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When the Universe Cooled

An international team has found evidence that the Universe broke its rising “fever” and began to cool about 11 billion years ago.

The study, published in the Monthly Notices of the Royal Astronomical Society, measured the temperature of the Universe when it was 3–4 billion years old, when many extremely active galaxies were “switching on” for the first time and heating their surroundings. “However, 11 billion years ago this ‘fever’ seems to have broken and the Universe began cooling down again,” said lead researcher Elisa Boera from Swinburne’s Centre for Astrophysics and Supercomputing.

An earlier study found that the Universe caught this fever early in its history. Its authors used a new “thermometer” – the imprint of the intergalactic medium on the light emitted by distant quasars.

In the new study, Boera collected the bluest light that the Earth’s atmosphere transmits – harsh ultraviolet (UV) light from 60 quasars – and used the same method as the earlier study. This UV light comes from slightly later in the Universe’s development, allowing the new temperature measurement.

“The quasar light suggests that the Universe had cooled by about 1000°C within one billion years after reaching its maximum of 13,000 degrees,” Boera said. “This cooling trend has probably continued to the present day.”

Why did the Universe’s fever break? “We think the answer is helium,” said Boera’s co-author, A/Prof Michael Murphy. “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.

“Once all the helium was ionised, the radiation would simply pass through the gas without heating it. Then, as the Universe expands the gas cools down.”