Cosmology

12 Key excerpts on "Cosmology"

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  A First Course in General Relativity

  • Bernard Schutz(Author)
  • 2022(Publication Date)
  • Cambridge University Press

    (Publisher)

  13 Cosmology 13.1 What is Cosmology? The Universe in the large Cosmology is the study of the Universe as a whole: its history, evolution, composition, dynamics. The primary aim of research in Cosmology is to understand the large-scale structure of the Universe, but Cosmology also provides the arena, and the starting point, for the development of all the detailed small-scale structure that arose as the Universe expanded away from the Big Bang: galaxies, stars, planets, people. The interface between Cosmology and other branches of astronomy, physics, and biology is therefore a rich area of scientific research. Moreover, as astronomers have begun to be able to study the evidence for the Big Bang in detail, Cosmology has begun to address deeply fundamental questions of physics: what are the laws of physics at the very highest possible energies, how did the Big Bang happen, what (if anything) came before the Big Bang, how did the building blocks of matter (electrons, protons, neutrons) get made? Ultimately, the origin of every system and structure in the natural world, and possibly even the origin of the physical laws that govern the natural world, can be traced back to some aspect of Cosmology. Our ability to understand the Universe in the large depends in an essential way on general relativity. It is not hard to see why. Newtonian theory is an adequate description of gravity as long as, roughly speaking, the mass M of a system is small compared to its size, R : M/ R  1. We must replace Newtonian theory with GR if the system changes in such a way that M/ R gets close to 1. This can happen if the system’s radius R becomes small faster than M does, which is the domain of compact or collapsed objects: neutron stars and black holes have very small radii for the mass they contain. But we can also get to the relativistic regime if the system’s mass increases faster than its radius.

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  Relativity In Our Time

  • Mendel Sachs(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  23 Cosmology The title of this chapter comes from the two Greek words, ‘Cosmos’— meaning the universe—and ‘logos’—meaning reason. Thus, Cosmology refers to the reason, or order, that underlies the entire universe. The aim of the part of astrophysics that deals with the subject of Cosmology is then to gain some comprehension of the physical laws that underlie the behaviour of the universe, as a whole. In Chapter 22, in discussing the Hubble law, we covered the class of present- day astronomical observations that would describe the universe as a whole— though the actual job of interpreting these data is indeed quite complicated. Whatever theory of the universe, whether it is Einstein’s theory of general relativity or Newton’s universal gravitation, applied to the universe, or some new innovation, it would be required to explain and predict these data successfully, as well as making new predictions of yet unobserved phenomena in the cosmological domain. When the entire mass of the universe is involved, then according to the theory of general relativity, results interpreted with the formulations of the classical theories could be entirely fallacious, since the space and time of the universe as a whole is curved, globally, and the Hubble Law, as well as the intensity-distance relation, that we discussed previously, should be consistent with their formulations in terms of the riemannian geometrical system. In principle, then, one should start out to describe the universe in global terms. Astrophysicists have studied a few different models of the universe, the most popular one today being the ‘big bang model’ 1 , which assumes that at some initial time, all of the matter of the universe was so close together that there were no individual atoms or nuclei. According to George Gamow’s model 2 , all matter was initially a charge-independent neutron fluid. It was so dense that there was no empty space to talk about.

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  Principles of Physical Cosmology

  • P. J. E. Peebles, P. James E. Peebles(Authors)
  • 2020(Publication Date)
  • Princeton University Press

    (Publisher)

  I. The Development of Physical Cosmology I . The Standard Cosmological Model Physical Cosmology is the attempt to make sense of the large-scale na-ture of the material world around us, by the methods of the natural sciences. It is to be hoped that those who love physical science will take pleasure in Cosmology as an example of the art. It operates under the special restrictions of astronomy that allow one to look but not touch, but according to the rules and procedures that have proved to be so wonderfully successful in sister fields from stellar as-tronomy to particle physics. I will argue throughout this first part of the book that Cosmology as an enterprise in physical science really has made substantial progress, though the advances certainly have moved around considerable gaps in our understanding, as will be discussed in part 3. Behind physics is the more ancient and honorable tradition of attempts to un-derstand where the world came from, where it is going, and why. Cosmology inherits this tradition, in part by design, in larger part because that is where the as-tronomy and physics have led us. We have believable evidence that the universe is expanding, the space between the galaxies opening up, and that this expan-sion traces back to a hot dense phase, the big bang. The expansion may reverse in the future, and the world as we know it end in a collapse to a hot dense big crunch. An alternative is a universe that continues to expand indefinitely, to ar-bitrarily low mean density, but with most of the matter trapped in galaxies and clusters of galaxies that eventually contract (through loss of energy by evapora-tion of stars and gravitational radiation) and end up as black holes, in a series of little crunches. This is exciting stuff, and it has served a useful purpose in physical Cosmology in keeping us all occupied with speculations on how the world ought to begin and end, as we sort through the evidence of what really is happening.

