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Precise and Accurate

Precise and Accurate

Recent measurements using the Hubble Space Telescope help to resolve a scientific dispute that impacts how old the universe appears. Most readers will recall that the initial WMAP results gave an age for the universe of 13.7 ±0.2 billion years. After more data accumulation, the quoted date changed to 13.73 ±0.15 billion years. Yet, other scientists quote an age for the universe of 15.1 billion years with similar error bars. How can these two clearly discrepant results be resolved?

First, one must realize that the error bars quoted include only the statistical errors, not the systematic errors. The former most closely relate to the precision of the experiment and give a measure of how likely redoing the same experiment will produce the same results. Systematic errors relate to the accuracy of the results and quantify how close the results reflect reality. Look here for a good discussion of precision and accuracy.

The age issue above provides a good example of the importance of accounting for systematic errors because the difference in ages arises from two different techniques of determining distances to faraway objects. However, recent results affirm the WMAP dates and bring the 15.1-billion-year date into closer agreement by correcting for a systematic error.

While a paper by G. Fritz Benedict et al. describes the details, the essential point relates to how the astronomers use a particular type of variable star, called a Cepheid, to measure distances. The luminosity of these stars directly depends on the period of their variability, but that relationship must be calibrated. The work by Benedict et al. shows that the calibration used to determine the 15.1-billion-year age of the universe systematically overestimated distances to Cepheid variables. This overestimation leads to the larger, and presumably less accurate, value for the age of the universe.

While the accuracy and precision lesson provides an interesting reminder of its importance, the bigger picture coming from this work is that the conclusions regarding the age and history of the universe, and their implications for supernatural creation, rest on stronger scientific footing.