Starts With A Bang

Ask Ethan: How Do We Know The Universe Is 13.8 Billion Years Old? (Synopsis)

As we peer back to greater and greater distances, we see farther back in time, too. How far must we go back to reach the birth of our Universe? Image credit: ESA/Hubble & NASA; Acknowledgement: Judy Schmidt.

“The oldest and strongest emotion of mankind is fear, and the oldest and strongest kind of fear is fear of the unknown.” -H.P. Lovecraft

There’s a unique relationship between everything that exists in the Universe today — the stars and galaxies, the large-scale structure, the leftover glow from the Big Bang, the expansion rate, etc. — and the amount of time that’s passed since it all began. When it comes to our Universe, there really was a day without a yesterday, but how do we know exactly how much time has passed between then and now?

The Universe’s expansion rate is determined by the various types and percentages of matter and energy present within it. Image credit: NASA, ESA, and A. Feild (STScI).

There are two ways: one complex and one simple. The complex way is to determine all the matter and energy components making up the Universe, to measure how the Universe has expanded over the entirety of its cosmic history, and then, in the context of the Big Bang, to deduce how old the Universe must be. The other is to understand stars, measure them, and determine how old the oldest ones are.

The most ancient globular clusters known contain stars up to 95% of the Universe’s age. Image credit: ESA/Hubble & NASA.

The complex answer is more accurate, but more importantly, they both agree with each other. Get the details on this week’s Ask Ethan!