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One Hundred Years of the Cosmological Constant: from 'Superfluous Stunt' to Dark Energy

We present a centennial review of the history of the term known as the cosmological constant. First introduced to the general theory of relativity by Einstein in 1917 in order to describe a universe that was assumed to be static, the term fell from favour in the wake of the discovery of the expanding universe, only to make a dramatic return in recent times. We consider historical and philosophical aspects of the cosmological constant over four main epochs: (i) the use of the term in static cosmologies (both Newtonian and relativistic); (ii) the marginalization of the term following the discovery of cosmic expansion; (iii) the use of the term to address specific cosmic puzzles such as the timespan of expansion, the formation of galaxies and the redshifts of the quasars; (iv) the re-emergence of the term in today's Lamda-CDM cosmology. We find that the cosmological constant was never truly banished from theoretical models of the universe, but was sidelined by astronomers for reasons of convenience. We also find that the return of the term to the forefront of modern cosmology did not occur as an abrupt paradigm shift due to one particular set of observations, but as the result of a number of empirical advances such as the measurement of present cosmic expansion using the Hubble Space Telescope, the measurement of past expansion using type SN 1a supernovae as standard candles, and the measurement of perturbations in the cosmic microwave background by balloon and satellite. We give a brief overview of contemporary interpretations of the physics underlying the cosmic constant and conclude with a synopsis of the famous cosmological constant problem.


Introduction
First introduced one hundred years ago, the mathematical entity known as the cosmological constant plays a central role in modern cosmology. However, the term has had a chequered career over the century. Initially added by Einstein to the field equations of general relativity in order to allow a consistent model of a universe that was assumed to be static (Einstein 1917a), it was gradually sidelined following the emergence of evidence for an expanding universe in the 1930s. Yet the term was invoked on several occasions over the next few decades as a tool to address diverse empirical puzzles such as the timespan of cosmic expansion, the formation of galaxies and the redshifts of quasars. The term also found application in alternate cosmologies such as the steady-state model of the universe, and more recently, in the theory of cosmic inflation. Later still, it was invoked in the 1990s to address puzzles concerning anisotropies in the cosmic microwave background (CMB) and problems concerning the cold dark matter (CDM) model of structure formation. With the discovery of an acceleration in cosmic expansion, sophisticated measurements of cosmic geometry, and new advances in the study of galaxy formation and galaxy clustering, the term returned to the centre stage of modern cosmology in the early years of the 21 st century.
The aim of this article is to provide a comprehensive historical review of the story of the cosmological constant. Our review will draw on primary materials made available online by the Einstein Papers Project 1 and on original papers in cosmology, astronomy and astrophysics. We will also draw on classic reviews of the cosmological constant such as (Zel'dovich 1968;McCrea 1971;Weinberg 1989;Carroll, Press and Turner 1992) and more recent 'post-dark energy' discussions such as (Straumann 1999;Earman 2001;Rugh and Zinkernagel 2002;Carroll 2001;Peebles and Ratra 2003;Ellis 2003;Turner and Huterer 2007). While some material is technical by nature, an effort will be made to present a coherent narrative that is comprehensible to physicists and historians alike.
Our article begins with a brief description of the use of a term similar to the cosmological constant in Newtonian cosmology. In section 3, we recall Einstein's use of the term in his 1917 'static' model of the universe, the alternate model proposed by Willem de Sitter and the 1 The Einstein Papers Project is an invaluable online historical archive of primary sources provided by Princeton University Press in conjunction with the California Institute of Technology and the Hebrew University of Jerusalem. http://einsteinpapers.press.princeton.edu/. debate between the two. 2 The use of the term in the 1920s in the cosmologies of Alexander Friedman and Georges Lemaître is recalled in section 4; this section also includes a brief description of early ideas concerning the zero-point energy of the vacuum. In section 5, different attitudes to the cosmological constant following the discovery of cosmic expansion are described. In sections 6 and 7, we recall the use of the term to address various issues in theoretical and observational cosmology in the years 1940-1970, while the relevance of the term for inflationary cosmology is described in section 8. The gradual re-evaluation of the cosmological constant in the 1990s is discussed in section 9, and the emergence of the term as a centrepiece of today's Λ-CDM model of the universe is described in section 10. We give a brief review of contemporary interpretations of the physics underlying the cosmological constant in section 11 and conclude with a brief summary in section 12.

