On 7 August 1912, Victor Hess took a now famous balloon flight in which he observed a “clearly perceptible rise in radiation with increasing height” and concluded that “radiation of very high penetrating power enters our atmosphere from above”.
This issue of the CERN Courier marks this discovery of cosmic rays with a look at cosmic-ray research in the past as well as at its future directions.
“We took off at 6.12 a.m. from Aussig on the Elbe. We flew over the Saxony border by Peterswalde, Struppen near Pirna, Birchofswerda and Kottbus. The height of 5350 m was reached in the region of Schwielochsee. At 12.15 p.m. we landed near Pieskow, 50 km east of Berlin.”
The flight on 7 August 1912 was the last in a series of balloon flights that Victor Hess, an Austrian physicist, undertook in 1912 with the aid of a grant from what is now the Austrian Academy of Sciences in Vienna. The previous year, he had taken two flights to investigate the penetrating radiation that had been found to discharge electroscopes above the Earth’s surface. He had reached an altitude of around 1100 m and found “no essential change” in the amount of radiation compared with observations near the ground. This indicated the existence of some source of radiation in addition to γ-rays emitted by radioactive decays in the Earth’s crust.
For the flights in 1912 he equipped himself with two electroscopes of the kind designed by Wulf, which were “perfectly airtight” and could withstand the pressure changes with altitude. The containers were electrolytically galvanized on the inside to reduce the radiation from the walls. To improve accuracy the instruments were equipped with a new “sliding lens” that allowed Hess to focus on the electroscopes’ fibres as they discharged without moving the eyepiece and hence changing the magnification.
Hess undertook the first six flights from his base in Vienna, beginning on 17 April 1912, during a partial solar eclipse. Reaching 2750 m, he found no reduction in the penetrating radiation during the eclipse but indications of an increase around 2000 m. However, on the following flights he found that “the weak lifting power of the local gas, as well as the meteorological conditions” did not allow him to ascend higher.
So, on 7 August he took off instead from Aussig [today Ústí nad Labem in the Czech Republic], several hundred kilometres north of Vienna. Although cumulus clouds appeared during the day, the balloon with Hess and the electrometers were never close to them; there was only a thin layer above him, at around 6000 m. The results of this flight were more conclusive. “In both γ-ray detectors the values at the greatest altitude are about 22–24 ions higher than at the ground.”
Before reporting these results, Hess combined all of the data from his various balloon flights. At altitudes above 2000 m the measured radiation levels began to rise. “By 3000 to 4000 m the increase amounts to 4 ions, and at 4000 to 5200 m fully to 16 to 18 ions, in both detectors.”
He concludes: “The results of the present observations seem to be most readily explained by the assumption that a radiation of very high penetrating power enters our atmosphere from above … Since I found a reduction … neither by night nor at a solar eclipse, one can hardly consider the Sun as the origin.”
Although continuing research discovered more about the particles involved, the exact location of the source remains a mystery that continues to drive adventurous research in astroparticle physics.
• The extracts are from a translation of the original paper by Hess, taken from Cosmic Rays by A M Hillas, in the series “Selected readings in physics”, Pergamon Press 1972.
Sixty years ago, in September 1952, two young researchers at the UK’s Atomic Energy Research Establishment went out on a moonless night into a field next to the Harwell facility equipped with little more than a standard-issue dustbin containing a Second World War parabolic signalling mirror only 25 cm in diameter, with a 5 cm diameter photomultiplier tube (PMT) at its focus, along with an amplifier and an oscilloscope. They pointed the mirror at the night sky, adjusted the thresholds on the apparatus and for the first time detected Cherenkov radiation produced in the Earth’s atmosphere by cosmic rays (Galbraith and Jelley 1953).
William (Bill) Galbraith and John Jelley were members of Harwell’s cosmic-ray group, which operated an array of 16 large-area Geiger-Müller counters for studying extensive air showers (EAS) – the huge cascades of particles produced when a primary cosmic particle interacts in the upper atmosphere. Over several nights, by forming suitable coincidences between the Geiger-Müller array and their PMT, Jelley and Galbraith demonstrated – unambiguously – a correlation between signals from the array and light pulses of short duration (<200 ns) with amplitudes exceeding 2–3 times that of the night-sky noise. By cross-calibrating with alpha particles from a 239Pu source, they were further able to estimate that they were detecting three photons per square centimetre per light flash in the wavelength range of 300–550 nm. A new age of Cherenkov astronomy was born.
