Princeton Series in Physics – serie

The Dutch scientist Hendrik Kramers (1894-1952) was one of the greatest theoretical physicists of the twentieth century--and one of a mere handful who have made major contributions across the whole field. Physicists know his name from, among other things, the Kramers dispersion theory, the Kramers-Heisenberg dispersion formulae, the Kramers opacity formula, the Kramers degeneracy, and the Kramers-Kronig relations. Yet few people know more than the name, or recognize the full depth and range of his contributions. In this book, D. ter Haar seeks to change that. He presents for the first time anywhere a comprehensive discussion of Kramers's scientific work, and reprints twelve of his most important papers. The author shows us that Kramers's remarkable and diverse work makes him at least the equal of such celebrated physicists as Fermi and Landau. He takes us through Kramers's groundbreaking research in such subjects as quantum theory, quantum electrodynamics, statistical mechanics, and solid-state physics.The papers he reprints include Kramers's derivation of the dispersion formulae that led to Heisenberg's matrix mechanics; his classic paper on the Brownian-motion approach to chemical reactions; a pioneering paper on polymers; and a paper on renormalization, a concept first introduced by Kramers and now one of the basic ideas of modern field theory. This book will change how we view the course of twentieth-century science and will show that Kramers was indeed one of the masters of modern physics.

Like its predecessor, this book by the renowned physicist Sir Rudolf Peierls draws from many diverse fields of theoretical physics to present problems in which the answer differs from what our intuition had led us to expect. In some cases an apparently convincing approximation turns out to be misleading; in others a seemingly unmanageable problem turns out to have a simple answer. Peierls's intention, however, is not to treat theoretical physics as an unpredictable game in which such surprises happen at random. Instead he shows how in each case careful thought could have prepared us for the outcome. Peierls has chosen mainly problems from his own experience or that of his collaborators, often showing how classic problems can lend themselves to new insights. His book is aimed at both graduate students and their teachers. Praise for Surprises in Theoretical Physics: "A beautiful piece of stimulating scholarship and a delight to read. Physicists of all kinds will learn a great deal from it."--R. J. Blin-Stoyle, Contemporary Physics

This widely acclaimed introduction to N = 1 supersymmetry and supergravity is aimed at readers familiar with relativistic quantum field theory who wish to learn about the supersymmetry algebra. In this new volume Supersymmetry and Supergravity has been greatly expanded to include a detailed derivation of the most general coupling of super-symmetric gauge theory to supergravity. The final result is the starting point for phenomenological studies of supersymmetric theories. The book is distinguished by its pedagogical approach to supersymmetry. It develops several topics in advanced field theory as the need arises. It emphasizes the logical coherence of the subject and should appeal to physicists whose interests range from the mathematical to the phenomenological. In praise of the first edition: "A beautiful exposition of the original ideas of Wess and Zumino in formulating N = 1 supersymmetry and supergravity theories, couched in the language of superfields introduced by Strathdee and the reviewer...[All] serious students of particle physics would do well to acquire a copy."--Abdus Salam, Nature "An excellent introduction to this exciting area of theoretical physics."--C. J.Isham, Physics Bulletin

During the course of this century, gauge invariance has slowly emerged from being an incidental symmetry of electromagnetism to being a fundamental geometrical principle underlying the four known fundamental physical interactions. The development has been in two stages. In the first stage (1916-1956) the geometrical significance of gauge-invariance gradually came to be appreciated and the original abelian gauge-invariance of electromagnetism was generalized to non-abelian gauge invariance. In the second stage (1960-1975) it was found that, contrary to first appearances, the non-abelian gauge-theories provided exactly the framework that was needed to describe the nuclear interactions (both weak and strong) and thus provided a universal framework for describing all known fundamental interactions. In this work, Lochlainn O'Raifeartaigh describes the former phase. O'Raifeartaigh first illustrates how gravitational theory and quantum mechanics played crucial roles in the reassessment of gauge theory as a geometric principle and as a framework for describing both electromagnetism and gravitation.He then describes how the abelian electromagnetic gauge-theory was generalized to its present non-abelian form. The development is illustrated by including a selection of relevant articles, many of them appearing here for the first time in English, notably by Weyl, Schrodinger, Klein, and London in the pre-war years, and by Pauli, Shaw, Yang-Mills, and Utiyama after the war. The articles illustrate that the reassessment of gauge-theory, due in a large measure to Weyl, constituted a major philosophical as well as technical advance.

