When did water first appear in the universe? This vital element, so necessary for life as we know it, didn’t exist in the early universe.
Hydrogen is the single most abundant element in the universe. For a simple reason: the simplest of atoms, a single proton and an electron, hydrogen was the first type of matter to condense out of the primordial ‘particle soup’ of the early universe. From the Big Bang, the universe was so hot and dense that elementary particles couldn’t come together to form atoms.
Then about 380,000 years after the Big Bang, a remarkable thing happened. The universe, which had been an opaque soup of particles, suddenly became transparent, as it cooled enough for protons and electrons to bind together as hydrogen. But it needs another type of atom to bind with hydrogen to make water: oxygen.
Roughly 13.8 billion years ago, the universe was essentially just hydrogen, helium and a little bit of lithium. It took stars to make the rest.
Stars are factories for elements heavier than the first three on the Periodic Table. As they age, the fusion reaction that drives stars fuses particles into heavier elements. When a star goes supernova, it spews its core of heavier elements into space.
Some midweight elements, such as carbon and oxygen, are fused inside of stars as they age. Others are forged in stellar deaths, such as explosive supernovas or the violent mergers of neutron stars. However, for more complex molecules to form in significant quantities, relatively dense and cool conditions, ideally less than a few thousand degrees Celsius, are needed.
So, when did oxygen first join with abundant hydrogen, to make water? According to new research, surprisingly early.
The first generation of stars in the universe could have produced significant amounts of water upon their deaths, just 100 million to 200 million years after the Big Bang.
How do scientists know this? Because astronomy is a form of time travel. Light takes time to travel through space. The light from the Sun is nine minutes old by the time it reaches us. From Alpha Centauri, the nearest star, about four and a half years. The deeper a telescope peers into space, the further back in time. Light from the furthest stars is light from the earliest universe. And it tells us some remarkable things.
Signatures of water have previously been observed some 780 million years after the Big Bang. But now, computer simulations suggest that this essential condition for life existed far earlier than astronomers thought, researchers report March 3 in Nature Astronomy.
“The surprise was that the ingredients for life were all in place in dense cloud cores [leftover after stellar deaths] so early after the Big Bang,” says astrophysicist Daniel Whalen of the University of Portsmouth in England.
Analysing ancient light from the distant reaches of the universe, scientists are able to study its spectra and find telltale lines left by specific elements. But the new claims are not entirely based on observation.
To see if there could have been water in the infant universe, Whalen and his colleagues ran computer simulations of the lives and deaths of two first-generation stars. Because astronomers think early stars were much larger and had shorter lifespans than modern stars, the team simulated one star with 13 times the mass of the sun and another 200 times the sun’s mass. At the end of their short lives, these behemoths exploded as supernovas and flung out a shower of elements, including oxygen and hydrogen.
The simulations showed that as the supernovas’ ejected matter expanded and cooled, oxygen reacted with hydrogen and dihydrogen, or two joined hydrogen atoms, to make water vapor in the growing debris halos.
This chemical proceeded on what seems to us a glacial scale: millions to tens of millions of years. But in 14-billion-year-old universe, that’s just a tick of the clock.
The dusty central cores of the supernova remnants had cooled enough for water to form. Water began amassing rapidly there since the densities were high enough for atoms to meet.
“[The water’s] concentration in dense structures, that to me is the game changer,” Whalen says. “The total overall mass of water being formed, it’s not that much. But it becomes really concentrated in the dense cores, and the dense cores are the most interesting structures in the remnant, because that’s where new stars and planets can form.”
Including watery, life-sustaining planets like our own.
At the end of the simulations, the smaller supernova produced a mass of water equivalent to a third of Earth’s total mass while the larger one created enough water to equal 330 Earths. In principle, Whalen says, if a planet were to form in a core leftover from the larger supernova, it could be a water world like our own.
The hope that springs eternal for astronomers is that, where there’s water, there’s life.
