Rachel Courtland asks a deep question in New Scientist:
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Ever since Arthur Eddington travelled to the island of Príncipe off Africa to measure starlight bending around the sun during a 1919 eclipse, evidence for Einstein’s theory of general relativity has only become stronger. Could it now be that starlight from distant galaxies is illuminating cracks in the theory’s foundation?
Everything from the concept of the black hole to GPS timing owes a debt to the theory of general relativity, which describes how gravity arises from the geometry of space and time. The sun’s gravitational field, for instance, bends starlight passing nearby because its mass is warping the surrounding space-time. This theory has held up to precision tests in the solar system and beyond, and has explained everything from the odd orbit of Mercury to the way pairs of neutron stars perform their pas de deux.
Yet it is still not clear how well general relativity holds up over cosmic scales, at distances much larger than the span of single galaxies.