I am trying to understand what is meant when people say space itself is expanding. When I hear that, I imagine a 3-d Cartesian coordinate system in which every axis gets scaled at a given rate (like in an animation). What I don't understand is, if everything gets scaled, then how can anyone even tell that scaling is occurring? Let's say I looked at a meter stick. Then let's double all lengths in all 3 dimensions. If I look at the meter stick again, I would think that I wouldn't be able to physically tell any difference between this case and the first... Not only has the meter stick doubled length in all directions, but so have all my sense organs (and anything else) which I use to measure it (funny since the meter stick itself is a measuring device). This tells me that not everything gets scaled... Or at least... not scaled equally. So the next natural question is, what determines scaling differences for larger structures (whole universe) vs. smaller structures (single galaxies, solar systems, meter sticks, variance (RMS) of the probability density of the position of an electron, ...etc) Looking at this example: http://www.astro.ucla.edu/~wright/Balloon2.html The smaller structures in that example expanding universe don't seem to themselves be expanding that much... so why are they not getting scaled? And if smaller structures are not getting scaled, is it really valid to say space itself is expanding, or is it more valid to say that things are just flying away from one another? - I know the answer that will be given to me here, but not the intuition. Here's a quote from a wikipedia page: https://en.wikipedia.org/wiki/Metric_expansion_of_space "In principle, the expansion of the universe can be measured by taking a standard ruler and measuring the distance between two cosmologically distant points, waiting a certain time, and then measuring the distance again." So my question is, why doesn't the "standard ruler" itself talked about here also expand, such that the same measurement will be made over and over again. Thanks!