The sky at night
Five years before this observation, at a meeting of the Royal Society’s Gassiot Committee in July 1947 on “The emission spectra of the night sky and aurorae”, Patrick Blackett had presented a paper in which he suggested, for the first time, that Cherenkov radiation emitted by high-energy cosmic rays should contribute to the light in the night sky. Blackett estimated the contribution of cosmic-ray-induced Cherenkov light to be 0.01% of the total intensity, concluding: “Presumably such a small intensity of light could not be detected by normal methods.” Blackett’s work went largely unnoticed until a chance meeting at Harwell in 1952, which Jelley later recounted (Jelley 1986): “… hearing of our work on Cherenkov light in water, [Blackett] quite casually mentioned that … he had shown that there should be a contribution to the light of the night sky, amounting to about 10–4 of the total, due to Cherenkov radiation produced in the upper atmosphere from the general flux of cosmic rays.” Jelley continued: “Blackett was only with us a few hours, and neither he nor any of us ever mentioned the possibility of pulses of Cherenkov light, from EAS. It was a few days later that it occurred to Galbraith and myself that such pulses might exist and be detectable.”
The work of 1952 demonstrated the presence of short-duration pulses of light in coincidence with EAS but it did not prove that the light was, indeed, Cherenkov radiation. In particular, Galbraith and Jelley were aware that the light that they had observed could be also be produced either by bremsstrahlung or by recombination following ionization in the atmosphere. Thus, in the summer of 1953, they set out to establish the Cherenkov nature of the light pulses that they had observed.
Daunted by the vagaries of the British weather, they headed to the Pic du Midi observatory in France where, over six moonless weeks in July to September 1953, they carried out a series of experiments to determine the polarization and directionality of the light and also performed a rudimentary wavelength determination. This time they were equipped with four mirrors and two types of PMT. Conscious that the light-pulse counting rate would change with the noise level of the night sky, which in turn would depend on which part of the sky they were looking at, they devised a method of keeping the mean PMT current and, hence the noise, constant by using a small lamp next to the mirror.
Experimental conditions at the top of the mountain were challenging. EAS correlations were provided by requiring coincidences of signals from the PMTs with those from a linear array of five trays of Geiger-Müller counters, each tray 800 cm2 in area and aligned over almost 75 m – the positioning of these units was somewhat limited by the available space on the mountain (Galbraith and Jelley 1955). PMT pulses were recorded on an oscilloscope and subsequently photographed. Evidence for polarization of the observed light, a known characteristic of Cherenkov radiation, was clearly established by taking readings of a PMT with a polarizer placed over the PMT’s photocathode and calculating the ratio of the number of events seen when the polarizer was aligned parallel or perpendicular to the Geiger-Müller array. The result was a ratio of 3.0±0.5 to 1 for events seen in coincidence with two Geiger-Müller counter trays (Jelley and Galbraith 1955).
The two researchers also investigated the directionality of the observed light by plotting the coincidence rate of pulses seen in two light receivers (normalized accordingly) as a function of the angle between the two receivers. This experiment was done using pairs of receivers 1 m apart and was repeated with mirrors having different fields of view. The results fell between the two theoretical curves for Cherenkov and ionization light but they gave additional support for the premise that the light being observed was, indeed, Cherenkov light. In addition, the use of wide-band filters enabled Galbraith and Jelley to demonstrate that the light contained more blue light than green, which was another expected feature of Cherenkov radiation.
During their studies on the Pic du Midi, Jelley and Galbraith went on to explore the relationship between the light yield in the atmosphere and the energy of the shower, confirming, as expected, that larger light pulses were correlated with showers with higher particle densities. Finally, aware that their light receivers had both a considerable effective area and good angular resolution, they went on to search for possible point sources of cosmic rays in the night sky. The search yielded no statistically significant variations, and Galbraith and Jelley subsequently estimated that the receiver was sensitive to showers of energies of 1014 eV and above.
