Tag Archives: Big bang

Cosmogenesis (9) : The Big Bang Discovery

Sequel of the preceding post Cosmogenesis (8) : The Nebular Hypothesis

Star Clusters and Nebulae. This page from "Telescopic views of Nebulae and Clusters by the Earl of Rosse and Sir J. Herschel" (1875) includes a variety of drawings of nebulosities by different observers. There are star clusters and gaseous nebulae (now known to belong to our own galaxy) as well as other galaxies. Observational techniques of the time were unable to distinguish between these very different types of objects.
Star Clusters and Nebulae. This page from “Telescopic views of Nebulae and Clusters by the Earl of Rosse and Sir J. Herschel” (1875) includes a variety of drawings of nebulosities by different observers. There are star clusters and gaseous nebulae (now known to belong to our own galaxy) as well as other galaxies. Observational techniques of the time were unable to distinguish between these very different types of objects.

In the first quarter of the 20th century cosmology became a distinct scientific discipline, thanks in part to the theoretical advance made in 1915 by Einstein with his theory of general relativity and in part to the revolution in observational techniques which revealed the true extent of the universe. Having at last been able to measure the distance of certain spiral nebulae, Edwin Hubble could confirm in 1925 that there existed other galaxies like our own.

His colleague Vesto Slipher had previously discovered that the radiation from these galaxies was constantly shifting towards the red end of the optical spectrum, which suggested that they were moving away from us at great speed. This movement was not understood until scientists came to accept an idea based on the theory of general relativity and first proposed by Alexandre Friedmann in 1922 and independently Georges Lemaître in 1927: that space was constantly expanding and consequently increasing the distance between galaxies. This idea proved to be one of the most significant discoveries of the century[i].

Alexander Friedmann in 1922
Alexander Friedmann in 1922

In an article which appeared in 1922, entitled “On the Curvature of Space“, Friedmann took the step which Einstein had balked at: he abandoned the theory of a static universe, proposing a “dynamic” alternative in which space varied with time. For the first time the problem of the beginning and the end of the universe was couched in purely scientific terms. Friedmann suggested that the universe was several tens of billions of years old, much older than the earth (then estimated to be about one billion years old) or the oldest known celestial objects. It was a remarkable prediction, the most recent estimate for the age of the universe being between 10 and 20 billion years.

In 1927, in a seminal article entitled “A Homogeneous Universe of Constant Mass and Increasing Radius Accounting for the Radial Velocity of Extra-Galactic Nebulae“, Lemaître explained the observations of Hubble and Slipher by interpreting them, within the context of general relativity, as manifestations of the expansion of the universe. This expansion was taking place uniformly across the entire universe (which might be finite or infinite), not outwards from a particular point (in this sense the often quoted analogy of a balloon being inflated is misleading). It was not a case of matter moving within a fixed geometric framework, but of the framework itself dilating, of the very “fabric” of space-time stretching. Continue reading

The Rise of Big Bang Models (4) : Lemaître

Sequel of previous post : Dynamical solutions

In this series of posts about the history of relativistic cosmology, I  provide an epistemological analysis of the developments of the field  from 1917 to 2006, based on the seminal articles by Einstein, de Sitter, Friedmann, Lemaître, Hubble, Gamow and other main historical figures of the field. It appears that most of the ingredients of the present-day standard cosmological model, including the accelation of the expansion due to a repulsive dark energy, the interpretation of the cosmological constant as vacuum energy or the possible non-trivial topology of space, had been anticipated by Lemaître, although his papers remain mostly  unquoted.

The discovery of expanding space

The 1920’s were precisely the time when the experimental data began to put in question the validity of static cosmological models. For instance, in 1924 the British theorist Arthur Eddington pointed out that, among the 41 spectral shifts of galaxies as measured by Vesto Slipher, 36 were redshifted ; he thus favored the de Sitter cosmological solution while, in 1925, his PhD student, the young Belgian priest Georges Lemaître, proved a linear relation distance-redshift in de Sitter’s solution. The same year 1925, Edwin Hubble proved the extragalactic nature of spiral nebulae. In other words, he confirmed that there existed other galaxies like our own, and the observable Universe was larger than previously expected. More important, the radiation from the faraway galaxies was systematically redshifted, which, interpreted as a Doppler effect, suggested that they were moving away from us at great speed. How was it possible ?

