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. Photo: NASA, ESA, and Johan Richard (Caltech, USA) TEXT searchers wantedtousetheserecord rather atelescopeandcamera. There- ther ablanketnorballwith them,but mer ArthurStanleyEddington hadnei- the weatherturnedouttobe miserable! ty ontheislandofPrincipe.Andthen of Einstein’s ofrelativi- generaltheory would bepossibletotestthisprediction path. Onthisday, May29,1919,it locity, followamoreorlesscurved course and,dependingonitsinitialve- will deviatefromitsoriginallystraight then rolledtowardtheball,marble ball onarubberblanket.Ifmarbleis ilar totheindentationmadebyalead massive objectshouldbendspace,sim- cording tothistheory, thegravity ofa Einstein hadrecentlypublished.Ac- German physicistbythenameofAlbert thata less thantoproveaboldtheory scientific history. Theywantednothing the GulfofGuinea.Theirgoal:towrite weeks toreachthevolcanicislandin land, themenhadbeentravelingfor T ominous substanceknownasdarkmatter. uses thisphysical phenomenon tosuchendsasweighing galaxy clustersandprobing that Max Planck Institutefor Astrophysics simulates themonacomputer:gravitational lenses. The academicstaffmemberatthe Einsteinpredicted them,moderngianttelescopes detectedthem–and Albert Cosmicof Light Ways Warped The But theexpeditionledbyastrono- HELMUT HORNUNG Having setoutfromEng- tation hadhitrockbottom. camp onthecoconutplan- cloudy andthemoodin he skyoverPrincipewas tiently atthecloud-covered,slowly instruments thathenowaimedimpa- ference fromtheSun,usingsame six monthsearlierwithoutanyinter- Eddington hadrecordedthestarfield true positioninthesky. To checkthis, ever soslightlywithrespecttotheir ity oftheblackSunshouldbeshifted points oflightintheimmediatevicin- straight course.Inotherwords,the ing lightfromstarstodeviateits then theSunmustalsocausepass- simple: ifamassiveobjectbendsspace, a totaleclipseoftheSun.Thetestwas as alegend. rect –andestablishedAlbert Einstein wascor- the generalrelativitytheory “shifted lightsinthesky”proved that York Times onNovember10,1919. The appeared onthefrontpageof theNew ages. “LightsAllAskewintheHeavens” Eddington managedtocapturetwoim- clouds partedforafewmomentsand his perseverance.Duringtotality, the And theastronomerwasrewardedfor INTO SPACE VIRTUAL TELESCOPESPEER darkening Sun. inGarching andatthe PHYSICS &ASTRONOMY University Observatory Munich University Observatory the Sunbendspathofstarlight,Al- other detourthroughhistory. Because to aglasslens.Inthisway, thelight straight lineinemptyspace –similar behind it–whichpropagates ina could diffractthelightfrom anobject journal S bert Einsteinhadspeculated inthe simulations, creatingvirtualtelescopes. Dolag doeshisresearch withcomputer doesn’t evenneedacloudlesssky:Klaus Munich the UniversityObservatory Planck InstituteforAstrophysicsandat Moreover, thestaffmemberatMax the hiddenmatterinuniverse. weigh galaxyclustersortotrackdown and appliesthephysicalprincipleto sort. Hesimplyworkswiththeeffect– concern himselfwithevidenceofany mains. SoKlausDolagdoesn’t haveto perfectly confirmed–notadoubtre- since beenmeasuredmanytimesand nitude. Butthegravitationaleffecthas samemag- of, asitsohappens,thevery ently theresultofaninstrumenterror the prediction,butthattheyareappar- at thattimedid,indeed,correspondto findings showthatthevaluesmeasured More recentanalysesofEddington’s To understandthis,letustake an- CIENCE Klaus Dolag 1 | 11 in1936thatastar _Gravitational Lenses MaxPlanckResearch 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