Following these studies in the early 1950s, it soon became apparent that use of the atmosphere as a Cherenkov radiator was a viable experimental technique. By the end of the decade, Cherenkov radiation in the atmosphere had been developed further as a means for studying cosmic rays – far away from the generally unsuitable British climate. In the Soviet Union, Aleksandr Chudakov and N M Nesterova of the Lebedev Physical Institute deployed a series of large-area Geiger counters along with eight light receivers at 3800 m in the Pamir Mountains to detect the lateral distribution of the Cherenkov light and thereby study the vertical structure of cosmic-ray showers. In Australia, around the same time, Max Brennan and colleagues of the University of Sydney used two or more mis-aligned light receivers to demonstrate the effects of Coulomb scattering of the charged particles in the cosmic-ray shower.
Meanwhile, at the International Cosmic Ray Conference in Moscow in 1959, Giuseppe Cocconi made a key theoretical prediction – that the Crab Nebula should be a strong emitter of gamma rays at tera-electron-volt energies. This stimulated further work, both by a British–Irish collaboration that included Jelley, and by Chudakov and his colleagues. The work at the Lebedev Physical Institute led in the early 1960s to the construction of the first air-Cherenkov telescope, with 12 searchlight mirrors, each 1.5 m in diameter and mounted on railway cars at a site in the Crimea close to the Black Sea.
The legacy
So, just a decade after the initial pioneering steps by Galbraith and Jelley, the first operational air-Cherenkov telescope had been built, setting in motion a chain of events that would ultimately lead in 1989 to the observation of gamma rays from the Crab Nebula by Trevor Weekes and colleagues at the Whipple telescope in the US. This breakthrough came nearly 25 years after Weekes had worked with Jelley in a collaboration between AERE and the University College Dublin, making the first attempts to detect gamma rays from quasars – a feat achieved only recently by the MAGIC air-Cherenkov telescope in the Canary Islands. Now, researchers around the world are teaming up to build the most sensitive telescope of this kind yet – the Cherenkov Telescope Array (Cherenkov Telescope Array is set to open new windows).
In writing only a few years ago about the work at Harwell, Weekes stated: “The account of these elegant experiments is a must-read for all newcomers to the field” (Weekes 2006). He also summed up well that first experiment by Galbraith and Jelley: “It is not often that a new phenomenon can be discovered with such simple equipment and in such a short time, but it may also be true that it is not often that one finds experimental physicists with this adventurous spirit!”
By Claude Leroy and Pier-Giorgio Rancoita World Scientific
Hardback: £153 $232
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Like its predecessors, this third edition addresses the fundamental principles of the interaction between radiation and matter and the principles of particle detection and detectors in a range of fields, from low to high energy, and in space physics and the medical environment. It provides abundant information about the processes of electromagnetic and hadronic energy deposition in matter, detecting systems, and performance and optimization of detectors, with additional information in the third edition. A part of the book is also directed towards courses in medical physics.
By Michael Mark Woolfson Imperial College Press
Hardback: £65 $98
Paperback: £32 $48
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The range of imaging tools, both in the type of wave phenomena used and in the devices that utilize them, is vast. This book illustrates this range, with wave phenomena that cover the entire electromagnetic spectrum, as well as ultrasound, and devices that vary from those that simply detect the presence of objects to those that produce images in exquisite detail. The aim also is to give an understanding of the principles behind the imaging process and a general account of how those principles are utilized, without delving into the technical details of the construction of specific devices.
By Fayyazuddin and Riazuddin World Scientific
Hardback: £54 $82
The Pakistani brothers, who were both students of Abdus Salam, wrote the first edition of their book in 1992, based on lectures given in various places. Aimed at senior undergraduates or graduate students, it provides a comprehensive account of particle physics. Having first been updated in 2000, this latest edition contains many revised chapters, in particular those that cover subjects such as heavy flavours, neutrinos physics, electroweak unification, supersymmetry and string theory. Another addition is a substantial number of new problems. This self-contained book covers basic concepts and recent developments, as well as overlaps between astrophysics, cosmology and particle physics.
In 2009, CERN’s Proton Synchrotron (PS) reached its half century, having successfully accelerated protons to the design energy for the first time on 24 November 1959. Still in operation more than 50 years later, it is not only a key part of the injection chain to the LHC but also continues to supply a variety of beams to other facilities, from the Antiproton Decelerator to the CERN Neutrinos to Gran Sasso project. During its operation, the PS witnessed big changes at CERN; at the same time, particle physics itself advanced almost beyond recognition, from the days before quarks to the current reign of the Standard Model.
