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. At that time the temperature was an astonishing 13,000°C, hotter than the surface of the sun.
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.".
Knowing when and why the universe started to cool is a first step to understanding many other things, according to Murphy. “We know some atoms cool down and make their way into galaxies. Somehow you have to get that gas from the intergalactic medium and how that happens is something we don't know a lot about. We'd like to know if the temperature of the medium changes how the gas or when the gas gets down into galaxies, and that is the part we are just starting to understand.”
In Monthly Notices of the Royal Astronomical Society, Boera and Murphy reached the conclusion that the turning point was between 3 and 4 billion years after the universe formed. They also think they know why.
“We think the answer is helium,” says 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.”
However, once the helium ionized completely, so that it had no more electrons to lose, the radiation would simply pass through the gas without heating it.” At this point the cooling effect of expansion would have taken over.
Boera says their measurements suggest “The Universe had cooled by about 1000 degrees within 1 billion years after reaching its maximum.”
While the team expect that this cooling would have continued, we don't know the current temperature of the intergalactic medium, nor what it has been for billions of years. Murphy explains that the measurements for the temperature of the intergalactic medium require us to sample light that, when emitted, had wavelengths too short to penetrate our atmosphere. Murphy says that the space telescopes currently in existence are also too small to take the measurements required. However, the radiation from the early universe has been red-shifted “into our optical window” says Murphy, so that it can be measured with ground based telescopes. To find when the temperature peaked Boera took measurements in the harsh ultraviolet from 60 quasars. However, studying more recent temperatures will require new instruments.
Murphy doubts that the extraordinary temperatures between the galaxies would affect the prospects of planetary formation. Nevertheless, from our biased position we are naturally more interested in the minority of atoms that exist within galaxies than those that lie outside, and he says “Gravity funnels mass down into galaxies in the form of quite fine, cold filaments. The process of star formation once they reach the galaxy is probably not very sensitive to how hot this once was, but the process of getting it going probably is.”
Swinburne animation of Quasar light ionizing helium to heat the intergalactic medium up, and then passing through without effect once the gas begins to cool down.
