Calibration plays a key role in our driving habits. A poorly calibrated speedometer leads to more frequent encounters with law enforcement personnel. A poorly calibrated fuel dispenser might extract extra money from our wallets. Similarly, a scientific experiment using poorly calibrated equipment leads to useless results.
One piece of "equipment" astronomers use to measure the expansion rate of the universe is Type Ia supernovae. These supernovae occur in binary systems where a white dwarf star accretes matter from a companion. When the mass of the white dwarf exceeds a threshold, it undergoes catastrophic nuclear burning that leads to the supernova event. The constancy of the mass threshold for all white dwarfs ensures that each supernova exhibits a uniform brightness. Astronomers rely on this uniform brightness to determine the distance to the supernova. If the brightness varied during the history of the universe, then scientists would draw wrong conclusions about the expansion rate of the universe.
Fortunately, astronomers have devised a number of ways to "calibrate" these supernovae to ensure valid results. As the universe ages, stellar burning enriches the initial elemental composition of hydrogen and helium with a growing abundance of elements heavier than helium. Because of the changing composition that later stars incorporate, it is possible that the brightness of Type Ia supernovae might change in an unpredictable way–thus rendering them useless for distance measurements.
A team of international astronomers sought to investigate the reliability of supernova brightness. Using a sample of Type Ia supernovae spanning the last 9 billion years, the researchers analyzed the spectrum of light emitted by each. They determined that the compositions of the supernovae did vary with time as expected. Additionally, the team reproduced this variation with a rather simple model, demonstrating that any changes in light output caused by composition changes could be corrected. In other words, even though the light output from Type Ia supernovae does vary as the universe ages, astronomers can independently measure the changes and calibrate the brightness of each individual supernova.
The measured expansion history of the universe reveals a great deal of fine-tuning for life, reflecting a purpose for the universe. Thus, this proper calibration of the most prominent tool used to measure the expansion, namely Type Ia supernovae, affirms the validity of the conclusion that this universe was engineered to support life.