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.

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

Even if we accept the Big Bang, how confident are we that we have its age right?

Ethan Siegel
Sep 3, 2016 · 7 min read

“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

You’ve heard the story before: the Universe began with the Big Bang 13.8 billion years ago, and formed atoms, stars, galaxies, and eventually planets with the right ingredients for life. Looking at distant locations in the Universe is also looking back in time, and somehow, through the power of physics and astronomy, we’ve figured out not only how the Universe began, but its age. But how do we know how old the Universe is? That what Thys Hauptfleisch wants to know for this week’s Ask Ethan:

Ethan, how was the 13.8 billion years calculated? (In English please!)

There are actually two different, independent methods we have to measure this number, and while one is far more accurate to the other, the less precise method involves far fewer assumptions.

A depiction of our Universe’s cosmic history. Image credit: NASA / GSFC / Dana Berry.

The more accurate one is to think about the fact that our Universe is expanding and cooling today, and to recognize that it was therefore hotter and denser in the past. If we go back, to earlier and earlier times, we’d find that as the volume of the Universe was smaller, all the matter in it was not only closer together, but that the wavelengths of all the individual photons (particles of light) in it were shorter, as the Universe’s expansion has lengthened them to be as long as they are today.

Radiation gets redshifted as the Universe expands, meaning it was more energetic in the Universe’s past, with a greater amount of energy per photon. Image credit: E. Siegel.

Since a photon’s wavelength defines its energy and temperature, a shorter-wavelength photon is more energetic and higher in temperature. As we go back farther and farther in time, the temperature goes up and up, until at some point, we reach the earliest stages of the hot Big Bang. This is important: there is an “earliest stage” for the hot Big Bang!

If we were to extrapolate “infinitely” far back, we’d reach a singularity, where physics breaks down. With our modern understanding of the very early Universe, we know that an inflationary state preceded the hot, dense Big Bang, and that inflationary state was of an indeterminate duration. So when we speak of “the age of the Universe,” we’re talking about how much time has passed since the Universe could first be described by the hot Big Bang until the present day.

After cosmic inflation ends, we can describe our Universe as we do today, and start the clock ticking from the hot Big Bang. Image credit: Bock et al. (2006, astro-ph/0604101); modifications by E. Siegel.

Under the laws of General Relativity, if you have a Universe like ours, which is:

  • of uniform density on the largest scales,
  • which has the same laws and general properties at all locations,
  • which is the same in all directions, and
  • in which the Big Bang occurred at all locations everywhere at once,

then there is a unique connection between how old the Universe isand how it’s expanded throughout its history.

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).

In other words, if we can measure how the Universe is expanding today and how it has expanded throughout its entire history, we can know exactly what all the different components are that make it up. We learn this from a whole host of observations, including:

  • From direct measurements of the brightnesses and distances of objects in the Universe such as stars, galaxies and supernovae, allowing us to construct the cosmic distance ladder.
  • From measurements of large-scale-structure, the clustering of galaxies, and from baryon acoustic oscillations.
  • And from the fluctuations in the cosmic microwave background, a “snapshot” of the Universe when it was a mere 380,000 years old.
Three different types of measurements, distance indicators found in stars and galaxies, the large scale structure of the Universe, and the fluctuations in the CMB, tell us the expansion history of the Universe. Images credit: ESA/Hubble and NASA, Sloan Digital Sky Survey, ESA and the Planck Collaboration.

You put all of these things together, and you get a Universe that is made up,today, of 68% dark energy, 27% dark matter, 4.9% normal matter, about 0.1% neutrinos, about 0.01% radiation, and pretty much nothing else. But you throw in how the Universe is expanding today, and we can extrapolate this back in time, and learn the entire expansion history of the Universe, and hence, its age.

The breakdown of the various components contributing to the Universe’s energy density as a function of time. Image credit: E. Siegel.

The number we get — most precisely from Planck but augmented from the other sources like supernova measurements, the HST key project and the Sloan Digital Sky Survey — is that the Universe is 13.81 billion years old,with an uncertainty of just 120 million years. This means we’re confident in the age of the Universe to 99.1% accuracy, which is an amazing feat!

Yes, we have a number of different data sets that point to this conclusion, but in reality, it’s all the same method. We’re simply fortunate that there is a consistent picture that they all point towards, but in reality, any one of the constraints themselves is insufficient to say “this is exactly how the Universe is.” Instead, they all offer a variety of possibilities, and it’s only their intersection that tells us where we live.

The combination of the three main types of data sets, distance indicators such as supernovae, galaxy clustering and the CMB, point to exactly what makes up our Universe. Image credit: Suzuki et al. (The Supernova Cosmology Project), accepted for publication, Ap.J., 2011., via

If the Universe had the same current properties today but were made of 100% normal matter and no dark matter or dark energy, our Universe would be only 10 billion years old. If the Universe were 5% normal matter (with no dark matter or dark energy) and the Hubble constant were 50 km/s/Mpc instead of 70 km/s/Mpc, our Universe would be a whopping 16 billion years old. With the combinations of things we have today, however, we can confidently state 13.81 billion years is the age of the Universe, with a very small uncertainty. It’s an incredible feat of science.

And the entirety of that is legitimately one method. It’s the main one, it’s the best one, it’s the most complete one, and it’s got a ton of different pieces of evidence pointing towards it. But there is another, and it’s incredibly useful for checking our results.

The twinkling stars you see are evidence of variability, which is due to a unique period/brightness relationship. Image credit: Joel D. Hartman, Princeton University, via

It’s the fact that we know how stars live, burn through their fuel, and die. In particular, we know that all stars, when they’re alive and burning through their main fuel (fusing hydrogen into helium), have a specific brightness and color, and remain at that specific brightness and coloronly for a certain amount of time: until their cores start to run out of fuel.

At that point, the brighter, bluer and higher mass stars begin to “turn off” of the main sequence (the curved line on the color-magnitude diagram, below), evolving into giants and/or supergiants.

The life cycles of stars can be understood in the context of the color/magnitude diagram shown here. Images credit: Richard Powell under c.c.-by-s.a.-2.5 (L); R. J. Hall under c.c.-by-s.a.-1.0 (R).

By looking at where that turn-off-point is for a cluster of stars that all formed at the same time, we can figure out — if we know how stars work — how old those stars in the cluster are. When we look at the oldestglobular clusters out there, the ones lowest in heavy elements and whose turn-offs come for the lowest-mass stars out there, we find that they pretty consistently come in at an age of up to around 13.2 billion years, but not much older. (There are significant uncertainties of around a billion years on this, mind you.)

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

Ages of 12 billion years and up are very common, but ages of, say, 14 billion years and over are unheard of, although there was a period in the 1990s where ages of 14–16 billion years were often cited. (An improved understanding of stars and their evolution has bumped these numbers down.)

So all in all, we have two methods — one from our cosmic history and one from measuring local stars — that show us our Universe’s age is between 13 and 14 billion years old. It wouldn’t surprise anyone if we turned out to be as little as 13.6 or as much as 14.0 billion years old, but we’re not13.0 or 15.0 billion years old with extreme certainty. Say we’re 13.8 billion years old with confidence, and now you know how we’ve figured it out!

Submit your Ask Ethan questions for a chance to see them here by writing to startswithabang at gmail dot com!

This post first appeared at Forbes, and is brought to you ad-free by our Patreon supporters. Comment on our forum, & buy our first book: Beyond The Galaxy!

Starts With A Bang!

The Universe is out there, waiting for you to discover it.

Ethan Siegel

Written by

The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist.

Starts With A Bang!

The Universe is out there, waiting for you to discover it.

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