When the universe was born 13.8 billion years ago, it was filled with hot gas comprising ions of hydrogen and helium whizzing around with electrons.
But after 400,000 years or so, the universe cooled. Its soup of positively charged atoms and negatively charged electrons paired off, and the universe was awash with neutral hydrogen.
It stayed this way, cold and dark, for hundreds of millions of years – but eventually, cold gas clumped together and the first generation of stars began shining.
They emitted powerful blasts of radiation that knocked electrons from those neutral atoms to ionise the gas and start forming heavier elements, such as oxygen and carbon, as light elements fused.
This “cosmic ionisation” changed the universe and seeded formation of more stars and galaxies.
But what kinds of stars triggered the initial flurry of activity which dragged the universe out of its early dark ages?
Studying the heavy elements created during this early period can give astronomers clues. The Large Magellanic Cloud, a nearby galaxy and hotbed of vigorous star formation, houses plenty of ionised oxygen.
Previous telescope observations of the Large Magellanic Cloud showed wavelengths emitted by ionised oxygen to be particularly bright, so any ionised oxygen billions of light-years away could feasibly be detected from Earth.
A team of researchers, led by Akio Inoue from Osaka Sangyo University in Japan, used the Atacama Large Millimetre-submillimetre Array (ALMA) telescope in Chile to peer at the galaxy SXDF-NB1006-2 13.1 billion light-years away – and, in doing so, glimpsed what the universe was like at a mere 700 million years old.
They published their work in Science.
The ancient galaxy, discovered in 2012, was observed with the ALMA telescope in June 2015 after the researchers ran simulations to show they could pick up the faint glow of ionised oxygen from such a distance.
Indeed, ALMA picked up the signature of doubly ionised oxygen – that’s oxygen atoms with two electrons knocked out – but only a little: around a 10th the total amount found in our sun.
This is expected, says study co-author Naoki Yoshida, “because the universe was still young and had a short history of star formation at that time”.
But the small amount shows that brilliant stars, larger than the sun, were blazing away and energetic ultraviolet photons flying from the stars ionised – knocked electrons off – those oxygen atoms.
The galaxy was also pretty much devoid of light-blocking dust and carbon. The ultraviolet light was free to escape and ionise gas outside the galaxy.
The team is already planning another observation with the ALMA telescope and hopes to capture a high-resolution image of the ionised gas’ distribution and motion within the galaxy. And after launch in 2018, the James Webb Space Telescope will see farther back in time still.
Little by little, then, the early universe’s secrets will be revealed.