Relativity Lite

Cosmology | 67 CHAPTER 5 Cosmology THE BIG BANG One of the more interesting applications of the general theory of relativity is in cosmology. In 1927, Georges Lemaître published a paper attempting to reconcile the massless cosmo- logical solution of general relativity by Willem de Sitter, which explained “the observed receding velocities of extra-galactic nebulæ,” and Einstein’s own solution containing mass but including a cosmological constant to make for a static universe. Lemaître’s solution did reconcile them and provided a value for the relation between the recessional velocities of the galaxies and their distances, 625 km per second for every 1 million parsecs the galaxies are away from us. * That is, nearby galaxies are sluggishly moving away from us, while the farthest galaxies are sprinting. Two years later, Edwin Hubble (after whom the space telescope is named) used his own improved observations to derive a more accurate constant. Hubble’s work became widely known, while Lemaître’s prior explication of what became known as the Hubble law did not, in part because Lemaître left his value out of the 1931 English translation, since he felt that better values had subsequently been offered. † It was only in 2018 that the Interna- tional Astronomical Union (IAU) recommended that the law henceforth be known as the Hubble–Lemaître law. This law suggests that we may be in the middle of some cosmic explosion of galaxies. In 1936, Robertson and Walker found a solution to Einstein’s equations in which the uni- verse could have expanded outward from an infinitely dense state. ‡ In 1948, Ralph Alpher and Robert Herman showed that as this expanding universe cooled from its inconceivably high initial temperature to a mere billion degrees in the first 200 seconds or so, about 1/4 of the hydrogen nuclei would have been transformed into helium nuclei. This is exactly * G. Lemaître, Annales de la Société Scientifique de Bruxelles A47 , 49–59 (1927). † The relevant passage in the original is “Utilisant les 42 nébuleuses figurant dans les listes de Hubble et de Strômberg ( 1 ), et tenant compte de la vitesse propre du soleil (300 Km. dans la direction α = 315°, δ = 62°), on trouve une distance 0,95 millions de parsecs et une vitesse radiale de 600 Km./sec, soit 625 Km./sec à 10° parsecs ( 2 ).” Abbé G. Lemaître, MNRAS 91 , 483–90 (1931). ‡ H. P. Robertson and A. G. Walker, Proc. London Math. Soc. 42 , 90 (1936).