How did the CIAR come to be involved in this field, what are the intellectual challenges of the field, and where is it going? These are the questions the following report will answer.
A decision was also made at that time to concentrate on theory. There were a number of reasons for this decision. The key scientific ferment at the time was in theory, and especially in the impact of ideas from particle physics on the early universe. Theory is also on the whole much cheaper than experimental or observational science. Furthermore, theorists tend to much more mobile in that they are not tied to labs and equipment. Finally, Canada had a severe shortage of theorists, not only in Cosmology, but all of astronomy. While Canada, through the Hertzberg Institutes of the NRC and through the Astronomy departments across Canada, had developed a cadre of observational astronomers of significant international stature, it was very weak in the theorists. This had already been recognised by the Canadian Astronomical Society which was advocating the establishment of a center for Theoretical Astrophysics, a center which was finally founded at the University of Toronto as CITA (Canadian Institute for Theoretical Astrophysics). The initial study group at CIAR recognised that its program in theoretical Cosmology and this new institute could profitably grow together. Thus we felt that the CIAR could make it greatest and quickest impact on the Canadian and the world scene by concentrating on theory.
The initial scientific thrust of the program was directed into two areas. The first was to understand how gravity, quantum mechanics, and particle physics played their role in the birth and early development of the universe. The second was to understand how the observations which we can be made today can tell us about those earliest days in the life of the universe.
The initial group of fellows of the CIAR program grew by attracting a number of top young international people to Canada. At the University of Alberta, we were able to bring in Don Page, a professor from the University of Pennsylvania, and Valery Frolov, a Senior Researcher at the Lebedev Institute in Moscow. This created at Alberta a world center in gravitation physics. We were also able to attract Nick Kaiser, a young researcher from Cambridge University, to CITA, making that one of the world's centers for physical cosmology. The financial difficulties which hit both the universities and the CIAR in the past 5 years have limited our ability to add more funded Fellows to the program, but they have not stopped our growth through the addition of Associates.
Throughout this time, we have demanded that all of our Fellows be of the highest international calibre. This has resulted in the standards for admission to the program being higher at the program level than at the Research Council level, and this is a situation which we are determined to maintain.
The program is one of the smallest in the CIAR, but it has also been very strong. To accomplish this we have made use of the strengths of the international community through the appointment of Associates. These Associates are of necessity loosely associated with the program, with a commitment arising primarily through their interactions with the Fellows. For example, it became clear over the past five years that observational techniques had made a sudden jump in sensitivity. Rather than appointing new Fellows at a time when financial constraints were severe, we appointed a number of observational cosmologists as Associates. We were in this way able to bring into the program people who form the elite of the field from around the world, people whom we could never hope to attract to Canada. It is indicative of the reputation of the program that not only has no-one we have approached ever turned us down, but that a number have requested to join the program.
The current composition of the program is as follows. This list includes a very brief description of the Fellows' and the Associates' research interests.
The at--first--unwelcome conclusion that General Relativity demanded a dynamic universe, with an origin at a finite time in the past, gave us the first model in which one could discuss the idea of the origin of the universe as a whole within the context of the theory itself. The observations by Hubble that the predicted dynamics actually did occur in our universe meant that one could now study not just the history of the entities which make up the universe, but the history of the whole. If the universe came into being some 10 billion years ago, one could now ask what were the features of that young universe which led to our existence. Are the conditions necessary for the support of life with forms like ours improbable or are they generic?
Key impediments to a picture of the development of the universe were both theoretical and experimental. Einstein's theory predicted that the universe must once have originated in a singularity, a region of zero volume. However the theory gave no indication of the features that that initially tiny universe must have in order to describe the present. Furthermore, the observational technology was sufficient to tell only the bare fact of the expansion of the universe. To become more than an intellectual curiosity, cosmology required both a sufficiently sophisticated theory to describe which of the currently observable features are remnants from those earliest stages, and the observational techniques sufficiently sensitive to see those features. Both of these conditions began to develop slowly after the war. In the late 40s came the prediction of the Cosmic Microwave Background Radiation (CMBR) by Gamov and colleagues. These predictions were developed in more detail by the group at Princeton around Dicke, including Peebles, as a prelude to a search for that radiation. They were scooped by the serendipitous observations by Penzias and Wilson of this radiation in the 60s. Here finally was a ``fossil" left over from the universe a brief few tens of thousands of years after its formation. This radiation thus gives us a glimpse of the state of the universe mere millenia after its birth, and the view is one of almost unrelieved boredom and uniformity. Everywhere the radiation was the same.
