How do we know the age of the Universe?
13.8 billion years is a great number. But how sure are we of this, and how did we get there?
“Nobody grows old merely by living a number of years. We grow old by deserting our ideals. Years may wrinkle the skin, but to give up enthusiasm wrinkles the soul.” -Samuel Ullman
In an ideal world, when we want to uncover the answer to a question like “what is the age of the Universe,” we’d 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.
The “good” one is to think about the fact that our Universe is expanding and cooling today, and to recognize that it was therefore hotter and denser in the past. If we go back, to earlier and earlier times, we’d find that as the volume of the Universe was smaller, all the matter in it was not only closer together, but that the wavelengths of all the individual photons (particles of light) in it were shorter, as the Universe’s expansion has lengthened them to be as long as they are today.
