When you start thinking about the universe, the questions start getting mind-bending really fast. What’s beyond our universe? What came before our universe? How old is the universe? Well, while a lot of these questions require a Ph.D. in astrophysics to even scratch the surface, we actually have a relatively simple answer to the last one. Of all the mysteries of the universe, we know how old it is with remarkable accuracy.
It's All Relative
But first, how did we get there? That was a pretty long and complicated road. Early scientific thought believed that the universe had no age; it simply was. They called it a steady state universe. This was what Albert Einstein himself believed when he came up with his theory of general relativity.
The only problem was, Einstein’s work didn’t quite make sense in a steady state universe. He actually had to come up with something called a “cosmological constant” to make the math check out. But the universe is not static like Einstein thought. It’s constantly expanding, and has been ever since it formed.
The Growing Universe
Not long after Einstein published his theories, other scientists used his work to prove that the universe couldn’t possibly be static. According to the math, the universe should have been expanding. Around the same time, astronomers discovered that distant galaxies weren’t sitting still in the sky—they were actually moving away from the Earth. Einstein realized his mistake and abandoned the cosmological constant: the Universe was officially expanding.
It may not seem like the biggest bombshell in the world, but the knowledge that the universe was expanding led us not only to the Big Bang theory, but to realize that the universe has a finite age—and that we can calculate it.
How Old is Our Universe?
Since the early 20th century, scientists have had two surprisingly simple means of aging the universe: by studying the oldest things in it, and by measuring the rate at which it’s expanding (also known as the Hubble constant).
Over the years, we’ve gained more and more accurate ways of aging the oldest stars, and of measuring the Hubble constant. Today, taking all available information into account, scientists have determined that the universe is 13.8 billion years old, give or take 20 million years. While 20 million years may sound like a big margin of error, but on the time scale we're talking about, that number is accurate to within a fraction of a percent. As far as astrophysics goes, that's a remarkably accurate number.
So while many questions about the universe will still break our brains for years to come, thanks to your local astrophysicist, at least we can say how old it is.