Einstein's standard and battle-tested geometric theory of gravity--spacetime tells mass how to move and mass tells spacetime how to curve--is expounded in this book by Ignazio Ciufolini and John Wheeler. They give special attention to the theory's observational checks and to two of its consequences: the predicted existence of gravitomagnetism and the origin of inertia (local inertial frames) in Einstein's general relativity: inertia here arises from mass there. The authors explain the modern understanding of the link between gravitation and inertia in Einstein's theory, from the origin of inertia in some cosmological models of the universe, to the interpretation of the initial value formulation of Einstein's standard geometrodynamics; and from the devices and the methods used to determine the local inertial frames of reference, to the experiments used to detect and measure the "dragging of inertial frames of reference." In this book, Ciufolini and Wheeler emphasize present, past, and proposed tests of gravitational interaction, metric theories, and general relativity.They describe the numerous confirmations of the foundations of geometrodynamics and some proposed experiments, including space missions, to test some of its fundamental predictions--in particular gravitomagnetic field or "dragging of inertial frames" and gravitational waves.

In the 1930s, physics was in a crisis. There appeared to be no way to reconcile the new theory of quantum mechanics with Einstein's theory of relativity. Several approaches had been tried and had failed. In the post-World War II period, four eminent physicists rose to the challenge and developed a calculable version of quantum electrodynamics (QED), probably the most successful theory in physics. This formulation of QED was pioneered by Freeman Dyson, Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga, three of whom won the Nobel Prize for their work. In this book, physicist and historian Silvan Schweber tells the story of these four physicists, blending discussions of their scientific work with fascinating biographical sketches. Setting the achievements of these four men in context, Schweber begins with an account of the early work done by physicists such as Dirac and Jordan, and describes the gathering of eminent theorists at Shelter Island in 1947, the meeting that heralded the new era of QED.The rest of his narrative comprises individual biographies of the four physicists, discussions of their major contributions, and the story of the scientific community in which they worked. Throughout, Schweber draws on his technical expertise to offer a lively and lucid explanation of how this theory was finally established as the appropriate way to describe the atomic and subatomic realms.

The interpretation of quantum mechanics has been controversial since the introduction of quantum theory in the 1920s. Although the Copenhagen interpretation is commonly accepted, its usual formulation suffers from some serious drawbacks. Based mainly on Bohr's concepts, the formulation assumes an independent and essential validity of classical concepts running in parallel with quantum ones, and leaves open the possibility of their ultimate conflict. In this book, Roland Omnes examines a number of recent advances, which, combined, lead to a consistent revision of the Copenhagen interpretation. His aim is to show how this interpretation can fit all present experiments, to weed out unnecessary or questionable assumptions, and to assess the domain of validity where the older statements apply. Drawing on the new contributions, The Interpretation of Quantum Mechanics offers a complete and self-contained treatment of interpretation (in nonrelativistic physics) in a manner accessible to both physicists and students. Although some "hard" results are included, the concepts and mathematical developments are maintained at an undergraduate level.This book enables readers to check every step, apply the techniques to new problems, and make sure that no paradox or obscurity can arise in the theory. In the conclusion, the author discusses various philosophical implications pertinent to the study of quantum mechanics.

The past century has seen fantastic advances in physics, from the discovery of the electron, x-rays, and radioactivity, to the era of incredible solid state devices, computers, quarks and leptons, and the standard model. But what of the next? Many scientists think we are on the threshold of an even more exciting new era in which breakthroughs in a startling variety of directions will produce significant changes in our understanding of the natural world. In this book, a group of eminent scientists define and elaborate on these new directions. Ed Witten and Frank Wilczek discuss string theory and the future of particle physics; Donald Perkins describes the search for neutrino oscillations; Alvin Tollestrup reveals dreams of a muon collider at Fermilab to probe the heart of "elementary" particles; and Robert Palmer anticipates a new generation of particle accelerators. Thibault Damour reviews classical gravitation and the relevant new high-precision experiments; Kip Thorne describes the exciting future for gravitational wave astronomy; and Paul Steinhardt examines the recent breakthroughs in observational cosmology and explains what future experiments might reveal.James Langer explores nonequilibrium statistics and relates it to the origins of complexity; Harry Swinney takes an experimentalist's view of the emergence of order in seemingly chaotic systems; and John Hopfield describes an extremely unusual dynamical system--the human brain. Bruce Hillman, M. D., discusses the recent developments in imaging techniques that have brought about outstanding advances in medical diagnostics. T.V. Ramakrishnan looks at high-temperature superconductors, which could eventually revolutionize the solid-state technology on which society is already highly dependent.