In the early 70's, Peebles realised that the largest scale structures in the universe could also be regarded as a remnant from those very earliest stages. The distribution of matter on those largest scales would not have had time to change very much since the birth of the universe. What changes there were could be calculated on the basis of the known laws of gravity and of matter. Thus current measurements of those structures could be used to ``predict" what the universe must have been like in its earliest stages. These two features, the large scale structure and the CMBR, still form our chief windows back to those earliest times.
The development of cheap computer power also had a profound effect. For the first time one could hope to use the laws of gravity to determine how the initial fluctuation in the universe could grow through gravitational collapse to form the galaxies. A problem far too difficult for analytic treatment looked ripe for attack by computer. The pioneering studies by Efstathiou and colleagues showed how that the current distribution of the galaxies could place strong constraints on the form of the matter in the universe. Studies such as theirs finally placed the large- scale-structure on a firm footing as a genuine window into the early universe.
On the observational side, similar revolutions were taking place. The application of the CCD chip (developed commercially for video cameras) changed completely the use of large telescopes for probing the most distant reaches of the universe. Where film was ultimately limited by the brightness of the night sky, CCDs, because of their highly linear response to light, allow one to probe sources whose brightness was over a hundred times dimmer than the night sky. They allowed us to see galaxies completely inaccessible to previous techniques, and to probe into the farthest reaches and thus earliest times in the life of the universe.
Space-borne instruments became a possibility and began to be designed and launched. Although Hubble is the most public example, it is probably not the most important. The atmosphere of the earth is opaque to many areas of the spectrum and the new space-borne instruments allowed us to see the universe in new ways.
It was these developments, especially the theoretical ones, which led to the formation of the CIAR Cosmology program. As mentioned, the concentration was in the first instance on theory, rather than observation, because it was felt that a larger and more rapid impact on the field could be made in that way.
The program took as its range the entire field of theoretical cosmology, from the birth of the universe in quantum uncertainty to the development of observational tests. Due to historical accident, the division of the program between the earliest theoretical stages and the later observational stages also occurred along geographic lines, with Toronto the location of the physical cosmologists (ie, cosmology directly related to observations), and the west (UBC and Alberta) the location of the early theoretical particle physicists and cosmologists (primarily concerned with gravity as it dominates the behaviour in the earliest stages of the universe).
The largest change in the field over the lifetime of the program has been
in the explosion of observational data. Although it has been difficult
for us to respond through the appointment of a fellow, we have used our
influence and our Associate positions to move the program closer to
the observational areas. We sponsored a number of key meetings which revived
the Canadian Network for Observational Cosmology, a consortium of Canadian
observers. They have been very successful at getting observing time on
the best telescopes (eg CFH) for their program of cosmological observation.
We have formed links with the Sudbury Neutrino Detector. We have also appointed
4 observational Associates to the program, as well as having one
of our Fellows
(Kaiser) become closely involved himself in an observational program, and
another (Bond) spend much af his time analysing CMBR data.
Areas of Research
I am not going to try to describe all of the areas of research we have been
engaged in over the past ten years. Rather I will choose a few topics to give
a feeling for the types of questions which we tackle, and their relation
to the field as a whole.
What is the universe made of and how much matter is there? The difficulty in answering this question is that there is clearly no way of directly observing or measuring matter which is far away from us. We are bound to the earth and its immediate vicinity, and must observe the rest by the means nature has given us. The first tool is clearly the use of light. Most of the matter in our solar system is contained in the central star, our sun. If this is true of all of the matter in the universe then by observing the light given off by the stars, we can estimate the total amount of matter. Unfortunately, this is clearly not true. An examination of the velocity with which stars revolve around the centers of galaxies indicates that there is more matter than light in the outer reaches of those galaxies. Furthermore, the motion of the galaxies in clusters of galaxies shows that such clusters contain far more matter than is indicated by the starlight. This is the missing mass, the dark matter, and what it is is one of the great questions in Cosmology. Furthermore, since the visible matter is about 1% of that needed to eventually stop the universe from expanding, and the know dark matter is about 10 times more than this, is there enough dark matter eventually to stop the expansion of the universe? If not, why is the amount so very close to the right amount?