A cosmological constant in Newtonian cosmology
In modern usage, the term 'cosmological constant' refers to a mathematical term added to the field equations of general relativity in order to give a consistent model of the universe.
However, it is worth recalling that a similar entity was employed in Newtonian cosmology, long before the advent of the general theory of relativity.
Towards the end of the 19th century, the eminent German astronomer Hugo von Seeliger noted that, assuming a cosmos of infinite spatial extent 3 and a non-zero, uniform density of matter on the largest scales, the Newtonian gravitational force at any point is indeterminate (Seeliger 1895(Seeliger , 1896. 4 The problem had been noted previously, but Seeliger was the first to address the paradox in a systematic manner (North 1965, pp 16-17;Norton 1999). As he later remarked: "About two years ago, I drew attention to certain difficulties arising out of the attempt to extend Newton's law of gravitation to infinite space. The considerations then adduced showed the necessity of choosing between two hypotheses, viz: 1. The sum total of the masses of the universe is infinitely great, in which case Newton's law cannot be regarded as a mathematically exact expression for the attractive forces in operation. 2. The Newtonian law is rigourously exact, in which case the infinite spaces of the universe cannot be filled with matter of finite density. Inasmuch as I am wholly unable to find adequate reasons for the second of these assumptions, I have, in another place, decided in favour of the first" (Seeliger 1898a).
Seeliger's solution was to modify the familar Newtonian force with an exponential decay factor that would become significant only at extremely large distances. Expressing his argument in terms of the gravitational potential where G is Newton's gravitational constant and is the mean density of matter in a volume element of space dV, Seeliger noted first that the Newtonian potential would not be defined as the distance r tended to infinity. His suggestion was to redefine the potential according to where λ was a decay constant sufficiently small to make the modification significant only at extremely large distances. In terms of classical mechanics, this modification amounted to replacing the well-known Poisson equation with the relation where 2 is the Laplacian operator. A similar modification of the Newtonian potential -albeit in a slightly different context -had earlier been suggested by physicists such as Pierre-Simon Laplace (Laplace 1846) and Carl Neumann (Neumann 1896 pp 373-379). 5 Thus, we note that the concept of a cosmological constant -in the sense of a constant decay term added to standard gravitation theory in order to give a consistent model of the universe -was a feature of theoretical cosmology before the advent of relativistic models of the universe. It is also interesting to note that Seeliger suggested that an estimate of λ might 5 See (Ray 1990) or (Norton 1999 (1) (2) be obtained from astronomical measurement of anomalies in the orbits of the planets (Seeliger 1895), although this proposal was not successful (Seeliger 1896;Norton 1999).

The cosmological constant in the 1910s: the cosmologies of Einstein and de Sitter
In November 1915, Einstein published the field equations of the general theory of relativity.
This was a set of covariant equations that related the geometry of a region of space-time to the distribution of matter/energy within it, according to Here, is a four-dimensional tensor representing the curvature of space-time (known as the Ricci curvature tensor), is a four-dimensional tensor representing energy and momentum, T is a scalar, and is the Einstein constant 8 G 2 ⁄ (Einstein 1915a). A few years later, it was realised that the general field equations could be written in simpler form as where G (= ) is a scalar known as the Ricci curvature scalar. 6 Thus, in the general theory of relativity, the flat 'Minkowskian' space-time interval of special relativity 2 = − 2 − 2 − 2 + 2 2 is replaced by a space-time interval of curved geometry given by 2 = ∑ 3 , =0 6 We employ the nomenclature used by Einstein in the years [1915][1916][1917]. Nowadays, the Ricci curvature tensor and Ricci scalar are denoted by and R respectively.