Problems in theoretical physics often lead to paradoxical answers; yet closer reasoning and a more complete analysis invariably lead to the resolution of the paradox and to a deeper understanding of the physics involved. Drawing primarily from his own experience and that of his collaborators, Sir Rudolf Peierls selects examples of such "surprises" from a wide range of physical theory, from quantum mechanical scattering theory to the theory of relativity, from irreversibility in statistical mechanics to the behavior of electrons in solids. By studying such surprises and learning what kind of possibilities to look for, he suggests, scientists may be able to avoid errors in future problems. In some cases the surprise is that the outcome of a calculation is contrary to what physical intuition seems to demand. In other instances an approximation that looks convincing turns out to be unjustified, or one that looks unreasonable turns out to be adequate. Professor Peierls does not suggest, however, that theoretical physics is a hazardous game in which one can never foresee the surprises a detailed calculation might reveal.Rather, he contends, all the surprises discussed have rational explanations, most of which are very simple, at least in principle. This book is based on the author's lectures at the University of Washington in the spring of 1977 and at the Institut de Physique Nucleaire, University de Paris-Sud, Orsay, during the winter of 1977-1978.

Stochastic mechanics is a description of quantum phenomena in classical probabilistic terms. This work contains a detailed account of the kinematics of diffusion processes, including diffusions on curved manifolds which are necessary for the treatment of spin in stochastic mechanics. The dynamical equations of the theory are derived from a variational principle, and interference, the asymptotics of free motion, bound states, statistics, and spin are described in classical terms. In addition to developing the formal mathematical aspects of the theory, the book contains discussion of possible physical causes of quantum fluctuations in terms of an interaction with a background field. The author gives a critical analysis of stochastic mechanics as a candidate for a realistic theory of physical processes, discussing measurement, local causality in the sense of Bell, and the failure of the theory in its present form to satisfy locality.

This book presents articles written by leading experts surveying several major subfields in Condensed Matter Physics and related sciences. The articles are based on invited talks presented at a recent conference honoring Nobel laureate Philip W. Anderson of Princeton University, who coined the phrase "More is different" while formulating his contention that all fields of physics, indeed all of science, involve equally fundamental insights. The articles introduce and survey current research in areas that have been close to Anderson's interests. Together, they illustrate both the deep impact that Anderson has had in this multifaceted field during the past half century and the progress spawned by his insights. The contributors cover numerous topics under the umbrellas of superconductivity, superfluidity, magnetism, electron localization, strongly interacting electronic systems, heavy fermions, and disorder and frustration in glass and spin-glass systems.They also describe interdisciplinary areas such as the science of olfaction and color vision, the screening of macroions in electrolytes, scaling and renormalization in cosmology, forest fires and the spread of measles, and the investigation of "NP-complete" problems in computer science. The articles are authored by Philip W. Anderson, Per Bak and Kan Chen, G. Baskaran, Juan Carlos Campuzano, Paul Chaikin, John Hopfield, Bernhard Keimer, Scott Kirkpatrick and Bart Selman, Gabriel Kotliar, Patrick Lee, Yoshiteru Maeno, Marc Mezard, Douglas Osheroff et al., H. R. Ott, L. Pietronero et al., T. V. Ramakrishnan, A. Ramirez, Myriam Sarachik, T. Senthil and Matthew P. A. Fisher, B. I. Shklovskii et al., and F. Steglich et al.

In the Shadow of the Bomb narrates how two charismatic, exceptionally talented physicists--J. Robert Oppenheimer and Hans A. Bethe--came to terms with the nuclear weapons they helped to create. In 1945, the United States dropped the bomb, and physicists were forced to contemplate disquieting questions about their roles and responsibilities. When the Cold War followed, they were confronted with political demands for their loyalty and McCarthyism's threats to academic freedom. By examining how Oppenheimer and Bethe--two men with similar backgrounds but divergent aspirations and characters--struggled with these moral dilemmas, one of our foremost historians of physics tells the story of modern physics, the development of atomic weapons, and the Cold War. Oppenheimer and Bethe led parallel lives. Both received liberal educations that emphasized moral as well as intellectual growth. Both were outstanding theoreticians who worked on the atom bomb at Los Alamos. Both advised the government on nuclear issues, and both resisted the development of the hydrogen bomb.Both were, in their youth, sympathetic to liberal causes, and both were later called to defend the United States against Soviet communism and colleagues against anti-Communist crusaders. Finally, both prized scientific community as a salve to the apparent failure of Enlightenment values. Yet, their responses to the use of the atom bomb, the testing of the hydrogen bomb, and the treachery of domestic politics differed markedly. Bethe, who drew confidence from scientific achievement and integration into the physics community, preserved a deep integrity. By accepting a modest role, he continued to influence policy and contributed to the nuclear test ban treaty of 1963. In contrast, Oppenheimer first embodied a new scientific persona--the scientist who creates knowledge and technology affecting all humanity and boldly addresses their impact--and then could not carry its burden. His desire to retain insider status, combined with his isolation from creative work and collegial scientific community, led him to compromise principles and, ironically, to lose prestige and fall victim to other insiders.Schweber draws on his vast knowledge of science and its history--in addition to his unique access to the personalities involved--to tell a tale of two men that will enthrall readers interested in science, history, and the lives and minds of great thinkers.

