Mirages in the Universe

INTERNATIONAL SPACE SCIENCE INSTITUTE SPATIUM Published by the Association Pro ISSI No. 32, November 2013 Editorial

When it comes to astrophysics, one blur reality, they have evolved in Impressum can’t go past Albert Einstein. His the hands of astrophysicists into a overwhelming intellectual author- powerful tool to detect the pres- ity endowed science with a series ence and distribution of dark of landmarks, of which the Gen- matter or to estimate the age of eral Theory of Relativity perhaps the Universe, just to name two SPATIUM stands out most. Upon publishing notable examples. Published by the the Special Theory of Relativity in Association Pro ISSI 1905 he embarked on an eight-year All those exciting topics made up search for a relativistic theory of the milestones of the talk on gravity. After numerous detours Mirages in the Universe by Profes- and false starts, his work culmi- sor Georges Meylan, Director of nated in the presentation to the the Laboratory of Astrophysics, Association Pro ISSI Prussian Academy of Science in École Polytechnique Fédérale de Hallerstrasse 6, CH-3012 Bern November 1915 of what are now Lausanne for the Pro ISSI audience Phone +41 (0)31 631 48 96 known as the Einstein field equa- on 22 March 2012. It is with great see tions. These equations specify how pleasure that we publish herewith www.issibern.ch/pro-issi.html the geometry of space and time is an issue of Spatium entirely de- for the whole Spatium series ruled by matter and gravity, and voted to one of astronomy’s most form the core of his General The- intriguing aspects, the mirages in President ory of Relativity. the Universe. Prof. Nicolas Thomas, University of Bern Even though the theory is clearly Hansjörg Schlaepfer superior to Newtonian gravity, Brissago, November 2013 Layout and Publisher General Relativity remained some- Dr. Hansjörg Schlaepfer thing of a curiosity among physi- CH-6614 Brissago cal theories for several decades. This didn’t change even when the Printing deflection of starlight by the Sun, Stämpfli Publikationen AG as predicted by General Relativity, CH-3001 Bern was observed by Sir Arthur Stan- ley Eddington on an expedition during the total solar eclipse of 29 May 1919: Einstein became in- stantly famous, while his theory remained a Sleeping Beauty.

This did change, however, in the 1960’s and 1970’s when ever more precise solar system tests confirmed its predictive power. One of the theory’s most popular forecasts re- fers to gravitational lensing, the effect observed by Eddington in 1919. Even though gravitational lenses tend to create mirages that

SPATIUM 32 2 Mirages in the Universe1 by Prof. Georges Meylan, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL)

Introduction

Mirages in the Universe? Well, we have heard about mirages on Earth, such as the Fata Morgana, the flickering apparitions in the hot desert. But mirages in the Uni- verse? Do such elusory features ex- Fig. 1 (left): The Moon setting in Fig. 2 (right): The image of the set- ist in space too, and if this is the Casco Bay in Maine. The Moon’s shape ting Sun is perturbed in a similar way. is disturbed near the horizon as a con- This image displays even the elusive case, how do they form, and fur- sequence of refraction effects by the green flash at the top of the solar disk. ther: are they more than just an Earth’s atmosphere. The image of a (Credit: Karina Hamalainen) exotic oddity amusing astro- second Moon rises slowly towards the physicists? first, both eventually merging during setting. (Credit: John Stetson). The answer is yes, of course, and the present issue of Spatium aims age of a second Sun or Moon even- duced by the presence of huge ag- at providing our readers with an tually merging with the first during gregations of mass. What makes insight into this fascinating field of setting. This may lead to the fancy them most valuable for scientists is astronomy. In the framework of Etruscan Vase moonset as shown in the fact that not only the mass of his General Relativity Theory in Fig. 1 and the colourful sunset of visible matter, but also that of in- the mid-1910’s Albert Einstein2 Fig.. 2 visible dark matter gives rise to postulated the existence of mirages mirages. Hence, mirages help open long before they were actually ob- In the Universe, other effects give the window towards the dark served. Today, scientists do not rise to mirages. Here, it is the cur- Universe. only enjoy their beauty but also vature of space-time, a phenome- have learned to exploit their scien- non described first by Albert Ein- Fig. 3: The Horseshoe Einstein ring. tific potential as a powerful tool to stein, that distorts our vision. This The gravity of the luminous yellow galaxy in the foreground has distorted observe the Universe. First, let us may take such fascinating forms as the light coming from a much more dis- contemplate some mirages we are the famous Einstein ring of which tant blue galaxy. In this case, the align- all familiar with. an example is shown in Fig.. 3 The ment is so precise that the background amazing blue ring is not a real galaxy is imaged as a nearly perfect ring or a horseshoe. As such lensing effects On Earth, mirages may appear physical entity in space, but rather were predicted by Albert Einstein, rings when light is bent by layers of air the image of one single object be- like this are now known as Einstein with different temperatures over hind the yellow galaxy in the fore- Rings. (Credit: NASA/ESA) land or over the sea. Light propa- ground. Its enormous mass causes gates in air with a speed that de- the light rays from the blue back- pends on the air density and hence ground object to be bent in such a its temperature. Warm air imme- way as to form a regular, nearly diately above the ocean or the perfect ring around the galaxy. terrain may cause the Sun or Moon’s rays to be mirrored up- We conclude that mirages exist in wards producing the inverted im- the Universe and that they are in-

