Creating a good snowball requires good technique. After scooping the right amount of snow, one must squeeze the fluffy white pile to make the snowball. The pressure decreases the snow’s volume, which raises the temperature and melts some of the snow. After releasing the pressure, the newly formed water expands and refreezes, resulting in a ready-to-throw snowball. The same principle––things cool as they expand––applies to measuring the temperature of the universe as it continues to expand after the initial big bang. Scientists seek ways to measure the universe’s temperature—though difficult to carry out—and expect results that affirm inflationary big bang cosmology, otherwise known as the “standard model.”
The universe’s vast size means that light takes a large amount of time to propagate from its origin point to astronomers’ telescopes. Thus, to measure the temperature at a time in the past (such as 7 billion years ago), astronomers must find an object from which light takes 7 billion years to reach Earth. They also need a more distant light source to shine light through that object. One such alignment occurs where a distant quasar (named PKS 1830-211) illuminates the gas in an unnamed galaxy 7.2 billion light-years away. The cosmic background radiation (leftover from the big bang) provides the only source of heating for this gas. Consequently, measuring the temperature of the gas also gives the universe’s temperature 7.2 billion years ago.
The gas in the closer galaxy absorbs light from the more distant quasar and the temperature of the gas affects the absorption. Astronomers made careful measurements of the absorbed quasar light and found a temperature of 5.08 +/- 0.10 K (or, approximately –450.5°F).1 Today, the background radiation (as it continues to cool) exhibits a temperature of 2.725 K (or 2.72548 ± 0.00057 K for technical accuracy),2 but the radiation started with a temperature of 3,000 K (super hot) when the universe was just 380,000 years old. As the universe expanded, the radiation cooled and the standard model predicts a temperature of 5.14 K for the cosmic background 7.2 billion years ago.
The close match of the measured temperature with the standard model not only excites astronomers but also it eliminates more exotic options—such as ones where dark energy decays over time—that predict different cooling rates for the universe. Such a measurement might not seem significant, but any data that affirms the standard model also buttresses the creation implications of the model. These include a Creator bringing the universe into existence, a divine Architect fashioning the universe and Earth to support life, and a Mind bringing humanity (and all life) into existence on Earth.
- S. Muller et al., “A Precise and Accurate Determination of the Cosmic Microwave Background Temperature at z=0.89,” Astronomy and Astrophysics 551 (March 2013): A109.
- D. J. Fixsen, “The Temperature of the Cosmic Microwave Background,” Astrophysical Journal 707 (December 20, 2009): 916–20.