By Carl Sagan, The Cosmic Evangelist

The universe is 13.8 billion years old. You have probably heard that number. You may have memorized it for a test. But do you know HOW we know? Do you know the detective story behind the number?

Because it is one of the greatest detective stories ever told. And it starts with a simple observation: the light from distant galaxies is redder than it should be.

Chapter One: The Redshift

In 1929, an astronomer named Edwin Hubble was measuring the light from galaxies. He used a technique called spectroscopy: splitting starlight into its component colors, the way a prism splits sunlight into a rainbow. Each chemical element absorbs light at specific wavelengths, leaving dark lines in the spectrum. Hydrogen always absorbs at the same wavelengths. Calcium always absorbs at the same wavelengths. These lines are as reliable as fingerprints.

But Hubble noticed something strange. The fingerprints were in the wrong place. The dark lines from distant galaxies were shifted toward the red end of the spectrum. The farther away the galaxy, the more the shift.

This is called redshift, and it means the galaxy is moving away from us. It is the same principle that makes a siren sound lower-pitched as an ambulance drives away from you. The sound waves stretch. With light, the waves stretch too, shifting toward red.

Hubble's discovery: almost every galaxy in the sky is moving away from us. And the farther away it is, the faster it is moving.

Chapter Two: Running the Film Backward

If everything is flying apart, then at some point in the past, everything must have been closer together.

Run the film backward. The galaxies get closer. The universe gets smaller. The matter gets denser, hotter. Keep running the film backward and everything converges toward a single point: unimaginably dense, unimaginably hot, a state of matter so extreme that the laws of physics as we know them break down.

That point is the beginning. The moment the expansion started. What we call the Big Bang (a term coined, ironically, by a physicist who did not believe in it, Fred Hoyle, on a BBC radio program in 1949).

If you know how fast the galaxies are flying apart and how far away they are, you can calculate how long the expansion has been running. That gives you the age. Hubble's first estimate was about two billion years, which was wrong because his distance measurements were off. But the METHOD was right. And better measurements were coming.

Chapter Three: The Afterglow

In 1964, two engineers at Bell Labs in New Jersey, Arno Penzias and Robert Wilson, were trying to calibrate a radio antenna. They kept picking up a faint hiss of microwave radiation. It came from every direction. It would not go away. They cleaned pigeon droppings off the antenna. The hiss persisted.

What they had found, almost by accident, was the cosmic microwave background (CMB): the afterglow of the Big Bang itself.

Here is what that means. In the first few hundred thousand years after the Big Bang, the universe was so hot and dense that light could not travel freely. Photons bounced off charged particles like pinballs. The universe was opaque, a fog of plasma.

Then, about 380,000 years after the beginning, the universe cooled enough for atoms to form. Suddenly the photons were free. They streamed outward in every direction. They have been traveling ever since, for 13.8 billion years, slowly cooling and stretching as the universe expanded.

That ancient light is still arriving. It fills the entire sky. It has cooled from thousands of degrees to 2.7 degrees above absolute zero. It is the oldest light in the universe, and Penzias and Wilson heard it as a hiss on a radio antenna in New Jersey.

Chapter Four: The Precision

In 2001, NASA launched a satellite called WMAP (the Wilkinson Microwave Anisotropy Probe). Its job: map the cosmic microwave background with unprecedented precision. Measure the tiny temperature fluctuations in the afterglow. Those fluctuations are the seeds of everything. The slightly denser regions became galaxy clusters. The slightly less dense regions became the voids between them. The pattern of hot and cold spots in the CMB is a baby picture of the universe.

From those measurements, using well-tested physics (general relativity, thermodynamics, nuclear physics), scientists calculated the age of the universe: 13.77 billion years, plus or minus about 40 million years.

In 2013, the European Space Agency's Planck satellite refined the measurement: 13.799 billion years, plus or minus 21 million years. That is a precision of about 0.15 percent. We know the age of everything to better than two tenths of one percent.

What This Actually Means

Here is the part I want you to carry with you.

We did not guess the age of the universe. We did not receive it from a holy book. We did not find it in a dream. We MEASURED it. With light. With patience. With instruments built by human hands and mathematics developed by human minds.

The method works like this: observe something (redshift). Form a hypothesis (the universe is expanding). Make a prediction (there should be an afterglow). Find the afterglow. Measure it precisely. Calculate the age. Check it against independent measurements. Refine.

Every step is checkable. Every step can be repeated by anyone with the right instruments and the right training. Every step invites you to prove it wrong, and nobody has.

That is what science IS. It is not a collection of facts. It is a method for finding things out that is honest about its own limitations and welcomes disproof. And this particular application of the method told us something no human being knew before the twentieth century: the universe had a beginning, and the beginning was 13.8 billion years ago, and the light from that beginning is still arriving, right now, filling the sky, and you can hear it as static on an old television set tuned to no channel.

The oldest light in the universe, arriving at your antenna, disguised as noise. That is the cosmos trying to tell you its own story. All you have to do is learn how to listen.

"The cosmos is all that is or ever was or ever will be. Our feeblest contemplations of the Cosmos stir us. We know we are approaching the greatest of mysteries."

Carl Sagan, The Cosmic Evangelist