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  Philosophy of Science: Teach Yourself

  • Mel Thompson(Author)
  • 2012(Publication Date)
  • Teach Yourself

    (Publisher)

  9 Cosmology In this chapter you will:

  • consider the ability of science to describe the origins and nature of the universe

  • examine the attempt to achieve a single explanatory theory

  • explore the human perspective on the universe.

  Understanding the nature of the universe as a whole has always been a central quest for both science and philosophy. As we saw in Chapter 2 , the rise of modern science, with its emphasis on observation and experiment and its use of mathematics, promoted a new approach to astronomy. As we look at the work of Copernicus, Galileo, Kepler, Newton and others, we see both the quest to understand the structure of the universe, but more specifically to discover the laws of physics which would account for the movement of bodies, both heavenly and terrestrial.

  In Cosmology, there are two interrelated sets of questions:

      1  The first question

   

  What is the structure of the universe? How did it originate? What is its future? How did it develop its present form? By what physical laws can we understand its workings?

   

  • Questions of this sort have led us from the Ptolemaic Earth-centred universe, through Copernicus to Newton, and on to the modern image of the world expanding outwards for the last 15 billion years from a spacetime singularity. They are concerned with order, structure and the process of development.

      2  The second question

  What is the simplest, most basic and most general thing we can say about reality? What lies beneath the multiplicity of what we see?

   

  • Questions of this kind started when Thales speculated that the world was essentially composed of water, and the atomists tried to find the basic building blocks of physical reality. They were implied within the world of Newtonian physics by the quest for ever more general and comprehensive laws of nature. More recently they have been expressed in the quest for a single ‘theory of everything’ that might show how the fundamental forces of the universe (electromagnetic, gravitational, strong and weak nuclear) relate to one another.

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  The Standard Model and Beyond

  • J D Vergados(Author)
  • 2017(Publication Date)
  • WSPC

    (Publisher)

  Chapter 10

  A Brief Introduction to Cosmology

  In this chapter we are going to discuss recent developments in Cosmology with emphasis on those aspects related to particle physics. As we will see the word, derived from the Greek word κo′σμoς, which means beautiful in the Greek view of the universe, indeed describes our current model of the universe 3000 years later.

  10.1Cosmological principles

  Before proceeding with consideration of the facts and observations of Cosmology, we are going to examine the basic underlying principles needed for a better understanding and organization of these facts. These are:

  i.First principle: The universe appears the same to all observers. ii.The universe is homogeneous and isotropic (at large enough scale)

  iii.The universe can be understood in physical terms, i.e. in terms of the laws governing the events in our microscopic laboratories and the theories explaining them. In particular in terms of the general theory of relativity.

  The first and the last principle are more or less obvious. One would like a picture of the universe, which is not observer dependent, especially in the absence of the ability to transform such information from one observer to the next. It is also obvious that one would like to have a picture of the universe, which incorporates as firm an information as our knowledge of the laws of nature permits.

  The second, principle, however seems to be counter-intuitive. The world around us, the planetary system, the structure of the galaxy and generally a look through the sky indicates that the density of matter and energy in the universe is not homogeneous and isotropic. Even the furthest segments we have been able to see are not quite isotropic and homogeneous, see Fig. 10.1 . And yet we accept this principle, crucial as it is in understanding the evolution of the universe, and expect it to hold, if we will be able to see even further.

