Expanding Universe

11 Key excerpts on "Expanding Universe"

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  Matter And Spirit In The Universe: Scientific And Religious Preludes To Modern Cosmology

  Scientific and Religious Preludes to Modern Cosmology

  • Helge Kragh(Author)
  • 2004(Publication Date)
  • ICP

    (Publisher)

  Now the universe came to be seen as a vast congregation of galaxies, somewhat analogous to a gas made up of molecules. Until Hubble’s determination of a linear relationship between velocities and distances, the recession of the galaxies was not seen as indicating a changing universe. A book on astrophysics from 1924 included one of the very few speculations that the observed redshifts might have something to do with the past of the universe. The author of Modern Astrophysics , Herbert Dingle (on whom more later), sug-gested that the recession might be “the legacy of a huge disruption, in the childhood of matter, of a single parent mass.” The suggestion was not unlike the one on which Lemaître would base his big-bang pic-ture seven years later, but Dingle did not seriously believe in an exploding universe of a finite age. He considered the possibility that “we exist at a special point of time in a Universe which had a begin-ning in time” only to turn it down as “an assumption which we are as loth to make as the corresponding one, that we are at a particular place in a finite space.” 2 The Copernican principle was assumed to apply not only spatially, but also temporally. Meanwhile, important developments had occurred in theoretical cosmology. The great breakthrough—effectively the foundation of modern cosmology—was Einstein’s application of his new theory of general relativity to the universe at large. Einstein’s universe of 1917 was static and spatially finite in spite of having no boundary—a result due to space being conceived as positively curved. In order to secure its static character, he modified his gravitational field equa-tions by adding a term proportional to the metrical tensor, the famous cosmological term characterized by the constant . The model uni-verse was homogeneously filled with dilute matter, and could be ascribed a definite radius ( R ), volume, and mass.

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  Universe And The Atom, The

  • Don B Lichtenberg(Author)
  • 2007(Publication Date)
  • World Scientific

    (Publisher)

  In order to make his theory compatible with a static universe, Einstein modidfied his theory by adding a constant term, called the “cosmological constant,” that could be ad-justed to counteract the expansion or contraction and make the uni-verse static. It was not many years after Einstein introduced the cosmological constant that the American astronomer Edwin Hubble (1889–1953) discovered that the light from distant galaxies was redshifted. This fact implied that the far-off galaxies were not only moving but that their motion was not random—they were all receding from us. Fur-thermore, their speed was approximately proportional to their dis-tance away from us. These facts could be explained if all the material we observe was concentrated at one point (or a very small region) in space and then at one time suddenly exploded. If so, the faster the particles moved, the farther away they would be. The astronomer Fred Hoyle, who did not believe that the universe began in an ex-plosion, called it mockingly, “the big bang,” and the name has stuck. When Einstein heard that the universe was expanding he said his introduction of the cosmological constant was his biggest mistake. 19.1. EXPANSION OF THE UNIVERSE 271 However, he was premature in discarding the constant, as later de-velopments have shown. We shall discuss these developments later in this chapter. We have to discuss several issues in connection with the big bang. First, if the distant galaxies are all receding from the earth, does that make the earth the center of the universe? The answer is no—the galaxies are all receding from one another. We can get help in un-derstanding this fact by means of an analogy. Consider a balloon with dots marked on its surface. Then blow up the balloon. As it expands, every dot gets farther away from every other dot, so no dot can be considered as the center of the expansion of the balloon.

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  Introduction to High Energy Physics

  • Donald H. Perkins(Author)
  • 2000(Publication Date)
  • Cambridge University Press

    (Publisher)

  10 Particle physics and cosmology In this chapter, we discuss the connection between particle physics and the physics of the cosmos. This is not a text on cosmology or astrophysics, and all that we shall do here is reproduce a few of the essential features of the ‘Standard Model’ of the early universe, insofar as they affect and are affected by particle physics. The presently accepted cosmological model rests on four main pieces of experi-mental evidence: (i) Hubble’s law; (ii) the cosmic microwave background radiation; (iii) the cosmic abundances of the light elements; (iv) anisotropies in the background radiation, of the right magnitude to seed the formation of large-scale structure (galaxies, clusters, superclusters etc.). 10.1 Hubble’s law and the Expanding Universe As described in Section 1.9, Hubble in 1929 observed that spectral lines from distant galaxies appeared to be redshifted and interpreted this as a result of the Doppler effect associated with their velocity of recession v = β c , according to the formula λ = λ ( 1 + β)/( 1 − β) = λ( 1 + z ) (10.1) where λ is the wavelength in the rest frame of the source, and z = λ/λ is the redshift parameter, which has currently been measured up to values of z 5. Hubble deduced that for a particular galaxy, the velocity v is proportional to the distance r from Earth, v = Hr (10.2) where H is the so-called Hubble constant. Figure 1.11 shows the evidence supporting Hubble’s law. Although, for z 1 (10.1) can indeed be interpreted as a 303 304 10 Particle physics and cosmology Doppler shift, the factor 1 + z more generally describes an overall homogeneous and isotropic expansion of the universe (analogous to the stretching of a rubber sheet in the two-dimensional case), which expands all lengths – be they wavelengths or distances between galaxies – by a time-dependent universal factor R ( t ) .

