Hubble’s Law

Hubble’s Law is the hypothesis that the red-shifts of distant galaxies are directly proportional to their distances from Earth. The American astronomer Edwin Powell Hubble first proposed this relationship in 1929. It can be expressed as

V = H₀ D,

where V is the recessional velocity of a distant galaxy, H₀ is the Hubble constant, and D is its distance from Earth. Assuming expansion at a steady state, H₀ is the constant of proportionality in the relationship between the relative recessional velocity (V) of a distant galaxy or a very remote extragalactic object and its distance (D) from Earth. Unfortunately, there is still much debate and controversy among astrophysicists and cosmologists as to what the value of the Hubble constant really is (based on measurements of distant receding galaxies). An often-encountered value is 70 kilometers per second per megaparsec (km s–1 Mpc–1), although values ranging from 50 to 90 km s–1 Mpc–1 can be found in the technical literature.

The inverse of the Hubble constant (1/H₀ = Hubble time) has the dimension of time, and astronomers consider it one measure of the age of the universe, especially if a constant expansion rate is assumed. For example, if H₀ is given a value of 80 km s–1 Mpc–1, this would suggest that the universe is between 8 and 12 billion (109) years old. However, if H₀ is given a value of 50 km s–1 Mpc–1, then the universe has an estimated age of about 20 billion years. The smaller the value assigned to the Hubble constant, the older the estimated age of the universe, that is, the time that has passed since the primordial big bang.

Astronomers and astrophysicists currently think the universe is between 12 and 14 billion years old, with 13.7 billion years as a popular consensus value. Astrophysical observations of distant supernovas made in the late 1990s now suggest that the rate of expansion of the universe is actually increasing. These observations once confirmed and widely accepted within the astrophysical community, will cause a reexamination of Hubble’s law and its inherent assumption of a linear rate of expansion since the big bang. Of course, nothing is being taken away from the brilliant work of Edwin Hubble and his pioneering advocacy of the modern cosmological model based on an expanding universe. Scientific progress is made through refinements and improvement in models of the physical universe, so Hubble’s law, which has served the astronomical community well for more than 70 years, will undergo some form of evolutionary refinement and modification to embody the latest discoveries in observational astronomy.

Questions to Ponder

• If a galaxy is observed to have a velocity V₁, what would its velocity have been a billion years ago?
• What observations constitute Hubble’s Law? How is it interpreted to describe the universe?