  The observation that Type Ia supernovae are fainter than expected given their
red shifts has led to the conclusion that the expansion of the universe is
accelerating. The widely accepted hypothesis is that this acceleration is
caused by a cosmological constant or, more generally, some dark energy field
that pervades the universe. This hypothesis presents a challenge to physics so
severe that one is motivated to explore alternative explanations. In this
paper, we explore whether the data from Type Ia supernovae can be explained
with an idea that is almost as old as that of the cosmological constant,
namely, that the strength of gravity varies on a cosmic timescale. This topic
is an ideal one for investigation by an undergraduate physics major because the
entire chain of reasoning from models to data analysis is well within the
mathematical and conceptual sophistication of a motivated undergraduate.