Arthur Eddington (1882-1944)
Arthur Eddington (1882-1944)

Young-Lemaitre
The young Georges Lemaître

It was Lemaître who solved the puzzle. In his 1927 seminal paper Un univers homogène de masse constante et de rayon croissant, rendant compte de la vitesse radiale des nébuleuses extragalactiques, published in French in the Annales de la Société Scientifique de Bruxelles, Lemaître calculated the exact solutions of Einstein’s equations by assuming a positively curved space (with elliptic topology), time varying matter density and pressure, and a non-zero cosmological constant. He obtained a model with perpetual accelerated expansion, in which he adjusted the value of the cosmological constant such as the radius of the hyperspherical space R(t) constantly increased from the radius of the Einstein’s static hypersphere RE at t = – ∞. Therefore there was no past singularity and no « age problem ». The great novelty was that Lemaître provided the first interpretation of cosmological redshifts in terms of space expansion, instead of a real motion of galaxies : space was constantly expanding and consequently increased the apparent separations between galaxies. This idea proved to be one of the most significant discoveries of the century. Continue reading

The Rise of Big Bang Models (3) : Friedmann’s Dynamical solutions

Sequel of previous post : Static Solutions

In this series of posts about the history of relativistic cosmology, I  provide an epistemological analysis of the developments of the field  from 1917 to 2006, based on the seminal articles by Einstein, de Sitter, Friedmann, Lemaître, Hubble, Gamow and other main historical figures of the field. It appears that most of the ingredients of the present-day standard cosmological model, including the accelation of the expansion due to a repulsive dark energy, the interpretation of the cosmological constant as vacuum energy or the possible non-trivial topology of space, had been anticipated by Lemaître, although his papers remain mostly  unquoted.

The Friedmann’s pioneering work

expanding-friedmannIn an article which appeared in 1922, entitled On the Curvature of Space (see Luminet 2004 for reference and translation), the Russian physicist Alexander Friedmann took the step which Einstein had balked at : he abandoned the theory of a static universe, proposing a “dynamic” alternative in which space varied with time. As he stated in the introduction, “the goal of this notice is the proof of the possibility of a universe whose spatial curvature is constant with respect to the three spatial coordinates and depend on time, e.g. on the fourth coordinate.

friedmann-equation
The Friedmann’s Equation. R is the curvature radius of space, rho the mass density, Lambda the cosmological constant, k the sign of the space curvature, G the gravitational constant, c the speed of light

Thus he assumed a positively curved space (hypersphere), a time variable matter density and a vanishing cosmological contant. He obtained his famous “closed universe model”, with a dynamics of expansion – contraction. Continue reading

The Rise of Big Bang Models (2) : Static solutions

Sequel of previous post :  From Myth to Science

In this series of posts about the history of relativistic cosmology, I  provide an epistemological analysis of the developments of the field  from 1917 to 2006, based on the seminal articles by Einstein, de Sitter, Friedmann, Lemaître, Hubble, Gamow and other main historical figures of the field. It appears that most of the ingredients of the present-day standard cosmological model, including the accelation of the expansion due to a repulsive dark energy, the interpretation of the cosmological constant as vacuum energy or the possible non-trivial topology of space, had been anticipated by Lemaître, although his papers remain mostly  unquoted.

The History of Relativistic Cosmology can be divided into 6 periods :

– the initial one (1917-1927), during which the first relativistic universe models were derived in the absence of any cosmological clue.

– a period of development (1927-1945), during which the cosmological redshifts were discovered and interpreted in the framework of dynamical Friedmann-Lemaître solutions, whose geometrical and mathematical aspects were investigated in more details.

– a period of consolidation (1945-1965), during which primordial nucleosynthesis of light elements and fossil radiation were predicted.

– a period of acceptation (1965-1980), during which the big bang theory triumphed over the « rival » steady state theory.

– a period of enlargement (1980-1998), when high energy physics and quantum effects were introduced for describing the early universe.

