If the Universe is expanding, we can understand why distant galaxies recede from us as they do. But then why aren’t stars, planets and even atoms expanding, too? Image credit: C. Faucher-Giguère, A. Lidz, and L. Hernquist, Science 319, 5859 (47).

Ask Ethan: If the Universe is expanding, why aren’t we?

Space is getting bigger, but atoms, humans, Earth and our Milky Way stay the same size. How is this possible?

“The Universe is expanding the way your mind is expanding. It’s not expanding into anything; you’re just getting less dense.” -Katie Mack

One of the biggest scientific surprises of the 20th century was the discovery that the Universe itself is expanding. Distant galaxies recede from us and from one another more quickly than the nearby ones, as though the fabric of space itself is being stretched. On the largest scales, the matter and energy densities of the Universe has been dropping for billions of years, and continues to do so as time goes on. And if we look to large enough distances, we’ll find galaxies that are being pushed away so rapidly by the expansion of space that nothing we send out today will ever reach them, not even at the speed of light. But doesn’t that create a paradox back here? That’s what Kent Hudson wants to know:

If the universe is expanding at rates in excess of the speed of light, why does it not appear to affect our solar system and the planetary distances from the sun, etc.? And why would the relative distances of stars in our galaxy not appear to be increasing… or are they?

Kent’s hunch is right, and the solar system, planetary and stellar distances all aren’t increasing as the Universe expands. So what’s actually expanding in the expanding Universe? Let’s find out.

The original conception of space, thanks to Newton, as fixed, absolute and unchanging. It was a stage where masses could exist and attract. Image credit: Amber Stuver, from her blog, Living Ligo.

When Newton first conceived of the Universe, he pictured space as a grid. It was an absolute, fixed entity filled with masses that gravitationally attracted one another. But when Einstein came along, he recognized that this imaginary grid wasn’t fixed, wasn’t absolute and wasn’t at all like Newton had imagined. Instead, this grid was like a fabric, and the fabric itself was curved, distorted and forced to evolve over time by the presence of matter and energy. Moreover, the matter and energy within it determined how this spacetime fabric was curved.