When Did the Universe Flood With Water?

By studying ancient molecular clouds in our galaxy, astronomers have revealed that the universe’s reservoir of water likely appeared much earlier than thought — only a billion years after the Big Bang.

The challenge facing water formation, a molecule composed of two hydrogen atoms and an oxygen atom, is that any element heavier than helium had to have been formed in the cores of stars and not by the Big Bang itself.

The earliest stars would have taken some time to form, mature and die, so elements as heavy as oxygen would have emerged from its furnace through stellar winds and supernovae some time later. With this delay in mind, and the time it would have taken for these oxygen atoms to disperse throughout the cosmos and attach to hydrogen, astronomers have long thought that H2O appeared throughout the universe rather late.

But according to new research published in the journal Astrophysical Journal Letters, this may not have been the case. In fact, there was likely an abundance of water only a billion years after the universe was born.

“We looked at the chemistry within young molecular clouds containing a thousand times less oxygen than our sun. To our surprise, we found we can get as much water vapor as we see in our own galaxy,” said Avi Loeb, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics (CfA), Mass.

The first stars to pop into existence around 100 million years after the Big Bang were massive and unstable. They quickly burnt through their supply of hydrogen fuel, exploding as supernovae. These stellar explosions seeded the universe with heavier elements. The result was pockets of gas rich in heavy elements — but “rich” is a matter of perspective; compared with the oxygen content of our modern galaxy, these early gas clouds were very oxygen-poor.

But despite the low levels of oxygen, the environment at that time would have been ideal to “cook up” water molecules. Temperatures of 80 degrees Fahrenheit (300 Kelvin) would have been perfect to combine what oxygen that was available to the abundant hydrogen atoms.

“These temperatures are likely because the universe then was warmer than today and the gas was unable to cool effectively,” said co-investigator Shmuel Bialy of Tel Aviv University.

“The glow of the cosmic microwave background was hotter, and gas densities were higher,” added Amiel Sternberg, a co-author also from Tel Aviv University.

Read more at Discovery News