There are two ways of identifying the dark matter. One is to use the current theories of the elementary particles to see if there are some type of elementary particle which could form the bulk of the matter in the universe. Unfortunately our theories present us with too many candidates, and our knowledge of particle-physics is insufficient to decide between them (or to give any of them credence). The second approach is by direct observation, but this must be a form of observation not dependent on the light given off by the matter itself. The only aspect of nature which is universal, which responds to all types of matter equally, is gravity. The only way of measuring the gravity of far distant objects is by examining the orbits of other matter near the object. Determining the orbit of conventional matter, like stars or whole galaxies is difficult---one needs to make a number of assumptions about the orbits of those stars to use them as probes of the gravitational field. However, as was shown by the British expedition of 1919, light is also affected by gravitational fields, and one can use the orbits of light rays to measure the strength of the distant gravity. This is an idea which has been implicit in General Relativity since early in this century, but it was recognised by Tyson as practical, and developed into a detailed working technique by Kaiser only in the past 5 years. As shown in Figure 1, a superb example of the effects of gravitational lensing by a cluster of galaxies, the gravitational field of such a cluster can severely distort the shapes of the even more distant galaxies. It was only with the development of the CCD camera, and its ability to see the shapes of galaxies lying far beyond the clusters in question, that one could measure the distortions caused by gravitational lensing. From those distortions one can determine the amount of matter in the cluster of galaxies.
This technique is less than 5 years old, but is already being used by more than three groups around the world. They finding clusters in which the amount of matter measured by lensing corresponds well with other estimates of the mass in the cluster. But they have also uncovered regions with little or no starlight which nevertheless appear to contain a sizable concentration of matter. Whether these indicate a totally new phenomenon (dark clusters) or represent a problem with the technique is still an open question. But for the first time, largely due to the work of a member of the CIAR program, we have a tool to measure directly the total amount of matter in regions far from us.
Another area of observational advance has been in measuring the Cosmic Microwave Background Radiation. While earthbound observations measured the temperature of this radiation, the opacity of the atmosphere at wavelengths shorter than a few millimeters made it difficult to obtain more detailed information. Five years ago the COBE satellite, and the UBC-COBRA rocket spectrometer, showed that this radiation has all of the properties of thermal radiation to extremely high precision. No source for such thermal radiation other than as a remnant of the early universe has survived examination. Furthermore, the COBE satellite, through its observation of this radiation over the whole sky, demonstrated that there exist fluctuations in this uniform sky-glow, fluctuations in temperature on the level of a few tens of microdegrees (in the overall mean temperature of 2.7 degrees). These fluctuations correspond directly, but on a larger scale, to the fluctuations which must have grown to form the galaxies and clusters of galaxies. From the known distribution of galaxies the size of these fluctuations was predicted, and the measured value is within a factor of five of the expected result. However that difference indicates that the simplest models for the matter in the universe (the cold dark matter scenario) does not appear to work. The distribution of these fluctuations, especially on smaller scales than those measured by COBE, will give us detailed knowledge about the type and amount of dark matter in the universe. It has been largely through the work of Bond that we understand how to use this fluctuation spectrum to understand the constituents of the universe.
In the very earliest stages of the universe (10^-44 to 10^-30 seconds after the beginning,) one would expect not only gravity, but also quantum phenomena to play a critical role. Unfortunately, all attempts at a theory of ``quantum gravity" have failed. Hawking and Unruh discovered in the 1970s that there were regimes in which gravity could have surprising effects on quantum phenomena. Unruh showed that under conditions of extreme accelerations, our concepts of what a particle is break down. Accelerated detectors in regions with no particles responded as though immersed in a thermal bath of particles. Hawking showed that black holes were not black, but rather suffered from a quantum instability which caused them to gradually decay via the emission again of a thermal spectrum of particles. This union of gravity, quantum mechanics and thermodynamics is one of the greatest surprises in theoretical physics in the 20th century. It is highly suggestive that ``quantum gravity" will lead to deep and unexpected relations between disparate physical principles, but so far no one has been able to follow these clues to a solution.
Black holes thus form an ideal theoretical laboratory for the testing of our concepts of quantum gravity, and have been one of the key areas of research of the ``Western Wing" of the CIAR program. The understanding of what happens near black holes promises also to cast light on what must have happened at the birth of the universe. The research ranges from that of Israel, who has done more than anyone in the world to describe the ultimate fate of the interior of a black hole, to Frolov, who has made key contributions to understanding the origin of the entropy of a black hole.
