Since the big bang, the universe is expanding, it doesn’t mean new space is being inhabited that was previously empty, its the existing space spreading out. How measurable is this on the human scale (meaning human dimensions and human timeline)?
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>How measurable is this on the human scale (meaning human dimensions and human timeline)?
Expansion does not occur near or within human-like structures at all.
We know space is expanding because light from distant objects is red-shifted at a very predictable rate proportional to the distance of the object. We do not, however, observe this same red shift in bodies within our galaxy or in our local group. (To be clear: you can find plenty of red-shifted objects near us, but they won’t follow this same red-shift/distance relationship that we see with distant galaxies.)
Digging into the math, what appears to be happening is that the baseline metric of spacetime is gradually changing in such a way that space expands. In relativity, an object called a “metric tensor” is used to describe the structure of a given point of spacetime. You can think of this as an array of values that describe the structure of spacetime (from which we can define things like distance, time, etc.). The “baseline” metric is the metric describing (practically) empty, gravitationally flat space, and that’s what appears to be expanding, in both our models and our observations.
And indeed we would not expect an expansion of the baseline metric to have any significance on the structure of something like a human or a galaxy. Spacetime in these structures is decidedly *not* empty or gravitationally flat, so we can’t use the baseline metric to describe it. The local metric is going to be completely dominated by local gravity (even in conditions that we humans would consider gravitationally weak). So whether your system is a baseball or a galaxy cluster, it’s not going to experience any expansion because it is gravitationally bound.
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