Help Understanding the Hubble Constant's Units

[skynr13]

TL;DR

If the Hubble Constant is 45 miles (73 kilometers) per second per megaparsec. What is this converted to miles (or required specific length) per second, minute or hour?

If the Hubble Constant is 45 miles (73 kilometers) per second per megaparsec. What is this converted to miles (or required specific length) per second , minute or hour? The idea of this question is to find a basic speed of the Hubble Constant that anyone can relate to. Like if I was to say to someone the Hubble Constant is 45 miles (73 kilometers) per second per megaparsec, no one can easily understand how fast the Universe is expanding, because a Megaparsec is just too hard to conceive. But if someone in this Physics forum convert 45 miles (73 kilometers) per second per megaparsec into something more conceivable like one foot an hour then it is more understandable for a common non-scientist to understand.
Thanks for your help, kevin.

 

[Hill]

TL;DR: If the Hubble Constant is 45 miles (73 kilometers) per second per megaparsec. What is this converted to miles (or required specific length) per second, minute or hour?

If the Hubble Constant is 45 miles (73 kilometers) per second per megaparsec. What is this converted to miles (or required specific length) per second , minute or hour? The idea of this question is to find a basic speed of the Hubble Constant that anyone can relate to. Like if I was to say to someone the Hubble Constant is 45 miles (73 kilometers) per second per megaparsec, no one can easily understand how fast the Universe is expanding, because a Megaparsec is just too hard to conceive. But if someone in this Physics forum convert 45 miles (73 kilometers) per second per megaparsec into something more conceivable like one foot an hour then it is more understandable for a common non-scientist to understand.
Thanks for your help, kevin.

It can't be done because the units of the Hubble Constant are not the units of speed, but rather the units of 1/time.

 

[Filip Larsen]

The linear expansion rate corresponds to around 7.1⋅10^-11 1/year so you can get the "equivalent speed = distance/year" if you multiply this number by a distance. For instance, at the Moons distance from Earth the Hubble expansion would correspond to a speed of around 2 cm/year. (Note that this is just a change of scale and does not as such mean linear Hubble expansion is applicable to the Earth-Moon system or anything local. The Moon actually recedes from the Earth but due to other reasons).

 

[Ibix]

If the Hubble Constant is 45 miles (73 kilometers) per second per megaparsec. What is this converted to miles (or required specific length) per second , minute or hour?

It doesn't work like that. How it works is that something one megaparsec away recedes from us at 73km/s. Something two megaparsecs away recedes at 2 × 73 = 146km/s. Something ten megaparsecs away recedes at 10 × 73 = 730km/s. Recession speed increases with distance, so there isn't a single speed value that can be stated.

Note that there is significant random motion of galaxies on top of this pattern, and bound systems such as individual galaxies don't expand at all.

 

[Bandersnatch]

As has been said, you can't get a velocity out of the Hubble constant, because its dimension is that of a frequency. I can appreciate that it's not very helpful as far as making it relatable, though.

But maybe this will be - since H0 is stated as some speed per some distance, it tells you how long it'd take to cover that distance at the given speed. Let's call this time t.
So the 70-ish km/s/Mpc tells you how long it would take to cover a megaparsec while travelling at 70 km/s.
And, since a longer/shorter distance has a proportionally faster/slower speed associated with it - as per the Hubble law - that time t is the same for any distance in the universe.
2 Mpc at 140 km/s and 10 Mpc at 700 km/s and 0.1 Mpc at 7 km/s and 100 light years at 21 km/s - all these pairs of distances and speeds have the same t associated with them. That is because any and all distances in the universe change at the same rate, at a given time in its history, due to expansion. This includes whatever distance you pick for the size of the universe.

Which is to say, Hubble constant tells you how long it'd take for the universe to grow from 0 to its current size, if the rate of expansion had always been the same as today (it hadn't). If you were to run the numbers, the value of the Hubble constant in units of 1/t is something like 1/14.4 billion years.
I.e. it tells you that at this rate it'd take 14.4 billion years to grow/shrink the universe from/to zero size.