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  The Universe

  A Historical Survey of Beliefs, Theories, and Laws

  • Britannica Educational Publishing, Erik Gregersen(Authors)
  • 2009(Publication Date)
  • Britannica Educational Publishing

    (Publisher)

  XPANDING STUDY OF THE UNIVERSE

  W hen the universe was viewed in the large in the early 20th century, a dramatic new feature, not present on small scales, emerged—namely, the cosmological expansion, as defined by the Hubble law. Hubble’s original value for the constant was 150 km (93 miles) per second per 1 million light-years. Modern estimates, using measurements of the cosmic microwave background (CMB), place the value at between 21.5 and 23.4 km (13.3 and 14.5 miles) per second per 1 million light-years. The reciprocal of Hubble’s constant lies between 13 billion and 14 billion years, and this cosmic timescale serves as an approximate measure of the age of the universe.

  THE NATURE OF SPACE AND TIME

  To speak of the expansion of space and time raises the question of what are space and time? What are their properties? Are they finite or infinite?

  An issue that arises when one contemplates the universe at large is whether space and time are infinite or finite. After many centuries of thought by some of the best minds, humanity has still not arrived at conclusive answers to these questions. Aristotle’s answer was that the material universe must be spatially finite, for if stars extended to infinity, they could not perform a complete rotation around Earth in 24 hours. Space must then itself also be finite because it is merely a receptacle for material bodies. On the other hand, the heavens must be temporally infinite, without beginning or end, since they are imperishable and cannot be created or destroyed.

  Except for the infinity of time, these views came to be accepted religious teachings in Europe before the period of modern science. The most notable person to publicly express doubts about restricted space was the Italian philosopher-mathematician Giordano Bruno, who asked the obvious question that, if there is a boundary or edge to space, what is on the other side? For his advocacy of an infinity of suns and earths, he was burned at the stake in 1600.

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  Modern General Relativity

  Black Holes, Gravitational Waves, and Cosmology

  • Mike Guidry(Author)
  • 2019(Publication Date)
  • Cambridge University Press

    (Publisher)

  As Hubble demonstrated by observing the wavelength of light from distant galaxies, the distance  between conserved particles is changing according to the Hubble law 1 The cosmological principle should not be confused with the perfect cosmological principle, which was the underlying idea of the steady state model. According to the perfect cosmological principle the Universe is homogeneous in both space and time; thus it looks the same not only from any place, but from any time. The steady state theory and perfect cosmological principle fell prey to various observations indicating that the early Universe looked different from the present one, invalidating the idea that the Universe is constant in time. 327 328 The Hubble Expansion Box 16.1 What is Expanding? The Hubble expansion is a cosmological effect observed in the large-distance behavior of the Universe. Small objects, such as our bodies, are held together by chemical (electrical) forces. They do not expand with the Universe. Even larger objects like planets, solar systems, and galaxies are also held together by forces, in this case gravitational in origin. They do not expand with the Universe either. It is only on much larger scales – beyond superclusters of galaxies – that gravitational forces among local objects are sufficiently weak to cause negligible perturbation on the overall Hubble expansion. v ≡ d dt = H 0 , (16.1) where H 0 is the Hubble constant and v is an apparent recessional velocity obtained by interpreting (incorrectly, as we shall see) the spectral shift as a Doppler shift. As a legacy of some past controversy over the value of H 0 , any uncertainty is sometimes absorbed into a dimensionless parameter h by expressing H 0 as H 0 = 100h km s −1 Mpc −1 = 3.24 × 10 −18 h s −1 , (16.2) where h is of order one.

