What "the universe is expanding" actually means
In the 1920s, Edwin Hubble noticed that almost every distant galaxy is moving away from us, and the farther away it is, the faster it's receding. The first instinct is to think Earth is at the center of an explosion. It's not. Space itself is stretching. Every galaxy is moving away from every other galaxy because the space between them is getting bigger. There's no center; the expansion happens everywhere at once.
Run that backwards in time and the expansion contracts. The galaxies converge. Eventually all the matter and energy of the universe collapses into a state of unimaginable density and temperature. We don't have physics for the very first instant, but we have it from about 10⁻³² seconds onwards. That state is what we call the Big Bang. It happened ~13.8 billion years ago.
Three things this is, and isn't
- It is the best-tested scientific theory of cosmic origins, supported by an enormous web of evidence.
- It isn't an explanation of where the energy/matter came from "before" — "before" the Big Bang isn't well-defined in current physics. Time itself begins (in any usable sense) at the Big Bang.
- It isn't incompatible with religion or philosophy in any forced way. It's a description of how the universe evolved from an early hot dense state, not a metaphysical claim about why anything exists.
Three pillars of evidence
- Hubble's redshift. Every distant galaxy's light is shifted toward red — the wavelengths stretched as space expanded between us and them. The farther the galaxy, the more redshift. Maps the expansion.
- Cosmic microwave background. About 380,000 years after the Big Bang, the universe cooled enough for atoms to form, and light could travel freely. That ancient light is still around, redshifted to microwave wavelengths, filling all of space at almost exactly 2.7 Kelvin (~-270°C). The CMB was predicted in the 1940s, accidentally detected in 1965, mapped to extraordinary precision since. It's the universe's baby photo.
- Element abundance. The Big Bang's nucleosynthesis predicts ~75% hydrogen, ~24% helium, traces of lithium. That's exactly what we observe in the most pristine, distant gas clouds. Heavier elements came later, from stars (counting track lesson 4).
What the cosmos looks like
The observable universe contains roughly 100-200 billion galaxies. Each galaxy contains 100 billion to a trillion stars. Each star, on average, has a few planets. The numbers are absurd; the structure is layered (galaxies cluster into superclusters, which form filaments and walls, separated by vast voids). The deepest pictures we have show this structure stretching as far as instruments can see — and there's reason to think there's more beyond what we can see, expanded out of view by the speed of light's bound.
You're an emergent layer in a 13.8-billion-year-old, expanding cosmos containing hundreds of billions of galaxies. The math says the universe is the size it is because it's been expanding for that long; the galaxies cluster the way they do because slight density fluctuations in the early universe grew over billions of years; you're made of atoms forged inside earlier generations of stars (counting). It all hangs together.