1 The present issue of Spatium reports on the lecture given by Prof. Meylan for the Pro ISSI Association on 22 March 2012. The notes were taken by Dr. Hansjörg Schlaepfer. 2 Albert Einstein, 1879, Ulm – 1955, Princeton, New Jersey, Swiss-US physicist, Nobel prize laureate in physics, 1922.

SPATIUM 32 3 Stars come also in entire families The Mechanism as in the case of the cluster of stars of Mirages of the Pleiades that contain a few thousand stars, Fig. 5. As this is a young cluster of stars, there is still much remaining gas in between. Let us now come to the facts that help us to understand the physical Other stars come in entire globu- processes that create mirages in the lar clusters that contain not a few Universe and to appreciate their thousand but rather a few million value for astrophysicists. To this Fig. 4: Beta Albireo. The two bright stars, such as Omega Centauri, end, we begin with enumerating stars of Albireo in the Constellation of Fig. 6. the Swan (Cygnus) are some 380 light- the forms and types of matter in years away. It is the fifth brightest star the Universe. system in the sky and easily visible to Much visible matter in the Uni- the unaided eye. They circle around verse is also contained in dust and each other in about 75,000 years. gas in large picturesque structures, (Credit: Giuseppe Donatiello) Matter in Space such as for instance the Orion nebula, Fig. 7 on the opposite page. From our everyday experience, we axies. A good example of a star is know what astrophysicists call (or- of course our Sun, but also Beta Fig. 7: The Great Orion Nebula is an immense, nearby star-birth region, lo- dinary) baryonic matter. It consti- Albireo in the constellation of cated in the same spiral arm of our tutes the visible performers of the Cygnus, see Fig.. 4 It is actually a Galaxy as the Sun. The various colours theatre of the Universe. Those are double star where the two partners highlight the emission of different gases. the stars with their planets, clusters have different temperatures and The nebula contains many stellar nurseries where gas and dust are cur- of stars, galaxies with a lot of gas hence different colours in the rently forming hot young stars3. (Credit: and dust and entire clusters of gal- visible. NASA/ESA)

Fig. 5: The Pleiades can be seen with the naked eye in the Fig. 6: Omega Centauri: the globular star cluster NGC 5139 Orion Constellation. It is one of the brightest clusters in the is over 10 billion years old. It holds nearly than ten million sky counting over 3,000 stars. Huge clouds of dust and gas stars in a sphere some 150 light-years in diameter. (Credit: surround the stars constituting the raw material that eventu- ESA/NASA) ally will build up new stars. (Credit: NASA/ESA)

