The standard interpretation of Hubble constant ≈70 km/s/Mpc
means that each mega-parsec of distance adds 70 km/s
to a galaxy recession velocity from us (or to a space expansion rate), but when expressing Hubble constant in SI units directly one gets about 2.27×10^(-18) Hz
In: 0
d distance
t time
d/t/d = d/t × 1/d = 1/t
Hz=1/s
You’d get that expressing the constant in SI so (m/s)/m = m/sm = 1/s = Hz
The reason why the large scale unit is a bit more faithful at expressing whats going on, is because expansion is the property of space on the largest of scales where matter distribution is homogeneous. In that case space is described by the FLRW metric and it expands.
But assume you are in empty space and you also don’t have mass to change the spacetime metric with your friend (also massless) in that case the Hz unit literally means that if your friend is 1 meter away they “dirft” away at 2.27×10^(-18) m/s.
It means that, if Hubble Parameter was always constant and no other phenomenon (like inflation etc.) ever existed, galaxies would recede 2.27 x 10^-18 of the way to their current position in one second. Because the inverse is the Hubble Time (i.e. the estimate of the age of the universe based on the same assumptions).
But that’s in reality a meaningless interpretation, because neither of those assumptions is true as far as the current consensus stands.
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