What is the Hubble Constant?

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Until just a century ago, our galaxy was considered the only star family to occupy 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 on empirical data and therefore could not be proven.

This began to change during the 1920s and 1930s, as astronomer Edwin Hubble looked to other galaxies, using the 2.5-meter (100-inch) telescope recently built on Mount Wilson in the south. from California. For the first time, Hubble was able to clearly see the individual stars in M31 – the Andromeda Galaxy. For the first time, star families existed beyond the Milky Way.

Hubble also discovered something else: almost all galaxies are moving away from each other at tremendous speeds. He also discovered that these star clusters travel at a rate depending on their distance from us – more distant galaxies are moving away from us faster 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 answered precisely today – is how fast do they travel?

The expansion has started – WAIT!

Since the Big Bang, the Universe has expanded.

Edwin Hubble set out to measure the speed at which galaxies are moving away from each other. He discovered that galaxies obey a relationship, now known as Hubble’s Law, showing a linear relationship between the distance to a galaxy and its rate of recession. This speed is simply the result of measuring the distance to a galaxy and multiplying it by the Hubble constant.

The value of the Hubble constant is usually given in odd units, which may seem unknown – kilometers per second per megaparsec (km / s / Mpc). Let’s start at the end – a parsec is a unit of distance roughly equal to 3.26 light years. Therefore, one megaparsec (one million parsecs) is a distance equal to about 3.26 million light years.

If the Universe’s rate of expansion were 70 km / sec / Mpc, then a galaxy 10 megaparsecs away from us would – theoretically – be racing at (70 times 10, or) 700 kilometers per second. (It’s actually so close that the local gravitational effects would be significant, but this example shows the math). A galaxy twice this distance would have a recession rate twice as fast, and so on.

Then the question becomes – what is the value of the Hubble constant? It is one of the most important questions in cosmology and astrophysics today.

Ask the right question

To measure the Hubble constant, it is necessary to accurately measure distances to distant objects.  Here is an overview of the methods used by astronomers to measure these enormous distances.
Credit: NASA / ESA / A. Feild (STScI)