Assumption of strong priors

Calculating the age of the universe is only accurate if the assumptions built into the models being used to estimate it are also accurate. This is referred to as strong priors and essentially involves stripping the potential errors in other parts of the model to render the accuracy of actual observational data directly into the concluded result. Although this is not a valid procedure in all contexts (as noted in the accompanying caveat: "based on the fact we have assumed the underlying model we used is correct"), the age given is thus accurate to the specified error (since this error represents the error in the instrument used to gather the raw data input into the model).

The age of the universe based on the "best fit" to WMAP data "only" is 13.69±0.13 Ga^[1] (the slightly higher number of 13.73 includes some other data mixed in). This number represents the first accurate "direct" measurement of the age of the universe (other methods typically involve Hubble's law and age of the oldest stars in globular clusters, etc). It is possible to use different methods for determining the same parameter (in this case – the age of the universe) and arrive at different answers with no overlap in the "errors". To best avoid the problem, it is common to show two sets of uncertainties; one related to the actual measurement and the other related to the systematic errors of the model being used.

An important component to the analysis of data used to determine the age of the universe (e.g. from WMAP) therefore is to use a Bayesian Statistical analysis, which normalizes the results based upon the priors (i.e. the model).^[9] This quantifies any uncertainty in the accuracy of a measurement due to a particular model used.^[10][11]

History

The concept that the Universe may have a finite age is relatively modern, dating back only to the 1920's. The concept that the age of the Earth was millions, if not billions, of years old began to be realized in the 18th century. However, most scientists throughout the 19th century and into the first decades of the 20th century presumed that everything else in the Universe was Steady State and eternal, with maybe stars coming and going but no changes occurring at the largest scale known at the time.

The first scientific theories indicating that the age of the Universe might be finite were the studies of thermodynamics, formalized in the mid 19th century. The concept of Entropy dictates that if the Universe (or any other closed system) was infinitely old than everything inside would be at the same temperature, and thus there would be no stars and no life. No scientific explanation for this contradiction was put forth at the time. In 1915 Albert Einstein published the theory of General Relativity.^[12] This theory clearly showed that the Universe cannot be static and must be either expanding or contracting. Einstein himself did not believe this result and so he added what he called a cosmological constant to his equations in an unsuccessful attempt to produce a theory consistent with a Steady State Universe.

The first direct observational evidence that the Universe has a finite age came from the observations of astronomer Edwin Hubble published in 1929.^[13] Earlier in the 20th century, Hubble and others resolved individual stars within certain nebula thus determining that they were galaxies, similar to, but external to, our Milky Way Galaxy. In addition, these galaxies were very large and very far away. Spectra taken of these distant galaxies showed a red shift in their spectral lines presumably caused by the Doppler effect, thus indicating that these galaxies were speeding away from the Earth. In addition, the further away these galaxies seemed to be, the greater the redshift and thus the faster they seemed to be speeding away. This was the first direct evidence that the Universe is not static but expanding. By calculating when all of the objects must have started speeding out from the same point yielded the first estimate of the age of the Universe. Hubble's initial value for age was very low as the galaxies were assumed to be much closer than later observations found them to be.

The first reasonably accurate measurement of the rate of expansion of the Universe, a numerical value now known as the Hubble constant, was made in 1958 by astronomer Allan Sandage.^[14] His measured value for the Hubble constant yielded the first good estimate of the age of the Universe, coming very close to the value range generally accepted today.

However Sandage, like Einstein, did not believe his own results at the time of discovery. His value for the age of the Universe was too short to reconcile with the 25 billion year age estimated at that time for the oldest known stars. Sandage and other astronomers repeated these measurements numerous times, attempting to reduce the Hubble constant and thus increase the resulting age for the Universe. Sandage even proposed new theories of cosmogony to explain this discrepancy. This issue was finally resolved by improvements in the theoretical models used for estimating the ages of stars. Presently, using these new models for stellar evolution, the estimated age of the oldest known star is about 13.2 billion years.^[15]

The discovery of microwave cosmic background radiation announced in 1965^[16] finally brought an effective end to the remaining scientific uncertainty over the expanding Universe. The space probe WMAP, launched in 2001, produced data that determines the Hubble constant and the age of the Universe independent of galaxy distances, removing the largest source of error.