– the present period of high precision experimental cosmology, where the fundamental cosmological parameters are now measured with a precision of a few %, and new problematics arise (nature of the dark energy, topology of the universe, new cosmologies in quantum gravity theories, etc.)

Let us follow chonologically the rather hectic evolution of the ideas in the field. Continue reading

Luminet’s Illuminations

Luminet’s reinstated visualization of a finite Universe, albeit one from which we can exit through one face and simultaneously enter through the opposite one, relies upon a keplerian form of mental sculpture that may be described as plastic rather than algebraic. Luminet’s characteristic lithograph, Big Bang, exploits the spatial vocabulary of perspective to evoke realms beyond the three dimensional. Whereas Escher relied on contradictions and oscillating ambiguity in his graphic art, Luminet suggests plunging, interpenetrating and matter organizes itself into structures on the right; the tumbling dice on the left imply irreversible disorganization arising from chance. The remarkable range of Luminet’s creativity in art and science is integral to his agenda to recreate what he calls a « humanism of knowledge » — not that the arts and sciences are somehow to be conflated, because they work in very different ways, with illogical and logical means. But Luminet argues that they well up from the same instincts and intuitions: « I do not believe that we acquire at the beginning the ‘heart of an artist’ or the ‘heart of a scientist’. There is simply within oneself a single devouring curiosity about the world. This curiosity pushes us to explore it through various languages and modes of expression, » he says.

Martin Kemp, professor of the history of art at the University of Oxford. Excerpt from “Luminet’s Illuminations : Cosmological Modelling and the Art of Intuition”, Nature, 20 november 2003, vol. 426, p. 232

The Rise of Big Bang Models (1) : from Myth to Science

In this series of posts about the history of relativistic cosmology, I’ll  provide an epistemological analysis of the developments of the field  from 1917 to 2006, based on the seminal articles by Einstein, de Sitter, Friedmann, Lemaitre, Hubble, Gamow and other main historical figures of the field. It appears that most of the ingredients of the present-day standard cosmological model, including the accelation of the expansion due to a repulsive dark energy, the interpretation of the cosmological constant as vacuum energy or the possible non-trivial topology of space, had been anticipated by Lemaitre, although his papers remain mostly  unquoted.

 From Myth to Science

What are the origins of the universe, of the stars, of the earth, of life, of man? These questions have given rise to many different myths and legends, and today they are more than ever the subject of intensive research by astrophysicists, biologists and anthropologists. What were once fanciful stories are now scientific models; but whatever form they take, ideas about the origins of the universe both reflect and enrich the imagination of the people who generate them. Every society has developed its own stories to explain the creation of the world; all of them are ancient myths rooted in religion.

Whereas in monotheistic religions God is believed to have existed before the Creation, in most other kinds of religion the gods themselves are thought to originate from a creative element such as Desire, the Tree of the Universe, the Mundane Egg, Water, Chaos or the Void.

Tiamat-Marduk
Marduk slays the chaos dragon, Tiamat, in the Babylonian creation epic (British Museum, London)

A belief in some such primordial element, of which there are traces in every culture, underlies man’s thinking about the history of the cosmos like a primitive universal symbol buried in the collective subconscious. This may explain the vague links that can always be discerned between this or that creation myth and modern scientific descriptions of the origin of the universe –for example, big bang theory. There is therefore nothing mysterious or surprising about these correspondences other than that certain ways of thinking about the world should be so ingrained in the human mind.

An interesting approach, by the scientist and philosopher Wolfgang Smith (published in 2012)
An interesting approach, by the scientist and philosopher Wolfgang Smith (published in 2012)

In fact scientific and mythical explanations of the origins are neither complementary not contradictory; they have different purposes and are subject to different constraints. Mythical stories are a way of preserving collective memories, which can be verified neither by the storyteller nor by the listener. Their function is not to explain what happened at the beginning of the world but to establish the basis of social or religious order, to impart a set of moral values. Myths can also be interpreted in many different ways. Science, on the other hand, aims to discover what really happened in historical terms by means of theories supported by observation. Often considered to be anti-myth, science has in fact created new stories about the origin of the universe: big bang model, the theory of evolution, and the ancestry of mankind. It is therefore hardly surprising that the new creation stories developed by scientists tend to be regarded by the general public as modern myths.

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