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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. minimally different temperatures; the gravitational lens is composed of they were emitted 380,000 years after two merging clusters. This gives the re- INFLATION IS DRIVING THE the Big Bang and are being picked searchers insight into the structural for- UNIVERSE APART up today by satellites such as WMAP mation of the universe. and Planck. This aspect is of great importance: Conversely, the astronomers can draw The universe has been expanding “One of the things cosmology has to conclusions about the distribution of since inflation, but for the past ap- explain is why, after the Big Bang, such the masses within a gravitational lens, proximately five billion years, the rate things as galaxies and extended galaxy and distinguish the invisible and visi- has been much faster than expected. clusters in which stars and planets ag- ble components from one another. The mysterious driving force acts fur- glomerated emerged from what was “Gravitational lenses are the only di- tively and makes up around 73 percent originally quite an even distribution of rect method for determining the proof the energy density of space. A dis- matter,” says Klaus Dolag. Dark matter, portion of dark matter,” says Dolag. turbing concept: 73 percent dark ener- which makes up some 23 percent of And finally, gravitational lenses are gy, 23 percent dark matter – the cos- the universe, plays an important role precise tools for decoding the geome- mos is made up primarily of an here. The nature of this mysterious try of the cosmos. In the standard unknown substance. Baryonic matter substance is literally in the dark. model, space was born with a “bang” – making up our old familiar protons, As mentioned, the images also con- 13.7 billion years ago. Scientists prefer neutrons and electrons – accounts for

Photo: Axel Griesch tain information about the cluster act- to call this a singularity that can’t even just 4 percent; we see only this tiny

1 | 11 MaxPlanckResearch 67 MAPPING THE SKY

The universe is full of matter, both visi- the astronomers need the light from observe and numerous. For this reason, ble and, above all, invisible. On its way 200 different galaxies. This means that Ben Metcalf and Simon White from the to us, the light from distant galaxies the smallest area on which the “lensing” Max Planck Institute for Astrophysics in must traverse space and is diffracted by mass can be determined corresponds to Garching proposed radio waves from a the gravity of the matter contained a size of 0.2 percent of the surface area time when the galaxies weren’t yet even therein. This kind of gravitational lens- of the full moon. born. It seems this radiation really does ing effect distorts the images – the cos- That may seem like little, but trans- exist: some 100 million years after the mic landscape looks as if it were being lated to the dimensions of space, such Big Bang, the first stars and galaxies viewed through a carelessly polished a section shows only very coarse struc- formed from inhomogeneities in the piece of glass. When as many distor- tures, such as the largest galaxy clus- neutral hydrogen gas. Their UV light tions as possible have been observed, ters. Furthermore, it becomes that heated up the gas, which then emitted conclusions can be drawn about, for in- much more interesting for the cosmol- radio waves with a wavelength of 21 cen- stance, the mass of the matter in space ogists the further back in time they can timeters. Since then, space has expand- between us and the distant galaxies. look. To do this, however, the back- ed – and with it, the signals should have The more of these light sources are ground objects must be as far away as grown to wavelengths of 2 to 20 meters. observed, the more accurate this tech- possible, which means that they have According to the standard model of nique becomes. The best telescopes re- only a very weak glow and are thus dif- the Big Bang, the pregalactic hydrogen veal some 100,000 background galax- ficult to observe. exhibited various structures, such as ies in a section of sky the apparent size An alternative to background galax- node-like swellings from which galax- of the full moon. To reliably determine ies should therefore meet three condi- ies later formed. The pattern of the the image distortion caused by gravity, tions: they should be distant, easy to structures imprinted itself on the signals: according to Metcalf and White, there should be as many as 1,000 such structures at various distances. A radio telescope ought to be able to distinguish them and to create, from the dis- Matter causes tortions of the signals, a map of the image distortions large-scale distribution of cosmic matter. Its resolution would be 20 times better than the maps obtained using

Pregalactic 10-35 background galaxies. hydrogen Radio waves There’s just one catch: the radio telescope would have to be very large and occupy an area of 1,000 square kilo- meters – about 100 times as much as the central receiver of the new electronic telescope LOFAR (MaxPlanckResearch 4/2010, page 6). But in a few years, “fic-

Big Bang tion” could become “science”: as Ben Metcalf and Simon White have shown, the planned SKA radio system, with a collection surface of one square kilo- Light from meter, should already be able to inves- galaxies tigate, for example, the mysterious dark energy with greater precision than any instrument before it. Maybe the researchers would then be close to solv- Today Messages from the beginning of the world: ing one of the greatest cosmological A large radio telescope could record the mysteries and would know what is driv- electromagnetic waves of pregalactic hydrogen and provide highly precise maps of the ing the world apart at its core. distribution of mass in the universe. Helmut Hornung Graphic: MPI for Astrophysics – Ben Metcalf

