The Hubble-Lemaître Law: recognising where credit is due in science

We interview Armagh Observatory and Planetarium Director, Professor Michael Burton, about the IAU’s renaming of the Hubble Law describing the Expansion of the Universe.

Questions by: Heather Alexander



The Hubble Ultra Deep Field shows the furthest galaxies in the universe. The further away they are, the redder they are. (Image credit: NASA/ESA)

When and where did the International Astronomical Union (IAU) Conference take place this year?

The International Astronomical Union (IAU) is the world governing body for professional astronomers.  It meets every three years for its General Assembly (or GA to the astronomers).  This year was the XXXth General Assembly.  It was held in Vienna in Austria.  GA’s have held in the British Isles in the past. The IInd was in Cambridge in 1925, the IXth in Dublin in 1955, the XIVth in Brighton in 1970 and the XXIVth in Manchester.  I in fact helped organise the XXVth in Sydney in 2003 when I was living in Australia!  The IAU celebrates its centenary next year, 2019.  We plan on bringing a special exhibition (“Under One Sky”) to Armagh next July to celebrate – watch this space for more details!

What was the main discussion regarding the Hubble Law?

Edwin Huddle looking through the Mount Willson 100-inch telescope he used to make his startling measurements about recession speeds of distant galaxies. Credit: NASA

Hubble’s Law is one of the cornerstones of modern astrophysics, indeed it underlies scientific conception of our place in the Cosmos.  To paraphrase it slightly, the law states that the further an object is away from our Galaxy the faster [in general] it will be moving away from us.  In fact the speed of recession was found to be directly proportional to distance.  In other words, the universe is expanding!  The law is named after the American astronomer Edwin Hubble who published this result in 1929 in a paper in the Proceedings of the National Academy of Science in the USA entitled A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae.

However, as is often the case in science, the discovery involved insights gleaned from the work of several people, in particular the contribution made by the Belgian George Lemaître in 1927.  The discussion at the IAU GA in 2018 centred on whether astronomers should now also recognise Lemaître’s insight by renaming the law the Hubble-Lemaître law?

Who was George Lemaître?

George Lemaître was a Belgian astronomer who, in 1927, published a paper (in French) entitled Un Universe homogeneous de masses constant et de

Portrait of George Lemaitre. Credit: Wikipedia.

rayon croissant rendant de la Vitesse radiate des nébuleuses extra-galactiques. In English this is “A homogeneous Universe of constant mass and growing radius accounting for the radial velocity of extragalactic nebulae”.  Essentially this is the same result as Hubble’s Law – the further an object is away the faster it moves away from us!  In it he actually provided the first estimate of what we now call Hubble’s constant, which measures the rate of expansion, based on the (rather limited) observations of galaxies then available.

Lemaître was also a Catholic priest, as well as a Professor of Physics at the Catholic University of Leuven, Belgium’s oldest university.  He would come to scientific conferences dressed in his priest’s cassock, so clearly stood out in any conference photographs taken.

Why was George Lemaitre’s work not taken seriously at the time?

This paper appeared in the Annals de la Société Scientifique de Bruxelles, which unfortunately is a little read journal by scientists outside of Belgium. So the news of Lemaítre’s stunning insight was not heard about by the scientific community.

The equations of Einstein’s famous General Theory of Relativity have a solution that permit an expanding universe.  Einstein actually realised this but did not think this was physically possible.  This lead him to propose an additional term to his equations, the “cosmological constant”, which prevented this expansion – a static universe.  He later regretted this addition (“my biggest blunder”), but that is a whole other story which we won’t get into here!

In Lemaítre’s paper he actually starts from Einstein’s equations for a finite size, static universe, and then presents his ideas for an expanding universe.

Einstein heard of this, but refused to accept the idea that the universe could be expanding. Lemaître recalled him commenting “Vos calculs sont corrects, mais votre physique est abominable” (“Your calculations are correct, but your physics is atrocious.”).  Lemaître later took his own idea further, proposing that the universe started from a single point (the “primeval atom”), a “Cosmic Egg” exploding from the moment of creation.  This essential concept underlies what we call the “Big Bang” theory today.

Image of Albert Einstein

Time Lord: Albert Einstein during a lecture in Vienna in 1921 (Image credit: Photograph by Ferdinand Schmutzer).

However at the time Einstein’s view prevailed, perhaps not unsurprisingly given his scientific stature.  However, one wonders how much unconscious bias might have played a role in Einstein dismissing this idea, coming from a priest proposing what is in essence a genesis event for the universe??

