Why it’s taking a century to pin down the speed of the universe

The Hubble constant, a set number that connects a galaxy’s speed to its distance from Earth and tells us how fast the universe is expanding, was first described more than a hundred years ago – but astronomers have debated it ever since

By Jim Baggott

3 June 2025

This Hubble Space Telescope image is cluttered with myriad galaxies

NASA, ESA, B. Sunnquist, J. Mack, J. Lotz (STScI), HFF Team

The following is an extract from our Lost in Space-Time newsletter. Each month, we hand over the keyboard to a physicist or mathematician to tell you about fascinating ideas from their corner of the universe. You can sign up for Lost in Space-Time here.

If you look up on a clear night, chances are you’ll be able to identify the constellation Orion, most likely from the line of three stars that form its belt. These are, from left to right in the northern sky, Alnitak, Alnilam and Mintaka. Which is brightest?

Read more

How nothing could destroy the universe

The answer depends on what you mean by “brightest”. All three appear to have similar visual brightness, or what astronomers call apparent magnitude, but this is an illusion. They are all at different distances from Earth, and the further away a star (or an entire galaxy) is, the dimmer it appears.

In terms of intrinsic brightness, also known as absolute magnitude, the middle star Alnilam is the brightest. At about 1340 light years from Earth, it is also the furthest away.

But how do astronomers measure such distances?