What is The Age of The Universe and How Do We Know This?
A Nebulous Starting Point.
Because we don't exactly know what preceded the Big Bang singularity and indeed do not even know if the word time has a meaning at that point, what is meant by the age of the universe has to be somewhat conditional. Our understanding of the big bang and what, if anything, preceded it is so unclear that we must extrapolate forward a few millions of a second to our first understood state of the universe and technically it is this time to the present that the Lambda-CDM Correspondence Model measures as 13.789 Billions years (+ or minus - .037 Billion years or 37,000,000) it goes without saying that this has little effect on the answer to this question, but how did we come by it??
It is worth noting that the steady state model of the universe was accepted until the early part of the 20th century as a result of Einstein's and later Georges Lemaitre's work. The Universe was thought to be eternal, to have always existed.
Based on the General Theory of Relativity, Lemaitre concluded that the universe had to be expanding or contracting. Indicating an universe that either must have begun or would definitely have an end.
The first positive proof that the universe had a beginning came with the discovery of the red light shift, an observation made by Edwin Hubble in 1929 while observing galaxies other than the milky way. Hubble saw that these galaxies had red spectral lines, (a result of the Doppler Effect) which indicated that these galaxies were moving away from each other and that the furthest ones were moving away from each other the fastest. Determining the age of the Universe now became a matter of mathematically extrapolating backwards from this observed shift. Because Hubble assumed the galaxies were much closer than they actually were his estimate was considerably low.
In 1958, Allan Sandage came up with a model resulting in an age of the universe that is very close to the accepted age today. This time frame conflicted with the presumed age of the oldest star, which was over 20 million years. The model used to calculate the age of stars has since been refined and these irreconcilable results have been resolved into a coherent model of expansion and of the age of the universe.
The discovery of microwave cosmic background radiation, the energetic echoes of the big bang, cemented the concept of an expanding universe in 1965. (This radiation can be observed on a TV with no signal displaying, "snow." A percentage of this snow is an actual echo of the big bang.)
By use of the Cosmological Constant, Density Parameters, Hubble Parameters, and The Friedman Equation the age of the universe can be calculated by the integration of the resulting formula.
This yielded an age for the universe of 14.4 Billions years
NASA's Wilkinson Microwave Anisotrophy Probe (WMAP) after collecting data for nine years estimated the age of the universe at 13.772 Billion years in 2012.
This year the Planck Spacecraft of The European Union estimated the age of the universe to be 13.8 Billion years.
By combining these estimates our current best guess is 13.789 Billion years. While this may be subject to some minuscule revision, this estimate is thought to be very accurate. It is truly a testament to human ingenuity and the scientific method that we could determine such a thing to such a degree of accuracy.
It is worth noting that because the expansion of the universe is accelerating a day will come when all remnants of the big bang will be lost from view here on Earth and none of these models will be left with any observable evidence to support them.