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What is the Hubble Constant?

Until just a century ago, our galaxy was thought to be the lone family of stars occupying the Cosmos. Philosophers, notably Immanuel Kant in the 18th Century, postulated the existence of other families of stars beyond our own. Unfortunately, their postulations — although correct — were not based in empirical data, and so could not be proven.

This began to change during the 1920s and 30s, as astronomer Edwin Hubble set his sights on other galaxies, using the 2.5-meter (100-inch) telescope recently constructed on Mount Wilson in southern California. For the first time, Hubble was able to clearly see individual stars within M31 — the Andromeda Galaxy. For the first time, families of stars were known to exist beyond the Milky Way.

Hubble also discovered something else — nearly every galaxy races away from each other at tremendous speeds. He also found that these groupings of stars travel at a rate dependent on their distance from us — more distant galaxies race away from us more quickly than local bodies. (Incidentally, there is nothing special about our position in the Cosmos. This same effect would be seen from any location in the expanding Universe).

The big question — which still needs to be precisely answered today — is how fast are they traveling?

Expansion started — WAIT!

Since the Big Bang, the Universe has been expanding.

Edwin Hubble set to measure the speed at which galaxies are racing apart from each other. He found galaxies obey a relationship, now known as Hubble’s Law, showing a linear relationship between the distance to a galaxy and its recessional velocity. This velocity is simply the result of measuring the distance to a galaxy, and multiplying it by Hubble’s constant.

The value of Hubble’s constant is typically shown in odd units, which may seem unfamiliar — kilometers per second per megaparsec (km/sec/Mpc). Let’s start at the end — a parsec is a unit of distance roughly equal to 3.26 light years. Therefore, a megaparsec (a million parsecs) is a distance equal to around 3.26 million light years.

If the expansion rate of the Universe were 70 km/sec/Mpc, than a galaxy 10 megaparsecs away from us would — theoretically — be racing away at (70 times 10, or) 700 kilometers per second. (This is actually so close, local gravitational effects would be significant, but this example shows the math). A galaxy at twice that distance would have a recessional velocity twice that speed, and so on.

Then, the question becomes — what is the value of Hubble’s constant? This is one of the most important questions in cosmology and astrophysics today.

Asking the right question

Credit: NASA/ESA/A. Feild (STScI)