3 See Spatium no. 6: From Dust to Planets by Willy Benz, October 2000.

SPATIUM 32 4

Then, we have galaxies of all types: came aware of this problem and Fig. 9: Andromeda: Our nearest mixed forms like the stunning was among the first to stipulate the large galactic neighbour. Two ESA 5 observatories have combined forces to Sombrero M104 (Fig. 8) or beauti- existence of dark matter , a strange show the Andromeda Galaxy in a new ful spiral galaxies, like Andromeda form of matter that cannot be seen light. The Herschel spacecraft’s tele- (Fig. 9, at right). yet produces gravitational attrac- scope sees rings of star formations in tion like ordinary matter, enough this, the most detailed image of the Andromeda Galaxy ever taken at infra- Now, all these wonderful celestial to keep the stars in the galaxies on red wavelengths shown here in redish/ objects are made of visible bary- their regular orbit. We have, thus, yellow hues. The XMM-Newton’s tele onic matter, the same as you and two components in the Universe scope shows dying stars shining x-rays me. Yet, careful analysis of the that generate gravity fields and into space in bluish tones. (Credit: ESA) dynamics of galaxies provides the hence possibly give rise to mirages: puzzling result that the visible mat- visible baryonic matter and dark ter contained in the galaxy cannot matter. Yet, to understand the generate enough gravitational pull mechanism fully, we have to look to counteract the centrifugal force at the behaviour of space in the vi- acting on the stars that circle cinity of very massive aggregations around the galactic centre. It was of matter, where gravity fields the Swiss astrophysicist Fritz reach extremely high strengths. Zwicky4 who in the late 1940’s be-

Fig. 8: The M104 Galaxy (Sombrero) ters. Billions of stars cause the diffuse very centre of the Sombrero glows is named for its hat-like resemblance. It glow of the extended central bulge. across the entire electromagnetic spec- features a prominent dust lane and a M104’s spectacular dust rings harbour trum and is thought to house a large bright halo of stars and globular clus- many younger and brighter stars. The black hole6. (Credit: NASA/ESA)

4 Fritz Zwicky, 1898, Warna, Bulgaria – 1974, Pasadena, USA, Swiss astronomer and physicist. 5 See Spatium no. 7: In Search of the Dark Matter in the Universe by Klaus Pretzl, May 2007. 6 See Spatium no. 28: How Black Are Black Holes? by Marizio Falanga, December 2011.

SPATIUM 32 6

The Curvature of Space-Time Testing Einstein’s General slightly deferred. According to Relativity Theory Einstein, however, the solar mass It was Albert Einstein who, in his causes enough gravity to curve the General Relativity Theory recog- Upon publication in 1915, the space-time in the Sun’s vicinity nized that very massive objects, General Relativity Theory did not and hence to bend the light rays such as the Sun or entire galaxies, readily find approval by the scien- from the stars behind the Sun, this tend to curve the space time geo tific community. For example, in with an intensity twice as large as metry. This means that the path of 1922 the Nobel prize committee in the case of the Newtonian light does not follow a straight line preferred to assign Einstein the dynamics. from its origin down to the ob- award for other findings7. Yet it server but rather is bent by the stirred up enough interest for sci- It was the British scientist, Sir Ar- gravity of such aggregations of entists to conceive elaborate tests thur Eddington8 who was commis- mass. Fig. 10 depicts this effect sche- to check the theoretical predic- sioned to undertake an expedition matically reduced to two dimen- tions. The first occasion came on to the Principe Island in the Afri- sions: the light rays from the back- 29 May 1919 when a complete so- can Gulf of Guinea, where the ground object A are bent by the lar eclipse was forecast that could eclipse would be complete. Com- Sun’s gravity field at the centre. be seen in parts of the world. Dur- The observer at point B will see ing an eclipse, the Moon’s disk the background object twice at dif- covers the disk of the Sun thereby ferent places, of which neither one blocking its overwhelming lumi- Fig. 10: Following Einstein’s Gen- is correct: those are the mirages in nosity. This allows scientists to ob- eral Relativity, space-time is curved by the Universe. We have, hence, to serve the fainter stars in the back- massive objects such as the Sun. This conclude that it is the curvature of ground of the Sun. Now, according graph shows the effect reduced to two space-time that generates the mi- to classical Newtonian physics, the dimensions: an observer at B will observe the background object not at its rages in the Universe. light rays from stars in the back- true place A, but rather at two distinct, ground pass by the Sun and are apparent places.

7 Albert Einstein was named Nobel prize laureate in 1922 honouring his theory on the photoelectric effect published 17 years earlier. 8 Sir Arthur Stanley Eddington, 1882, Kendal, Great Britain – 1944, Cambridge, British astrophysicist.

