How Hot Was the Universe 7 Billion Years Ago? Scientists Now Have an Answer

A group of Japanese astronomers from Keio University collaborated with Japan's NAOJ to find out that the universe was about twice as hot 7 billion years ago as today. Researchers led by Tatsuya Kotani and Tomoharu Oka of Keio University measured a temperature of roughly 5.13 K (−268°C) for that epoch, compared to the current 2.7 K. These observations of the cosmic microwave background (CMB) indeed confirm predictions that cosmic expansion cools the universe over time.

Taking the Universe's Temperature

According to the new study, Keio University scientists (Kotani, Oka, and colleagues) collaborated with Japan's NAOJ to analyse archival data from the ALMA radio telescope array in Chile. As Universe Today explains, this team examined light from a distant quasar that “interacted with the cosmic background radiation, leaving telltale signatures.” By modelling those spectral shifts, astronomers calculated the CMB temperature 7 billion years ago to be about 5.13 K (±0.06)– roughly double the present 2.7 K background.

Confirming Cosmological Predictions

In the standard Big Bang cosmology (ΛCDM model), the CMB temperature scales with (1+z) as the Universe expands. By this theory, the CMB was ~3000 K at recombination (380,000 years after the Big Bang) and 2.7 K today, implying ~5.13 K at z≈0.89. The new result matches this prediction exactly: as Universe Today notes, Kotani's measurement “matches those predictions almost perfectly”. SpaceDaily likewise reports that it “demonstrates the CMB temperature increases predictably with redshift”. This precise new point is the most accurate mid-epoch measurement yet, effectively filling a gap between early-Universe and present data. In short, the finding reinforces confidence in the Big Bang model and rules out any unexpected cooling behaviour in the universe's history.