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left If researchers feed their computers with such data as energy and matter content, density and expansion rate, then over time, their programs produce a filament-like network that resembles the structure of real space. right Such condensations of matter in the computer-generated cosmic network are considered to be seeds of galaxies and galaxy clusters.

portion with the naked eye or with tel- he takes not only masses and density Because the foreground galaxies act as escopes, as planets and stars, gaseous fields into account, but also dark mat- a gravitational lens, they influence the nebula and galaxies. ter and baryonic gas. From these ingre- images of the background galaxies; dients, his software reconstructs the their shape depends on the structure A NETWORK OF INTRICATE FIBERS development of large-scale structures of the lens. In this way, Dolag obtains EMERGES ON THE COMPUTER and the evolution of galaxies. Dolag’s a virtual gravitational lens that can be colleagues use a similar approach, for played with. He can then study how The researchers describe the universe as example in their famous millennium the images change, for example when a whole using a set of parameters that simulation. There, to a virtual cube of more or less mass is assumed, or the characterize the amount of energy and space having a side length of 2.1 bil- distances between the galaxies are matter, the density and the expansion lion light-years, they added no fewer changed. rate. A further key value is the cosmo- than ten trillion solar masses of dark logical constant, a form of time-invari- matter, spread among ten billion units THE LENS STRUCTURE ant dark energy. In the department of of virtual matter. INFLUENCES THE IMAGES Director Simon White, a group at the “Our simulations use even more Max Planck Institute for Astrophysics is physics,” says Klaus Dolag. “Because we Klaus Dolag shows images of his simu- using the ingredients mentioned above also take baryonic matter into account, lations, which seem so deceptively real to create the universe on a computer we can simulate the appearance of gal- that a layperson can’t even distinguish (MAXPLANCKRESEARCH 4/2006, page 46 axies and galaxy clusters in various them from images taken with large tel- ff.). To do this, the cosmologists start a wavelength ranges, such as in X-ray escopes. In fact, the programs also in- few hundred thousand years after the light.” The artificial cosmos is formed clude instrument data: the diameter of Big Bang and watch how, over time, from essentially two layers: a few hun- the telescope mirror, spectral ranges the computer spins a network of intri- dred to a thousand foreground galaxies and filter properties, and exposure cate fibers and pearl-like bulges, the and countless background galaxies dis- times for the images. seeds of galaxies. tributed according to a model that cor- All this effort is not an end in itself. Klaus Dolag uses simulations, too. responds to nature. Superimposing the One group, of which Klaus Dolag was That is, he uses computer programs to two layers on the computer yields an also a member, simulated observations

Photos: Klaus Dolag (2) build gravitational lenses. To do this, image of a virtual section of sky. of a galaxy field such as might one day

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be possible with the space telescope galaxies in a cluster, there is hot gas clusters were distributed evenly; the Euclid planned by the European Space that emits X-rays. The observations more “streaks” there were in the lens- Agency (ESA). One of the scientists’ can be used to create a mass profile of es, the larger the errors became. Also, aims was to measure arcs – their length, the galaxy cluster – assuming that the the true structure of the galaxy clusters thickness and curvature. It was hoped distribution of the intergalactic gas in space – in other words, their shape that these virtual gravitational lensing can be described with simple models in all three dimensions – likewise influ- effects would offer valuable informa- and that it is in hydrostatic balance, enced the results; for instance, the mass tion about the effects actually ob- or in other words, that gravity keeps of a lens is generally overestimated served in space. it in form. when its optical axis points directly to- “We found interesting connec- ward the observer. “Using such simulations,” says Dolag. “For example, the ERRORS INCREASE NEAR tions, we test how the models for de- length-to-width ratio of the arcs is a THE EDGE OF THE CLUSTER scribing the distribution of matter in good indicator and permits conclu- gravitational lenses need to be im- sions about the distribution of mass in “In practice, there are problems, since proved in order to approximate reality a gravitational lens.” The simulations there are often discrepancies between as closely as possible,” says Klaus Dolag. also showed that observation effects the results of the two methods,” says But, thanks to their simulations, can influence the measurement of the Dolag. His team thus created three gal- the researchers also found a few error arcs. Dolag says: “We will have to take axy clusters and first simulated obser- sources in the X-ray observations. If, that into account when interpreting vations with an artificial X-ray tele- for example, the gas between the gal- future observations.” scope and then images with a axies was not exactly in the aforemen- So it’s about testing models on re- gravitational lens. In the center of the tioned hydrostatic balance, then the ality. Or comparing different measure- cluster, strong lensing effects appeared results deviated from reality by as ment methods with each other. In an that made it possible to determine the much as 20 percent. essay that recently appeared in the mass fairly precisely. However, extrap- “Comparing the two methods on a journal ASTRONOMY & ASTROPHYSICS, olating from the measurements near case-by-case basis is indeed rather Klaus Dolag and his colleagues address the edges of the cluster, where weak complicated. Nevertheless, observing the issue of determining the mass of lens effects appeared, led to relatively many clusters will give us valuable galaxy clusters. There are two main large errors. clues about the physics of the gas be- options available: gravitational lenses Furthermore, with weak lensing ef- tween the galaxies. Not least, we and X-ray observations. The latter are fects, the measurements worked best should be able to calibrate the X-ray based on the fact that, between the when the masses in the lensing galaxy measurements based on the results

