The theory that dark energy is Einstein's Cosmological Constant has received a boost by the neutron star merger observed on 17 August 2017 (see below). Dark energy is the name given to whatever is driving the accelerated expansion of the Universe. Gravitational waves are subject to the expansion of space, but to what degree varies from theory to theory. For example, one class of theory posits that the strength of dark energy can change across time and space, and the more complex theories among this class predict that dark energy would cause gravitational waves to take longer to reach us than light On the other hand, the theory that dark energy is a cosmological constant with the same strength across all of space and time, predicts that gravitational waves and light will be affected equally.
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