Gravity: The Popper Problem

Gravity: The Popper Problem

David Merritt, Astrophysicist and professor at the Rochester Institute of Technology

The universe is expanding, and Einstein’s theory of gravity makes a definite prediction about how the expansion rate should change over time: it should decrease, since the gravitational attraction between all the matter in the universe continually opposes the expansion.

The first time this prediction was observationally tested, around 1998, it was found to be spectacularly in error. The expansion of the universe is accelerating, not decelerating, and the acceleration has been going on for about six billion years.

How did cosmologists respond to this anomaly? If they adhered to the ideas of philosopher Karl Popper, they would have said: “Our theory of gravity has been conclusively disproved by the observations; therefore we will throw our theory out and start afresh.” In fact, they did something very different: they postulated the existence of a new, universe-filling substance which they called “dark energy”, and endowed dark energy with whatever properties were needed to reconcile the conflicting data with Einstein’s theory.

Philosophers of science are very familiar with this sort of thing (as was Popper himself). Dark energy is an example of what philosophers call an “auxiliary hypothesis”: something that is added to a theory in order to reconcile it with falsifying data. “Dark matter” is also an auxiliary hypothesis, invoked in order to explain the puzzling behavior of galaxy rotation curves.