> As a kid, my notion was that spacetime was "infinite", that galaxies floated in spacetime, and were moving apart. Gradually I became (a bit) more comfortable

> As a kid, my notion was that spacetime was "infinite", that galaxies floated in spacetime, and were moving apart. Gradually I became (a bit) more comfortable with the notion that spacetime is somehow created by "stuff", and that a location in spacetime that has never seen a photon isn't a location in spacetime at all. Neither is especially accurate. Spacetime is not a substance, and it's not created or destroyed. It's the thing physics happens on, for lack of a better non-mathematical description. > But what does it mean for time to expand? The very idea of expansion is of a thing that grows relative to time. Same thing as for space: that component of the metric - the shape of spacetime - is an increasing function of coordinate time. But you can always pick a coordinate system where only the spatial components are expanding. It's not physically meaningful either way. The metric is the real object.