where the elements of the space-time tensor are determined by the distribution and flux of matter/energy. In his seminal papers of 1915 and 1916, Einstein noted that general relativity passed an important empirical test; as well as furnishing a description of the orbits of the planets that was compatible with astronomical observation, the new theory accounted for a well-known anomaly for the case of the motion of Mercury (Einstein 1915b(Einstein , 1916. Another important test for the general theory of relativity was the issue of a consistent cosmology, i.e., whether the theory could give a consistent description of the universe as a whole. As Einstein later remarked to the Dutch astronomer Willem de Sitter: "For me, though, it was a burning question whether the relativity concept can be followed through to the finish, or whether it leads to contradictions. I am satisfied now that I was able to think the idea through to completion without encountering contradictions" (Einstein 1917b Einstein's starting point for his 1917 model of the cosmos was the assumption of a static, non-zero distribution of matter, and therefore a static metric of positive spatial curvature. As he remarked in the paper: "The most important fact that we draw from experience as to the distribution of matter is that the relative velocities of the stars are very small as compared with the velocity of light. So I think that for the present we may base our reasoning upon the following approximate assumption. There is a system of reference relatively to which matter may be looked upon as being permanently at rest" (Einstein 1917a). This assumption was reasonable at the time; many years were to elapse before the discovery of a linear relation between the recession of the distant galaxies and their distance (Hubble 1929), the first evidence for a non-static universe.
A second assumption was that of a uniform distribution of matter (Einstein 1917a). This assumption implied a universe that was both isotropic and homogeneous on the largest scales and was later named the 'Cosmological Principle' (Milne 1935 p24). One reason for the principle was simplicity, as it greatly simplified the business of solving the field equations. A deeper reason was that the principle chimed with the spirit of relativity and with a Copernican approach to cosmology. After all, to assume otherwise was to posit a universe in which all viewpoints were not equivalent; indeed, it's worth noting that the Cosmological Principle was originally named "the extended principle of relativity" (Milne 1933 Here the represent the familiar components of the spacetime metric and is a universal constant that became known as the cosmological constant (Einstein 1917a (Einstein 1917a). 8 The only constraint was that the new constant had to be small enough to make the modification significant only at extremely large distances, in order that the field equations remained compatible with observations of the motion of the planets in our solar system: "This field equation, with λ sufficiently small, is in any case also compatible with the facts of experience derived from the solar system" (Einstein 1917a).
Einstein then showed that, for the case of the universe as a whole, the modified field equations (9) have the solution = 2 = 1 2 where represents the cosmological constant, ρ is the mean density of matter and R represents the radius of the cosmos (Einstein 1917a). Thus, his model gave a satisfactory 7 Einstein's interpretation of Mach's Principle in these years implied that space could not have a physical reality an infinite distance from matter. This created a puzzle regarding the correct boundary conditions for his cosmology, a puzzle that was only resolved by the postulate of a closed spatial geometry for the universe (Realdi and Peruzzi 2009;Smeenk 2014;O'Raifeartaigh et al. 2017).

8
It is sometimes stated that Einstein first introduced the cosmological term in a footnote to section 14 of his 1916 'Grundlage' paper (Harvey 2012a;Straumann 2013 p80;Kragh and Overduin 2014 p48). This is not quite correct as Einstein's footnote concerned the Ricci scalar rather than an additional term in the field equations (Einstein 1916 p33).
