As space expanded, the radiation and particles that permeated the Universe became more dilute and cooled down. They behaved like hot air trapped inside a balloon, which becomes less dense and cooler as the balloon expands. Because each energy component’s gravitational influence affects the expansion differently, the study of the Universe’s expansion over time allows astronomers to disentangle the separate contributions of radiation, matter, and dark energy. Matter and radiation dilute with the expansion but radiation, which redshifts to lower energy—much like a siren decreases in frequency as
...more
