The Pleasures of Model Drift

24 April 2010


Yesterday in Twenty Years of the Hubble Space Telescope I briefly touched on the discovery roughly ten years ago that the expansion of the universe is accelerating rather than decelerating. We ought to consider ourselves fortunate to be living in a time of genuine model drift in cosmology. One might pass one’s entire life in a period of normal science in which revolutions do not occur, and that would not be nearly as interesting as witnessing first hand the emergence of anomalies in a major scientific theory.

Model drift is one of the stages in the Kuhnian cycle such that it has been recognized that anomalies have emerged within an established scientific paradigm, and this paradigm is subject to drift, or change, as a result of anomalies that cannot be explained in an obvious way by the model. Model drift can be resolved in some cases so that the theory is changed but not abandoned. In other cases, in cases of a genuine scientific revolution, the previous model is abandoned once the anomalies accumulate to the point of triggering a model crisis.

Once Hubble demonstrated that the universe was expanding, the “great idea” of twentieth century cosmology was simply to reverse the expansion of the universe and collect the scattered galaxies at a single point of origin. This is the Big Bang Theory, and this is the model that is currently adrift. The most famous cosmologist of our time, Stephen Hawking, made some of his contributions to cosmology by modeling the big bang on lesser cosmic explosions such as novae and supernovae, as well as conceptualizing the big bang in terms of reversing the collapse of a star into a singularity. This opened the way to thinking of the point of origin of the universe as the “initial singularity” as it is sometimes called.

We know now that the universe is not only expanding, but that its expansion is accelerating.

Intuitively, the big bang is like an enormous explosion. Or, in other words, the idea of an ordinary explosion gives us a kind of intuitive grounding for our theory. And if the Big Bang was an enormous explosion, then in stands to reason that after that explosion the remnants continue to fly outward in all directions, but they slow down as they do. But, of course, there are problems with this intuitive model. Was there a flash of light when the big bang went bang? Was there an enormous “KABOOM!”? These are, at least in part, philosophical questions, like whether a tree falling in a forest with no one present makes a sound.

If we make an effort to insulate ourselves from the philosophical difficulties implied by the intuitive basis of the big bang theory (and science usually bends every effort to avoid the philosophical problems implicit in its formulations), we can avoid some of the difficulties of the theory. But other anomalies have now emerged. It is more difficult to conceptualize the big bang as an enormous explosion if we must picture to ourselves the remnants flying outward at ever-increasing speeds. So if the big bang didn’t result in decelerating debris and it didn’t make a big sound and it didn’t make a flash of light, we have to ask ourselves if it really was any kind of explosion at all. And if it was not an explosion, what was it?

I have no answer to this question. Moreover, I don’t think that anyone yet has a fully satisfactory answer to this, or even a sketch of an answer. What is needed is some kind of unpredictable intuitive breakthrough that will allow us to re-conceive the origins of the universe as it is now known to science, which means that any re-conceptualization must not fly in the face of established data like the cosmic microwave background radiation. Such data might be re-interpreted, but it cannot be wished away. To date, the big bang theory has adequately captured our intuitions about the naturalistic origins of the universe as well as explaining the data, but it is now a model experiencing drift. Whether the drift marks a correction in the model or the first indications of the model crisis we have yet to see.

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