Age of The Universe - History

History

In the 18th century, the concept that the age of the Earth was millions, if not billions, of years began to appear. However, most scientists throughout the 19th century and into the first decades of the 20th century presumed that the Universe itself 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) were infinitely old, then 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. Based on Einstein's theory, Mgr. Georges Lemaître's work 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. Earlier in the 20th century, Hubble and others resolved individual stars within certain nebulae, 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 moving away from the Earth. In addition, the farther away these galaxies seemed to be, the greater the redshift and thus the faster they seemed to be moving away. This was the first direct evidence that the Universe is not static but expanding. The first estimate of the age of the Universe came from the calculation of when all of the objects must have started speeding out from the same point. 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. 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.

The discovery of microwave cosmic background radiation announced in 1965 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.