relation between the size of the cosmos and the matter it contained. Indeed, in his correspondence around this time, Einstein attempted a rough estimate of the size of the universe (and thus of the cosmological constant) from estimates of the density of matter in the Milky Way, although he later realised that such calculations were unreliable (O'Raifeartaigh et al. 2017). He also suggested that a value for the cosmological constant might be estimated by searching for departures from Newtonian predictions in the motion of astrophysical objects (in a manner similar to Seeliger's earlier suggestion), but found the method unsuccessful due to uncertainties in observation (Einstein 1921a(Einstein , 1921b where λ denotes a universal constant. If 0 be the uniform density of a distribution of mass, then = − (4 ) ⁄ independently by Einstein 13 as a "foil for what is to follow" (Einstein 1917a). However, Einstein's modification of the general field equations in the paper was not "perfectly analogous" to his modification of Newtonian gravity, contrary to his claim. As pointed out by several analysts, 14 the modified field equations (9) do not reduce in the Newtonian limit to the Seeliger-Poisson equation (4), but to a different relation given by 2 + 2 λ = 4 G This error may be significant as regards Einstein's interpretation of the cosmic constant. After all, the later view of the cosmological constant term as representing a tendency for empty space to expand would have been deeply problematic for Einstein in 1917, given his view of Mach's Principle at the time (Smeenk 2014;O'Raifeartaigh et al. 2017). It appears that he was shielded from this interpretation by a slight mathematical error, at least in the early years.
Another strange aspect of Einstein's 1917 memoir was his failure to consider the stability of his static model. After all, equation (10) draws a direct relation between a universal constant , the radius of the universe R, and the density of matter ρ. But the quantity ρ represented a mean value for the latter parameter, arising from the theoretical assumption of a uniform distribution of matter on the largest scales. In the real universe, one would expect a natural variation in this parameter (giving rise to the formation of structure), raising the question of the stability of the model against perturbations in density. It was later shown that the Einstein World is generally unstable against such perturbations: a slight increase in the density of matter would cause the universe to contract, become more dense and contract further, while a slight decrease in density would result in a runaway expansion (Eddington 1930: Eddington 1933. It is strange that Einstein did not consider this aspect of his model in 1917; some years later, it was a major reason for rejecting the model, as described below.

The de Sitter universe
In July 1917, the Dutch astronomer and theorist Willem de Sitter noted that Einstein's modified field equations allowed an alternate cosmic solution, namely the case of a universe 13 In a paper of 1919 (Einstein 1919b), Einstein remarked that he was unaware of Seeliger's modification of Newtonian gravity when writing his cosmological memoir in 1917. He cited Seeliger scrupulously after this point (Einstein 1918d p123;Einstein 1931a;Einstein 1933). 14 See for example (Rindler 1969 p223;Norton 1999;Harvey and Schucking 2000;Earman 2001). (11) with no matter content (de Sitter 1917). Approximating the known universe as an empty universe, de Sitter set the energy-momentum tensor in Einstein's modified field equations (9) to zero according to and showed that these equations have the solution = 0; = 3 2 a result he dubbed 'Solution B' to Einstein's 'Solution A' (de Sitter 1917). In this cosmology, Einstein's matter-filled three-dimensional universe of closed spatial geometry was replaced by an empty four-dimensional universe of closed spacetime geometry.
Not surprisingly, Einstein was greatly perturbed by de Sitter's empty universe. Quite apart from the fact that the model was physically unrealistic, the existence of a vacuum solution for the cosmos was in direct conflict with his understanding of Mach's Principle in these years (Smeenk 2014;O'Raifeartaigh et al. 2017 (Schulmann et al. 1988 pp 351-352). After much confusion, it was eventually realised that the de Sitter solution is not truly a static solution.
Indeed, it was shown that the de Sitter metric can be represented by the simple line element 2 = √ (− 2 − 2 − 2 ) + 2 2 (12) where a and λ are arbitrary constants (Lemaître 1925). Presumably, this aspect of the de Sitter universe would have made Einstein even less trusting of the model; it is telling that he did not cite the de Sitter solution in any of his reviews of cosmology around this time (Einstein 1918d(Einstein pp 116-118, 1921a(Einstein , 1922a. It is also interesting to speculate that the existence of a mathematically viable vacuum solution to the modified field equations may have marked the beginning of Einstein's distrust of the cosmological constant.

