So the discovery of gravitational waves observed a contraction and stretching of space-time, and I've been trying to understand how the expansion of the universe means that space itself is growing. I want to understand how this actually works in relation to 'things' like a photon and an atom. Like take an atom at a fixed point in time. I understand that a photon has no concept of time from it's reference frame. But as space expands how does this affect 'distance' as we understand it in relation to the atom? I imagine that sub-atomic particles are of a fixed size and do not grow with space-time as the universe expands. So does a planck length inside an atom grow, or stay the same? Because as far as I'm aware light gets stretched as the universe expands, and I know that a planck length is based off light and time, but I don't know the end ramifications. Thanks!