How is new space created as the universe expands? Einstein discovered that empty space is not nothing, so what is responsible for new space appearing into existence, when energy and matter cannot?

Question

[Nasa.gov](https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy) explains dark energy like a property of space, and go on to say:

> Albert Einstein was the first person to realize that empty space is not nothing. (…) The first property that Einstein discovered is that it is possible for more space to come into existence.

But they never elaborate on how Einstein knows this. I know this is a field without concrete answers, but I’m curious about Einstein’s explanation.

In: Physics

Answer 1

Think of it like a balloon of air. If I bring a cold balloon into a warm room it expands. There aren’t any more nitrogen atoms in the balloon, but they have more energy. This means that the pressure they exert on the membrane requires the balloon’s fabric to stretch to contain it.

Most of space is empty of nitrogen atoms, so that’s not exactly what’s happening. But space is full of quantum field. The field itself is expanding, moving the atoms slight apart. Some atoms are clumped together by gravity into planets, so the planet isn’t expanding because the gravity keeps pressing the atoms together.

However, when things get far enough apart there isn’t a force to clump them. The empty space between galaxies expands, and the galaxies move (because of their momentum).