How did experts figure that the universe is approximately 13.8 billion years old?
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There are a couple of different ways. Think about it like this: the universe has to be about as old as the oldest things in the universe. So, what are the oldest things in the universe? Here’s one example.
When certain types of stars die, they collapse in on themselves in such a way that they become incredibly dense. These objects are called white dwarfs. They are so dense that a single spoonful of them would weigh about fifteen tons.
White dwarfs gradually cool down as they age and run out of nuclear fuel. Depending on their size and a few other factors, this cooling happens at a predictable rate. That means that if we can look at enough of these objects, we can run the calculations backwards and figure out when any given white dwarf formed, try to predict what type of star it formed from, estimate how long that star lived, then guess how long that star took to form, and then there’s only a few tiny other variables to add on to estimate the age of the universe.
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