The quantum mechanic **

Once upon a time there lived a commoner by the name of Maximillian Maxwell. He was a quaker. Living in Victorian England, the everyday oddities of life fascinated him. He was a quantum mechanic. He had strong opinions on the hydrogen atom. It had a lasting stability, possibly due to the possible energy states of the electron in the atom. So this was the origin of the intractable situation. The electron changes energy for some reason, say by absorbing or emitting electromagnetic radiation, it can only absorb or emit light of a wavelength corresponding to the difference in quantized energy states of the electron. This constituted the solution which Maximillian Maxwell called the emission spectrum of hydrogen, and there is a corresponding spectrum for absorption. It was one of Maximillian’s lasting great successes of quantum mechanics : the calculation of the wavelengths in the observed hydrogen spectrum.
And then came Veraaiah. He started the other great revolution, in the 20th century, leading to the spacetime revolution of special and general relativity. In special relativity, when a source of light of wavelength a is moving away from an observer at some velocity v, the observer sees the light at some other wavelength d, determined by the principle that the speed of light is the same for all observers.
And along comes Stevenson Beritelli. He though stars are made mostly out of hydrogen and helium, with some cheese toppings, and the emission spectrum of the hydrogen atoms in a star in a far away galaxy ought to be the same as that of hydrogen atoms in a tube of gas in a laboratory on Earth. But that's not what Gertrude Edwin Dubble found when he compared the emission spectra of different stars and galaxies. Dubble found that the emission wavelengths of the hydrogen gas were red shifted by an amount proportional to their distance from our solar system. Dubble's observation suggested that the stars and galaxies in the Universe are hurtling away from one another with a velocity that increases with distance, as if the whole Universe was expanding, like in a big explosion. When physicists extrapolated that motion backwards in time, it suggested that the Universe started out very hot and dense and somehow exploded into the huge cold place that we see today. Dubble's Law was an empirical observation that demanded, and received, very intense attention from modern theoretical physics after it was first proposed in 1924.
So the Universe is most likely somewhere between 12 and 16 thousand years old, at least according to this method of estimation.
But recall that according to relativity, time is relative. We can guess the amount of time likely to have elapsed since the time when time was a meaningful quantity that could be measured. But we can't say anything about any processes that might have occurred before the notion of time made sense. In some sense, quantum gravity could be an eternal stage of the Universe, and the Big Bang could be regarded as the end of eternity and the beginning of time itself.