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The Warped Ways of Cosmic Light

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The Warped Ways of Cosmic Light BIOLOGIE & MEDIZIN_Infektionsbiologie “Around that area there are indeed numerous small nebulous patches so close together that, upon seeing the region, one is positively startled by the strange appearance of this ‘nebula cluster’.” This was the sentiment expressed by Heidelberg-based astronomer Max Wolf in 1901 about the galaxy cluster in the constellation Coma Berenices. The cluster Abell 2218 presents a similar appearance to the viewer today on this image from the Hubble Space Telescope. The “strange appearance” – the many blue and orange arcs – owes, in this case, to the effect of a gravitational lens. PHYSICS & ASTRONOMY_Gravitational Lenses The Warped Ways of Cosmic Light Albert Einstein predicted them, modern giant telescopes detected them – and Klaus Dolag simulates them on a computer: gravitational lenses. The academic staff member at the Max Planck Institute for Astrophysics in Garching and at the University Observatory Munich uses this physical phenomenon to such ends as weighing galaxy clusters and probing that ominous substance known as dark matter. TEXT HELMUT HORNUNG he sky over Principe was a total eclipse of the Sun. The test was More recent analyses of Eddington’s cloudy and the mood in the simple: if a massive object bends space, findings show that the values measured camp on the coconut plan- then the Sun must also cause the pass- at that time did, indeed, correspond to tation had hit rock bottom. ing light from stars to deviate from its the prediction, but that they are appar- Having set out from Eng- straight course. In other words, the ently the result of an instrument error T land, the men had been traveling for points of light in the immediate vicin- of, as it so happens, the very same mag- weeks to reach the volcanic island in ity of the black Sun should be shifted nitude. But the gravitational effect has the Gulf of Guinea. Their goal: to write ever so slightly with respect to their since been measured many times and scientific history. They wanted nothing true position in the sky. To check this, perfectly confirmed – not a doubt re- less than to prove a bold theory that a Eddington had recorded the star field mains. So Klaus Dolag doesn’t have to German physicist by the name of Albert six months earlier without any inter- concern himself with evidence of any Einstein had recently published. Ac- ference from the Sun, using the same sort. He simply works with the effect – cording to this theory, the gravity of a instruments that he now aimed impa- and applies the physical principle to massive object should bend space, sim- tiently at the cloud-covered, slowly weigh galaxy clusters or to track down ilar to the indentation made by a lead darkening Sun. the hidden matter in the universe. ball on a rubber blanket. If a marble is Moreover, the staff member at the Max then rolled toward the ball, the marble VIRTUAL TELESCOPES PEER Planck Institute for Astrophysics and at will deviate from its originally straight INTO SPACE the University Observatory Munich course and, depending on its initial ve- doesn’t even need a cloudless sky: Klaus locity, follow a more or less curved And the astronomer was rewarded for Dolag does his research with computer path. On this day, May 29, 1919, it his perseverance. During totality, the simulations, creating virtual telescopes. would be possible to test this prediction clouds parted for a few moments and To understand this, let us take an- of Einstein’s general theory of relativi- Eddington managed to capture two im- other detour through history. Because ty on the island of Principe. And then ages. “Lights All Askew in the Heavens” the Sun bends the path of starlight, Al- the weather turned out to be miserable! appeared on the front page of the New bert Einstein had speculated in the But the expedition led by astrono- York Times on November 10, 1919. The journal SCIENCE in 1936 that a star mer Arthur Stanley Eddington had nei- “shifted lights in the sky” proved that could diffract the light from an object ther a blanket nor a ball with them, but the general relativity theory was cor- behind it – which propagates in a rather a telescope and camera. The re- rect – and established Albert Einstein straight line in empty space – similar Photo: NASA, ESA, and Johan Richard (Caltech, USA) searchers wanted to use these to record as a legend. to a glass lens. In this way, the light 1 | 11 MaxPlanckResearch 65 Apparent location of the source GRAVITATIONAL FIELD Electromagnetic waves Object with large mass Actual position Observer of the source Apparent location of the source from this object should be amplified, the effect – and especially the ring that Take, for example, the galaxy cluster much like the Sun’s rays passing was named after him – was too small: Abell 2218, which is some two billion through a magnifying glass; and fur- “Of course we have no hope of observ- light-years away in the constellation thermore, multiple images of one and ing the phenomenon directly.” In Draco. On images from large tele- the same object might be created. 