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Taking the temperature of the Universe

28 janvier 2013 Taking the temperature of the Universe

This is the best ever measurement of how the universe has cooled since the Big bang. Astronomers have put the Big bang theory through a tough new test - by measuring the temperature of the universe when it was half as old as it is now. An international team of scientists, including astronomers from Paris Observatory, has made the most precise measurement of how the universe has cooled down during its 13.77 billion year history. They studied molecules in clouds of gas in a distant galaxy, so far away that its light has taken half the age of the universe to reach us. To make the measurement they used the CSIRO Australia Telescope Compact Array, an array of six 22-metre radio telescopes in eastern Australia. The article has just been published in Astronomy and Astrophysics.

The astronomers used a clever new method to measure the temperature of the cosmic microwave background — the very weak remnant of the heat of the Big bang that pervades the entire universe. They observed radio waves from a remote quasar, which are absorbed by molecules in a galaxy located in front of the quasar, on the same line of sight. The galaxy is so far away that its light has taken 7.2 billion years to reach us. The intervening galaxy is so well aligned along the line of sight towards the remote quasar, that it deflects and amplifies its light, like a gravitational lens, and therefore the phenomenon is much brighter to detect.

Radio waves (yellow lines) from a bright quasar pass through a galaxy where they are absorbed by molecules in a cold cloud of gas. Astronomers used radio telescopes to measure the temperature of the universe 7.2 billion years ago, by measuring the signatures of the molecules in the radio waves.
(Onsala Space Observatory/R. Cumming/S. Muller)

The gas in this galaxy is so rarefied that the only thing keeping its molecules warm is the cosmic background radiation — what’s left of the Big bang. Taking advantage of a lucky alignment, the team measured light from an even more distant source behind the galaxy, a quasar known as PKS 1830-211. The quasar is a young galaxy which shines brightly because of jets from a supermassive black hole at its centre.

Astronomers analysed radio waves from the quasar which had passed through the gas in the galaxy. The radio waves allowed to identify traces of many different molecules. Using a sophisticated computer model, the astronomers used these molecular signatures, left like fingerprints in the light from the quasar, to measure the temperature in the gas clouds in the intervening galaxy.

The temperature of the cosmic background radiation they measured was 5.08 Kelvin (+/- 0.10 Kelvin). This is extremely cold, but significantly warmer than the temperature which scientists measure in today’s universe, 2.73 Kelvin. Scientists measure temperatures in Kelvin above absolute zero (0 Kelvin = -273 degrees Celsius). One Kelvin is the same size as one degree Celsius.

Radio image of PKS1830-211
The galaxy plays the role of a gravitational lens, and produces an Einstein ring and two main images of the remote quasar PKS1830-211, at cm radio wavelengths. The molecules absorb the radiation coming from the two images, corresponding to the two red points, separated by one arcsecond in the sky. This photo has been obtained at 5GHz with the interferometer MERLIN (UK).

The temperature of the cosmic background radiation in the past has been measured before, at even larger distances. But this is the most precise measurement yet of the ambient temperature when the universe was younger than it is now. According to the Big bang theory, the temperature of the cosmic background radiation drops smoothly as the universe expands. That’s just what the astronomers measure. The universe of a few billion years ago was a few degrees warmer than it is now, exactly as the Big bang theory predicts.

Reference
A precise and accurate determination of the cosmic microwave background temperature at z=0.89, S. Muller, A. Beelen, J.H. Black, S.J. Curran, C. Horellou, S. Aalto, F. Combes, M. Guelin, C. Henkel
Astronomy & Astrophysic, in press

Contact
Francoise Combes
Observatoire de Paris-LERMA, CNRS

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