system
Given the following post, respond as if you're commenting on Reddit:
human
If everything in the Universe is constantly expanding, can/do we measure changes in size/space on an atomic level?
gpt
No. The expansion only applies on the largest scales. On smaller scales where you have things like galaxies and planets, there is no real expansion.

But be careful, because this is the sort of question that often gets people saying things which are completely incorrect. As a rule of thumb, if someone says that things on Earth (say) don't expand because intermolecular bonds or gravity or whatever "overcome" the expansion, don't listen to them. With one notable exception (which I'll mention soon), the expansion is *not* some universal force which fills everywhere and everything. It's an effect which occurs on the largest scales, where the Universe appears to be uniform, and is more like the upward motion of a ball thrown into the air than the upward motion of a propelled rocket. Once the ball turns around and starts to fall, there's no more "upward force" trying to bring it up, and similarly when a part of the Universe starts to collapse and form galaxies, there's no residual "expansion force" doing things on smaller scales.

There is one exception to this, which is the cosmological constant that may be causing the expansion of the Universe to accelerate. If the acceleration is due to a cosmological constant, that actually modifies Newton's gravitational force so that there is indeed some effect felt on small scales. In some more exotic cases, the cosmological "constant" can grow stronger with time, resulting in a "big rip" that eventually tears apart objects on the smallest scales. There are also models in which the accelerated is caused by something which wouldn't affect anything here on Earth, though, so we await more data to know for sure!