  Fig. 10.1: Even at the scale of 1000 Mpc, approximately 2 × 109 light years the universe is not completely homogeneous and isotropic as seen by the 2df Galaxy Redshift Survey of the galaxy cluster Coma. One still sees regions of high density and voids extending up 50 Mpc or 160 × 106

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  Theoretical Concepts in Physics

  An Alternative View of Theoretical Reasoning in Physics

  • Malcolm S. Longair(Author)
  • 2020(Publication Date)
  • Cambridge University Press

    (Publisher)

  Case Study VII Cosmology AND PHYSICS Famously, Ernest Rutherford, Cavendish Professor of Experimental Physics and Head of the Cavendish Laboratory, stated: Don’t let me catch anyone talking about the Universe in my department. 1 He made this remark despite the fact that already in 1904 he had made the first unambiguous estimate of the age of the Earth, and consequently the Universe, from the study of radioactive decay chains. Understandably, in the early twentieth century, there were neither the observations nor the theoretical tools which could have made the large- scale structure of the Universe and its evolution an area of serious study for physicists. The many observational discoveries since the 1960s throughout the electromagnetic spectrum have completely changed this perspective. The physics of the large-scale structure and geometry of the Universe are now just as much integral parts of physics as is its structure on the microscopic scales of atoms, nuclei, elementary particles and so on. We now have a clear physical picture of the processes involved in the evolution of the Universe from the time it was of the order a microsecond old to the present era – this story is full of splendid physics and illustrates how the laws established in the laboratory can be applied successfully on the scale of the very large, as well as the very small. This new understanding has also posed a number of fundamental problems for physicists and cosmologists and these will undoubtedly become part of the new physics of the twenty-first century. Fortunately, the essential physics necessary to understand what is involved in reaching the present level of understanding can be developed rather precisely using the tools of an undergraduate physics course, combined with ideas from elementary general relativity discussed in Chapter 19. The two chapters of this case study concern the standard model of Cosmology, often referred to colloquially as the Big Bang.

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  Science and Christianity

  An Introduction to the Issues

  • J. B. Stump(Author)
  • 2016(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  94 Science and Christianity: An Introduction to the Issues, First Edition. By J. B. Stump. © 2017 John Wiley & Sons, Ltd. Published 2017 by John Wiley & Sons, Ltd. Questions to be addressed in this chapter: 1. What is the Big Bang? 2. How should we respond to the apparent fine tuning we observe in the universe? 3. What can we conclude about multiverses? CHAPTER 8 Cosmology T he study of Cosmology has long been fodder for dialogue and argu- ment between science and religion. As far back as Plato’s Timaeus in the 4th century BCE we find tension between what we today call a natural explanation of the existence of the cosmos and a supernatural explanation. We might expect that as science advances and develops more accurate theories, theological explanations would recede—the way they have for explanations of weather-related phenomena. But this expectation does not hold in Cosmology. The fantastic discoveries of 20th-century Cosmology—general relativity, the Big Bang, stellar nucleosynthesis, black holes, and so on—have not chased away those who see the hand of God at work in the universe. And in the case of fine tuning, a better understanding of the science of the cosmos has reinforced or even resurrected a theological impulse among some thinkers. In this chapter, we attempt to understand the connection between Cosmology and theology by looking at some of their central points of contact today: the Big Bang, fine tuning, and the multiverse hypothesis. 1. Big Bang Cosmology What does it mean? The term “Big Bang” refers to a couple of different scientific concepts, and it is important to keep them straight. Sometimes the term “Big Bang” is used as a label for the model of our current understanding of the development of the cosmos. As such, it refers to the entire history of the expansion of the universe. Alternatively, Cosmology 95 if we talk about the “Big Bang” as a singular event, things get more confusing because we don’t really know what that refers to.