SPATIUM 32 8 paring the image of the stars be- From Glass Lenses to Now, scientists are inventive: in- hind the Sun during eclipse with Gravitational Lenses stead of coming to terms with those their image taken when the Sun limitations, they try to exploit their was elsewhere would reveal the ef- The effect of a gravitational lens scientific potential. The Laboratory fect of light bending as stipulated as sketched in Fig. 11 strongly re- of Astrophysics at the EPFL9 has put by Einstein. The sensation in the sembles the bundling of light rays gravitational lensing into the focus world’s science community – and by an ordinary lens. In both cases of one of its research fields. Despite not only – was complete when Ed- shown in Fig. 11 incoming light rays their poor imaging properties, dington confirmed Einstein’s pre- are deviated from their original gravitational lenses are scientifically dictions. That was the very first path. In the case of the lens, the dif- attractive: as outlined above, it is not experimental validation of the ference of the refraction index be- only the visible baryonic matter that General Theory of Relativity after tween the lens material and that of may give rise to gravitational lens- the solution of the precession of air gives rise to the deflection. In ing but dark matter as well. This Mercury’s perihelion. The shift of space, aggregations of mass curve allows scientists to observe parts of the light rays near its limb is equal space-time which in turn leads to the invisible Universe. Even more: to 1.75 seconds of arc, which is the deviation of the light rays in a while ordinary visible matter ac- small but was already measurable very similar way. This is why the counts for a mere 5% of the matter at that time. One arcsec compares effect is called gravitational lensing, in the Universe, dark matter with a Swiss franc at a distance of a term first introduced by Albert amounts to some 27%, conse- 4.7 km. Einstein. Any massive aggregation quently gravitational lensing pro- of mass, be it a star like the Sun, or vides the key to fundamentally new So, we conclude that Eddington a spiral galaxy, or any other large observations from which we can was the first to witness mirages in object, will act as a gravitational draw important cosmological con- the Universe and that the Sun was lens. However, in contrast to glass clusions that cannot be made based the first gravitational lens ever optics that may be of excellent on observing visible matter alone. observed. quality, gravitational lenses possess Though, before entering this as- more than just one focal point in pect, we will address some histori- general and will hence produce cal aspects of gravitational lensing. blurred images. This makes their interpretation a little more compli- cated than those of an optical lens.

Fig. 11: From glass lenses to gravitational lenses. In a glass lens, paral- lel rays of light from a distant object are bundled and brought to a focus. In a gravitational lens, a massive aggregation of mass, such as a spiral galaxy, causes the curvature of space-time which in turn leads to the focusing of the incoming light from a distant object.

9 École Polytechnique Fédérale de Lausanne.

SPATIUM 32 9 The History of much stronger and more frequent and spectra. “Difficulties arise in de- Gravitational Lensing than the alignment of two stars. scribing them as two distinct objects,” This brought him to the conclu- the researchers argued. “Gravita- sion, that the case of a star lensing tional lensing offered a more likely ex- After successfully observing the another star offers an ideal but use- planation: light from a single source, af- Sun’s gravitational lensing effect, less test of General Relativity, but ter travelling along distinct, bending the question arose whether other he thought it highly probable that paths through the gravitational field of such examples might exist in the the effect would be observed in- some large intervening object, was ar- Universe. The answer is yes, of volving two galaxies. He was per- riving at the Earth from two slightly course; and they were soon inves- fectly right. Yet, partly as a conse- different directions. tigated by Orest Khvolson10 in quence of World War II, science 1924 and Einstein 1936. Einstein needed another 60 years to find the Fig. 12 shows the underlying idea: was considering the lensing effect second example of a gravitational the observer at left looks at the qua- between two stars in perfect align- lens. sar QSO at right. A galaxy in be- ment with the observer which tween acts as a lens constructing ideally would lead to the so-called In the meantime, technology had two images of the quasar. The Einstein ring (Fig. 3). In this ar- made major progress: Charge Cou- spectra of both images are identi- rangement, the background object pled Devices (CCDs)11 came up cal, which proves that the two im- appears as a ring around the fore- that replaced the photographic ages originate from the same ob- ground star provided that the mass plates for astronomical observa- ject. Initially, part of the scientific distribution of the lensing object is tions. Furthermore, in the early community was sceptical about this fully symmetric around the opti- 60’s, quasars12 were discovered, revolutionary interpretation, today, cal axis. Yet, he concluded that that are very bright nuclei of very however, we know, that QSO there is no hope of observing this distant galaxies. With these assets phenomenon, as the angle covered at hand, the probability became by the ring would be of the order high that further gravitational of some milli-arcseconds which lenses could be observed. In fact, was far beyond the technological this happened in 1979 when the capabilities at that time. Conse- first extragalactic gravitational lens quently, he argued that the proba- at cosmological distances was bility of finding such a perfect found13 – by chance. Walsh, alignment was practically equal to Carswell and Weymann were ob- Fig. 12: A galaxy in the centre acts as a gravitational lens producing two im- zero. serving two quasars that looked ages of the quasar QSO 0957+561 in suspiciously similar. Known as the distant background. Note that the In 1937 while working at the Cali QSO 0957+561 A and B, and sep- different paths of the two images have fornian Institute of Technology, arated by only 5.7 arc seconds in different lengths in general giving rise to different time lapses between emis- Fritz Zwicky found that the effect the sky, the two objects had nearly sion at the quasar and arrival at the involving two galaxies should be identical magnitudes, redshifts14 observer.

