ugly bandages: inflation to cover horizon and flatness problems; overwhelming amounts of dark matter to provide internal structure; and dark energy, whatever that might be, to explain the seemingly recent acceleration. A skeptic is entitled to feel that a negative significance, after so much time, effort and trimming, is nothing more than one would expect of a folktale constantly re-edited to fit inconvenient new observations.
The historian of science Daniel Boorstin once remarked: "The great obstacle to discovering the shape of the Earth, the continents and the oceans was not ignorance but the illusion of knowledge. Imagination drew in bold strokes, instantly serving hopes and fears, while knowledge advanced by slow increments and contradictory witnesses." Acceptance of the current myth, if myth it is, could likewise hold up progress in cosmology for generations to come.