At the close of the anniversary year, CERN held a symposium in honour of the accelerator developments at CERN and the concurrent rise of the Standard Model: “From the PS to the LHC: 50 years of Nobel Memories in High-Energy Physics”. Fittingly, at the end of 2009, the LHC – the machine that everyone expects to take the first steps beyond the Standard Model – was just beginning to come into its stride after the first collisions in November.
Key players who had been close to all of these developments, including 13 Nobel laureates, came together for the symposium. Now, several of the talks have been written up and published in the latest edition of The European Physical Journal H – the journal launched in 2010 as a common forum for physicists, historians and philosophers of science. The edition also includes three additional articles that were invited to provide a more complete picture, by covering CERN’s Intersecting Storage Rings, the history of stochastic cooling and searches for the Higgs boson at the Large Electron-Positron (LEP) collider – which started up in 1989 and hence celebrated its 30th anniversary at the symposium.
Dip into the pages and you will find many gems: among the Nobel laureates, Jerome Friedman describes the work at SLAC that revealed the reality of quarks, which were unheard of in 1959; Jim Cronin revisits the early 1960s when he and his colleagues discovered CP violation; Jack Steinberger looks back at early experiences at CERN; Carlo Rubbia presents the story of the discovery of W and Z bosons at CERN; and Burt Richter recalls early ideas on LEP, from his days on sabbatical at CERN. On the accelerator side, the articles detail developments with the PS, as well as the highlights (and lowlights) of the construction and running of LEP. The invited article on stochastic cooling includes the work of Simon van der Meer, who shared the Nobel prize with Carlo Rubbia in 1984. Sadly, he was too ill to attend the symposium and passed away in March 2011.
All of the articles provide an interesting view of remarkable events through the reminiscences of people who were not simply “there”, but who played a big part in making them happen. They are a fascinating reminder of what particle physics was like in the past and well worth a read. They also reflect the different styles of the various individuals, but not so much, perhaps, as did the original presentations at the symposium. To get the full flavour, and to see all the participants, take a look at the recordings. There you will find still more gems.
Towards the end of July 1958, at a house in the hills south-east of Rome, three Italian scientists discussed key ideas that were to form the foundations of the European Space Agency (ESA). Edoardo Amaldi, who had been instrumental in the establishment of CERN four years previously, was with Giorgio Salvini – whose house it was – and Gino Crocco, who was Goddard Professor of Jet Propulsion at Princeton in the US. During their conversation, the old friends discussed how European countries, in particular Italy, could become involved in space research. Only the previous October, the Soviet Union had opened up the space age with the launch of the first artificial satellite, Sputnik 1. This had been followed in January 1958 by Explorer 1, launched by the US. So what could Europe do?
As Salvini recalls, the conversation was “long and animated”. While Crocco was sceptical about what Italy could achieve, Salvini was more optimistic, and Amaldi, with all of his experience in setting up CERN, saw the case for an organization that would enable European countries to work together on research in space. In particular, Amaldi insisted on two points: that there should be no military involvement and that such an organization should be based on the successful model that had given rise to CERN.
At the end of the year, Amaldi wrote to Crocco at Princeton, describing the contacts that he had made in the meantime with some influential scientists. In the letter, Amaldi went on to describe how he thought the project to launch a “Euroluna” (“Euromoon”) satellite for scientific research should take shape. The letter makes clear his insistence that the underlying organization should not be linked to the military but should be purely scientific and based on the same principles as CERN.
Amaldi insisted on two points: there should be no military involvement and the organization should be based on the model that had given rise to CERN.
As a starting point, Amaldi suggested that a small group of experts from the major European countries could prepare a plan for creating an appropriate organization. By early 1959 he had discovered an ally in an old friend, Pierre Auger, the French cosmic-ray physicist who had also been involved in setting up CERN. By May, after several interactions with Auger, Amaldi had written the first draft of his paper, Space Research in Europe, with the aim of stimulating discussions on the formation of a European organization for space research. A French version, together with supportive coments from several countries, was distributed in December (Amaldi 1959).
In Amaldi’s original vision, not only the development of the satellites – the “Eurolunas” – but also that of their launchers would be the responsibility of the organization, which would need experts in the technology and engineering of rockets as well as space scientists. The idea was to mirror CERN, which had accelerator physicists and engineers to build its own machines for the high-energy-physics community to use in scientific research. By collaborating at CERN, Europe’s scientists had access to accelerators that no country had the means to build on its own.
