Free Astronomy Magazine January-February 2024

7 JANUARY-FEBRUARY 2024 ASTRO PUBLISHING Lemaître and Edwin Hubble. Ac- cording to Zwicky, the phenomenon of the reddening of light (a shifting towards longer wavelengths com- pared to those of the light just emit- ted) was attributed to the fact that on their journey towards the ob- server, the photons slowly lose en- ergy by interacting with interga- lactic and interstellar matter and with other photons. Since the en- ergy loss corresponds to a reduction in frequency (Planck’s constant, 1900), the process produces a red- shift of the spectral lines that in- creases proportionally with the distance from the source. The “tired light” hypothesis (a term coined by Richard Tolman in the early 1930s) was not the only one to interpret the redshift with the pro- gressive loss of photon energy, but it was undoubtedly the best known and best developed alternative to the model of an expanding universe. At that time, the instruments avail- able to astronomers did not allow them to establish which of the two opposing models described reality, and although over the decades the idea of an expanding universe be- came increasingly established, until at least the 1980s, tired light cos- mology had remained difficult to disprove. Indeed, some scientists had meanwhile found new argu- ments in favor of the stationary uni- verse. An example is Halton Arp (famous for his catalog of peculiar galaxies), who argued that the red- shift of quasars was not attributable to their extreme distance in an ex- panding universe, but rather to local effects of the system consid- ered. In practice, Arp asserted that quasars were objects ejected from the nuclei of very active galaxies, such as those of Seyfert. Although controversial and opposed, this hy- pothesis also contributed to keep- F ritz Zwicky worked much of his life at the California Institute of Technology, where he made many important contributions to theoreti- cal and observational astronomy. The paternity of the concept of “tired light” is attributed to him. [Mount Wilson Institute] H alton Arp was one of the staunchest opponents of all models of an expanding universe. He argued that the redshift was not due to the physical motion of celes- tial objects, but instead must have a non-cosmological or “intrinsic” ori- gin. His alternative vision countered the standard Big Bang model for decades. [Jean-Pierre Jans] R ajendra Gupta, adjunct professor of physics in the Faculty of Sci- ence at the University of Ottawa. According to a hybrid cosmological model he developed, the universe would be twice as old as that ac- cepted so far. [Univ. of Ottawa]