1 Simulated quasar absorption spectrum
1 Simulated quasar absorption spectrum
Quasars are highly luminous objects that emit light over a wide range of wavelengths (red line), with peaks at several wavelengths due to emission by elements such as hydrogen, nitrogen, silicon, carbon and iron in the gas around the quasar. When light from the quasar passes near a galaxy on its way to Earth, the gas around the galaxy causes a distinct pattern of absorption lines in the quasar spectrum (green line). By measuring the wavelengths of the absorption lines due to heavy elements (on the right of the figure) we can determine both the redshift of the gas and the value of the fine-structure constant, α, at the time when the light from the quasar was absorbed. Such observations suggest that the value of α was slightly smaller billions of years ago. The plethora of absorption features at the shorter wavelengths on the left are due to hydrogen at different redshifts.