• Gravitational lenses. Galaxies. The term gravitational lens indicates an effect, described in General Relativity, according to which photons emitted by a light source, when passing near a very massive celestial body, are affected by its gravitational attraction. Close to this body, space-time is modified and it becomes curved. This perturbation translates into a change in the light path compared to the original route; this gives rise to the formation of multiple images of the astronomic source (see left). Therefore, massive celestial bodies act like gravitational lenses.

  When the gravitational lens is made up of a galaxy or a cluster of galaxies, the image of the source results in either one or more "point-objects"or large bright arches (Fig. 4 & 5). By measuring the angle by which the light is bent or the degree of distorsion of the image, we can estimate the mass of the lens (deflector). In most cases it has been found that this mass is much greater than that associated with visible matter.

Fig. 3: Double image to show effect of gravitational lens. The photo, taken by the Hubble Space Telescope (HST 14164+5215)shows in the centre the "lens"galaxy placed between earth and the source. (Credits: Kavan Ratnatunga (Carnegie Mellon Univ.) and NASA)		Fig. 4: The Abell 2218 cluster of galaxies. This cluster is so dense and compact that light coming from very far away, situated beyond the cluster in the direction of observation, is amplified and distorted in the form of luminous arches. (Credits: NASA, Andrew Fruchter and the ERO Team [Sylvia Baggett (STScI), Richard Hook (ST-ECF), Zoltan Levay (STScI)] (STScI))

• Gravitational lenses. MACHO. When the lens is made up of an astronomical body with a mass less than that of the sun (we talk in this case of a microlens, and these celestial bodies are called MACHO), as in the case of an isolated planet, for example, the angular separation of the images is too small to be able to see the double image. However, if the source is fixed and the lens is moving in relation to the star, we can observe apparent variations of luminosity in the light emitted by the source star: luminosity is best when the source, lens and observer are all in line; luminosity is reduced (and this is the true luminosity) when the lense is moved out of line. By measuring how much the light of the source has been amplified, we can "weigh" the non-visible object that generates the gravitational lens.

  Fig. 5: Artist's impression of a gravitational lens produced by a MACHO. Click on figure to see animation. (If you can't see it, download Flash Player [qui]). (Credit: Davide Centomo, Dip. di Fisica Università di Bologna)

  There has been a systematic analysis of the light variations of the stars in the Grande Nube di Magellano, a small satellite galaxy in our Milky Way . In some cases, light variations in the stars have been observed and interpreted as being due to lenses made up of invisible objects with a mass similar to that of large planets which, by moving on the edge of our galaxy, intercept the light of certain stars. A certain number of these bodies has been identified, but their mass and number contribute only in a small way to dark matter.

Gravitational lenses help us to obtain information about the presence of non-visible matter located in specific places, such as large planets, black holes, and galaxies. This dark matter should be of the same type as visible matter (ordinary matter, baryonic matter).