By Carl Sagan, The Cosmic Evangelist

There is a light that fills the universe. It is everywhere. It is in this room. It is between the galaxies. It is inside your body right now, passing through you at the speed of light, 400 photons per cubic centimeter, remnants of a fire that burned 13.8 billion years ago.

It is the cosmic microwave background — the oldest light in the universe. And it is the single most important piece of evidence in all of cosmology.

The Discovery

In 1964, two radio engineers at Bell Laboratories in New Jersey — Arno Penzias and Robert Wilson — were trying to calibrate a horn antenna for satellite communications. They kept picking up a faint hiss of microwave radiation. It came from every direction. It did not change with the time of day or the season. It was not coming from the Milky Way. It was not coming from any known source.

They checked for equipment errors. They cleaned pigeon droppings from the antenna (this is not a joke — the pigeons were a genuine concern). The hiss persisted. It was 3.5 Kelvin — three and a half degrees above absolute zero. A faint warmth, everywhere, uniform, inexplicable.

Forty miles away, at Princeton University, a theoretical physicist named Robert Dicke had predicted exactly this radiation. He had calculated that if the Big Bang theory was correct — if the universe had once been incredibly hot and dense — then the light from that early fireball should still be detectable today, cooled by 13.8 billion years of cosmic expansion to a temperature of a few degrees above absolute zero.

Penzias and Wilson had found it. By accident. They won the Nobel Prize in 1978. The pigeons did not.

What the Light Shows

The cosmic microwave background is not just evidence that the Big Bang happened. It is a photograph.

380,000 years after the Big Bang, the universe cooled enough — to about 3,000 Kelvin — for electrons to combine with protons to form neutral hydrogen atoms. Before that moment, the universe was opaque. Photons could not travel far without being scattered by free electrons. The universe was a fog.

At 380,000 years, the fog cleared. Light was free to travel. And the light that was released at that moment — the flash of the universe becoming transparent — has been traveling ever since, stretching with the expansion of space, cooling from 3,000 Kelvin to 2.725 Kelvin, shifting from visible light to microwaves.

That light is what Penzias and Wilson detected. It is the first light that was ever free to travel across the universe. It is a snapshot of the cosmos at age 380,000 years — a baby picture of everything.

The Temperature Map

The cosmic microwave background is almost perfectly uniform. From every direction, the temperature is 2.725 Kelvin. The universe at age 380,000 years was astonishingly smooth — the same temperature everywhere, to a precision of one part in 100,000.

But not perfectly smooth. Those tiny fluctuations — one part in 100,000, hot spots and cold spots differing by millionths of a degree — are the seeds of everything. Every galaxy, every star, every planet, every person exists because of those fluctuations. The slightly denser regions had slightly stronger gravity. They pulled in more matter. They grew. Over billions of years, they became galaxies and clusters and the great cosmic web.

The temperature map of the cosmic microwave background is the blueprint of the universe. Every structure that exists today was encoded in those faint ripples 13.8 billion years ago. We can read the blueprint. We can predict what it should look like based on our models of physics. And the prediction matches the observation to extraordinary precision.

This is not a loose agreement. This is one of the most precise confirmations of any theory in the history of science. The standard model of cosmology — the proportions of ordinary matter, dark matter, and dark energy; the rate of expansion; the geometry of space — is determined by fitting the temperature fluctuations of the cosmic microwave background. And it fits.

What It Tells Us

The cosmic microwave background tells us:

The age of the universe. 13.797 billion years, plus or minus 20 million. Not "about 14 billion." 13.797. The precision comes from the microwave background.

The composition of the universe. 4.9 percent ordinary matter. 26.8 percent dark matter. 68.3 percent dark energy. The universe is mostly made of things we do not understand. But we know their proportions to the tenth of a percent, because the microwave background encoded them.

The geometry of space. Flat. Not curved like a sphere, not curved like a saddle. Flat, to within 0.4 percent. The universe obeys Euclidean geometry on the largest scales. Parallel lines never meet. The angles of a cosmic triangle add up to 180 degrees.

The initial conditions. The fluctuations are nearly scale-invariant — the same strength at every size. This is a prediction of cosmic inflation, the theory that the universe underwent an exponential expansion in its first fraction of a second. The microwave background confirms it.

All of this, from a faint hiss of microwaves that two engineers found while cleaning pigeon droppings from an antenna.

The Light That Surrounds Us

I want you to understand something about this light. It is not distant. It is not remote. It is not "out there." It is here. Now. Everywhere.

Four hundred photons of the cosmic microwave background pass through every cubic centimeter of space, every second. They are passing through your body right now. They have been traveling for 13.8 billion years. They are the oldest things you will ever touch.

When you tune an old analog television to a channel with no signal, about one percent of the static on the screen is caused by cosmic microwave background photons. The snow on the dead channel is the afterglow of creation. You have been watching the Big Bang your whole life. You just did not know it.

The universe is not something that happened a long time ago in a place far away. The universe is happening now, here, at the speed of light, passing through you, surrounding you, filling every gap between every atom in your body with the oldest light there is.

You are immersed in the echo of the beginning. You are bathing in the first light. The cosmic microwave background is not a distant signal. It is the medium you live in.

"We are a way for the cosmos to know itself."