The inconstant constant

Georges Lemaître

In a previous post, The Hubble-Lemaître Law, I explained how Georges Lemaître discovered in 1927 the expansion of the universe, but as he published in a French-language journal, it didn't make a great impact, and for almost a century the discovery was attributed to Edwin Hubble, who published in 1929 in a much more widely circulated English-language journal. This injustice was corrected on October 29, 2018, by the International Astronomical Union, and I echoed the renaming of the law in my post, published three days later in this blog.

The Hubble-Lemaître Law says this: The farther away a galaxy is, the faster it is receding from us. Its recessional velocity is proportional to its distance. The constant of proportionality is called the Hubble constant, which has the dimension of 1/time. In the International System of Units, this dimension would be expressed as seconds⁻¹ or 1/second, but in practice, its definition (speed/distance) is used, with the following units: km/s/Mpc, which means: the increase in the recessional velocity of a galaxy (in km/s) as its distance from us increases (measured in Megaparsecs). One Megaparsec (Mpc) is one million parsecs, and one parsec is equal to 3.2616 light-years.

The problem with the Hubble constant

Cosmic Microwave Background Radiation
NASA-WMAP

The Hubble constant, which measures the speed of expansion of space in the universe, has very curious properties. For instance, although we call it constant, it turns out that it is not a constant, as it varies over time. That is why its current value is represented by the symbol H₀, but since its value was different at other times, it can be represented by other symbols, such as H_CMBR, which refers to its value at the time when the cosmic microwave background radiation originated, about 13.7 billion years ago.