The discovery that our universe is accelerating poses fascinating challenges for physics and astronomy. Vacuum energy, more generally called dark energy, is a possible explanation for the observed cosmic acceleration. Modifications to Einstein's general relativity are being explored as alternatives to dark energy. I will describe the theoretical motivations and experimental tests of gravity theories and other scenarios that go beyond the standard model of cosmology. On large scales, they require a different take on cosmological observations such as gravitational lensing and large-scale structure in the universe. On much smaller scales the new tests of gravity use pulsating stars, rotating disk galaxies and other astronomical phenomena.