10 Orest Danilovich Khvolson, 1852, Saint Petersburg – 1934, Leningrad, Russian physicist. 11 Charge-Coupled Devices are light sensitive electronic sensors that are used to generate electronic images. The first such devices were built in 1970. 12 The name quasar stems from the compound “quasi-stellar radio source”. 13 Walsh, Carswell and Weymann in Nature 279, 381–384 (1979). 14 When a light emitting body recedes fast enough, an observer will see its light shifted towards the red as a consequence of an effect similar to the Doppler effect. This is called redshift. The redshift is a measure for the speed at which a body apparently moves away from the observer.

SPATIUM 32 10 0957+561 A and B constitute a We have shown two different cases one looks very carefully at this im- most valuable confirmation of Ein- of an alignment of the observer, age one finds several thousand such stein’s General Relativity Theory. the lens and the source. If the dis- images with a great variety of tribution of matter of the lensing shapes. One is even creating parts Since 1979, when this second ob- galaxy is symmetric along the op- of an Einstein ring. Abell 1689 is servation of a gravitational lens was tical axis, the image will take the of utmost scientific value as one made, scientists have accumulated form of an Einstein ring around the can measure all the images to re- a few hundred of such observa- lensing galaxy. If, however, there construct the distribution of mat- tions. One of the most famous ex- is a slight asymmetry, then the re- ter causing the observed images. amples is the Einstein Cross shown sult will be an Einstein Cross. Yet, the theoretic modelling pro- in Fig., 13 where the nucleus of a cess turns out to be quite demand- spiral galaxy at a distance of 8 bil- Another beautiful example of a ing: it can be compared with the lion light years is imaged four times gravitational lens is the cluster of problem of identifying the speed by a massive lensing galaxy located galaxies called Abell 1689. The and the mass of a car after an acci- 400 million light years away in the bright galaxies of this cluster are dent by looking at the damaged car foreground. As all four images massive and hence their sum – the and analysing the damage caused have exactly the same spectra, the cluster – constitutes a powerful by another car. Careful analyses of hypothesis that they are four dif- gravitational lens. In Fig. 14 (on the Abell 1689 have provided a clear ferent objects is completely ruled next page) the galaxies appear yel- understanding of what is happen- out. It is therefore a beautiful ef- low, while the blue galaxies are the ing out there. In addition, it is one fect of gravitational lensing. images of background galaxies. If of the best examples to prove the presence of dark matter: without injecting dark matter in the theoretical model one cannot repro- duce the effects of gravitational lensing as we can observe them.

So, we come to the conclusion, that the Sun is not the only case of a gravitational lens, but rather that the Universe is full of such lenses. In every direction one looks out into space, there is matter at different distances distorting our vision of the background. The cartoon in Fig. 15 brings it to the point: While at first it seems a bit exaggerated (which is the right of a good cartoon), it is an astronomer’s daily reality. The whole sky as we can see it, is completely perturbed by

Fig. 13: The Einstein Cross QSO 2237+0305. A galaxy in the foreground creates four different images A, B, C and D of the same distant quasar in the background. (Credit: NASA/ESA)

