How Do We Know the Age of the Universe?

A recent Forbes article, written by the astrophysicist Ethan Siegel, had the provocative title, "How Do We Know the Age of the Universe?" Though there is nothing new in this article, it is a good lay-level discussion of the current understanding of the age of the universe within the big bang model. Since Forbes is a well-read source, many people who regularly visit the Answers in Genesis website may wonder what the biblical response is; this is a good opportunity to offer some thoughts. Siegel began his piece with the comment that, in a question of this nature, it would be best to have “an incredible number of independent lines of evidence, all converging to the same answer. But in reality, there are only two good ones, and one is better than the other.” This is a refreshingly candid observation; all too often these sorts of articles have far more certainty than is warranted.

Methods for Measuring the Age of the Universe

The better method that Siegel discussed is the combination of several different kinds of data, such as the current measurement of the expansion rate of the universe, changes in the rate of expansion from type Ia supernovae, clumping of matter on large scales, and fluctuations in the cosmic microwave background. When these data are combined with a model of the big bang, cosmologists can compute an age for the universe. It is important to note that the computed age for the universe depends upon the particular version of the big bang model that is assumed: if the model changes, the age changes. The current estimated age is 13.81 billion years, with an uncertainty of 120 million years. One hundred twenty million years may sound like a lot of time, but when compared with 13.81 billion years, it is within one percent. In commenting on this precision, Siegel wrote, “We have a number of different data sets that point to this conclusion, but in reality, it’s all the same method.” Again, this is unusually honest because many others attempt to expand this to be several different methods.

Siegel discusses using the ages of globular star clusters as a second, less accurate method of measuring the age of the universe. A globular cluster is a centrally condensed, radially symmetric, gravitationally bound group of stars that contains between 50,000 and a half-million stars. Our galaxy, the Milky Way, contains about 200 globular clusters that orbit in the halo, a spherically symmetrical region around the galaxy. Globular clusters are thought to consist of some of the oldest stars in the galaxy, dating back to the beginning of the Milky Way, shortly after the big bang. The age estimates of globular clusters come from comparing observations of their stars (what astronomers call color-magnitude diagrams) to the predicted behavior of stars over time. Stars are lit by thermonuclear reactions deep within them. As stars age, their interior composition changes. These changes in composition alter the structure of stars, bringing about gradual changes in the gross properties of stars, which ought to show up in color-magnitude diagrams. By comparing observed color-magnitude diagrams of globular clusters with calculated changes over time from models of stellar evolution, astronomers expect the best fit between observations and theoretical models to reveal the ages of globular clusters. As with age estimates of the universe within the big bang model, the ages of globular clusters are model dependent. The current estimate of the age of globular clusters is about 13.2 billion years, though there is uncertainty of a billion years or so. Since this age is 600 million years less than the big bang age of 13.81 billion years, this is thought to be good confirmation of the universe’s age.

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