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CP2

1 Color composite of a simulated galaxy cluster. For this photo, scientists reconstructed the instrument properties of the Subaru telescope at the Mauna Kea Observatories in Hawaii. 2 The same virtual galaxy cluster as in photo (1), Rc this time viewed through the – likewise simulated – CP1 CC Hubble Space Telescope. 3 The strong gravitational lensing effect distorts the images from background galaxies into arcs. Their shape permits conclusions about the lenses (usually galaxy CP3 clusters). Such images produced on the computer 3 help researchers better assess and analyze the data obtained in nature.

from gravitational lensing effects,” the development of the universe as a searchers don’t have to travel to exot- says Klaus Dolag, summarizing the whole will become. Even now, the ic locations as they did in 1929 to outcome of their work. physics of gravitational lenses is show- make big discoveries, exploring space ing initial approaches to examining is still an adventure – even if doing so COSMIC ZOOM different models and testing cosmo- requires only the right algorithms and ON DISTANT OBJECTS logical parameters. Although the re- fast computers.

The more closely the astronomers scan the universe with their telescopes, the more gravitational lenses they find. GLOSSARY Special search programs like the Cosmic Evolution Survey (COSMOS) and the Cosmological constant Planck Sloan Lens ACS Survey (SLACS) are un- A positive constant introduced by Albert The European probe that was covering dozens of new objects for the Einstein (1879 to 1955) in his gravitational launched in May 2009 together field equations to prevent the contraction with the infrared Herschel observa- scientists. These also find such exotic of the universe due to gravitational attrac- tory is expected to closely examine specimens as the three different color- tion. In 1998, (lambda) experienced a the cosmic microwave background, ed arcs that are grouped around a gal- renaissance when the rapid expansion of but with much greater precision axy; in this system, it is possible that space was discovered. It is still not clear than WMAP. The first complete two lenses are acting together and what causes . image of the sky was published in summer 2010. zooming in to the image of an im- Millennium simulation mensely distant galaxy. This would al- Researchers use supercomputers to investi- WMAP low the astronomers to glimpse that gate the question of how today’s galaxies The US satellite Wilkinson Microwave and stars were able to form from what Anisotropy Probe was launched in much further back into the past (see was, directly after the Big Bang, a formless 2001 and transmitted data to the box on page 68). universe. With the appropriate ingredients, Earth up until a few months ago. In parallel with improving the de- such as dark matter, the simulation does The aim of the mission was to inves- tection methods, scientists such as indeed show the emergence of major irreg- tigate cosmic background radiation, ularities when small manipulations are in- and especially irregularities stem- Klaus Dolag are fine-tuning their sim- troduced. Cosmologists from Germany, the ming from large-scale cosmic ulations. The more exact their compu- UK, Canada, Japan and the US are partici- structures. The famous WMAP map ter reproductions of the cosmos are, pating in the project, which is being led by shows the universe as it appeared the more accurate their statements the Max Planck Institute for Astrophysics. 380,000 years after its birth.

Photos: Massimo Meneghetti – Osservatorio astronomerico di Bologna (3) about the evolution of the galaxies and

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