1979, researchers discovered the first scopes, dozens of small arcs in differ- quasar whose light was split off into ent colors and shapes tell of galaxies EINSTEIN HIMSELF DIDN’T multiple images by the gravitational that lie more than six billion light- BELIEVE IT COULD BE PROVEN field of a galaxy lying on the line of years beyond Abell 2218 and that, sight with the Earth. Quasars are the without its effect, wouldn’t be visible In 1937, Swiss astronomer Fritz Zwicky active cores of young galaxies. Driven at all. In short, gravitational lenses act introduced the possibility of a galaxy by massive black holes, they have a as natural telescopes. The powerful acting as a lens that creates distorted very high energy output. magnifying effect of the Abell 1689 images of more distant galaxy systems The type and shape of the lensed cluster led, for instance, to the discov- lying beyond it. Different images are images follow the laws of optics and ery of one of the most distant galaxies created depending on the spatial orien- gravitation. Thus, similar to classical known to date. We see it today in the tation of observer, lens and object. If optics, it is possible to draw important light it emitted 700 million years after all three lie exactly in a line (the opti- conclusions from the cosmic images, the Big Bang. cal axis), a ring is formed. If they lie to about, for instance, the lens material one side of the optical axis, the images or the composition of the medium – TIME MACHINES SHOW THE of the background sources appear as that is, about how mass is distributed DEVELOPMENT OF THE GALAXIES more or less strongly curved arcs. It is within the galaxies, or the structure of not just individual objects such as gal- the space. Galaxy clusters are particu- Because the speed of light is finite, the axies that act as lenses, but frequently larly interesting for researchers like observation of distant objects is simul- galaxy clusters – accumulations of Klaus Dolag: “They are the largest sys- taneously a journey back in time. The enormous concentrations of matter tems in the universe that are bound to- universe has changed significantly that we observe as many, relatively gether by gravity. That is why they since its birth 13.7 billion years ago. tightly clustered galaxy systems. have especially marked gravitational The galaxies, for example, have since Regarding the practical proof of the lensing effects.” This also explains developed in a certain way. Because gravitational lensing effect, Albert Ein- why they benefit science in so many telescopes (and gravitational lenses) stein remained skeptical. He said that ways. And just what is the benefit? act like time machines, they offer as- Graphic: designergold, based on original material from Horst Frank 66 MaxPlanckResearch 1 | 11 PHYSICS & ASTRONOMY_Gravitational Lenses left side: A gravitational lens functions essentially according to laws similar to those that apply in optics. In this case, however, the “glass” is a large mass – such as a galaxy or a galaxy cluster. left: Experimenting with space on the computer: Astrophysicist Klaus Dolag. tronomers outstanding insights into ing as a lens. For example, it quickly be described with physical laws. Ac- the evolution of space. becomes apparent that the visible mass cording to the model, at the age of 10-35 In the case of Abell 2218, massive contained in galaxies is not even close seconds, the cosmos expanded by 50 elliptical galaxies crowd together in the to being sufficient to hold together orders of magnitude, from the diame- center; photos show, a bit outside of the such a gigantic cluster as Abell 2218, ter of a proton to that of an orange. middle, a further small cluster core. Ap- with its several hundred members. Fur- That moment, known as inflation, in- parently two galaxy clusters are merg- thermore, the visible matter is not creased the bubbling in the quantum ing together. This fact alone is an im- nearly adequate to produce the multi- soup to macroscopic dimensions. portant indication for astronomers that ple arc-shaped images of the objects ly- These structures are visible in the baby such mergers shape the appearance of ing beyond it. After all, the optical photos of space, in the light of micro- space. It seems that a particularly large properties of a gravitational lens are de- waves, as color-coded spots having amount of luminous arcs appear when termined by its surface density.
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