It’s either late Winter or early Spring, the weather can’t make up its mind. The geese don’t seem to approve of my walk around the park’s lake but then I realize it’s not me they object to. “Hey, Moire, wait up, I got a question for you!”
“Good morning, Mr Feder. What can I do for you?”
“This Big Bang thing I been hearing about. How can it make everything from nothing like they say?”
“You’re in good form, Mr Feder, lots of questions buried within a question.”
“Oh yeah? Seems pretty simple to me. How do we even know it happened?”
“Well, there you go, one buried question up already. We have several lines of evidence to support the idea. One of them is the CMB.”
“Complete Monkey Business?”
“Very funny. No, it’s the Cosmic Microwave Background, long‑wavelength light that completely surrounds us. It has the same wavelength profile and the same intensity within a dozen parts per million no matter what direction we look. The best explanation we have for it is that the light is finally arriving here from the Big Bang roughly 14 billion years ago. Well, a couple hundred thousand years after the Bang itself. It took that long for things to cool down enough for electrons and protons to pair up as atoms. The photons had been bouncing around between charged particles but when the charges neutralized each other the photons could roam free.”
“Same in all directions so we’re in the center, huh? The Bang musta been real close‑by.”
“Not really. Astronomers have measured the radiation’s effects on a distant intergalactic dust cloud. The effect is just what we’d expect if the cloud were right here. We’re not in a special location. From everything we can measure, the Bang happened everywhere and all at once.”
“Weird. Hard to see how that can happen.”
“We answered that nearly a century ago when Edwin Hubble discovered that there are other galaxies outside the Milky Way and that they’re in motion.”
“Yeah, I heard about that, too, with everything running away from us.”
“Sorry, no. We’re not that special, remember? On the average, everything’s running away from everything else.”
“Whaddaya mean, ‘on the average‘? Why the wishy-washy?”
“Because things cluster together and swirl around. The Andromeda galaxy is coming straight toward us, for instance, but it won’t get here for 5 billion years. The general trend only shows up when you look at large volumes, say a hundred million lightyears across or bigger. The evidence says yeah, everything’s spreading out.”
“But how can everything be moving away from everything? You run away from something, you gotta be running toward something else.”
“That’d be true if your somethings are all confined in a room whose walls don’t move. The Universe doesn’t work that way. The space between somethings continually grows new space. The volume of the whole assemblage increases.”
“Is that why I just hadda buy new pants?”
“No, that’s just you gaining weight from all that beer and bar food. The electromagnetism that holds your atoms and molecules together is much stronger than what’s driving the expansion. So is the gravitation that holds solar systems and galaxies together. Expansion only gets significant when distances get so large that the inverse square laws diminish both those forces to near zero.”
“What’s this got to do with the CMB?”
“The CMB tells us that the Bang happened everywhere, but expansion says that at early times when stars and galaxies first formed, ‘everywhere‘ was on a much smaller scale than it is now. Imagine having a video of the expansion and playing it backwards. Earendel‘s the farthest star we’ve seen, but if we and it existed 12 billion years ago we’d measure it as being close‑by but still all the way across the observable Universe. Carry that idea the rest of the way. The Big Bang is expansion from a super‑compressed everywhere.”
“Okay, what’s driving the expansion?”
“We don’t know. We call it ‘dark energy‘ but the name’s about all we have for it.”
“Aaaa-HAH! At last something you don’t know!”
“Science is all about finding things we don’t know and working to figure them out.”
~~ Rich Olcott
Hard Science Ain't Hard 