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  The Mystery of the Missing Antimatter

  • Helen R. Quinn, Yossi Nir(Authors)
  • 2010(Publication Date)
  • Princeton University Press

    (Publisher)

  To replace an eternal Universe with one with a fixed starting time for its history seemed very strange to them. Indeed it raises deep questions about what was before that beginning. As yet, and perhaps for ever, these questions lie outside the realm of testable science, so we will have little to say about them. To Fred Hoyle the idea seemed outrageous enough that, to mock it, Hoyle coined the term Big Bang for the start of Hubble’s evolving Universe. Nowadays, however, the term is used with respect, for the evidence continues to pile up on Hubble’s side of the dispute. The Big Bang is the moment at which the expansion begins that leads to our currently observed Universe. We now know, with some confidence, that this occurred almost fourteen billion years ago. The Hubble Space Telescope, appropriately named after Edwin Hubble, provided important parts of the data that make us so confident of this conclusion. The idea of an evolving Universe, together with the assumption of immutability of the underlying laws of physics, is the basis of the subject of scientific cosmology. The aim of this game is to find a model history for the Universe that emerges from the Big Bang with some simple initial conditions and that, following the known laws of physics, evolves from that instant to give the Universe as we observe it at present, a model consistent with all the observations we can make. In this sense, the Universe can be thought of as an experimental test of our theory. It is a unique experiment! In a standard experiment, we prepare our experimental setup in some initial state, let the system evolve 10 c h a p t e r 2 in time, and measure features of the final state. If we figured out the relevant laws of nature and can model their implications correctly, we should be able to predict the results of these measurements. Conversely, if we fail to do so, we may have to modify our understanding of the laws of nature. In cosmology, the job is to run this pattern backward.

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  New Cosmology, The - Proceedings Of The 16th International Physics Summer School, Canberra

  • Matthew Malcolm Colless(Author)
  • 2005(Publication Date)
  • World Scientific

    (Publisher)

  Finally, in 1929, Hubble presented data in support of an Expanding Universe, with a clear plot of galaxy distance versus r e d ~ h i f t ~ ~ . It is for this paper that Hubble is given credit for discovering the Expanding Universe. Within two years, Hubble and Humason had extended the Hubble law out to 20000 km/s using the brightest galaxies, and the field of measuring extragalactic distance, from a 21st century perspective, made little substantive progress for the next 30 and some might argue even 60 years. 2. The Cosmological Paradigm Astronomers use a standard model for understanding the Universe and its evolution. The assumptions of this standard model, that general relativity is correct, and the Universe is isotropic and homogenous on large scales, are not proven beyond a reasonable doubt - but they are well tested, and they do form the basis of our current understanding of the Universe. If these pillars of our standard model are wrong, then any inferences using this model about the Universe around us may be severely flawed, or irrelevant. The standard model for describing the global evolution of the Universe is based on two equations that make some simple, and hopefully valid, assumptions. If the universe is isotropic and homogenous on large scales, the Robertson-Walker metric, ds2 = dt2 - a(t) [g dr2 + r2d02] . gives the line element distance (s) between two objects with coordinates r,8 and time separation, t. The Universe is assumed to have a simple topology such that if it has negative, zero, or positive curvature, k takes the value -l , O , 1, respectively. These universes are said to be open, flat, or closed, respectively. The dynamic evolution of the Universe needs to be input into the Robertson-Walker metric by the specification of the scale factor a(t), which gives the radius of curvature of the Universe over time - or more simply, provides the relative size of a piece of space at any time.

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  Gravity from the Ground Up

  An Introductory Guide to Gravity and General Relativity

  • Bernard Schutz(Author)
  • 2003(Publication Date)
  • Cambridge University Press

    (Publisher)

  The Hubble law would still apply, as measured by an observer on any dot. And still no dot would be in a special position. Cosmologists make a distinction between local and global properties of the uni- verse. Local properties are measurable directly. Global ones are properties of the Universe as a whole; as we emphasized earlier, these are usually more hypotheti- cal. The Hubble expansion, for example, does not define the global structure of our rubber-band universe: it cannot tell us whether the Universe looks like a straight piece of rubber or a circular rubber band. It only tells us how it stretches, locally. In Chapter 25 and Chapter 26 we will take up the subject of what the Universe looks like in the large. 350 Chapter 24. Cosmology Our picture of the Universe as a homogeneous, expanding “gas” of galaxies is really very simple. What does this tell us about the Universe at earlier times? If we add to the picture our expectation that gravity is attractive, so that the different parts of the Universe will be pulling back on each other, then we expect that the expansion should be slowing down. If so, then the expansion rate in the past would actually have been faster than it is today, and at some finite time in the past all the gas and galaxies in the Universe would have been squeezed together to an infinitely high density. This moment of infinite density is another example of The homogeneity of the Universe makes this infinite density inevitable: everything in the Universe came together at exactly the same moment. a singularity, just as we found inside a black hole. Where the singularity in the black hole is the end of time for any particle that encounters it, the cosmological singularity is the beginning of time for all particles in the Universe. The expansion of the Universe away from this singularity is what we call the Big Bang.

