An artist’s impression of a supermassive black hole at the heart of a quasar. A correlation between emissions of ultraviolet light, generated in the disc surrounding the hole, and X-ray emissions from clouds of hot gas above and below the disc allow astronomers to use quasars as “standard candles,” measuring the effects of dark energy over cosmic time. Image: ESA–C. Carreau
X-ray observations of some 1,600 quasars, galaxies harboring active supermassive black holes in their cores, indicate that dark energy, the mysterious force causing the expansion of the universe to accelerate, may not be a “cosmological constant” after all. It may instead be growing stronger over cosmic time.
Dark energy was discovered about 20 years ago based on observations of type 1a supernovae, explosions in binary star systems that generate the same levels of brightness. By comparing a type 1a supernova’s apparent brightness with its known absolute luminosity, astronomers can use them as “standard candles” to measure the distance to the host galaxy.
After precisely measuring the distances and velocities of some four dozen type 1a supernovae, researchers concluded the expansion of the universe began speeding up about 6 billion years ago. The dark energy powering that acceleration was thought ...