Gravitation and Cosmology

Einstein's theory of gravitation languished on the sidelines for nearly half a century after the initial flurry of 1919, because relativistic deviations from Newtonian predictions were, in most cases, immeasurably small under all astrophysical conditions then conceivable. Only after 1960 was it recalled to center stage, when the discovery of X-ray sources, active galactic nuclei, quasars, pulsars and the Cosmic Microwave Background Radiation(CMBR) revealed the presence in our universe of strong-gravity regimes and spectacular relativistic effects.

Today the theory is central to our understanding of some of the most exotic realms of science, from black holes to the evolution of the cosmos fractions of a second after the big bang. Experimental confirmation of the theory has progressed enormously since 1915. A milestone was the 1974 discovery of the binary pulsar, a pulsar in an 8-hour non-circular orbit around another neutron star. The orbital period is decreasing at a rate that squares with the loss of energy by gravitational radiation as calculated from Einstein's quadrupole formula. This indirect but compelling evidence for the reality of gravitational waves lends encouragement to the intensive efforts currently under way to detect such waves directly (LIGO in the USA, Virgo in Italy, LISA a joint project of ESA and NASA). Observations with the Hubble Space Telescope and sophisticated ground based technology have provided irrefutable evidence for the presence of black holes in the nuclei of our own and other galaxies, with masses ranging from millions to billions times that of the sun. At a theoretical level, black holes represent one of the frontiers in trying to understand how quantum mechanics and gravity can coexist. In the 1970's, Bekenstein and Hawking inferred an intimate relationship between black holes and thermodynamics, and an understanding of the entropy of black holes remains one of the driving questions in current research.