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  A Fortunate Universe

  Life in a Finely Tuned Cosmos

  • Geraint F. Lewis, Luke A. Barnes(Authors)
  • 2016(Publication Date)
  • Cambridge University Press

    (Publisher)

  Thus, we expected distant 138 the universe is expanding supernovae to be of a particular brightness. Two groups of astrono- mers, led by Brian Schmidt, Adam Riess and Saul Perlmutter, observed that supernovae are actually dimmer than anticipated. In fact, they were so dim that the Universe doesn’t appear to be decelerating at all. Einstein’s equations tell us that the expansion must be accelerating. Here’s the conundrum. All the particles that you’ve ever seen, and that astronomers have ever seen through their telescopes, and that we’ve created in particle accelerators, cause the expansion of the Universe to slow down. The expansion of the Universe is speeding up. The stage is moving in ways that no known actor can cause. What’s going on out there? We need another casting call: quirky character required for starring role in accelerating spacetime. No time-wasters. We called this unfami- liar something dark energy. We’re-even-more-in-the-dark energy. What could make the expansion of the Universe accelerate? Einstein’s equations provide a potential solution, suggesting that this acceleration is due to a strange property of spacetime itself known as the cosmological constant. When Einstein derived his famous equations, he was presented with a few forks in the road. Being a good physicist, he took the simplest option. For example, he could have considered twisty spacetimes (technically known as tor- sion), but with no need to add this complication, he left it out. At another point, Einstein could have added an extra term to his equations that included the cosmological constant. Einstein’s first con- cern with his equations was to reproduce the familiar pull of gravity in the Solar System, and since the cosmological constant didn’t help with that, he left it out too.

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  Fundamentals of Astrophysics

  • Stan Owocki(Author)
  • 2021(Publication Date)
  • Cambridge University Press

    (Publisher)

  And as points were added at larger distances, they did indeed show the expected trend above this linear Hubble law, marked by the purple line in Figure 30.2. But in one the greatest surprises of modern astronomy, and indeed of modern sci- ence, such data points were found to generally lie below the black curve that represents a nearly-empty universe, with a constant expansion rate ˙ R = H o . This immediately rules out all the decelerating models that lie above this black curve representing constant-rate expansion. Instead it implies that the expansion of the universe must be accelerating! 244 31 Accelerating Universe 31.2 Cosmological Constant and Dark Energy For the universe’s expansion to be accelerating, it is required that, in opposition to the attractive force of gravity, there must be a positive, repulsive force that pushes galaxies apart. Ironically, in an early (∼1917) application of his General Relativity theory, Einstein had posited just such a universal repulsion term – dubbed the “cosmological constant,” and traditionally denoted Λ. This was introduced to balance the attractive force of gravity, and so allow for a static, and thus eternal, model of the universe, which was the preferred paradigm at that time. Then, after Hubble’s discovery that the universe is not static but expanding, Einstein completely disavowed this cosmological- constant term, famously calling it “his greatest blunder.” But nowadays, with the modern discovery that this expansion is actually accelerat- ing, the notion of something akin to the cosmological constant has been resurrected. The full physical bases and origin are still quite unclear, but the effect is often charac- terized as a kind pressure or tension of spacetime itself, with associated mass–energy density, dubbed dark energy, parameterized in terms of the fraction Ω Λ of the critical mass–energy density ρ co c 2 .

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  Precision Cosmology

  The First Half Million Years

  • Bernard J. T. Jones(Author)
  • 2017(Publication Date)
  • Cambridge University Press

    (Publisher)

  The uncertainty was considerable: it took many decades to sort this out. 16 1.3.2 The Physics of the Big Bang – George Gamow Not everyone, and notably Hubble himself, were happy with the Expanding Universe inter- pretation of the redshift–distance relation and there were important alternative theories. Notable among these was the ‘Steady State Theory’ which proposed that, instead of all the matter in the Universe being created at one instant in a cataclysmic event, the mate- rial would be created continuously at just the rate required to fill up the space left by the expansion. The Big Bang versus Steady State controversy raged for 20 years. The definitive evidence about the physical nature of the Universe came in 1965 with the discovery of the Cosmic Microwave Background Radiation (‘CMB’). This radiation field was interpreted as natural consequence of a Hot Big Bang theory in which the Universe was hotter in the past. 17 The idea that the Big Bang would have been hot enough to synthesise 15 The final sentence of this great paper is ‘It also seems desirable to express an open-minded position as to the true cause of the nebular red-shift, and to point out the indications that the spatial curvature may have to play a part in the explanation of the existing nebular data’. This was a statement that the Hubble diagram would eventually provide a measure of the geometric curvature of the Universe. 16 The discovery of Quasars in the early 1960s provided a flurry of excitement since they were bright objects with higher recession velocity than any hitherto measured galaxy. Schmidt (1965) measured the first redshift in excess of 1.0. However, they were manifestly not standard candles and since there was no way of estimating their intrinsic luminosity they added little to the story of the expansion of the Universe except perhaps to cause a certain amount of controversy, see Section 2.5.

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