It soon became clear that this vision was not to be, albeit not to begin with. There was too much political and commercial interest surrounding the construction of rockets. Governments, in particular the British and French, began the negotiations that would separate the business of building launchers from that of making the satellites for scientific research. On 29 March 1962 in London, seven countries – Belgium, France, Germany, Italy, the Netherlands, the UK and Australia (associate member) – signed the convention that created the European Launcher Development Organisation (ELDO). Three months later, on 14 June 1962 in Paris, Belgium, Denmark, France, Germany, Italy, the Netherlands, Spain, Sweden, Switzerland and the UK signed a different convention, in this case to create the European Space Research Organisation (ESRO).
The foundation of these separate bodies may have been counter to Amaldi’s vision for an organization similar to CERN but they were the forebears of ESA, which was established in May 1975. With the formation of ESA, the science and the means to do it were brought into the same fold.
Amaldi’s letter to Crocco, which is translated from Italian on the following pages, constitutes the first document in which a European space organization is mentioned. It is for this reason that 10 copies recently went into space on board a spacecraft taking essential supplies to the International Space Station (ISS). ESA’s 3rd Automated Transfer Vehicle (ATV), named in honour of Amaldi, arrived at the ISS on 29 March 2012, exactly 50 years to the day of the convention to create ELDO being signed in London. Appropriately, the ATV had been launched by an Ariane rocket built by ESA. The copies of the letter will be signed by the astronauts and brought back to Earth by a Soyuz spacecraft. One will be given to CERN.
Amaldi’s 1958 letter, translated
16 December 1958
Prot. No 4674/A
Distinguished Prof.
Gino Crocco
College Road 74
PRINCETON – N.J.
Dear Gino,
After our discussion at Salvini’s home in Rocca di Papa at the end of July, I thought over the possibility to develop an appropriate activity in Europe in the field of rockets and satellites. It is now very much evident that this problem is not at the level of the single states like Italy, but mainly at the continental level. Therefore, if such an endeavour is to be pursued, it must be done on a European scale as already done for the building of the large accelerators for which CERN was created.
The launch of one or more “Euroluna”, performed by a dedicated European organization, would definitely be of the highest importance, both moral and practical, for all the nations of the continent.
With these ideas in mind, at the end of July I wrote a letter to [Luigi] Broglio who replied, at the end of August, expressing his substantial agreement with the theoretical formulation of the problem but also a considerable scepticism with regards to the practical feasibility of an actual project.
During the Conference of Geneva, held in the first half of September, I had the opportunity to discuss it with [Isidor] Rabi who reacted very positively and stated that, if this would have developed further, he would have done everything possible for obtaining the support of the United States. Actually, himself being a representative of the United States in the NATO Science Committee, he thought that this could be the initiating body for this activity; however, I think this wouldn’t be appropriate, as I shall explain later.
In November I spoke to [Harrie] Massey of [University College] London who, however, was rather sceptical; though this is the normal British attitude in front of any continental initiative.
At the beginning of December I spoke about the matter with [Francis] Perrin who was very interested and convinced and he promised me to look for some competent people in this specific field in France that could flag the problem.
The idea I have about this organization is that, in addition to the six EURATOM nations, Britain and the Scandinavian countries should participate in the manufacturing of satellites. Britain would at first limit itself to sending some observers and would probably show some resistance, but would certainly end up contributing substantially, would the project start taking shape.
It should, in my opinion, proceed as follows: some authoritative expert in the field (Broglio I hoped, but he seems not to have the necessary enthusiasm) should start flagging the problem and obtaining some level of participation of one or two experts of the largest European countries. Some Italian, French and German experts would be needed to start. These five or six people should prepare, within a few months, a plan of technical development containing :
1) a very well defined scope which should be so ambitious to be comparable with the targets that the USA and the USSR have set for themselves in this field, and in order to justify the European character of the endeavour;
2) an assessment of the cost and its time distribution;
3) an assessment of the specialized workforce;
4) a realistic time frame.
Such programme should be submitted to the governments for approval and for the resulting creation of the final organization which should be provided with the necessary resources.
