The Hubble Constant is the unit of measurement used to describe the expansion [rate] of the universe. The cosmos has been getting bigger since the Big Bang kick-started the growth about 13.82 billion years ago. [I]n fact, it's getting faster in its acceleration as it gets bigger.
What's interesting about the expansion is not only the rate, but also the implications, according to NASA. If the expansion begins to slow down, that implies that there is something in the universe that is making the growth slow down — perhaps dark matter, which can't be sensed with conventional instruments. If the growth gets faster, though, it's possible that dark energy is pushing the expansion faster.
As of January 2018, measurements from multiple telescopes showed that the rate of expansion of the universe is different depending on where you look.
The nearby universe (measured by the Hubble Space Telescope and Gaia space telescope) has a rate of expansion of 45.6 miles per second per megaparsec, while the more distant background universe (measured by the Planck telescope) is a bit slower, expanding at 41.6 miles per second per megaparsec. A megaparsec is a million parsecs, or about 3.3 million light-years, so this is almost unimaginably fast.
Discovery by Hubble
The constant was first proposed by Edwin Hubble (the namesake for the Hubble Space Telescope). Hubble was an American astronomer who studied galaxies, particularly those that are far away from us.
In 1929 — based on a realization from astronomer Harlow Shapley that galaxies appear to be moving away from the Milky Way — Hubble found that the farther these galaxies are from Earth, the faster they appear to be moving, according to NASA.
While scientists then understood the phenomenon to be galaxies moving away from each other, today astronomers know that what is actually being observed is the expansion of the universe. No matter where you are located in the cosmos, you would see the same phenomenon happening at the same speed.
Hubble's initial calculations have been refined over the years, as more and more sensitive telescopes have been used to make the measurements, including Hubble and Gaia (which examined a kind of variable star called Cepheid variables) and other telescopes that extrapolated the constant based on measurements of the cosmic microwave background — a constant background temperature in the universe that is sometimes called the "afterglow" of the Big Bang. ..."
As I sit in my recliner pondering the speed of the earth around the sun which in turn is in rotation through the galaxy which in turn is speeding through the universe, I figure added all up I’m going about a million miles per hour which explains why I feel so tired at my age.