Gravitational lens

A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is "bent" around a massive object (such as a massive galaxy) between the source object and the observer. The process is known as gravitational lensing, and was one of the predictions made by Einstein's general relativity.

Table of contents

1 Description 2 History 3 Some Examples 4 Cosmological Applications 5 Astronomical Applications 6 References and external links

Description

Bending light around a massive object from a distant source. The orange arrows show the apparent position of the background source. The white arrows show the path of the light from the true position of the source

In a gravitational lens, the gravity from the massive object bends light as a lens might. As a result, the path of the light from the source is curved, distorting its image. For example, the apparent location of the source to the observer may be different from its actual position. In addition, the observer may see multiple images of a single source. If the source, massive object, and the observer lie on a line, the source will appear as a ring behind the massive object. This image is known as an Einstein's ring. More commonly, the massive galaxy is off-center, creating different numbers of images according to the relative positions of the source, lens, and observer, and the shape of the gravitational well of the lensing system.

There are three classes of gravitational lensing:

1. Strong lensing: where there are easily visible distortions such as the formation of Einstein rings, arcs, and multiple images
2. Weak lensing: where the distortions of background objects are much smaller and can only be detected by analysing large numbers of objects to find distortions of only a few percent
3. Micro-lensing: where no distortion in shape can be seen but the amount of light received from a background object can be changed.

Gravitational lensing - Click on the magnifying glass to see the lensing arcs

Gravitational lenses act on all kinds of electromagnetic radiation, not just visible light. Weak lensing effects are being studied for the cosmic microwave background and strong lenses have been observed in radio and X-ray regimes as well.

History

According to the general relativity, gravitational fields "warp" the space-time and therefore must also bend the light. His theory was confirmed in 1919 during a solar eclipse, when Arthur Eddington observed the light from stars passing close to the sun was slightly bent, so that stars appeared slighlty out of position.

Einstein realized that it was also possible for astronomical objects, like galaxies, to bend light, and that under the correct conditions, one would observe multiple images of a single source, called a gravitational lens or sometimes a gravitational mirage. But, he put off publishing his ideas for over twenty years, because he felt that these multiple images would be impossibles to see with the technology of the early part of this century.
With the invention of powerful radio telescopes and CCD optical detectors, astronomers can see farther and to resolutions far beyond Einstein's expectations.

The first gravitational lens ever discovered is called the "Twin Quasar" since it's image simply looks like two identical objects; it is officially named 0957+561 A & B. This gravitational lens was discovered accidentally by Dennis Walsh, Bob Carswell, and Ray Weymann using the Kitt Peak National Observatory 2.1 meter telescope in 1979 -- some 60 years after Einstein predicted this phenomena

The study of gravitational lenses is an important part of the future of astronomy and astrophysics.

Cosmological Applications

Gravitational lenses may be used to examine objects at distances at which they would not normally be visible, providing information from further back in time than otherwise possible (see below). Also, not just the object being lensed but the lens itself can provide useful information. By inverting the lens equations information can be gathered on the mass and distribution of the lensing body.

In weak lensing large scale maps of dark matter distributions may be produced, and these techniques are particularly important to cosmology as they provide a measure of the mass directly, without relying on any assumptions about the link between distributions of dark matter and visible. Lensing therefore can give a way of constraining the amount of dark matter in the universe and the manner in which it clusters together.

Another parameter that may come out of the study of gravitational lenses is Hubble's constant which encodes the age and size of the universe. It can be determined, in theory, by measuring two quantities: the angular separation between two images, and the time delay between these images.
There are two contributions to the time delay:

1. the first is the obvious delay due to the difference in optical path length between the two rays.
2. the second is a general relativistic effect, the Irwin Shapiro time-delay, that causes a change in the rate that clocks tick as they pass through a gravitational field.
   Because the two rays travel through different parts of the potential well created by the deflector, the clocks carrying the source's signal will differ by a small amount.

Researchers at Caltech have used the gravitational lensing afforded by Abell 2218 cluster of galaxies and the Hubble Space Telescope to detect the most distant galaxy known (February 15, 2004).

Gravitational microlensing can provide information on comparitively small astronomical objects, such as MACHOs within our own galaxy, or maybe even planets beyond the solar system.

References and external links

• Newbury, Pete, "Gravitational Lensing". Institute of Applied Mathematics, The University of British Columbia.
• Cohn, J., "Gravitational Lensing". Astron.berkeley.edu.
• "Q0957+561 Gravitational Lens". Harvard.edu.
• "Gravitational lensing". Gsfc.nasa.gov.
• Bridges, Andrew, "Most distant known object in universe discovered". Associated Press. February 15, 2004. (Farthest galaxy found by gravitational lensing, using Abell 2218 and Hubble Space Telescope.)