A Japanese team led by doctoral student Tatsuya Kotani and professor Tomoharu Oka from Keio University used data from the Atacama Large Millimeter/submillimeter Array (ALMA) to determine the temperature of the cosmic microwave background (CMB) at redshift z = 0.89. They reported a value of 5.13 ± 0.06 kelvin, nearly double the current temperature of 2.7 K.
Filling a Gap in the Universe’s Thermal Timeline
The cosmic microwave background is the faint afterglow of the Big Bang, a relic radiation that has cooled as the universe expanded. Measuring its temperature at different points in time allows scientists to test the accuracy of standard cosmological models.
While previous studies had successfully measured the CMB temperature at both very early epochs and the present day, intermediate measurements, especially those at z ≈ 0.89, corresponding to a time when the universe was less than half its current age, were still imprecise. This latest result, published in The Astrophysical Journal, provides a crucial data point that reinforces the predicted evolution of temperature over time.
Molecular Absorption Lines to Track Ancient Heat
To measure this ancient temperature, the team analyzed light from the distant quasar PKS1830–211, a bright source located far beyond the Milky Way. As this light passed through a foreground galaxy, it encountered cold gas containing hydrogen cyanide (HCN). The molecular gas absorbed specific frequencies of the background light, creating absorption lines that act as thermometers for the CMB at that time.
According to the Keio University research team, the analysis focused on four HCN rotational transitions (J = 2–1, J = 3–2, J = 4–3, J = 5–4). From these, they derived excitation temperature profiles by correcting for key uncertainties: the continuum covering factor, time variability in absorption strength due to a known flare event, and the non-uniform distribution of the absorbing gas. A Monte Carlo approach was used to quantify these uncertainties with 100,000 simulations per velocity bin.
The most reliable temperature values were taken from the spectral region between −12 and +13 km/s, where absorption was strongest and best constrained. The final averaged value, 5.13 K, was determined from this central component.
Confirming the Big Bang Model with Unmatched Precision
This measurement matches the standard Big Bang prediction that the CMB temperature scales with (1 + z). The expected value at redshift 0.89 is 5.14 K, which places the result by Kotani’s team in near-perfect agreement with the theoretical curve.
This is the most precise CMB temperature measurement ever recorded at intermediate redshift, reducing the uncertainty of previous estimates—such as the 2013 value of 5.08 ± 0.10 K by Muller et al.—by about 40%, according to the new study.
Moreover, the researchers avoided common pitfalls in past analyses, such as relying on over-simplified assumptions about gas distribution or ignoring the effects of absorption saturation. The use of high signal-to-noise ALMA data and a detailed uncertainty model gives this result a degree of confidence previously unattained at this epoch.
strength.(c) Dependence of the Cmb Temperature on Redshift. ©Keio University Press Release
Groundwork for Deeper Exploration into Cosmic Evolution
Although this study focused on a single redshift, its impact stretches far beyond. By demonstrating a reliable method for measuring the universe’s temperature in the past with such accuracy, it opens new avenues for examining whether the universe has always followed the same physical laws.
According to the Keio University press release, future observations targeting quasars at higher redshifts could push this method even further. Instruments like the Square Kilometre Array (SKA) and next-generation Very Large Array (ngVLA) are expected to improve sensitivity and expand the redshift range for CMB measurements.
For now, this precise measurement at z = 0.89 serves as a solid benchmark, reinforcing one of the most fundamental assumptions in modern cosmology: that as space expands, the universe cools predictably, just as theory says it should.
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