The classic introduction to physical cosmology from Nobel Prize–winning physicist P. J. E. PeeblesPrinciples of Physical Cosmology is the essential introduction to this critical area of modern physics, written by a leading pioneer who has shaped the course of the field for decades. P. J. E. Peebles provides an authoritative overview of the field, showing how observation has combined with theory to establish the science of physical cosmology. He presents the elements of physical cosmology, including the history of the discovery of the expanding universe; surveys the cosmological tests that measure the geometry of space-time, with a discussion of general relativity as the basis for these tests; and reviews the origin of galaxies and the large-scale structure of the universe. Now featuring Peebles's 2019 Nobel lecture, Principles of Physical Cosmology remains an indispensable reference for students and researchers alike.

The classic account of the structure and evolution of the early universe from Nobel Prize–winning physicist P. J. E. PeeblesAn instant landmark on its publication, The Large-Scale Structure of the Universe remains the essential introduction to this vital area of research. Written by one of the world's most esteemed theoretical cosmologists, it provides an invaluable historical introduction to the subject, and an enduring overview of key methods, statistical measures, and techniques for dealing with cosmic evolution. With characteristic clarity and insight, P. J. E. Peebles focuses on the largest known structures—galaxy clusters—weighing the empirical evidence of the nature of clustering and the theories of how it evolves in an expanding universe. A must-have reference for students and researchers alike, this edition of The Large-Scale Structure of the Universe introduces a new generation of readers to a classic text in modern cosmology.

A landmark book on the influential many-worlds interpretation of quantum mechanicsIn 1957, Hugh Everett proposed a novel interpretation of quantum mechanics—a view that eventually became known as the many-worlds interpretation. This book presents Everett’s two landmark papers on the idea—“‘Relative State’ Formulation of Quantum Mechanics” and “The Theory of the Universal Wave Function”—as well as further discussion of the idea in papers from a number of other physicists: J. A. Wheeler, Bryce DeWitt, L. N. Cooper and D. Van Vechten, and Neill Graham.In his interpretation, Everett denies the existence of a separate classical realm and asserts the propriety of considering a state vector for the whole universe. Because this state vector never collapses, reality as a whole is rigorously deterministic. This reality, which is described jointly by the dynamical variables and the state vector, isn’t the reality customarily perceived; rather, it’s a reality composed of many worlds. By virtue of the temporal development of the dynamical variables, the state vector decomposes naturally into orthogonal vectors, reflecting a continual splitting of the universe into a multitude of mutually unobservable but equally real worlds, in each of which every good measurement has yielded a definite result, and in most of which the familiar statistical quantum laws hold.Bryce S. DeWitt (1923–2004) was a prize-winning theoretical physicist and professor emeritus of physics at the University of Texas at Austin. Neill Graham (1941–2015) was a physicist and writer.

The physics of extended systems is a topic of great interest for the experimentalist and the theoretician alike. There exists a large literature on this subject in which solutions, bifurcations, fronts, and the dynamical stability of these objects are discussed. To the uninitiated reader, the theoretical methods that lead to the various results often seem somewhat ad hoc, and it is not clear how to generalize them to the nextthat is, not yet solvedproblem. In an introduction to the subject of instabilities in spatially infinite systems, Pierre Collet and Jean-Pierre Eckmann aim to give a systematic account of these methods, and to work out the relevant features that make them operational. The book examines in detail a number of model equations from physics. The mathematical developments of the subject are based on bifurcation theory and on the theory of invariant manifolds. These are combined to give a coherent description of several problems in which instabilities occur, notably the Eckhaus instability and the formation of fronts in the Swift-Hohenberg equation.These phenomena can appear only in infinite systems, and this book breaks new ground as a systematic account of the mathematics connected with infinite space domains. Originally published in 1990. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

