During a solar eclipse in 1919, Arthur Eddington observed light bending around the sun just as predicted by general relativity, Albert Einstein’s new theory of gravity. Since then, general relativity, which says that massive objects like stars warp the fabric of space-time around them, has passed increasingly precise tests. A year rarely goes by without a new experiment or observation confirming Einstein’s theory. But there’s a hitch.
Invisible substances known as dark matter and dark energy seem to make up some 95% of the content of the universe. The working assumption is that dark matter consists of nonluminous elementary particles, and that dark energy is the energy of space itself. But it’s also possible that they are illusions that appear because gravity works differently from how Einstein thought about it. “We’re invoking these mysterious things,” said the cosmologist Celia Escamilla-Rivera.“I am strongly convinced that alternative theories of gravity are needed.”
Escamilla-Rivera is searching for another, more complete theory. A bewildering array of alternatives to general relativity have been put forward over time, from “teleparallel gravity” to “complex quintessence” and “negative-mass cosmology,” but they long seemed like theoretical fancies. With cosmologists unable to create experiments that can distinguish these theories from general relativity, the ideas have gathered dust.
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