Friday, August 25, 2006

Gravitational Mirage

Simulated gravitional lensing

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 is one of the predictions of Albert Einstein's general relativity theory. It is sometimes known as the Einstein effect, although that is not the only meaning attributed to that term.

Above is a simulation of gravitational lensing caused by a Schwarzschild black hole going past a background galaxy. A secondary image of the galaxy can be seen within the black hole's Einstein ring on the side opposite the galaxy. The secondary image grows (remaining within the Einstein ring) as the primary image approaches the black hole. The surface brightness of the two images remains constant, but their angular sizes vary, hence producing an amplification of the galaxy luminosity as seen by a distant observer. Maximum amplification occurs when the galaxy (or in this case a bright part of it) is exactly behind the black hole.

Gravitational lensing

In galaxy clusters, the normal matter, like the atoms that make up the stars, planets, and everything on Earth, is primarily in the form of hot gas and stars. The mass of the hot gas between the galaxies is far greater than the mass of the stars in all of the galaxies. This normal matter is bound in the cluster by the gravity of an even greater mass of dark matter. Without dark matter, which is invisible and can only be detected through its gravity, the fast-moving galaxies and the hot gas would quickly fly apart.

In addition to the Chandra observation, the Hubble Space Telescope, the European Southern Observatory's Very Large Telescope and the Magellan optical telescopes were used to determine the location of the mass in the clusters.

This was done by measuring the effect of gravitational lensing, where gravity from the clusters distorts light from background galaxies as predicted by Einstein's theory of general relativity.

The hot gas in this collision was slowed by a drag force, similar to air resistance. In contrast, the dark matter was not slowed by the impact, because it does not interact directly with itself or the gas except through gravity. This produced the separation of the dark and normal matter seen in the data. If hot gas was the most massive component in the clusters, as proposed by alternative gravity theories, such a separation would not have been seen. Instead, dark matter is required.

NASA RELEASE: 06-297 CHANDRA reveals Dark Matter
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CHANDRA press Release by Sean Carroll @ Cosmic Variance

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 like a lens. As a result, the path of the light from the source is curved, distorting its image, and the apparent location of the source 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 straight line, the source will appear as a ring behind the massive object. This phenomenon was first mentioned by Chwolson in 1925, and quantified by Einstein in 1936. It is usually referred to in the literature as an Einstein ring, since Chwolson did not concern himself with the flux or radius of the ring image. More commonly, the massive galaxy is off-center, creating a number of images according to the relative positions of the source, lens, and observer, and the shape of the gravitational well of the lensing galaxy.

There are three classes of gravitational lensing:
Strong lensing:
where there are easily visible distortions such as the formation of Einstein rings, arcs, and multiple images.
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.
Microlensing: where no distortion in shape can be seen but the amount of light received from a background object changes in time. Typically, both the background source and the lens are stars in the Milky Way.

The effect is weak, such that (in the case of strong lensing) a galaxy having a mass of over 100 billion solar masses will produce multiple images separated by only a few arcseconds. Galaxy clusters can produce separations of several arcminutes. In both cases the galaxies and sources are quite distant, many hundreds of megaparsecs away from our Galaxy.
Gravitational lenses act equally on all kinds of electromagnetic radiation, not just visible light. Weak lensing effects are being studied for the cosmic microwave background as well galaxy surveys. Strong lenses have been observed in radio and x-ray regimes as well. If a strong lens produces multiple images, there will be a relative time delay between two paths: that is, in one image the lensed object will be observed before the other image.

According to general relativity, mass "warps" space-time to create gravitational fields and therefore bend light as a result. This 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 slightly out of position.

Einstein realized that it was also possible for astronomical objects 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. However, as he only considered gravitational lensing by single stars, he concluded that the phenomenon would most likely remain unobserved for foreseeable future. In 1937, Fritz Zwicky first considered the case where a galaxy could act as a lens, something that according to his calculations should be well within the reach of observations.

It was not until 1979 that the first gravitational lens would be discovered. It became known as the "Twin Quasar" since it initially looked like two identical quasars; it is officially named Q0957+561. This gravitational lens was discovered accidentally by Dennis Walsh, Bob Carswell, and Ray Weymann using the Kitt Peak National Observatory 2.1 meter telescope.

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


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