Dark Energy

Dark energy is the name physicists give to the mysterious something that appears to be driving the expansion of the universe to speed up over time. It is not "dark matter," a separate puzzle, but a property of space itself that pushes outward. Although it cannot be seen or directly detected, its effects suggest it accounts for roughly two-thirds of the total content of the universe, making it the dominant feature of the cosmos and one of the greatest unsolved problems in science.

Why it matters

Until 1998, most cosmologists expected the expansion of the universe to be gradually slowing under the pull of gravity. The shock discovery that it is in fact accelerating, made by studying distant exploding stars, forced the introduction of dark energy and earned a Nobel Prize. It dramatically changed the likely long-term fate of the universe, from a possible slowing or recollapse to an ever-faster, ever-colder expansion.

The connection to Hawking

Dark energy is closely related to an idea with a long and tangled history: the cosmological constant, a term Einstein once added to his equations of general relativity and later regretted. Hawking, working on the origin and overall shape of the universe, engaged with the question of why empty space carries energy at all, a problem that connects the largest scales of cosmology to the quantum physics of the vacuum he studied in his work on black holes. It remains unresolved.