How Hot is the Universe?

This Christmas you may decide to play a game of Trivial Pursuit with the family. Well, here's one question that you can add to the pack – How hot is the Universe? There are four choices: cold; cool; warm; or hot. You will be allowed to phone a friend, unless that person is our guest lecturer, one Professor Trevor Ponman from Birmingham University, who very kindly shared the answer with us. It turns out that it is – hot. At an average of over one hundred thousand degrees to be more precise.

The matter is not spread evenly throughout the Universe but has condensed under the force of gravity into a web-like filamentary structure. The majority of this gas is made up of Hydrogen atoms with some Helium. Dotted throughout the structure are clusters of galaxies that, as time passes, draw in more and more matter along the long tendril-like threads. The structure's pattern only becomes apparent on vast scales of hundreds of millions of light-years; where a light-year is the distance that light covers in a year.

However, the answer only relates to the bits of matter that make up a paltry six percent of our Universe; baryonic matter in the jargon. It may be only a small constituent in the cake mix but it is very important to us. This type of matter is what the Earth, our bodies, stars and galaxies are made of. The rest of the recipe involves 25 parts dark matter and then to finish off 69 parts dark energy.

These last two ingredients have only been discovered relatively recently and have most astronomers baffled. The term "dark matter" was coined by the eccentric astronomer Fred Zwicky in the 1930's. He was studying the motion of a distant group of galaxies know as the Coma Cluster. He estimated how much mass they had in them by looking at their brightness but when he saw how fast they were dancing around one another his estimate did not make sense. If they had so little mass and were moving so fast, by all rights they should be flung off in all directions. He concluded that there must be some unseen mass in the cluster that was acting like gravitational glue.

Dark energy came onto the scene later, when in 1998 a group surveying distant supernovae found out to their astonishment that they were much dimmer than expected. The supernovae that they looked at were type 1a. These are stars that become unstable and explode, attaining the brightness of entire galaxy in a matter of only a few days to weeks. They are known as "standard candles" as their peak light output is thought not to vary. The researchers concluded that the rate of expansion of the Universe must be speeding up in order to put more distance between their telescopes and the supernovae to make them appear dimmer.

To explain this extra acceleration scientists have come up with a new hypothesis that steps into the world of quantum theory. It is proposed that dark energy is an intrinsic property of space, or more correctly spacetime, itself. It is a force that opposes gravity and the more space there is the more dark energy exists. So, it appears that not only are we living in an expanding universe but someone has put their foot on the gas!