system
Given the following post, respond as if you're commenting on Reddit:
human
When astronomers say that "space is expanding," does that just mean that the things in space are moving away (say, from 3 to 7 cm on a ruler), or does it mean the unit length itself is being distorted (it's still 3 cm, but centimeters themselves are now longer)?
gpt
They mostly mean the former — the things in space are moving away (from 3 cm to 7 cm on a ruler).  A meter is still a meter.

However, there is a legitimate sense in which space is "scaling up" although our units of distance are not scaling up with them.  In terms of the formal mathematics of general relativity, space is equipped with a metric tensor which serves to provide a geometric notion of distance ... and over time, the scale factor of that metric tensor is increasing.  This is why the expansion of space is sometimes described as a "metric expansion" rather than an inertial expansion (like a conventional bomb) — physical distances are increasing even for objects which are initially at rest with respect to each other and upon which no forces are acting to drive them apart.

However, even though this is how our expansion of space is formally described, a meter is still a meter ... and something like a meterstick is not itself changing in length because electromagnetic forces still hold rigid objects at a characteristic size.  The consequence of the metric expansion of space is that distances between unbound objects are expanding — bound objects (including those bound by gravity, so including galaxy clusters and everything else smaller) are not expanding at all.  The expansion effect *only* happens across vast cosmological distances, where it is a real physical increase in distance (in whatever constant units you want to use).