SPATIUM 32 11 gravitational lensing. Gravitational Applications of trinsic luminosity of any distant lenses have become an interesting Gravitational Lensing background might be exploited to tool as we are going to show that end. Refsdal was thinking of below. the observation of Supernovae Let us now look at two examples since quasars had not yet been of studies undertaken in the Labo- discovered at that time. If a quasar ratory of Astrophysics at EPFL to can be observed behind a lensing exploit the effect of gravitational object, then its light will reach us lensing for astrophysical purposes. on different paths with different The first is the determination of lengths, as outlined in Fig. 12. the Hubble constant15, which is a Therefore, there will be a time de- measure of the expansion rate of lay between the light reaching us the Universe and its age. on the different paths. This time delay has two constituents: The Fig.14 at left: Abell 1689, one of the In the early 1960’s, the Norwegian geometric term tgeom represents the most massive objects in the Universe, is 16 also one of the most effective gravita- astrophysicist Sjur Refsdal estab- delay induced by the longer light tional lenses. (Credit: NASA/ESA) lished that any change in the in- path as compared to the shorter

one. The gravitational term tgrav Fig. 15: An astronomer’s nightmare. (Credit: Sidney Harris) represents the delay due to the relativistic time dilation induced by the gravitational field of the lensing object. This arrangement permits us to determine the absolute distance between the observer and the quasar in the background. Yet, this is not all: the quasar spec- trum will be characterized by a redshift caused by its recession speed relative to the observer. Therefore, one can, in addition to the quasar’s absolute distance, also measure its escape velocity. This in turn allows us to estimate the rate of expansion of the Universe. Ex- trapolating back in time gives a value for the age of the Universe (13.7 billion years17).

15 The Hubble constant H0 is named after the US-American astronomer Edwin Hubble. It constitutes one of the most fundamental parameters in cosmology describing the present rate of expansion of the Universe. Since its discovery and first measurement by Georges Lemaître in 1927 the value has continuously been changing as a consequence of the ever im- proving observational means. The reciprocal value 1/H0 is called Hubble time referring to the age of the Universe in the case of a void Universe. Normal (baryonic) matter, dark matter and dark energy may, according to their intrinsic nature and importance, accelerate or decelerate the expansion. 16 Sjur Refsdal, 1935, Oslo – 2009, Oslo, Norwegian astrophysicist. 17 See Spatium no. 1: Die Entstehung des Universums by Johannes Geiss, April 1998.

SPATIUM 32 13 The first quasar enjoying this pro- Initiated and led by EPFL, the in- benefit from the clear sky there cedure was the well-known QSO ternational Cosmograil18 pro- and our common research 0957+561. Fig. 16 shows the lumi- gramme is aimed at providing up- programme. nosity curves of the quasar’s two dated values for the age of the images as a function of time. After Universe. In this context, we are It is worth mentioning that such a relatively flat part, a huge in- currently observing about 30 such observations are very demanding crease can be seen followed by a time delays from very different and some may even turn out to be decrease of similar extent. The places such as Chile, La Palma, Uz- completely useless. Fig. 17 for in- lower line shows the luminosity of bekistan and Bangalore. For deter- stance shows a quasar with two im- image B, with very similar features mining the quasars redshift we ages. Yet, their luminosity curves to image A, but with a time delay use amongst others the Very Large do not reveal any similarity from of about 470 days. After careful Telescope19, the Keck Observa- which to determine a trustworthy analyses of this quasar’s data – and tory20, and for the clearest images time delay. The interpretation of no less complex modellization – of course the NASA/ESA Hubble this result is that unseen stars pass- scientists reached a value of H0 of Space telescope to identify the ing by the lensing galaxy cause dis- 67 km s–1 Mpsc–1 for the Hubble images exact positions. So, we have turbing micro-lensing effects that constant. colleagues in all these spots that completely destroy any informa- tion regarding the time delay. Af- Fig. 16: Luminosity over time of the two images of quasar QSO 0957+561. ter years of intense observation, Separated by a time delay of 470 days they are nearly identical. This time delay together with the quasar redshift allows for estimating the rate of ex pansion of the we had to discard all the data of a Universe and consequently its age. (Credit: Haarsma et al., 1997, ApJ, 479, 102) few quasars. Yet, it told us also that we have to be very careful while interpreting our observations and insist on the required long observation time to exclude potential mystifications. It is only after observing over a few seasons of the quasar, that such detrimental effects can be identified safely.

