Australian astrophysicists have shed new light on the state of the early universe in the few hundreds of millions of years after it formed in the Big Bang 13.8 billion years ago.
A team of physicists measured the density of carbon in the gas that surrounds ancient galaxies to build a picture of the cosmos when it was “only” 800 million years old.
“We found that the fraction of carbon in warm gas increased rapidly about 13 billion years ago, which may be linked to large-scale heating of gas associated with the phenomenon known as the ‘Epoch of Reionisation’,” says Dr Rebecca Davies, from Melbourne’s Swinburne University of Technology and a member of ARC Centre of Excellence project All Sky Astrophysics in 3 Dimensions (ASTRO 3D).
The Epoch of Reionisation took place one billion years after the Big Bang. Before this period of cosmic history, the universe was a dark, dense fog of gas. As the first massive stars formed, their light pierced through this veil, leading to a rapid heating of the surrounding gas causing the rise in warm carbon.
Warm carbon increased five-fold over only 300 million years, the researchers found. Previous studies have indicated a rise in warm carbon levels, but the new study’s larger sample size provides an accurate picture of its growth rate.
Two theories have been put forward to explain the increase of warm carbon.
The first is that carbon became more abundant purely because more was being formed through nuclear fusion in the young universe’s stars.
“During the period when the first stars and galaxies are forming, a lot of heavy elements are forming because we never had carbon before we had stars,” Dr Davies says. “One possible reason for this rapid rise is just that we’re seeing the products of the first generations of stars.”
But that doesn’t quite account for evidence in the study which shows that cool carbon decreased in the same period.
The researchers say this suggests two phases of carbon evolution – a rise during reionisation, followed by a plateau.
Coming by the measurements required for such a study of reionisation has been hard going, but vital to understand how the first stars formed and produced all the elements in the universe.
“The research led by Dr Davies was built on an exceptional sample of data obtained during 250 hours of observations on the Very Large Telescope (VLT) at the European Southern Observatory in Chile,” says Dr Valentina D’Odorico from the Italian Institute for Astrophysics, also an ASTRO 3D researcher. “This is the largest amount of observing time assigned to a single project carried out with the X-shooter spectrograph.
The team used light from distant quasars which have travelled 13 billion years across the universe.
As the light passes through galaxies on its journey, some photons are absorbed. By measuring which photons were absorbed, the astrophysicists can determine the chemical composition and temperature of the gas in distant galaxies.
“We increased from 12 to 42 the number of quasars for which we had high quality data, finally allowing a detailed and accurate measurement of the evolution of the carbon density,” says Dr D’Odorico.
But carbon, of course, is only one element among many. But astronomers are beginning to build a fuller history of the universe.
“Our results are consistent with recent studies showing that the amount of neutral hydrogen in intergalactic space decreases rapidly around the same time,” says Dr Davies. “This research also paves the way for future investigations with the Square Kilometre Array (SKA) which aims to directly detect emissions from neutral hydrogen during this key phase of the universe’s history.”
Professor Emma Ryan-Weber, also from Swinburne, says the research forms a part of our pursuit to understand our origins.
“It addresses this key goal: how did the building blocks of life – in this case carbon – proliferate across the universe?” Ryan-Weber comments. “As humans we strive to understand ‘where did we come from?’ It’s incredible to think that the barcode of those 13-billion-year-old carbon atoms were imprinted on photons at a time when the Earth didn’t even exist. Those photons travelled across the universe, into the VLT, and then were used to develop a picture of the evolution of the universe.”
The research is published in the Monthly Notices of the Royal Astronomical Society.