Home Sciences The early universe was seven times hotter than today’s

The early universe was seven times hotter than today’s


The early universe was seven times hotter than it is today, astrophysicists have found: They used a cloud of water vapor projected from a distant galaxy to observe the state of the Universe in its early stages. New door for the study of dark energy.

An international team of astronomers has been able to observe the state of the Universe very shortly after the Big Bang and confirm that it cooled very rapidly, opening up new perspectives for the study of elusive dark energy.

Thanks to Noema, the most powerful millimeter radio telescope in the northern hemisphere, installed in the French Alps, the team was able to measure the temperature of the microwave background radiation (CMB) as it was only 800 million years after the Big Bang.

The microwave background radiation is a form of electromagnetic radiation that fills the entire universe: it is one of the main pieces of evidence for the Big Bang cosmological model.

This is the first time that the temperature of the microwave background radiation, a relic of the energy released by the Big Bang, has been measured at such an early time in the Universe.

much hotter then

The feat was achieved by an international group of astrophysicists who observed with Noema a massive star-forming galaxy called HFLS3: it is so distant that its light takes 13 billion years to reach Earth.

HFLS3 is contained within a vast cloud of cold water vapor and is observed 880 million years after the Big Bang, at which time the Universe was 6% of its current age.

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The astrophysicists found that the cloud of cold water vapor present in HFLS3 cast a “shadow” on the microwave background radiation.

This shadow appears because the water vapor absorbs the warmer radiation from the cosmic microwave background on its way to Earth: its degree of darkness reveals the difference in temperature.

progressive cooling

Since the temperature of water can be determined from other properties, the difference indicates that the temperature of the relic radiation from the Big Bang was at that time about seven times higher than in the present Universe.

The prevailing cosmological model assumes that the Universe has cooled since the Big Bang and continues to do so. But so far, this cooling has only been confirmed for relatively recent cosmic times.

While the Universe today is bathed in cosmic radiation at a temperature of 2.7 Kelvin (-270.45°C), this temperature was around 20 K (-253.1°C) less than a billion years after the Big Bang.

So all the cosmic matter was exposed there, which implies that processes such as the evolution of galaxies must have been very different from today.

very young universe

In addition to proof of cooling, this discovery also shows that the Universe in its infancy had some quite specific physical features that no longer exist, explains lead author Professor Dominik Riechers of the Institute for Astrophysics at the University of Cologne in a release.

“Very early, around 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 observation window that opens only to a very young Universe”, he continues.

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In other words, if a galaxy with properties identical to those of HFLS3 existed today, the water shadow would not be observable because the required temperature contrast would no longer be available.

“This important milestone not only confirms the expected cooling trend for a much earlier epoch than was previously possible, but could also have direct implications for the nature of elusive dark energy,” adds Dr. Axel Weiß of the Institute. Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, the second author of the study.

Implications for dark energy

Scientists believe that dark energy is responsible for the accelerated expansion of the Universe over the past billion years, but its properties remain poorly understood because they cannot be observed directly.

However, these properties influence the evolution of the cosmic expansion, and therefore the rate of cooling of the Universe.

The precise measurement of the temperature of the cosmic background throughout the history of the Universe will allow, comparing the results obtained with the predictions of the Big Bang model, to trace and limit the effects of dark energy.

The research team intends to probe other cold water vapor clouds in distant galaxies to measure the cooling of the Universe through cosmic history using this new method and thus continue to explore the young years of the cosmos and better understand dark energy. .


Microwave background temperature at a redshift of 6.34 from H2O absorption. Dominik A. Riechers et al. Nature volume 602, pages58–62 (2022). DOI:https://doi.org/10.1038/s41586-021-04294-5

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