The last decade has seen striking progress in the subject of renormalization in quantum field theory. The old subject of perturbative renormalization has been revived by the use of powerful methods such as multiscale decompositions; precise estimates have been added to the initial theorems on finiteness of renormalized perturbation theory, with new results on its large order asymptotics. Furthermore, constructive field theory has reached one of its major goals, the mathematically rigorous construction of some renormalizable quantum field theories. For these models one can in particular investigate rigorously the phenomenon of asymptotic freedom, which plays a key role in our current understanding of the interaction among elementary particles. However, until this book, there has been no pedagogical synthesis of these new developments. Vincent Rivasseau, who has been actively involved in them, now describes them for a wider audience. There are, in fact, common concepts at the heart of the progress on perturbative and constructive techniques.Exploiting these similarities, the author uses perturbative renormalization, which is the more widely known and conceptually simpler of the two cases, to explain the less familiar but more mathematically meaningful constructive renormalization. Originally published in 1991. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

A timely addition to the literature, this volume contains authoritative reviews of three important areas in the physics of elementary particles. Sam B. Treiman, in "Current Algebra and PCAC," reviews the present state of the weak interactions. In "Field Theoretic Investigations in Current Algebra," Roman Jackiw deals with recent developments in current algebra and its applications, giving particular attention to anomalies. David J. Gross covers the high energy inelastic lepton-hadron scattering in his paper, "The High Energy Behavior of Weak and Electromagnetic Interactions." Originally published in 1972. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

This monograph combines a thorough introduction to the mathematical foundations of n-body Schrodinger mechanics with numerous new results. Originally published in 1971. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

Recently a symbiotic relationship between particle and nuclear physics has developed, with techniques and ideas from one field fertilizing developments in the other. This work outlines concepts from modern particle physics important to the current understanding of nuclear physics and reviews experiments involving nuclei which have influenced the present particle physics view of the weak interaction. In his discussion, the author summarizes both past and present activity in the field and identifies areas for future work. Familiarity with quantum mechanics is assumed in the presentation of ideas in this book intended for readers at the graduate level and beyond. A major goal of Weak Interactions in Nuclei is to encourage further activity at the intersection of particle and nuclear physics, two path-breaking areas of study in modern physics. Originally published in 1989. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press.These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

The physics of extended systems is a topic of great interest for the experimentalist and the theoretician alike. There exists a large literature on this subject in which solutions, bifurcations, fronts, and the dynamical stability of these objects are discussed. To the uninitiated reader, the theoretical methods that lead to the various results often seem somewhat ad hoc, and it is not clear how to generalize them to the nextthat is, not yet solvedproblem. In an introduction to the subject of instabilities in spatially infinite systems, Pierre Collet and Jean-Pierre Eckmann aim to give a systematic account of these methods, and to work out the relevant features that make them operational. The book examines in detail a number of model equations from physics. The mathematical developments of the subject are based on bifurcation theory and on the theory of invariant manifolds. These are combined to give a coherent description of several problems in which instabilities occur, notably the Eckhaus instability and the formation of fronts in the Swift-Hohenberg equation.These phenomena can appear only in infinite systems, and this book breaks new ground as a systematic account of the mathematics connected with infinite space domains. Originally published in 1990. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

Man's view of the universe is widening today, as it did once before in the early days of big telescopes and photographic plates. Modern man, by means of radio, infrared, optical, ultraviolet, and X-ray astronomy, can penetrate the universe to depths never before explored. P.J.E. Peebles has written a pioneering work in this newly defined area of investigation. Intended to bridge the chasm between classical textbooks on cosmology and modern developments, Physical Cosmology serves as a guide to current points of debate in a rapidly changing field. Originally published in 1972. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

This monograph combines a thorough introduction to the mathematical foundations of n-body Schrodinger mechanics with numerous new results. Originally published in 1971. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

Opinions on the large-scale structure of the early universe range widely from primeval chaos to a well-ordered mass distribution. P.J.E. Peebles argues that the evolution proceeded from a nearly uniform initial state to a progressively more irregular and clumpy universe. The discussion centers on the largest known structures, the clusters of galaxies, the empirical evidence of the nature of the clustering, and the theories of how the clustering evolves in an expanding universe. In Chapter One the author provides an historical introduction to the subject. Chapter Two contains a survey of methods used to deal with the Newtonian approximation to the theory of the evolution of the mass distribution. Recent progress in the use of statistical measures of the clustering is described in Chapter Three. Chapters Four and Five return to techniques for dealing with cosmic evolution, in the statistical measures of clustering and under general relativity theory. Lastly, in Chapter Six Professor Peebles assesses the progress in attempts to link theory and observation to arrive at a well established physical picture of the nature and evolution of the universe.