How to Explain How Old the Universe Is – For Dummies

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The Question: How Old Is the Universe, anyway?

The Universe is old.  Like really old.  Of course, we space nerds already know this – in fact, most of us know it’s about 13.8 billion years old – but how do we convince those neander-bros and non-nerds around us of this, and why should we even bother wasting our breath?  We’ll tackle the why first, then the how.

Why it matters:

Understanding the scale and scope of astronomical time is foundational to any sort of functional understanding of astronomy, physics, or pretty much any other form of legitimate scientific study.  Unfortunately, most people either don’t think about it or have been corrupted by misinformation about dinosaurs and arks.  After all, how does knowing how old the Earth and the Universe are, respectively, help me get to work every morning? 

But this stuff  – knowing how old our universe is – is important.  Policy decisions are often made based on the beliefs of a politician or her constituents, so it matters if your legislative representative believes that the Earth is only 6,000 years old, because that person probably doesn’t have a clue about longevity and the lasting impact of things like climate change, etc.

Then how do we make the case to people who are either uninterested or misinformed?

I’ve broken it down here into two broad categories that can help explain things to people with a minimal scientific background:

1. The logical implications of the things we can measure. 

  • Objects in Space

    • There are objects that we can measure which indicate that the universe is 13.8 billion years old.  The logic: a thing cannot exist within a universe that is older than the universe in which it exists.  So the argument goes like this: Using complex instruments and techniques, we can measure the mass of stars, and that data can help us to infer the age of the universe. 
  • Searching for the Earliest Stars
    • We know, based on data from myriad measurements across myriad studies (you may have to explain the word “myriad” or use a simpler term) that more massive stars burn faster than smaller stars.  Because of this, we know that the first stars that were formed after the big bang burned out relatively quickly, and scientists have been hunting for evidence of their existence for a long time.  According to a 2017 statement by the ESO, however, scientists have found that evidence in a bright galaxy, in the form of “the detection of dust in the early universe.”  The statement went on to say, “Determining the timing of this ‘cosmic dawn’ is one of the holy grails of modern astronomy, and it can be indirectly probed through the study of early interstellar dust.”
  • The summary:
    • Using measurements about the age of the oldest objects in the universe, we can determine a “minimum” age of the universe.  In other words, “The universe has to be at least this old because that’s how old the oldest things we can find are, and a universe has to be older than the things in it.
    • Possible follow-up questions from your audience:

      • Q: “Well, how do you know how old the stars and the space dust are?”
      • A: “There are a lot of different methods, including spectrography and radiocarbon dating.”
      • Q: “Well I heard from {insert unreliable source here} that radiocarbon dating is just a bunch of junk science. How can you trust that?”
      • A: “It’s not junk science.  Certain isotopes, like Carbon 14 in particular, decay at a very stable, measurable rate, and by measuring how much of that isotope is still in an object, we can tell how old it is.  Any cases “they” told you about where carbon dating said a cow bone was three-million years old or something like that were either hoaxes, logically fallacious arguments by special interests, or testing anomalies.  The overwhelming majority of radiocarbon dating methods are exceedingly accurate.”

2. The speed at which those things are expanding.

  • The Cosmic Speed Limit
    • Surely we can all agree that the speed of light is a thing, right?  The speed of light has been measured and is universally accepted by the scientific community to be around 186,000 miles per second.  It’s the fastest thing there is.  Sorry, Steve McQueen. 
  • The Formula
    • Well, even the biggest neander-bro of them all should be able to remember pre-algebra from high school (yeah, he took it in high school because he wasn’t ready for it in middle school) that distance equals rate multiplied by time. In other words, if a train leaves the station at a constant speed of 60 miles an hour and travels for one hour, how far has it gone?
  • The Application
    • Okay, so now let’s apply that to the universe.  Well… maybe we’d better start smaller.  Let’s apply it to the sun.  It takes light from the sun about 8.3 minutes to arrive at Earth.  We’ve measured that.  A lot.  It’s a thing.  Using that time, then, we can calculate the distance between Earth and the sun.  It goes like this: D = R(186,000 miles/sec) x T(498 seconds), therefore, D = approx. 92.96 million miles.If we apply that same logic to other objects in space, we can tell how far away they are, too.  Now, knowing their distance also means that we can measure how fast they’re moving away from us (that same D=RT formula can be transformed to be R=D/T).  Based on the speed of that expansion, we can tell how long it took the farthest objects we can observe to get where they are.  In the case of stars and other interstellar objects, this data can also be used to determine the age of those objects, and therefore the age of the universe.  
  • To summarize:
    • We can measure how far away objects are from us, and how fast they are moving away from us, therefore, we can measure the age of the universe based on how fast it’s expanding.  Those calculations tell us that the universe is about 13.8 billion years old.  Radiocarbon dating and other geological methods tell us that the Earth is about four and a half billion years old.
    • Possible Follow-up Questions from your Audience:

      • There shouldn’t be any follow up questions with this.  You can’t argue with math.  And if you’re arguing with someone who wants to argue with math, then you should cease your interaction with that person immediately because to continue would be like playing chess with a pigeon.  No matter how right you are, the bird is just going to crap all over the board and strut around as if it won anyway.

The Bottom Line

So there you have it.  The basics of how we know that we live in a really, really old universe.  Even though we simply can’t measure certain aspects of universal time due to its sheer scale, we can infer logically – and with solid evidence that is both verifiable and repeatable, by the way – the age of the universe.  

 

Brandon Humphreys

Brandon Humphreys

I'm a wizard. I write stuff and it goes from my head into yours - Magic! Apart from that, I am the Senior Editor for Space Porn, a veteran, a rock guitarist, and a teacher.

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