In this context, let me describe my own work. One of the greatest puzzles in understanding the influence of gravitational fields on matter is the ability of the gravitational field, through its creation of space, to take ultra-high-frequency ( and thus high-energy) phenomena and convert them to low-frequencies. However, no--one believes that the theory at those ultra high energies (energies such that each quantum contains more mass than the entire universe) is valid. Does this invalidate all of the conclusions, such as black hole evaporation (the Hawking process), that we derive from the theory? I discovered that there is an analog to black hole evaporation in fluid mechanics, where we completely understand the physics at all scales. While not conclusive, it seems that the evaporation process is not a high-frequency effect, as the straightforward calculations would indicate, but originates at low frequencies. This promises to give us an entirely different understanding of the origin of the radiation given out by the black holes, and of the creation of matter and particles in the very early universe as well. My result also ties in closely to one discovered by Frolov, who showed that the entropy of a black hole is also apparently independent of the high-frequency features of the theory. Both of these results hold hope of a new way of looking at the interactions between gravity and quantum mechanics, which may also lead to a new understanding of quantum gravity.
Another area of intense interest has been that of the role of time in a quantum theory of gravity. This is again a problem of long-standing interest, and one which has a long history of intractability. One of the advances was Hawking's demonstration of a simple model theory of quantum gravity in which the ``initial conditions" do not occur. As a part of its creation, the universe must begin in a unique state. This is probably the first time in the history of science in which a theory of the physical world has been proposed which did not naturally split up into the form of laws of dynamics plus initial conditions. It reminds one most of Leibnitz's monads, and his view of the universe as the best, or only, possible world, in which the microcosm and the macrocosm are intimately interrelated. Unfortunately, Hawking's proposal is difficult to implement in any realistic model for the universe. It cannot yet be used to make predictions about the actual behaviour of the universe. Both Page and I have spent time in trying to interpret these ideas and to understand whether or not they makes sense as a physical theory. It has led me to try to understand more deeply the role of time in ordinary quantum theory, and has led to insights into problems as diverse as the time required to tunnel through a barrier to the strengths and weaknesses of the newly discovered quantum computers.
Impact of the CIAR
The impact of the CIAR program on Cosmology and Gravity research in Canada
cannot be overstated. It is the CIAR, which has been directly responsible
for Canada's forefront role over the past 10 years. Before the program,
there were two people in Canada, Israel and Unruh, with some international
prominence. They worked completely independently. It was only because
of the program that the rest of the fellows came to Canada. Without the
continued existence of the program, it is also likely that many would leave.
This is a time of unprecedented excitement in the field, and a time in
which many universities around the world are looking to establish themselves
by hiring the best young people. Thus one role the CIAR now plays is to keep the
group we have gathered in Canada.
Secondly, the CIAR program has brought together the best people from not only across Canada but from around the world. There is no other forum in which the broad range of people, from string theorists like Susskind, to observers like Ellis, would be found together at the same meetings and be found discussing science together. There is no other forum where discussions about the use of Bayesian statistics to analysis the Cosmic Microwave anisotropies could help trigger an analysis of the timing errors on the Global Positioning System of satellites. It is not only that the program has incited active collaboration between the members, but that the ideas in one area excite complementary ideas in other areas, and that the enthusiasm and intelligence and ambition of the people in one area drive people in the other areas to equal or exceed them. While suffering from the lack of proximity between the fellows, the program has produced the type of intellectual ferment found at places like the Institute for Advanced Research in Princeton.
The program in Cosmology and Gravity will undergo some major changes in the immediate future. On the negative side, Ian Affleck's interests have shifted almost entirely to the area of high-temperature Superconductivity and condensed matter systems. He has been a valuable resource, and will continue as an associate of the program, but will devote his main efforts to the CIAR program on Superconductivity. In addition, Werner Israel is taking early retirement from the University of Alberta next year. Again, while we will continue our interaction with him and continue to make use of his expertise and sage insights, his commitment will probably decrease.
Fortunately, there is no paucity of exciting science for us to pursue. Cosmology is in a far more dynamic and exciting state today than it was 10 years ago. It is driven today largely by the wealth of experimental data that the new technologies are producing. The COBE satellite gave us a first glimpse of the possibilities for measurements by space-borne instruments. Over the next five to 10 years there are proposals for a number of new satellites to measure the CMBR to much greater accuracy, and on much smaller scales. In the USA, the MAP satellite, the PSI satellite (with Lubin a primary user), and the FIRE satellite have been proposed and one will be chosen on within the next month. The COBRAS/SAMBA satellite (for launch in 2005), with Efstathiou the key theoretical figure, will also be decided on by the European space agency in this year. Thus within 5-10 years there will be at least one new satellite producing much more detailed data. In the meantime, COBE continues to collect data and a number of ground- and balloon-based experiments are in progress and are reporting data (an example is the UK CAT ground based Interferometer which made the front page of the Independent recently.) The next five to ten years give promise of definitively determining many of the most fundamental parameters governing the evolution of the universe. In the midst of this wealth of data, the CIAR program is ideally placed to take advantage of it. Not only does the program have in Bond one of the world's foremost experts on the theory, but through our ability to choose Associates, we can ensure our participation in any of the most exciting CMBR experiments.
