Inquiring Minds

Physics Questions People Ask Fermilab

Dear Glyn,

This is a good question about an often-confusing topic! It's a question that even a lot of cosmology students don't think to ask. They often just accept the fact of the expanding universe and don't bother wondering why everything doesn't expand along with it.

First things first: the simple yes/no answers. Yes, the universe is in fact expanding. No, the planets aren't growing farther apart. Yes, the distance between galaxies is (mostly) increasing.

OK, now for a bit more explanation. The expansion of the universe, according to Einstein's general relativity, is actually a stretching of space itself. So the distance between any two "fixed" points in the universe will increase over time, because the space in between these points grows. Another way of saying this is that any two objects will grow farther apart, barring any other dynamic effects.

But the thing is, the universe is full of "other dynamic effects." The planets in our solar system aren't fixed objects; their orbits around the sun are determined by gravity. Your body is an enormous collection of atoms that stay together because of electromagnetic forces. The nuclei of those atoms are made of protons and neutrons stuck together by the strong force. On the distance scales of atoms, people, solar systems, or even galaxies, these forces are strong enough to counteract the expansion of the universe. It's only when you look at distances much larger than a galaxy that we begin to see the expansion of the universe "winning the tug-of-war."

Even at the larger galactic scales, the expansion has to fight against gravitational attraction (that's why I said "mostly" for the case of galaxies above). For example, even though most of the distant galaxies are receding from us, some of the closest ones are moving toward us or appear to be nearly stationary. (In fact, in five billion years or so our galaxy and the Andromeda galaxy will probably collide.)

One final remark: if somehow *everything* expanded along with the universe, we wouldn't notice it! That sounds strange, but it's perfectly reasonable, because if everything were to expand at the same rate, there'd be nothing of "fixed" size to compare the expansion to. Galaxies, people, and meter sticks would all grow, but measurements wouldn't change.

I hope this helps to answer your question. Please don't hesitate to write back if you have other questions about this.

Cheers,

- Craig Wiegert

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