# How Old is The Universe?

Astronomers have determined that our universe is 13.7 billion years old. How exactly did they come to this precise conclusion? how old is the universe tells the incredible story of how astronomers solved one of science’s most compelling mysteries, while also introducing readers to fundamental concepts and innovative advances in modern astronomy.

Scientists’ best guess is that the universe is about 13.8 billion years old. But like so many of the largest objects in the universe, we’re not entirely sure of its age. There are two competing estimates based on two competing measurements of the cosmos’ expansion rate, meaning it could be as young as 11.4 billion years.

So instead of determining the age of the universe based on the ages of the objects in it, we use the expansion of the cosmos itself and rewind that growth back to the big bang, when all the matter and energy that now exists exploded outward from a tiny point. The problem with this is that the expansion of the universe is not constant; If we look at the movements of distant objects, we can see that the expansion has accelerated over time.

To understand how old the universe is, we need to find out the rate of this acceleration, which is represented by a number called the Hubble constant. This is where the problem comes in. There are two ways to measure the Hubble constant.

One uses the cosmic microwave background (CMB), a relic of the very first light that shone after the Big Bang, as a sort of cosmic baby image. We can take this light and use our models of how the universe evolved to calculate what it should be like now, giving a measure of the Hubble constant.

The other method, called local measurement, is simpler. It uses observations of relatively close-by galaxies and supernovae to measure the universe’s expansion rate and how it changes between these objects. But those two measurements, the baby picture and the youngest, disagree on how fast the expansion of the universe is accelerating.