With the advent of the KECK telescope, the CFH telescope will be used primarily for long run cosmological observations. Through his collaboration with Lupino, the builder of the largest CCD camera in existence and a guaranteed user of the CFH telescope, Kaiser is in an ideal position to use the gravitational lensing technique to map out not only the mass distribution in clusters of galaxies, but possibly also the mass distribution over the whole sky. This will give us our first true measure of the location and amount of all the matter in the universe. This is an intensely competitive field, and we will need to ensure that we remain actively involved, primarily through the nomination of other key associates in this area.
In addition, with the incomparable collecting area and quality of the Keck and the future Gemini telescopes, the collection of data of cosmological importance will inundate the field. The Princeton/Fermilab collaboration on the Sloan Digital Sky survey (of which Fukugita is a member), mapping out not only the location but the red shift (and thus distance from us) of over a million galaxies will give us an unprecedented image of the large scale structure in the universe. From a field dominated by speculation, cosmology will over the next five years become a field dominated by data, and as usually happens in such cases, dominated by new problems that that data raises about our theoretical understanding.
We are also grasping a unique opportunity to extend the scope of the program. In Scott Tremaine of CITA, who has agreed to join the program as a Fellow, Canada has the world's top person in galactic and solar system formation and dynamics. In addition to links between physical cosmology and gravity which his work on black holes in galactic nuclei forges, his work also forms a bridge between the formation of the universe and the formation of the planets (especially the one we live on).
Another area of excitement is that of computational cosmology, gravity and astrophysics. Once one gets away from the most symmetrical situations, most theories cannot be solved analytically. Whether it is the motion of matter under the influence of gravity, or the emission of gravity waves by a black hole, only computer solutions will show us what the predictions of our theory actually are. And even here, the field demands considerable sophistication. While computer memory and speed increase yearly, the complexity of the problems seem to increase much more rapidly. In the discussion of the galaxy-formation scenarios discussed earlier, the dynamic range, the range over which one believes that the method gives reliable results, was less than a factor of a 100. This is far from enabling one to calculate individual galaxies at the same time as one is calculating the motion of clusters of galaxies. Since the work needed goes at least as the third power of the dynamic range, improvements are slow. Thus the key to enabling us to solve problems has largely been the discovery of new techniques rather than simply the use of faster computers. This area is one in which we have some expertise, from Tremaine's galaxy collision and solar-system-dynamics calculations, Bond's Smooth Particle Hydrodynamics simulations or Unruh's student's gravitation wave calculations. It is an area where the appointment of one or two associates or of a key Scholar could not only give the program a real edge, but also increase the interaction between the Cosmology program and other programs of the CIAR.
A final area of future observational excitement is gravity-wave detection. The US has funded a plan to build two gravity-wave detectors (LIGO) in the USA. In addition, there exist a variety of European initiatives-- both funded and proposed. The detection of such waves promises to open a totally new window on the universe. Gravity waves, precisely because of their weak interaction with matter, allow us to peer into the central core of stellar collapse. However, it has become clear that the theory of such sources is still badly understood, and the appropriate approximation techniques to calculate the radiation given off in such a collapse have still to be developed. The first generation detectors will be noise dominated, and to detect signals, the form of those signals must be understood accurately. Thus, the theoretical challenge is to understand the sources of such radiation before the detectors are actually put into operation.
Since such understanding will demand the intimate knowledge of the behaviour of gravity and the development of new mathematical approximation techniques, we feel that this is a suitable area for future expansion of the program. It will tie together the computational and the gravitational expertise in the program, and also bring in the detailed knowledge of astrophysical processes in the late life of giant stars or of neutron stars.
Thus to reiterate my opening statement, there is no doubt that Cosmology and Gravity is a field at the forefront of the natural sciences today. Through the CIAR Canada is a player on the world stage in one of the most exciting developments in science, the understanding of the very origins of the universe itself.
