An international team from Sweden, France, Germany and Australia used the Australia Telescope Compact Array in New South Wales to measure how warm the Universe was when it was half its current age.
"This is the most precise measurement ever made of how the universe has cooled down during its 13.77-billion-year history," said Robert Braun, chief scientist for astronomy at space science at the Commonwealth Scientific and Industrial Research Organization.
The astronomers looked at light coming from halfway across the universe, meaning the light started its journey in Earth's direction when the universe was half as old as it is today.
They studied gas in a galaxy 7.2 billion light-years away being kept warm only by the cosmic background radiation, the glow left over from the Big Bang.
The Big Bang theory of the birth of the universe holds that the temperature of cosmic background radiation should drop smoothly as the universe expands.
"That's just what we see in our measurements," said research leader Sebastien Muller of Onsala Space Observatory at Chalmers University of Technology in Sweden. "The Universe of a few billion years ago was a few degrees warmer than it is now, exactly as the Big Bang Theory predicts."