How the Universe Stretches: The Wild Expansion of Space Explained Simply

Every single second, a patch of empty space a million light-years across gets about fifteen miles wider. Wait, hold on. Fifteen miles wider? Every second? Every second. Which is just... wild. And it connects perfectly to what we talked about last time on the show—how all that empty space inside an atom is what actually makes your floor solid. Oh, right! Episode 14, why you don't fall through the floor. So you're saying empty space out THERE... between the stars... isn't really empty either? It's doing something? It's doing THE most important thing. It's stretching. A hundred years ago, scientists, even Albert Einstein, thought the universe was just...

static. A fixed, eternal container. So what changed their minds? An astronomer named Edwin Hubble. In the 1920s, he was looking at these faint, fuzzy smudges of light he knew were distant galaxies. And he noticed they all had this reddish tint. Like they were rusty? Not quite. It's more like the sound of a siren changing pitch as an ambulance drives away from you. The sound waves stretch out, and the pitch drops. For light, when an object moves away from you, its light waves stretch out, and it shifts toward the red end of the spectrum. It's called redshift. Okay, so these galaxies were all moving away from us.

Exactly. And here's the part that blew everyone's minds. The farther away a galaxy was, the FASTER it was moving away from us. Huh. That kind of makes it sound like we're at the center of a giant explosion and everything's flying away from us. It really does! But that's the illusion. The best way to picture it is to imagine a balloon with a bunch of dots drawn on it. You are one of those dots. I am a dot on a cosmic balloon. Got it. Now, start inflating the balloon. From your dot's perspective, what are all the other dots doing? They're... all moving away from me. Ohhh. And the ones that were already far away are moving away faster because there's more balloon stretching between us.

You got it! The dots—the galaxies—aren't running across the surface of the balloon. The surface itself—space-time—is expanding and carrying the galaxies along for the ride. There IS no center to the explosion. The expansion is happening everywhere. Okay, my brain is officially buzzing. So if you run that movie in reverse... if you deflate the balloon... You just figured out the Big Bang. That's the whole idea. If the universe is expanding, then in the past, it must have been smaller. And before that, smaller still. If you rewind all the way back, about 13.8 billion years... everything...

every single star and galaxy we can see... was compressed into a single point. So the Big Bang wasn't an explosion in space. It was the beginning of space itself. The moment the balloon started inflating. YES. That is the single most important takeaway. It's not a bomb going off in an empty room; it's the room itself suddenly appearing and expanding from a single point. So... is it going to stop? Is the balloon going to run out of air? That was the big question for decades. Everyone thought, you know, that gravity from all the galaxies would act like a brake. Maybe it would slow down and stop, or even collapse back in on itself in a "Big Crunch." That makes sense.

Gravity pulls things together. It does. Except, in the late 1990s, astronomers found something that made NO sense. The expansion isn't slowing down. It's speeding up. What? How? We have no idea. We gave our ignorance a cool name—"dark energy"—but that's really all it is. A name for the mysterious force that's pushing space-time apart, faster and faster every second. So the universe is just going to keep getting bigger, and colder... and emptier. It seems that way. It means that galaxies we can see in our telescopes tonight will, over billions of years, accelerate away from us so fast that their light will no longer be able to reach us.

They will literally disappear from our sky. Wow. That feels... lonely. It does. Or, you could see it another way. It means we live in this incredibly special, fleeting moment. We get to look up and see a sky full of neighbors, just before they all move away for good. We get to see the cosmic party, right at its peak.