Astronomy

How Gravitational Lensing Proves the Existence of Dark Matter



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For quite some time, astronomers have been chasing after evidence of one of the most elusive - but most prevalent - substances in the universe: an invisible, seemingly undetectable form of matter which they call dark matter. In recent years, that search has led them to a phenomenon which is equally quirky and, in its own way, mysterious: a process called gravitational lensing.

Dark matter began as nothing more than a placeholder, a hypothetical solution to a math problem repeatedly encountered by 20th-century astronomers. According to Ann Zabludoff of the University of Arizona, Fritz Zwicky proved in the 1930s that galaxies were moving much faster than they should be, given their visible mass in the form of stars and interstellar clouds of gas and dust. The missing mass was attributed to some invisible and as-yet-unknown substance, which was given the nickname "dark matter" because it must have mass (hence, it was some form of matter) but it must not emit or reflect light (hence, it must be "dark").

This, of course, said more about the shortcomings of modern astronomy than about dark matter itself, and more recently, many astronomers have been trying to find ways of observing dark matter indirectly, by studying some effects which it should cause if it is present, even if it cannot be seen visibly by traditional means. That's led some physicists to set up cutting-edge subterranean detectors to search for certain types of hypothetical particles. Others continue to look at the skies, but in new ways. That has led them to consider gravitational lensing.

Gravitational lensing occurs when light from a very distant source is bent or tugged onto a new trajectory by the gravitational energy of a mass which lies along its path. This creates a sort of optical illusion effect, in which light that reaches telescopes and the naked eye here on Earth wasn't actually emitted from anything in the precise direction from which it appears to be reaching the human eye. This phenomenon is already regularly used, and easily understood, when it comes to physical objects passing intense gravity wells. NASA and other space programs, for instance, regularly "slingshot" deep space probes past Jupiter and Saturn, allowing them to pick up speed as the massive gas giants tug them onto a slightly different course.

The speed of light is constant, but when light passes by an extremely powerful gravitational source, like a black hole or a galaxy, it can be affected in the same way as the space probe travelling past Jupiter. This can produce a number of strange optical effects when viewed from Earth, like Einstein's cross and the Twin Quasar. However, it can also provide scientists with an important clue about the presence of dark matter.

When light from a very distant galaxy passes a closer galaxy, it gets bent off course. According to the Berkeley Cosmology Group, scientists can estimate how massive a galaxy appears to be based upon visible matter, and estimate how distorted the passing light should be given this visible mass. Because of the hypothetical presence of dark matter, the light distortions will always be greater than they should be based solely upon calculations of visible, normal matter. The difference can then be attributed to dark matter. Using these comparisons, scientists have been able to establish that there is actually several times as much dark matter in the universe as there is normal matter. Galaxies, in addition to consisting of billions of stars and massive clouds of gas and dust, seem to be surrounded by even larger and more massive clouds of dark matter.

The gravitational lensing experiments have provided conclusive proof that dark matter really does exist - in some form. However, it is still another form of indirect observation. According to NASA, the new cutting edge in astronomy builds on the observations of gravitational lensing by looking for new ways that dark matter might be observed directly, like gamma rays released by colliding dark matter particles.

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