Thursday, November 1, 2018

The Hubble-Lemaître Law

Georges Lemaître
Let us look at a little history.
In various places in the sky, but especially in the constellation of Cepheus, where the first case was discovered, there are some stars whose light intensity varies regularly, and therefore are called variable Cepheids. In 1908, the American astronomer Henrietta Swan Leavitt discovered that the period of these stars is linked with their real luminosity: the greater the luminosity, the longer the period. Therefore, by measuring their period, their real luminosity can be deduced.
In 1913, the American astronomer Vesto Melvin Slipher obtained the spectrum of what was then called the Andromeda nebula (the giant galaxy closest to ours) and discovered a blue shift that indicated (according to the Doppler effect) that the nebula moves towards us with a speed of about 300 kilometers per second, much higher than expected. Slipher then studied the light of other spiral nebulae and made the unexpected discovery that most of them, unlike Andromeda, show redshifts, that is, they move away from the solar system with great speed. In fact, he measured speeds above 1000 kilometers per second.
In 1919, the American astronomer Edwin Powell Hubble used the Mount Wilson telescope to photograph several spiral nebulae, including Andromeda, and showed that, actually, they were not nebulae, as had been believed, but huge clusters of stars. From then on they were no longer called nebulae, but galaxies, in honor of our Milky Way, which also belongs to the class of spiral galaxies. Galactos in Greek means milk.
Hubble’s interest in the Andromeda galaxy went further. He also tried to calculate the distance separating it from us. To do this, he relied on Leavitt’s discovery about Cepheid stars. Some Cepheid stars are quite bright, and Hubble found about forty of them in the photographs of that galaxy. After measuring their period of variation, he applied the Leavitt relation to obtain their real luminosity. From the comparison of this result with their apparent luminosity, he could deduce their distance, since the apparent luminosity of an object decreases in inverse ratio to the square of the distance. His conclusion was that the Andromeda galaxy was a million light-years away from us.
Edwin Hubble
But there was a problem: there are several classes of Cepheid stars, each class with a different Leavitt relation, and Hubble did not apply the correct relation, so his computation of the distance of the Andromeda galaxy was wrong. The correct distance is more than double what he got.
As I mentioned in a previous post in this blog, in 1927 the Belgian priest and astronomer Georges Lemaître discovered Hubble’s law, which can be stated thus: The farther a galaxy is, the faster it gets away from us. Put in another way: the universe is in a state of expansion. Lemaître published his discovery in French, in a low-impact journal (Annales de la Société Scientifique de Bruxelles), which is the reason why many astronomers did not hear about it. In 1929, Hubble discovered the same law, and published it in English in the Proceedings of the National Academy of Sciences, received much more publicity and his name was associated with the discovery. Since then, this important law, on which modern cosmology depends, has been called the Hubble law.
This name (Hubble’s law) was a historical injustice. In science, the first discoverers of a law (or any other finding) give them their name. There is a very similar paradigmatic case in the history of biology. The Austro-Hungarian Augustinian friar Gregor Mendel discovered the laws of inheritance in 1865 and published them in an article entitled Experiments on Plant Hybridation (Versuche über Pflanzenhybriden) which was published in German in the proceedings of the Natural History Society of Brünn (Brno) The article went unnoticed.
In 1900, sixteen years after Mendel’s death, three botanists from different countries (Hugo de Vries, Dutch; Karl Correns, German; and Erich Tschermak, Austrian), in the course of their research, independently rediscovered the laws of Mendel. All three looked at the scientific literature, found the article of the Augustinian friar and recognized his priority. Thirty-five years late, the scientific world appreciated the importance of Gregor Mendel’s discoveries. Since then the laws of inheritance are called Mendel Laws.
In August 2018, during its XXX General Assembly in Vienna, the International Astronomical Union (IAU) decided to remedy the injustice and proposed that the Hubble Law should be called from now on the Hubble-Lemaître Law. The proposal was voted and accepted by the members of the IAU during October. The result of the voting was announced on October 29th 2018.
We must be happy with this attempt to correct an historical injustice, but my personal opinion is that the law should be called Lemaître-Hubble Law, to make the priority clear, and the order of its double discovery.

The same post in Spanish
Thematic thread on Standard Cosmology: Preceding Next
Manuel Alfonseca

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