The article by Joachim Wambsganss discusses the nature and functions of gravitational lenses that both help and challenge our understanding of the celestial bodies in the universe. As the author rightly puts it, the sky is filled with many mirages and mysteries to be explored. The gravitational lenses make the universe into a palace of glasses where one images reflects into multiple images creating an illusory effect. It is curious to know the nature and functions of these gravitational lenses that both help and defy our understanding of the universe. .
Gravitational lensing is a budding and promising area of study in astronomy. It helps astronomers to study the dark matters in the universe. It is also useful to investigate the structure of quasars, black holes and find earth like planets around other stars.
Gravitational light deflection was identified and accepted, though not accurately. Einstein was doubtful about it though he predicted that a foreground star could magnify the image of a background star as a result of gravitational lensing. More optimistic was the Swiss-American astrophysicist Fritz Zwicky with his predictions on lensing effects of galaxies. At last the speculation came to an end in 1979 when astronomers actually saw evidence of lensing.
Generally light that comes from a celestial body goes straight. But when there is any object in between it deflects and the deviation causes many distortions to what we see in the space. It is interesting to note that any thing that possesses mass can serve like a lens. It need not emit light on its own. Four consequences of gravitational lensing are identified. They are: 1.Change of Position, 2. Magnification and Demagnification, 3.Deformation, 4. Multiplication.
The perceptible location of star or galaxy changes because of the deflection of gravitational light
Secondly, it is also possible to see the magnification of brightness of a star or quasar because of the deflection of light. Sometimes the light demagnifies. Thirdly, galaxies can appear long-drawn-out into arcs or bananas because of the deflection of light. Finally, the multiple images are the result of strong gravitational lensing.
How Lensing Works
The gravitational –lens system has three components embedded in the space. The distance source of light, an intervening mass that acts as a lens and an observer on the earth are the three which form a straight line. The distant source of light could be a star, quasar, or galaxy. The intervening mass that acts as a lens can be anything from a planet to a black hole.
Light travels through the shortest distance, and it need not be a straight line always. Sometimes it can travel through a curve as well. The light bends when it reaches the curved space near a cosmic body. The degree of deflection depends on how close the rays get tot the body and on the mass of the body. The deflect ion angle is directly proportional to the mass and inversely proportional to the closest distance.
Gravitational lenses are different from ordinary lenses in many ways. The ordinary lenses have a well-defined focal point whereas the gravitational varieties produce focal lines or surfaces. The typical gravitational lens also causes light rays to experience smaller deflections. The gravitational lensing is achromatic. When the lens system is asymmetric, i.e. when source, lens and observer are not in alignment, the lens has an oblong mass distribution – and the resulting ring breaks up into discrete variegated images. The lens magnifies different parts of the source by different amounts and the highest magnification occurs at caustic. When the alignment is very far off or the lens mass distribution is very spread out, the lensing is very weak.
With the discovery of double quasar Q0957+ 561 gravitational lensing became an observable science. So far 64 double, triple and multiple quasars have been found. The CLASS (Cosmic Lens All-Sky Survey) project has mapped more than 10,000 radio sources and 17 multiply imaged systems.
To identify whether it is real quasar or an illusion, observes have developed a checklist. They see whether the quasars lie at the same distance, whether their spectra is similar, whether there is any potential lens between the observer and the quasar and whether the brightness of each quasar fluctuates in the same way.
When the galaxy lens is spherical it can distribute the light of background quasar or galaxy into a ring or circle called Einstein ring. About a dozen such rings are found.
The important application of quasar lensing is to measure Hubble constant, which is a measure of size and expansion rate of universe. Multiple quasars can give insight into cosmological parameter called cosmological constant. It is essential to explain why the expansion of universe appears to be accelerating. The more is the expansion accelerated, the bigger the volume of space. Cosmological constant can not be more than 62 percent of the energy density of the universe. If the density is more, there could be more number of quasars. It supports smaller values of the cosmological constant.
Quasars are unsteady by their nature and they tend to brighten and then dim on their own. To distinguish microlensing fluctuations and intrinsic variability astronomers monitor multiple quasar systems. In 1989 astronomers confirmed five multiple quasar systems. They conclude that innermost parts of quasar are hotter and bluer than outer parts.
If lens is not single galaxy, but a cluster of galaxies then the image can be a kaleidoscope of strongly distorted arcs and arc lets. Studies on clusters of galaxies reveal that clusters are dominated by unseen dark matter. On an extremely large scale, the vast galaxy clusters of matter tend to be powerful lens. The wide spread shearing of galaxy images support the view that universe is giant cobweb of matter interspersed with voids.
The article also focuses on MACHOs and Extra solar planets. MACHOs are collection of rogue planets, dead stars or black holes. However research says that the entire dark matter cannot be made entirely of MACHO. It also notes that stellar mass black holes may cause microlensing events. Stellar microlensing can even detect planets and the extra burst of brightening that might have been caused by planet candidates.
Thus, the article ‘Gravity’s Kaleidoscope’ narrates interestingly how the study of illusions can lead us to the discovery of truth relating to celestial bodies in the universe.