Gravitational Lens

Both Newtonian and Einsteinian theories of gravity predict that light will be bent in the presence of strong gravitational fields. Einstein's original prediction of such an effect and the value he obtained for its magnitude were confirmed in the 1960s by experiments in which radar signals were bounced off the planets. When a signal is reflected off Venus or Mercury while this planet is located on the other side of the Sun along the line of sight from the Earth to the Sun, the trajectory of the radar signal will be deflected by the Sun's gravitational field. This deflection is indicated experimentally by a slight change in the time it takes for the radar beam to reach the planet and return to the radio antenna on Earth. The results obtained have served to verify the general theory of relativity and enabled researchers to refine and calibrate its predictions.

As early as the eighteenth century, scientists hypothesized that the light from a distant star may be perturbed by the gravitational action of a closer star lying along the line of sight from the Earth to the distant star. With the advent of the general theory of relativity, there was renewed interest in the possible existence of such a phenomenon. In 1919 the British physicist Oliver Lodge (1851-1940) introduced the term "gravitational lens" to denote the closer star that acts gravitationally on the light from the more distant star. In 1924 Einstein calculated the magnitude of such an effect and concluded that it was too small to be observable. In the 1930s Fritz Zwicky proposed that gravitational lensing would be more likely to be observed in the case of extragalactic nebulae, what are today called galaxies. Because galaxies are very distant, there is a greater probability that an intervening massive body will lie somewhere along the long line of sight from us to any given galaxy. Furthermore, the large mass of an intervening galaxy or cluster of galaxies makes it a likely candidate to act as a lens.

In the 1960s, gravitational lens again became a subject of active theoretical interest, and several papers were published analyzing the optical properties of lens systems and producing calculations to measure their effects. Despite this interest, no systematic observational program emerged to detect such phenomena. In the 1970s a group of researchers led by Dennis Walsh (1933-2005) was attempting to correlate radio sources with objects observed through large optical telescopes. Walsh was working at the Jodrell Bank radio telescope in Britain and prepared a catalog of radio sources. One of these was 0957 + 561, so designated because it was located in Ursa Major at right ascension 9 hours and 57 minutes and declination 56 degrees, one minute. A preliminary survey placed this source very close to a blue double-stellar object of the 17th magnitude; its blueness made it a likely candidate to be a quasar. In 1979 Walsh and his collaborators, Robert Carswell and Raymond Weyman (1934—), examined the object with the two large reflecting telescopes at Kitt Peak Observatory in Arizona. To their surprise, they found that it was a double quasar with identical red shifts and similar emission and absorption spectra. They concluded that the two quasars were in fact one, its light being gravitationally lensed and split by an intermediate object, later to be identified as an elliptical galaxy (possibly part of a cluster of galaxies) located very close in position to one of the quasars. The light from the quasar passes close to the intermediate galaxy, which lies on the line of sight from the Earth to the quasar. The galaxy acts as a gravitational lens, producing the two images of the quasar. The quasar possesses a red shift z = 1.3, indicating that it is at a distance of about nine billion light-years.

It is worth noting that neither Walsh nor his collaborators had any prior involvement with the subject of lensing and that the 1979 discovery was essentially a serendipitous event. The realization that 0957 + 561 is a gravitational lensing system created a sensation and led to efforts to discover further such systems. Gravitational lensing became a very active topic in extragalactic astrophysics, a subject of investigation by the most advanced instruments of deep-space astronomy. By 2005, more than one 100 gravitational lensed quasars and galaxies had been detected. Pictures of lensed quasars and galaxies are among the most popular photographs released by the education office of the Hubble Space Telescope. A gravitational lens can act on the image of a distant object in various ways. The image may be distorted, bent into a curved form, or made to consist of multiple component images. Figure 10.2 depicts a distant quasar in the constellation Pegasus that has been lensed into four images by a galaxy that is relatively close to us. In recognition of Einstein's contributions to the theory of gravity, it is called "Einstein's Cross."

It is realized today that gravitational lens systems are not restricted to objects outside the galaxy, and there has been much interest in so-called minilensing events involving stars within the Milky Way system or the nearby Magellanic Clouds. However, from the viewpoint of cosmology it is distant extragalactic lenses that are of the most interest, and they have become an important tool of investigation in modern cosmology. If a lensed quasar undergoes a change in brightness, this change will be relayed to observers at slightly different times in the two images, reflecting the slightly different distances the light has to follow along the two optical trajectories. This difference and an analysis of the geometry of the lensing system allows one to determine the distance to the quasar more precisely and to obtain a more accurate value for Hubble's

Figure 10.2: Einstein's Cross, a gravitationally lensed quasar. NASA.

constant. Gravitational lenses also magnify the image of an object, opening up to investigation distant quasars and galaxies that would otherwise be unobservable. Indeed, the most distant objects in the universe, small galaxies at a distance of 13 billion light-years, have been sighted as part of lens systems, which magnify their brightness by as much as a factor of 100.

0 0

Post a comment