Current topics of interest and areas of research and discovery in Relativity include the following:
1) Black holes. Black holes were first recognized as a theoretical prediction of GR in 1939 by physicists J. Robert Oppenheimer and Hartland Snyder. Oppenheimer was convinced that they were a possible end state of the evolution of massive stars. Today it is recognized that stellar black holes are only one possibility. It now appears that the heart of every galaxy and at least some globular clusters may harbor black holes with masses millions of times that of the sun. Quasars and other active galaxies may only be cases where we see an active black hole at the center of a galaxy. For some reason they may not always be active. If there is one at the center of our galaxy then it is probably not active.
Galactic black holes raise a number of questions: Why are some of them active and some not? What are the physical processes that fuel their activity? How do they become so massive? Which comes first the black hole or the galaxy? Or are both the result of a third process which creates them? These questions, still to be answered, bear directly on how the universe got to be the way it is and maybe how we got to be here to see it.
For some of the latest information about black holes go to this Hubble Space Telescope web page.
2) Gravitational Lensing. One of the tests and proofs of relativity was the bending of starlight by the sun observed in 1919 by british astronomers under the leadership of Sir Arthur Eddington during an eclipse of the sun. Because a massive body can bend light it can act like a lens. Magnifying lenses operate in part by concentrating light from across their aperture to a point called the focal point. Astronomical masses like planets, stars and galaxies can do the same thing. The result can be seen as a brightening (and distortion since gravitational lenses are not made by opticians) of the distant object. Gravitational lensing is being pursued in a couple areas:
For more information about gravitational lensing see the Optical Gravitational Lensing Experiment (OGLE) web page or check out this bibliography.
- Micro-Lensing. It is possible to detect otherwise dark objects in our galaxy by the temporary brightening they cause in distant stars when they pass between us and those stars. Such dark objects include Brown Dwarfs -- so-called failed stars that are not massive enough to host thermonuclear reactions in their cores and "burn" like normal stars -- and black holes -- which arise from Supernovas and are the end states of some massive stars. A census of both of these types of objects would provide valuable information about the structure of our galaxy and be valuable in understanding stellar evolution. Both types of objects are also candidates for the "missing matter" which is supposed to dominate the universe.
- Cosmological Lensing. Galaxies and clusters of galaxies can also serve as gravitational lenses on a cosmological scale. Cosmological gravitational lenses occur when a galaxy or cluster of galaxies lies in line of sight with more distant galaxies. No relative motion is observed so it is not a matter of temporary brightening. The distant galaxy is brightened and imaged as a ring, a partial ring or multiple image. Because light in different parts of the image takes different paths to reach us cosmological gravitational lenses can provide information about the structure of spacetime between us and the distant source. The first gravitational lens to be recognized as such was cosmological.
3) Gravity Waves. In Newton's theory of gravity the mutual attraction between two masses is an instantaneous force. In GR the force of gravity propagates at a finite speed -- the Speed of Light. This one small difference in theory makes a big difference in reality because it leads inevitably to the prediction of gravitational radiation. The situation is analogous to Maxwell's prediction from theory that electromagnetic radiation (which he identified with light) is emitted by accelerated charged particles. GR predicts gravity waves are emitted if a mass is accelerated and propagate through its gravitational field as gravity waves.
Certain cosmological physical processes are thought to involve just such rapid acceleration of mass. Detectable processes are thought to include supernova core collapse, the revolution and merging of binary neutron stars and black holes and perhaps the remnants of gravitational radiation created by the birth of the universe itself. Detection of gravitational radiation from such phenomena would not only be yet another confirmation of GR, they would provide information from events that are otherwise obscured from our direct view.
Whenever in the past a new window of electro-magnetic radiation has been opened to us we have not only been able to make new discoveries about known phenomena but discovered new phenomena as well. We should certainly expect the same to happen once we are able to observe gravitational radiation.
The first attempts to detect gravitational radiation were experiments conducted by Joseph Weber in the 1960's using strain gauges attached to massive metal bars in hopes of detecting distortions in spacetime as gravity waves passed through the apparatus. These primitive early attempts were not successful. Theorists have since discovered that Weber's detectors were too small by several orders of magnitude to detect predicted phenomena.
The current Laser Interferometer Gravitational-Wave Observatory (LIGO) project is heroic in size and subtle in precision by comparison to Weber's experiment. It uses laser interferometers with crossed beams traveling through vacuum tubes 2 1/2 miles in length. The expected observable effect is just 10-16 centimeters, or one-hundred-millionth the diameter of a hydrogen atom over the length of one interferometer arm. Two such instruments located in Hanford, WA and Livingston, LA will be monitored to detect coordinated signals and weed out noise. They will eventually work with similar instruments in other counties in a world-wide system.
For more information about LIGO see the LIGO web page.
4) The Cosmological Constant. GR and Newton's gravitational theory contain a fundamental problem. If by gravity all matter in the universe is attracted to itself then what keeps it apart? Why doesn't or why hasn't all the matter in the universe collapsed in one place under its own gravity? The problem is worse if you assume that the universe is infinite for then the force of the universe's self-attraction should be infinite. Yet this is not what we observe. Until near the end of the first quarter of the 20th century the universe was even thought to be static. The vast galaxies spread over even vaster space showed no sign of rushing together under the influence of their mutual gravity.
To address this conundrum Einstein proposed that there was a repulsive force that keeps gravity at bay on the cosmic scale. This adjustment to GR is known as the Cosmological Constant (L). But soon thereafter in 1929 Astronomer Edwin Hubble discovered that the universe was not in fact static. Rather the galaxies appear to be generally moving apart with their speed of relative motion proportional to their separation. Hubble had discovered the cosmological red-shift and the Expansion of the Universe. With this discovery the conundrum of gravity was solved. And Einstein was moved to regret his introduction of the Cosmological Constant which disappeared from the GR equations as a result.
Of course the discovery of the Expansion of the Universe simply replaces one cosmological puzzle with another. If the universe is expanding then how long will it do so? The gravity of all its mass no doubt slows the expansion. Is there enough mass in the universe to stop it and cause it to collapse on itself? Or will it expand forever? Since matter curves space time according to GR this question is often stated in terms of the curvature of spacetime as a whole. If this curvature is positive then the expansion will slow, stop and reverse. If it is negative the expansion will go on forever.
And then there is the third possibility, that the expansion and gravitational attraction are so finely balanced that spacetime is flat. At first glance it appears that this third possibility is true. How else could the universe have existed for billions of years and neither dispersed or collapsed? But this -- like the pre-twentieth century idea that the universe is static -- is an illusion born of our limited perspective.
In 1998 two teams of astronomers studying supernovas in distant galaxies came to a startling conclusion: The Expansion of the Universe has sped up since the time the most distant supernovas had occurred. This is contrary to the way things should be and requires a previously unknown cause. The conclusion that there is a repulsive force that operates on the cosmological scale to speed up the Expansion of the Universe seems inescapable. This repulsive force has variously been referred to as "negative gravity" and "dark energy".
Suddenly Einstein's Cosmological Constant appears prescient.
As a result of this discovery we can conclude that spacetime almost certainly has a negative curvature and that the universe is open. Here's why.