Japanese astronomers from Keio University have conducted the most precise measurement to date of the Universe’s temperature from seven billion years ago—and found that it was almost twice as warm as it is now. The results, published on the university’s website, confirm the standard cosmological model and fill an important gap in the history of cosmic expansion.

How the Temperature of the Distant Universe Was Measured

To perform the calculations, the scientists used data from the ALMA radio telescope—the world’s largest array of 66 antennas, located 5,000 meters above sea level in the Atacama Desert, Chile. The dry air and lack of interference allow it to detect the faintest signals from the depths of the Universe.

The researchers analyzed light from a quasar—a super-bright core of a distant galaxy powered by a supermassive black hole. This light, having traveled for billions of years, passed through the cosmic microwave background (CMB)—the residual radiation from the Big Bang that uniformly fills space.

The interaction of quasar photons with the CMB slightly alters their spectrum. Based on these distortions, astronomers calculated the background temperature 7 billion years ago to be 5.13 K (±0.06 K). For comparison, today the CMB temperature is 2.7 K (–270.45 °C).

Why the Universe Cools Down

According to the Standard Cosmological Model, after the Big Bang the Universe has been expanding, causing its density and temperature to decrease. The CMB radiation “stretches” along with space—its wavelength increases while the energy of photons decreases.

• 13.8 billion years ago (Big Bang): ~3000 K
• 7 billion years ago: 5.13 K
• Today: 2.7 K

The new measurement is the most precise ever made for an intermediate stage between early (z = 2–3, about 10 billion years ago) and present-day data. It perfectly matches the predicted cooling curve.

The Significance of the Discovery

“This is the first measurement with such precision at a redshift of z ≈ 1.7,” said the study’s lead author. The results:
• Fill the “gap” in the history of the CMB.
• Confirm the ΛCDM model (dark energy + cold dark matter).
• Open the way to testing alternative theories (for example, variable speed of light).

In the future, ALMA and the upcoming CCAT-Prime telescope will allow measurements of the CMB at even greater distances.

In Short…

Seven billion years ago, the temperature of the Universe was 5.13 K—almost twice as high as today’s 2.7 K. Japanese researchers measured this using quasar spectra through ALMA, with an accuracy of ±0.06 K. The data perfectly confirm the cooling process during expansion and the Standard Model. For the first time, scientists have such an accurate “thermometer” halfway back to the Big Bang.