relativistic cosmology


Georges Lemaītre - Scientist of the Day

Georges Lemaītre, a Belgian priest and astronomer, was born July 17, 1894. It is often said that Edwin Hubble, in 1929, announced that the universe is expanding, certainly one of the great cosmological discoveries of the century. Except that Hubble didn’t proclaim any such thing. What Hubble discovered was that more distant galaxies have a larger red-shift than closer galaxies, which one might interpret as the result of an expanding universe, but Hubble did not do so. The man who first proposed the expansion of the universe was Lemaītre, in a paper of 1927, and he did so on no observational evidence at all, arguing instead that Einstein’s equations of general relativity were more compatible with an expanding universe than with a static one. When the relationship between galactic distances and their red-shifts was discovered, it was Lemaītre, not Hubble, who saw the important implication–that the universe must be expanding. And in 1931, when Lemaītre’s work finally became known to English cosmologists, Lemaītre further proposed that the universe must have had a beginning in time as an incredibly dense “primaeval atom”, and the expansion is the result of the explosion of that condensed atom. In our terminology, Lemaītre was the originator of the idea of the Big Bang. It is still common in popular literature to credit Hubble with the expanding universe and George Gamow for the Big Bang idea, but Lemaītre is increasingly getting his due as the originator of both concepts. In 1933, there were several photographs taken of Lemaītre with Einstein, on the occasion of their joint visit to the United States; we see one of them above. Einstein was not too impressed with Lemaītre in 1927, but by 1933 he had come to embrace the expansion of the universe as the only viable solution of his equations of general relativity.

The other image shows the dust jacket of the 1st English edition of Lemaītre’s Primeval Atom (1950).

Dr. William B. Ashworth, Jr., Consultant for the History of Science, Linda Hall Library and Associate Professor, Department of History, University of Missouri-Kansas City


Ask Ethan: How do black hole jets carve out bubbles in space?

“I really like the video [of the Illustris simulation], so much that I hunted down a description… which surprised me: “What appear to be explosions actually come from supermassive black holes blasting jets of material into intergalactic space, carving out huge bubbles.” This puzzles me because I expected jets to blast along a single axis; not as a sphere.”

One of the most remarkable features of a great number of giant, active galaxies are the presence of jets of hyper-accelerated matter, spanning thousands of light years. Correlated with feeding, supermassive black holes are these huge structures of light-emitting matter, identifiable from many millions of light years away. Yet our best simulations show that the gas temperature rises around them not in jet-like streams, but rather in spherical explosions around these supermassive black holes. Is there an incredible disconnect between the two pictures? There might have been if you had asked only 15 years ago, but the physics of radiative feedback has evolved tremendously thanks to advances in numerical simulations, allowing us to explain this phenomenon after all.

Thirteen years ago results were first presented indicating that most of the energy in our universe is not in stars or galaxies but is tied to space itself. In the language of cosmologists, a large cosmological constant is directly implied by new distant supernova observations. Suggestions of a cosmological constant (lambda) were not new – they have existed since the advent of modern relativistic cosmology. Such claims were not usually popular with astronomers, though, because lambda is so unlike known universe components, because lambda’s value appeared limited by other observations, and because less-strange cosmologies without lambda had previously done well in explaining the data. What is noteworthy here is the seemingly direct and reliable method of the observations and the good reputations of the scientists conducting the investigations. Over the past thirteen years, independent teams of astronomers have continued to accumulate data that appears to confirm the existence of dark energy and the unsettling result of a presently accelerating universe.

Image Credit: High-Z Supernova Search Team, HST, NASA