In the case of CERN, things essentially developed as mentioned above; however, that case took advantage of the existence of UNESCO which, by calling the representatives of the governments to a first conference, played the role of the mother and nurse of CERN. I do not know who could be the mother and nurse of the new organization; according to Rabi this could be the “Science Committee” of NATO, but I believe that it wouldn’t be the best mother for such organization. As a matter of fact, I think that it is absolutely imperative for the future organization to be neither military nor linked to any military organization. It must be a purely scientific organization open, like CERN, to all forms of co-operation both inside and outside the participating countries. I have the impression that all attempts to set up international organizations of a military nature have either failed or, if they didn’t fail, present such characteristics that do not minimally satisfy even their own promoters and managers.
The high level start-up project should include :
a) the construction of common European laboratories for solving the various major problems,
b) a related research programme to be run in the participating countries.
Through either one or the other of these activities, the individual countries would have all the technologies at their disposal, and therefore their scientific-technical structure would be greatly strengthened. Such strengthening would bring, evidently, great advantages also in the military sector in case the defence activity would be necessary but it wouldn’t make the realisation of the programme more difficult and complicated as would occur if the military, directly or indirectly, were the masters.
The financial problem, definitely irresolvable within the economy of one single country, could be solved in the context of the European continent.
The problem of the specialized workforce constitutes a second difficulty, but I believe that this could be solved in such a project; this would have the double advantage of attracting the liveliest part of the new generation and making it possible to recover academics who work outside Europe.
I would like to ask you to think about what I wrote here and to reply, as soon as possible, to the following questions which, in a more or less direct manner and on different levels, are related to the project mentioned above :
1) I would like to know whether you are interested and whether you would like to take an active role or even the leading role in it. Personally I don’t want to be involved in all of this except for launching the idea, at this stage, and later – in a few years – if the idea becomes reality, for participating in collecting the scientific data which can be obtained with this kind of activity;
2) I would like to know from you the names of the most competent and open persons in this field in Italy, France, Germany, Great Britain and in the Scandinavian countries. As I already told you, I contacted Broglio since July, but he seemed to be too sceptical for taking this route for the moment at least;
3) I would like to know which organizations, even of modest size, exist in Italy in this field and can provide an absolute guarantee of trust; for example, I came in contact with SAMI’s engineer Salvatore but I have no idea of neither the value and competence of this person nor the robustness of the company. The seriousness of the people is a very fundamental issue; this venture is destined to fail, if people who are not sufficiently trustworthy slip into the initial organization committee.
Furthermore, I would like to have von Karman’s address; Rabi asked me permission to speak to him about this and I agreed, but I don’t know if he actually did it and whether this would be of any help. I would like to have your opinion on this subject too; nevertheless, I think that an authoritative person like him could, if favourable, have a considerable influence.
I believe that you will be very much surprised by this letter of mine; it is based on my experience with CERN: in 1952 only three or four persons in the whole of Europe believed in the possibility of creating CERN, but in 1958 the laboratories in Geneva have exceeded 800 workers, the first machine has started running giving first class scientific results and the second machine will work before mid-1960.
I believe that, if the European experts in the field of rockets and satellites start moving already now, they will be in a condition, together with the American and Russian groups, to contribute very substantially to the study of space by 1965.
I take this opportunity for sending you and your wife my best wishes, including among them the wish for a Euroluna before 1965.
By Richard J Szabo Imperial College Press
Hardback: £42 $68
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Originally published in 2004, this book provides a quick introduction to the rudiments of perturbative string theory and a detailed introduction to the more current topic of D-brane dynamics. The presentation is pedagogical, with much of the technical detail streamlined. The rapid but coherent introduction to the subject is perhaps what distinguishes this book from other string-theory or D-brane books. This second edition includes an additional appendix with solutions to the exercises, thus expanding the technical material and making the book more appealing for use in lecture courses. The material is based on mini-courses in theoretical high-energy physics delivered by the author at various summer schools, so its level has been appropriately tested.
By David Goodstein (eds.) World Scientific
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This up-to-date collection of review articles offers a general introduction to cosmology by experts in various fields. It starts with “Galaxy Formation from Start to Finish” and ends with “The First Supermassive Black Holes in the Universe”, exploring in between the grand themes of galaxies, the early universe, the expansion of the universe, neutrino masses, dark matter and dark energy. Together the chapters provide a detailed view of what is known about the universe as well as what remains unknown. Students, researchers and academics interested in cosmology should find this book useful.
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