Hubble published his famous paper in 1929.  We do know that Hubble and Lemaître met at the 3rd meeting of the International Astronomical Union, which was held in Leiden in the Netherlands in 1928, and discussed the astronomical evidence suggesting the expansion of the universe.

When Lemaître heard about Hubble’s hypothesis of the expanding universe, following a paper Sir Arthur Eddington gave at the Royal Astronomical Society (RAS) in London in 1930, he wrote to him and told him about his prior work on the topic.  Eddington then invited Lemaître to republish the translation of his paper in RAS’s journal, the Monthly Notices of the Royal Astronomical Society.

In the meantime, Hubble had published new results on the expansion of the universe in the Astrophysical Journal.  This time the sample of galaxies used was larger, extending more than ten times further away than before, and clearly removing any ambiguity from the previous results.  They rendered the prior measurements obsolete.  So, when working on translating his original article, Lemaître removed some paragraphs where he estimated the rate of the expansion of the universe.

As a result, for those not familiar with his work (i.e. most scientists!), it looked like it was Hubble who was the first one to discover the expansion of the universe.  Lemaître was apparently not concerned with establishing priority for his original discovery.  Consequently, the formula that describes the present-day expansion rate bears the name of Hubble and not Lemaître.

How was the vote taken to change the name?

Within the IAU there has been a growing realisation that Lemaître’s contribution has not been properly recognised.  At the XXXth General Assembly a motion was therefore brought to the meeting, to recommend that from now on the expansion of the universe now be referred to as the “Hubble-Lemaître” law.

Naturally this caused considerable discussion amongst the world’s professional astronomers.  Mindful of the controversy generated through the re-classification of Pluto as a dwarf planet, an event that occurred during the IAU’s GA in Prague in 2006, many were worried of creating adverse publicity.   The most persuasive argument for many for not renaming Hubble’s Law was this would open a can of worms.  If we change Hubble, then what next?!  But this was also a counter argument for many.  If we made a mistake in the past then we should we not do our best to rectify it?  If changing Hubble leads to future calls to change the names of laws, then we need to deal with them as best we can when they come.

In the end the IAU decided to call upon a straw poll to get an indication of the strength of opinion on the matter.  Since there were only around 500 of the IAU’s global membership of 18,000 astronomers present, the straw poll would then be used to inform whether there was enough strength of feeling for the IAU to then use an electronic vote to poll the views of its entire membership.

In the straw poll approximately 80% of those present did call for a change in the naming of the law.  So a clear statement of intent, but by no means a unanimous verdict.  The IAU will now proceed with arrangements for a vote of its entire membership in order to formalise its decision on the recommendation.


Schematic showing the expansion of the Universe from the Big Bang to the present day.  The furthest back in time we can actually see is to the microwave background radiation, an echo imprinted from 400,000 years after the Big Bang.  Much of the history of the universe is dominated by the formation of galaxies, stars and planets during a period uniform expansion – the Hubble–Lemaître law.  Recently we have realised that the universe is not just expanding, but the rate of expansion is actually accelerating, which will require a revision to  the Hubble–Lemaître law.  However scientists have not yet determined what this should be (Image credit: NASA).

Was the Armagh Observatory and Planetarium for or against this change?

For scientific matters the individual members of the IAU get to vote.  It is not up to the institutions, or indeed the national representatives (who get to vote on governance matters) to register a vote.  So AOP, as an institution, does not register an opinion.  But as an individual, I chose to vote for the new resolution.  I feel that Lemaître deserves to be formally recognised for his contributions to one of the cornerstones of science today.  I will just have to change my lecture notes for the A-level physics course in the Armagh Planetarium when I give it next year, as this features the topic of redshift and expansion of the universe in it!

Do you think this change will have any impact on the wider astronomical community?

A very interesting question.  Certainly it will open our eyes to past practice, and whether we have neglected to properly accord recognition to contributions through scientific history?  It could indeed become a can of worms, as some worry, for very often it takes the contributions of many before an insight is made, not just the first author on the scientific paper whereby this insight becomes clear.  There will be no simple right or wrong, but yes, I suspect that in the years ahead there will be further discussion on the renaming of laws.

Of course, we should remember too that the naming of scientific laws actually have no legal basis.  They simply represent scientists encapsulating a concept with a shorthand description.  They are not always named after people anyway.  For instance, while we all know that Einstein proposed the Theory of Relativity, encapsulating our knowledge about the speed of light, space, time and the nature of gravity, we generally simply call this “Special Relativity” and “General Relativity”, without Einstein’s name in.


Acknowledgement:  The Wikipedia article on George Lemaître provided much of the background to his contributions.