Rather than cooling down, the universe heated up over its first few billion years. Astronomers now think they have identified the point where it moved from heating to cooling – 11 billion years ago.
Associate Professor Michael Murphy of Swinburne University, says, “When you are talking about the temperature of most things, like a room, you have to talk about different parts, the walls, the air, you. It is the same for the universe. Here we are talking about the temperature of the intergalactic medium. Most of the atoms in the universe are not in galaxies but in the medium between galaxies.”
As the universe expands one would expect it to cool, like “The cold gas sprayed from an aerosol can” but Swinburne PhD student Elisa Boera says in the first few billion years this was offset by extremely active galaxies “Switching on” and heating up the medium around them.
“However, 11 billion years ago, this fever seems to have broken and the Universe began cooling down again.”
“Fourteen per cent of the intergalactic gas is helium and 12 billion years ago it was absorbing the intense radiation from active galaxies, losing electrons in the process. The electrons whizz around, heating up the gas. It’s similar to the greenhouse effect on Earth: Carbon dioxide gas absorbs infrared radiation and heats our atmosphere.”
Boera says their measurements suggest “The Universe had cooled by about 1000 degrees within 1 billion years after reaching its maximum.”