How Old is the Universe?

Astrophysicists tell us the universe is 13.8 billion years old, but how do they know that? Everyday Einstein explains how we determine the age of our universe from ancient stars and relic radiation left over from the Big Bang.

Sabrina Stierwalt, PhD
4-minute read
Episode #146

The most massive stars still on the main sequence (i.e. still burning hydrogen) place a limit on the age of the cluster. The oldest observed star clusters have ages in the range of 11-13 billion years.

White Dwarfs

In a previous episode, we discussed the end stages of a dying star’s life and how a low mass star like our sun can evolve into white dwarf. White dwarfs are extremely dense objects that pack the equivalent of the Sun’s mass into the size of Earth. A teaspoon of white dwarf material weighs 15 tons!

Since white dwarfs are no longer burning elements through fusion to produce and emit radiation, they are instead left to cool much like the dying embers of a fire. The temperatures of white dwarf stars can thus tell us how long they have spent cooling and place a limit on their age. Observations with the Hubble Space Telescope find the oldest white dwarfs to be in the range of 12-13 billion years old.

The Cosmic Microwave Background

WMAP image of the CMB

While age-dating the universe through ancient stars is an important check, the most direct determination of its age comes from relic radiation left behind from the Big Bang, called the cosmic microwave background radiation, or CMB for short.

Simply put, our universe is expanding as time goes on, leaving more and more space between us and our extragalactic neighbors. We can turn back the clock however, by rewinding this expansion, through the help of information encoded in the CMB, to determine how long the universe has been expanding.

The CMB is radiation produced during the Big Bang, the singularity that in a fraction of a second began the combination of high densities, temperatures, and pressures that later expanded and cooled into the universe we observe today. The CMB radiation has cooled significantly as it has traveled, but it still encodes information from the Big Bang event. The CMB thus offers us the equivalent of a baby picture of our universe, a snapshot of what occurred in the beginning.

The age of the universe is tied to 3 cosmological parameters that together describe the expansion of the universe:

  1. the rate of expansion of the universe, known as the Hubble constant
  2. the density of both baryonic (normal) and dark matter in the universe (i.e. how much matter needs to be expanded)
  3. the cosmological constant, a parameter tied to the acceleration of that expansion


About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.

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