A second area where the Labora- tory of Astrophysics at EPFL is ac- tive relates to the forthcoming ESA Euclid mission scheduled for launch in 2020. It aims to measure the geometry of the Universe. It is now assumed that ordinary visible matter makes up only about 5% of the Universe, and that a further

18 COSMOGRAIL is an acronym for COSmological MOnitoring of GRAvitational Lenses: It involves eight teams grouped in five nodes depending on their respective country, namely Switzerland, Belgium, England, Uzbekistan and India. 19 The Very Large Telescope (VLT) is a set of four telescopes operated by the European Southern Observatory (ESO) of which Switzerland is one of the founding members. It is based on Cerro Paranal in the Atacama Desert of Northern Chile. Each telescopes has a primary mirror 8.2 m across. 20 The US-American W. M. Keck Observatory is placed on top of Hawaii Mauna Kea volcano 4,145 m above sea level. The two telescopes’ primary mirrors are 10 meters in diameter.

SPATIUM 32 14 27% is dark matter. The rest is be explained with current knowl- Conclusion thought to be dark energy. This edge of fundamental physics. This strange component had to be in- opens a wide field of activity for troduced in cosmological models future generations of space The phenomenon of gravitational some 15 years ago when, in con- scientists … lensing provides fascinating cosmic trast to what one would expect, it Euclid is to map the geometry of mirages. Yet, much more impor- was found that the rate of expan- the dark Universe. The mission tant, it provides a set of powerful sion of the Universe is accelerating. will investigate the distance-red- tools for studying the Universe. It Dark energy is assumed to be re- shift relationship and the evolution allows for cosmological measure- pulsive and hence may be the cause of cosmic structures. It achieves ments of fundamental parameters for the accelerated expansion. this by measuring shapes and red- related to dark matter and dark en- This leads to the uncomfortable shifts of about 1.5 billion galaxies ergy essential for furthering our fact that the mass-energy budget and clusters of galaxies out to red- understanding. Together with our of the Universe is dominated by shifts of about 2, which is equiva- partners in Switzerland and abroad dark energy and dark matter of lent to a look-back time of 10 bil- we are strongly committed to pro- which science has no sound obser- lion years. It will therefore cover viding major contributions to this vational explanation up to now. As the entire period over which dark fascinating field of science. these constituents account for 96% energy is thought to have played a of the energy and matter in the significant role in accelerating the Universe this is very unsatisfying. expansion. Together with national Even worse: the existence and the and international partners, the energy scale of dark energy, which Laboratory of Astrophysics, EPFL, accounts for the majority (68%) of is engaged in contributing to the the energy in the Universe, cannot Euclid mission. With more than 1,000 scientists now involved from Fig. 18. The luminosity evolution of across Europe and other parts of the four images of the quasar RXS Fig. 17: The luminosity evolution of the world, the Euclid Consortium J1131-1231. The high-quality photom- the quasar’s J0158-4325 two images etry over a period of 8 years combined shows the effect of micro-lensing from is the biggest astronomy collabora- with few fluctuations due to micro- unknown masses passing by in the tion ever created and is already big- lensing make this quasar an ideal case foreground. Such data cannot be used ger than those existing for any for time delay measurement and Hub- for determining the time delay, yet other ESA mission. ble constant determination. In the (fortunately) for other applications. framework of our EPFL-Cosmograil (Credit: EPFL-Cosmograil, in collab- collaboration, we have more than 20 oration with C.S. Kochanek, C. Mor- quasars with such exquisite light curves. gan, L. Hainline (OSU & USNA) (Credit: Tewes et al. 2013 A&A 556 22)

SPATIUM 32 15 SPATIUM

The Author

Georges Meylan completed his PhD thesis in astrophysics in 1985 at the Astronomical Observatory of the University of Geneva. Then, he spent some years as a postdoc at the University of California in Berkeley, USA, and at the Head- quarters of the European Southern Observatory (ESO) in Munich, Germany. He then held senior astronomer positions at ESO in Munich and at the Space Telescope Science Institute (STScI) in Balti- more, USA. Since 2004, he has occupied the chair of astrophysics at EPFL and is the director of the Laboratoire d’astrophysique of the Ecole Polytechnique Fédérale de Lausanne (EPFL).

His research interests are related to observational cosmology, includ- ing the phenomenon of gravitational lensing, quasars and their host galaxies, the formation and evolution of galaxies from the early Universe to the present time, stellar dynamics and stellar popula- tions from the nearby to the most distant galaxies.

Professor Meylan serves in many scientific organizations and com- mittees, not least ISSI’s science committee.