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How Fast Is the Universe Expanding?

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Written by
Erik Gregersen
Erik Gregersen is a senior editor at Encyclopaedia Britannica, specializing in the physical sciences and technology. Before joining Britannica in 2007, he worked at the University of Chicago Press on the...
Erik Gregersen
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Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.
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Galaxy clusters like Abell 2744 can act as a natural cosmic lens, magnifying light from more distant, background objects through gravity. NASA's James Webb Space Telescope may be able to detect light from the first stars in the universe if they are gravitationally lensed by such clusters. (astronomy, space exploration, galaxies)
NASA, ESA, J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

The rate at which the universe is expanding is called the Hubble constant, named after astronomer Edwin Hubble, who, with Milton Humason, showed convincingly that the velocity with which a galaxy was moving away from Earth was proportional to its distance. That is, velocity = Hubble constant × distance. Rearranging this equation gives Hubble constant = velocity ÷ distance, so the Hubble constant is expressed in units of kilometers per second (km/s) per megaparsec (Mpc), where a megaparsec is 3.26 million light-years.

So what is this constant? How fast is the universe expanding? In 1929 Hubble got a value of about 500 km/s/Mpc. Much more accurate measurements dropped this to about 100 km/s/Mpc by about 1960, but the astronomical community became divided into two camps, one championing 100 km/s/Mpc and the other at 50 km/s/Mpc. Solving this problem became one of the key projects of the Hubble Space Telescope (HST), and in 2001 the HST astronomers came to the conclusion of 72 km/s/Mpc.

Astronomers were finally happy, but their joy lasted only a few years. Recent accurate measurements of the cosmic microwave background give values of about 68 km/s/Mpc, while measurements using observations like that done by HST still give values of about 72 km/s/Mpc. This disagreement has been called the “Hubble tension” and has not yet been resolved.