How we know the Universe is 13.8 billion years old | by Ethan Siegel | Starts With A Bang! | Apr, 2024
The Universe is 13.8 billion years old, going back to the hot Big Bang. But was that truly the beginning, and is that truly its age?
According to the theory of the hot Big Bang, the Universe had a beginning. Originally known as “a day without a yesterday,” this is one of the most controversial, philosophically mind-blowing pieces of information we’ve come to accept as part of the scientific history of our Universe. Many detractors will reject it as being too in-line with certain religious texts, while others — perhaps more justifiably — note that in the modern context of cosmic inflation, the hot Big Bang only occurred as the aftermath of a previous epoch.
And yet, if you ask any cosmologist or astrophysicist who’s well-versed in the scientific story of our beginnings “How old is our Universe?” you always get the same answer: 13.8 billion years. Why is this, and when do we start counting? That’s what Denis Gaudet wanted to know, having written in to ask:
“Why do you start counting the age of universe after 380,000 years have elapsed after the Big Bang?”
The time “380,000 years after the Big Bang” is of particular interest, but very few people mark that as the beginning of the Universe; it is the beginning of something important, however. Here’s what we can say about how old our Universe truly is.
The first thing you need to understand is that there are two different ways of measuring the age of the Universe since the start of the hot Big Bang.
- We can find “the oldest thing we know how to measure its age” and conclude that the Universe must be at least that old.
- We can use what we know about the theory that governs the Universe, General Relativity, as well as our knowledge of what the Universe is made out of plus how fast it’s expanding today to calculate how long it’s been since the start of the hot Big Bang.
The first method isn’t exactly a measurement of how old the Universe is, but rather a sanity check…