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  Gravitation and Spacetime

  • Hans C. Ohanian, Remo Ruffini(Authors)
  • 2013(Publication Date)
  • Cambridge University Press

    (Publisher)

  9 Cosmology We are all in the gutter, but some of us are looking at the stars ... Oscar Wilde, Lady Windermere’s Fan. As we begin the study of the universe, we have to confront a fundamental question: Are the laws of physics that hold on or near the Earth also valid in distant regions of the universe? And are these laws of physics also valid at all times? When cosmologists look at distant regions of the universe, they see them as they were a long time ago – they see a galaxy at a distance of, say, 10 10 light-years as it was 10 10 year ago. To interpret the data collected in such observations, we need to know the laws of physics that govern these regions, far away in space and in time. Newton set a precedent for the universal validity of physical laws in his cosmological speculations. He conjectured that the (apparently) static distribution of the “fixed” stars was the result of an equilibrium of their mutual gravitational forces. He assumed that his inverse-square law for the gravitational force was also valid for distant stars, and that “the fixed stars, being equally spread out in all points of the heavens, cancel out their mutual pulls by opposite attractions” (Newton, 1713). 1 Einstein intended to follow Newton’s precedent by applying to the entire universe the field equation he had posited for the Solar System. But when he found that with these field equations he could not obtain a static solution for the mass distribution of the universe, he introduced new physics, in the form of a cosmological term added to the field equations. After the discovery of the expansion of the universe, Einstein promptly retracted his adoption of the cosmological term, and he thereafter strictly followed the example of Newton in treating the universe by the same laws as apply within the Solar System. In this chapter we will see that recent observational data on the acceleration of the expansion of the universe compel us to resurrect Einstein’s cosmological term.

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  The Blackwell Companion to Science and Christianity

  • J. B. Stump, Alan G. Padgett, J. B. Stump, Alan G. Padgett(Authors)
  • 2012(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Each of these possibilities necessarily leads to an engagement with science. Modern Cosmology attempts to come up with the most powerful and economical understanding possible of the universe that is consistent with observational data. It’s certainly conceivable that the methods of science could lead us to a self-contained picture of the universe that doesn’t involve God in any way. If so, would we be correct to conclude that Cosmology has undermined the reasons for believing in God, or at least a certain kind of reason?

  This is not an open-and-shut question. We are not faced with a matter of judging the merits of a mature and compelling scientific theory, since we don’t yet have such a theory. Rather, we are trying to predict the future: will there ever be a time when a conventional scientific model provides a complete understanding of the origin of the universe? Or, alternatively, do we already know enough to conclude that God definitely helps us explain the universe we see, in ways that a non-theistic approach can never hope to match?

  Most modern cosmologists are convinced that conventional scientific progress will ultimately result in a self-contained understanding of the origin and evolution of the universe, without the need to invoke God or any other supernatural involvement.1 This conviction necessarily falls short of a proof, but it is backed up by good reasons. While we don’t have the final answers, I will attempt to explain the rationale behind the belief that science will ultimately understand the universe without involving God in any way.

  The Universe We Know

  Cosmology studies the universe on the largest scales, and over large scales the most important force of nature is gravity. Our modern understanding of gravity is the theory of general relativity, proposed by Einstein in 1915. The key insight in this theory is the idea that space and time can be curved and have a dynamical life of their own, changing in response to matter and energy. As early as 1917, Einstein applied his new theory to Cosmology, taking as an assumption something we still believe is true: that on the largest scales, matter in the universe (or at least our observable part of it) is uniform through space. He also assumed, consistent with the apparent implication of observations at the time, that the universe is static. To his surprise, Einstein found that general relativity implied that any uniform universe would necessarily be non-static – either expanding or contracting. In response he suggested modifying his theory by adding a new parameter called the “cosmological constant,” which acted to push against the tendency of matter to contract together. With that modification, Einstein was able to find a static (but unstable) solution if the cosmological constant were chosen precisely to balance against the attraction of matter on large scales.

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  Modern Physics

  • Kenneth S. Krane(Author)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  530 Chapter 15 Cosmology: The Origin and Fate of the Universe 15.8 THE BIG BANG Cosmology The present universe is characterized by a relatively low temperature and a low density of particles. Its structure and evolution are dominated by the grav- itational force. Because the universe has been expanding and cooling, in the distant past it must have been characterized by a higher temperature and a greater density of particles. Let us imagine we could run the cosmic clock backward and examine the universe at earlier times, even before the forma- tion of stars and galaxies. At some point in its history, the temperature of the universe must have been high enough to ionize atoms; at that time the universe consisted of a plasma of electrons and positive ions, and the electro- magnetic force was important in determining the structure of the universe. At still earlier times, the temperature was hot enough that collisions between the ions would have knocked loose individual nucleons, so the universe consisted of electrons, protons, and neutrons, along with radiation. In this era, the strong nuclear force was important in determining the evolution of the universe. At still earlier times, the weak interaction played a significant role. If we try to go back still further, we reach a time when the matter of the uni- verse consisted only of quarks and leptons. Because we have never observed a free quark, we don’t know much about their individual interactions, and so we can’t describe this very early state of the universe. If someday we are able to understand the interactions of free quarks, we can penetrate this bar- rier and look to still earlier times. Eventually we reach a fundamental barrier when the universe had an age of only 10 −43 s, which is known as the Planck time (see Problem 26). Before this time, quantum theory and gravity are hope- lessly intertwined, and none of our present theories gives us any clue about the structure of the universe.

