In The Beginning…

We’ve been told, since we were young, that our universe began with an event known as “The Big Bang”. As with much of science, this serves as the kind of knowledge we’re told to accept as fact, despite our inability as laypeople to appreciate the reasoning behind it.

Yet the general idea of The Big Bang is so simple that anyone can understand it. I’m not talking about the what here, but the how; I’m talking about grasping the way in which human beings, stuck on Earth with some rudimentary technology, were able to determine how the universe began and how long ago it happened.

Kind of crazy that we can know that. And the very intuitive way some very smart people figured it out is simple and beautiful in the way the best scientific concepts are.

The Doppler Effect

We’re all familiar with the concept of the Doppler Effect, even if we don’t know it by name. If an ambulance with an active siren or a car sounding its horn has ever sped past you, you’ve heard the passing sound drastically drop in pitch, like this:

Here’s why this happens: As the car races toward you, the sound waves it emits get compressed (the car is shrinking the distance between them). As the car zooms away, the sound waves get stretched. At the moment it passes you, the pitch drops from the first category to the second.

What Does This Have To Do With Space?

Just like a car, any object that emits waves while in motion falls victim to the Doppler Effect. Sound is one kind of wave. Light is another.

And because light is a wave, as an object that emits light careens through our universe, the “pitch” of the light changes. If the object is moving toward us, an observer would perceive its light at a higher frequency (bluer, hence the name “blue shift” given to the phenomenon). If the object is moving away, its light’s frequency would appear lower (i.e. redder, hence the name “red shift”).

About a hundred years ago, as physicists began pointing more and more advanced machinery at the sky, they noticed something interesting. The stars shining light throughout space overwhelmingly had their light “red shifted.” Rather than observing these stars as they appeared, we were seeing light that had a lower frequency than what we knew the stars to be emitting.

“That’s odd,” thought scientists like Edwin Hubble (after whom the Hubble Space Telescope is named). “That seems to suggest that everything in space is moving away from us.” (He likely didn’t say any of this; I’m taking artistic license.)

Hubble and his contemporaries were right. In fact, from the perspective of anywhere in space, the rest of space appears to be moving farther and farther away.

Then things got even more surprising. It turns out that the speed with which stars were moving away from us (as measured by how “red shifted” they were) was directly proportional to how far away they were. A star that was 200 million lightyears away receded exactly twice as fast as one that was 100 million lightyears away.

It was actually a Catholic priest named Georges Lemaître who is credited with first realizing what this meant, back in 1927. “What if the reason things are all moving away from other things is that the universe is expanding?”

Picture dots drawn with marker on a deflated balloon. Then picture blowing air into the balloon to expand it. Every single dot is farther away from every other dot.

Blow it up even more. The dots are now even farther apart. Etc.

Running It Backward…

Lemaître kept digging. “What if we thought about how this all began by thinking of time running backward. Like when you play a VHS tape in reverse—” (and this I’m certain he never said, since VHS wasn’t invented until 1976) “—you should see the universe of stars getting closer and closer together. And because the ones farther away are moving faster, you should see those rushing in at higher speed than the closer ones.”

This realization was a “big bang” of innovation, a turning point for all of modern cosmology.

Picture two friends off in the distance, racing in your direction. One is 100 meters away from you, and one is 200 meters. Recall Doppler’s discovery that the one at 200 meters is moving twice as fast as the one at 100 meters. And so if we ran Lemaître’s VHS backward, you’ll see Ms. 200 Meters sprinting toward you, and Mr. 100 Meters jogging at half that speed.

If they’re moving at speeds in exact proportion to their distance, when will they arrive at your location?

At exactly the same time.

Put differently: If you run the tape backward, you realize that every star out there must have been shot out into space from some common origin, billions of years ago, all at the same exact moment.

We call that moment The Big Bang.

Isn’t that beautiful?