Australasian Science: Australia's authority on science since 1938

Universe Is Cooling as Expected

By Stephen Luntz

The universe is cooling down, and the most precise measurement confirms that it is happening at the rate expected by our models of the universe’s expansion.

The Big Bang created a very hot environment, but as space expanded the temperature cooled so that now our microwave measurements of the background radiation left over give a temperature of 2.73°K (–270.42°C). “We know this figure to four or five decimal places,” says the Chief Scientist of CSIRO Astronomy and Space Science, Dr Robert Braun.

An interesting feature of this measurement is that certain gases in our galaxy exist at this temperature, even when exposed to heating from stars or other objects that could warm them up. Among these gases are silicon monoxide and a dozen more exotic molecules.

Dr Sebastien Muller of the Onsala Space Observatory in Sweden led a team that used the CSIRO Australia Telescope’s Compact Array to measure the temperatures of the same molecules in a galaxy 7.2 billion light years away. Astronomers see the galaxy as it was when the universe was just under half its current age, and it happens to be handily backlit by a quasar that allows them to observe the effects of the molecules on the passing light.

If the universe is expanding smoothly, the background temperature at the time should have been 5.08°K. “That’s just what we see in our measurements,” says Muller. “The universe of a few billion years ago was a few degrees warmer than it is now, exactly as the Big Bang theory predicts.”

According to Braun, similar measurements have been taken many times before, but these have either looked at far less distant galaxies or had margins of error that were large enough to make the results unconvincing.

Braun says that the team used a “combination of careful measurements with high sensitivity and the use of about ten different molecular transitions. Each transition serves as a thermometer, and putting together all these independent estimates greatly improves the precision.”

Two points would not normally be considered enough to verify a curve, but Braun says the relationship between the universe’s size and temperature is “such a simple theoretical expectation” that even two points provides strong verification.