Water reveals the universe’s temperature 12.9 billion years ago

After propagating through the expanding universe for the past 13.8 billion years, the cosmic microwave background (CMB) radiation has cooled to a temperature of roughly 2.73 K. If the standard model of cosmology, ΛCDM (lambda–cold dark matter), is valid, then astronomers should see that temperature increase linearly with the redshift z as they look farther back in time. But few model-independent measurements of the CMB’s temperature exist beyond z = 1, corresponding to about 8 billion years ago, when the CMB was barely double its current temperature.

Now Dominik Riechers of the University of Cologne in Germany and colleagues have developed a method to estimate CMB temperature at high redshift. The thermometer they used to take the universe’s temperature 900 million years after the Big Bang comes in the form of water molecules.

The researchers focused on the massive star-forming galaxy HFLS3, which Riechers and another team discovered in 2013 at z = 6.34. When analyzing a microwave spectrum of the galaxy obtained with the Northern Extended Millimeter Array, located in the French Alps, the researchers noticed a deep absorption line at around 76 GHz associated with a specific energy transition of ortho-water (distinguished from para-water by the nuclear spins of its hydrogen atoms).

Their interpretation of the feature, which they support with simulations, is that the water molecules are first excited by CMB photons and then by the radiation from galactic dust. As a result, the molecule falls out of thermal equilibrium and no longer has a single excitation temperature for its energy-level transitions. Crucially, the excitation temperature for the 1₁₀–1₀₁ transition drops below that of the CMB. The result is the observed absorption line (shown outlined in red in the figure), essentially a shadow as compared with the warmer, brighter CMB. Based on the depth of the line, the researchers calculate a CMB temperature at z = 6.34 of 16.4–30.2 K. That’s consistent with the ΛCDM prediction of about 20 K.

Measuring the CMB temperature at high redshifts is not the only way to chart the universe’s expansion, nor is it the most precise. For example, the Dark Energy Survey, which is tracking galaxies and supernovae, was designed to uncover any potential new physics, such as a change in the strength of dark energy over time (see the article by Josh Frieman, Physics Today, April 2014, page 28 ). But the new technique could complement other methods, says Jeremy Darling of the University of Colorado Boulder, and water is abundant regardless of where and when you look. Plus, detecting the absorption feature depends only on the warmth of the CMB at a given redshift and the ability to obtain a spectrum. “Where water vapor exists and encounters the right physical conditions,” he says, “it should show this effect.” (D. A. Riechers et al., Nature 602, 58, 2022 .)