The universe's temperature just 880 million years after the Big Bang has been taken by astronomers. This was possible thanks to a space water cloud that, about 12.9 billion years ago, absorbed some light from the Cosmic Microwave Background (CMB) – the first light able to freely move in the Universe.
The average temperature of the observable universe derived from the CMB is currently about 2.73 Kelvin (-270.42 °C /-454.756 °F), just a few degrees above absolute zero. The universe started in a hot dense state and it has been cooling ever since.
As reported in the journal Nature, the temperature 880 million years ago is now estimated to be between 16.4 and 30.2 Kelvin.
This is consistent with a temperature of 20 Kelvin predicted for the universe at the time by the Standard Model of cosmology, our leading theory of how the universe at large scales works. It requires the existence of two mysterious components: dark matter and dark energy, which have measurable effects we can see but that astronomers currently haven’t been able to prove exist.
Alternative theories and modifications can sometimes predict different temperatures for different times. Some of them have different types of dark energy – in others, dark energy doesn’t exist. This is the furthest back in time that the temperature has been measured, and allows researchers to remove some of the possible alternative explanations.
"If there were to be any deviations from the expected trends, this could have direct implications for the nature of the elusive dark energy." Dominik Riechers, the lead author of the University of Cologne, said in a statement.
The cloud in question is located in the starburst galaxy HFLS3 and the observations were possible thanks to the IRAM NOEMA telescope array in the French Alps. This is a radio observatory that can study the universe at millimeter wavelengths, ideal to determine such signals.
The photons (particles of light) of the CMB at that time were still energetic enough to interact with water molecules creating such a signal. As the universe expanded, the CMB photons have lost energy so they can’t create such interactions anymore.
The team hopes that this first distant measurement is going to be the first of many.
"This new technique provides important new insights into the evolution of the universe, and shows us that the universe in its infancy had some unusual properties quite unlike today,” co-author Fabian Walter, from the Max Planck Institute for Astronomy, stated.