Light (and radio waves) from the distant galaxies has taken time to travel to Earth, allowing astronomers see the galaxies as they were between three and five billion years ago. Astronomers have known for at least 15 years that the rate of star formation peaked when the Universe was only a few billion years old and has declined steeply ever since.
According to Dr Robert Braun (CSIRO Astronomy and Space Science): “Our result helps us understand why the lights are going out. Star formation has used up most of the available molecular hydrogen gas.”
Stars form from clouds of molecular hydrogen. The less molecular hydrogen there is, the fewer stars will form. After stars form, they shed gas during various stages of their lives, or in dramatic events such as explosions (supernovae). This returns some gas to space to contribute to further star formation.
“But most of the original gas—about 70%—remains locked up, having been turned into things such as white dwarfs, neutron stars and planets,” Dr Braun commented, “So the molecular gas is used up over time. We find that the decline in the molecular gas is similar to the pattern of decline in star formation, although during the time interval that we have studied, it is declining even more rapidly.”
Dr. Braun noted: “Ultimately, the real problem is the rate at which galaxies are ‘refueled’ from outside. Gas falls into galaxies from the space between galaxies, the intergalactic medium. Two-thirds of the gas in the universe is still found in the intergalactic medium and only one third has already been consumed by previous star formation in galaxies, astronomers think. The drop-off in both gas availability and star formation seems to have started around the time that Dark Energy took control of the Universe.”
Up until that time, gravity dominated the Universe, so the gas was naturally pulled in to galaxies, but then the effect of Dark Energy took over and the Universe started expanding faster and faster. This accelerating expansion will have made it increasingly difficult for galaxies to capture the additional gas they need to fuel future generations of star formation, Dr Braun added.