Why Doesn't the Universe Stretch Us? - Why Bound Systems Do Not Grow with the Cosmos | Sleepy Facts About the Universe

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The balloon analogy for cosmic expansion provides a useful but unsettling image. If space is stretching, causing distant galaxies to drift apart, why doesn't that same expansion stretch the Earth, enlarge the atoms in our bodies, or pull the solar system wider? The question sounds simple, but the answer requires a careful look at what we actually mean by expanding space and how physical laws operate at different scales.

This episode examines the Friedmann-Lemaître-Robertson-Walker metric, the mathematical model describing cosmic expansion. The model assumes matter is spread evenly across vast distances, but real space is clumpy. Within galaxies, solar systems, and atoms, local forces completely dominate. Gravitational binding sets planetary orbits, electromagnetic interactions establish the fixed size of a hydrogen atom, and quantum mechanics dictates the length of a chemical bond. These bound systems continually respond to local physics, overwhelming the negligible effect of cosmic expansion to the point where it leaves no measurable trace.

I wanted to understand why a meter stick remains stable while the light from distant galaxies stretches into redshift. It turns out the stability of local structures is not a contradiction of cosmic expansion, but the very condition that makes it observable. By exploring the boundary between bound systems and unbound regions, we can see exactly why we do not grow with the cosmos.