New method to measure the cosmic microwave background’s temperature

Early cooling of our universe shortly after the Big Bang.


The cosmic microwave background (CMB) is leftover electromagnetic radiation from the earliest cosmological epoch, i.e., Big Bang. It gives a snapshot of the early Universe.

Over the past two decades, measurements of the cosmic microwave background’s temperature have provided profound insight into the nature of the Universe. Detailed information about the composition and evolution of the Universe is encoded in the temperature and polarization anisotropy of the CMB.

The previous cosmological model assumes that the Universe has cooled off since the Big Bang — and continues to do so. The model also describes how the cooling process should proceed, but so far, it has only been directly confirmed for relatively recent cosmic times.

An international group of astrophysicists has discovered a new method to estimate the cosmic microwave background’s temperature. This is the first time that the cosmic microwave background radiation temperature has been measured at such an early epoch of the Universe.

The discovery not only sets a very early milestone in the development of the cosmic background temperature but could also have implications for the enigmatic dark energy.

Using the NOEMA observatory in the French Alps, the most powerful radio telescope in the Northern Hemisphere, scientists observed a massive starburst galaxy HFLS3. The galaxy is located at a distance corresponding to only 880 million years after the Big Bang. They discovered a screen of cold water gas that casts a shadow on the cosmic microwave background radiation.

The shadow results from the absorption of the warmer microwave radiation by the colder water on its path towards Earth, and its darkness reveals the temperature difference. As the water temperature can be determined from other observed properties of the starburst, the difference indicates the temperature of the Big Bang’s relic radiation.

Lead author Professor Dr. Dominik Riechers from the University of Cologne’s Institute of Astrophysics said, “Besides proof of cooling, this discovery also shows us that the Universe in its infancy had some quite specific physical characteristics that no longer exist today. Quite early, about 1.5 billion years after the Big Bang, the cosmic microwave background was already too cold for this effect to be observable. We have therefore a unique observing window that opens up to a very young Universe only.”

“In other words, if a galaxy with otherwise identical properties as HFLS3 were to exist today, the water shadow would not be observable because the required contrast in temperatures would no longer exist.”

Co-author Dr. Axel Weiss from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn said, “This important milestone not only confirms the expected cooling trend for a much earlier epoch than has previously been possible to measure but could also have direct implications for the nature of the elusive dark energy.”

“Based on this experiment, the properties of dark energy remain for now consistent with those of Einstein’s ‘cosmological constant. That is to say, an expanding Universe in which the density of dark energy does not change.”

Riechers said, “Having discovered one such cold water cloud in a starburst galaxy in the early Universe, the team is now setting out to find many more across the sky. They aim to map out the cooling of the Big Bang echo within the first 1.5 billion years of cosmic history. This new technique provides important new insights into the evolution of the Universe, which are very difficult to constrain otherwise at such early epochs.”

Co-author and NOEMA project scientist Dr. Roberto Neri said“Our team is already following this up with NOEMA by studying the surroundings of other galaxies. ‘With the expected improvements in precision from studies of larger samples of water clouds, it remains to be seen if our current, basic understanding of the expansion of the Universe holds.”

Journal Reference:

  1. Riechers, D.A., Weiss, A., Walter, F. et al. Microwave background temperature at a redshift of 6.34 from H2O absorption. Nature, 2022 DOI: 10.1038/s41586-021-04294-5


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