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  Physics : Imagination And Reality

  • P R Wallace(Author)
  • 1991(Publication Date)
  • World Scientific

    (Publisher)

  The geometry of the universe should, then, also be homogeneous on that scale. On smaller scales, however, it will have local deviations of its curvature on the galactic scale, and smaller, more local ones on the scales of star clusters and individual stars. Energy in the form of radiation should also be expected to be inhomogeneous on these scales. The theory of Cosmology then describes the structure of the universe only on the large scale. All of the above must be read as an exercise in hindsight. When the first relativistic cosmologies were being proposed, the very no-tion of galaxies had not yet emerged. Astronomers observed, how-ever, that the universe appeared to be statistically much the same wherever one looked. Another concept which survived from ancient times concerned the apparent unchangeability of the firmament. Astronomers from time immemorial had charted the motion of the same stars at the same relative positions in the sky; this was one of the most significant proofs of the local motions of earth and planets. Even in 176 Phytict: Imagination & Reality Einstein's early years, the heavenly landscape seemed fixed and con-stant. The new relativistic Cosmology, however, did not accommodate itself to this constancy. Einstein's equations only allowed universes undergoing expansion or contraction. This disturbed Einstein, and led him to modify his equations to include a cosmological term designed to restore stability. Intuitively it would seem that an ex-panding universe should be decelerated by the mutual gravitational attraction of its parts. If the speed of expansion were great enough, it might expand forever, though always at a constantly decreasing rate. If the expansion were slower it might ultimately stop and the universe might start to recontract; such contraction would then be expected to continue at an accelerating rate. Since neither of these alternatives seemed palatable, something additional was needed to give the cosmos its presumed stability.

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  A First Course in General Relativity

  • Bernard Schutz(Author)
  • 2009(Publication Date)
  • Cambridge University Press

    (Publisher)

  12.5 Further reading The literature on Cosmology is vast. In the body of the chapter I have given the principal references to original results, so I list here some recommended books on the subject. Standard Cosmology is treated in great detail in Weinberg ( 1972 ). Cosmological models in general relativity become somewhat more complex when the assumption of isotropy is dropped, but they retain the same overall features: the Big Bang, open vs. closed. See Ryan and Shepley ( 1975 ). A well-balanced introduction to Cosmology is Heidmann ( 1980 ). Early discussions on physical Cosmology that remain classics include Peebles ( 1980 ), Liang and Sachs ( 1980 ), and Balian et al. ( 1980 ). More modern is Liddle ( 2003 ). An important current research area is into inhomogeneous cosmologies. See MacCal-lum ( 1979 ). Another subject closely allied to theoretical Cosmology is singularity theory: Geroch and Horowitz ( 1979 ), Tipler et al. ( 1980 ). See also the stimulating article by Penrose ( 1979 ) on time asymmetry in Cosmology. For greater depth on physical Cosmology, see the excellent text by Mukhanov ( 2005 ). For a different point of view on ‘why’ the universe has the properties it does, see the book by Barrow and Tipler ( 1986 ) on the anthropic principle. For popular-level Cosmology articles written by research scientists, see the Einstein Online website: http://www.einstein-online.info/en/ . 370 Cosmology 12.6 Exercises 1 Use the metric of a two-sphere to prove the statement associated with Fig. 12.1 , that the rate of increase of the distance between any two points as the sphere expands (as measured on the sphere!) is proportional to the distance between them. 2 The astronomer’s distance unit, the parsec, is defined to be the distance from the Sun to a star whose parallax is exactly one second of arc.

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