We cannot go back in time and stick a thermometer in the early universe, but astronomers have done the next best thing, using an indirect technique to find out what the universe's temperature was 11 billion years ago. It was a chilly 9 K (-264 °C) back then, which is still warmer than today's prevailing temperature of less than 3 K (-270 °C).
Some of the coldest objects in the universe are gas clouds that fill the space between stars and galaxies. But even these are warmer than absolute zero, or 0 K. That is because they are heated by radiation leftover from the universe's earliest times.
Called the cosmic microwave background (CMB), this radiation was emitted by the hot plasma that filled the universe a mere 380,000 years after the big bang, which took place an estimated 13.7 billion years ago.
But as the universe expanded, the electromagnetic waves that comprise this radiation were stretched to longer wavelengths and lower energies, decreasing the radiation's temperature. Today, that temperature is just 2.7 K.
Now, a team led by Raghunathan Srianand of the Inter-University Centre for Astronomy and Astrophysics in Pune, India, has measured what the CMB temperature was 11 billion years ago, when the universe was just a fifth its current age.
They found it to be 9.15 K back then, with an uncertainty of 0.7 K in either direction. That is "in excellent agreement" with the 9.3 K temperature predicted in the big bang scenario, says team member Patrick Petitjean of the Institut d'Astrophysique in Paris, France.
The astronomers arrived at their figure by a very indirect route. What they actually measured was the temperature of carbon monoxide gas in a galaxy about 11 billion light years away.
The gas was detected by the way it intercepts light from an even more distant object called a quasar – a bright galaxy whose central black hole is consuming its surroundings.
The team used the Very Large Telescope (VLT) array in Paranal, Chile, to measure the wavelengths where the carbon monoxide absorbs the quasar's light. The wavelengths affected depend on the temperature of the galaxy's gas, whose heat is thought to come from the CMB.
Charles Bennett, chief scientist for NASA's Wilkinson Microwave Anisotropy Probe (WMAP) mission, which measures the CMB, says it is important to make such measurements to test scientists' expectations. "It's nice to see consistent things in different ways," he told New Scientist.
Journal reference: Astronomy & Astrophysics (vol 482, p L39)
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