The cosmological constant in the 1920s
In 1922, the Russian physicist Alexander Friedman suggested that non-static solutions of the Einstein field equations should be considered in relativistic models of the cosmos (Friedman 1922). Starting from the modified field equations (9) and assuming a positive spatial curvature for the cosmos, he derived the two differential equations Few physicists paid attention to Friedman's time-varying cosmology, possibly because the work was quite technical and made no connection to astronomy. Worse, Einstein publicly criticized the paper on the basis that it contained a mathematical error (Einstein 1922b).
When it transpired that the error lay in Einstein's criticism, it was retracted a year later (Einstein 1923a). However, an unpublished draft of Einstein's retraction demonstrates that he did not consider Friedman's cosmology to be realistic: "to this a physical significance can hardly be ascribed" (Einstein 1923b). 16 A few years later, the Belgian physicist Georges Lemaître independently derived differential equations for the radius of the cosmos from Einstein's modified field equations.
Aware of astronomical observations of the redshift of light from the spiral nebulae by V.M.
Slipher (Slipher 1915(Slipher , 1917 and of emerging evidence of the extra-galactic nature of the spirals (Hubble 1925), Lemaître suggested that the recession of the nebulae was a manifestation of the expansion of space from a pre-existing static cosmos of radius 0 = 1 √ ⁄ (Lemaître 1927). 17 This work also received very little attention at first, probably because it was published in a little-read Belgian journal. Lemaître himself brought the work to Einstein's attention in conversation, only to hear the latter dismiss expanding cosmologies as "abominable" (Lemaître 1958). Describing the meeting many years later, Lemaître recalled an impression that Einstein's stance stemmed from a lack of knowledge of developments in astronomy: "Je parlais de vitesses des nébeleuses et j'eus l'impression que Einstein n'était guère au courant des faits astronomiques" (Lemaître 1958).

Zero-point energy and the quantum vacuum
In 1911, the great German theorist Max Planck proposed that the lowest value of the energy of an oscillator of frequency ʋ would not be zero, but 1 2 ℎʋ, where h is Planck's constant (Planck 1911). The concept, known as Nullpunktsenergie or zero-point energy, was controversial for some years, but received a solid theoretical foundation with the advent of formal quantum mechanics in the mid 1920s. 18 In 1916, the German chemist Walther Nernst suggested that, assuming empty space was filled with electromagnetic radiation, a zero-point energy associated with the vacuum could prevent the heat death of the universe (Nernst 16 A detailed account of this episode can be found in (Nussbaumer and Bieri 2009 pp 91-92). 17 It is now known that Lemaître considered many other expansion models in the draft of this paper, but selected the one closest to empirical observation  1916). Noting that the energy associated with this phenomenon would in principle be infinitely large, Nernst estimated a value of = 150 / 3 for the energy density of the cosmic vacuum from the Rayleigh-Jeans law of radiation by imposing a cut-off for frequencies above 10 20 Hz (Nernst 1916;Kragh 2012). It should be noted that Nernst's calculations were based on the concept of the ether; in his view, atoms of the chemical elements appeared out of the fluctuations of the ether, and these atoms and their radioactive decay products would later disappear in the zero-point energy of the ethereal sea (Nernst 1921;Kragh 2012).
In the 1920s, Nernst's idea of a zero-point energy of the vacuum was considered by a number of scientists. In particular, Wilhelm Lenz applied the concept to the Einstein Universe, and found that the phenomenon implied a nonsensical value for the size of the cosmos: "If one allows waves of the shortest observed wavelengths of λ ≅ 2 × 10 −11 cm ….and if this radiation, converted to material density (u/c 2 ≅ 10 6 ) contributed to the curvature of the world-one would obtain a vacuum energy density of such a value that the world would not reach even to the moon " (Lenz 1926;transl. Kragh 2012). A similar calculation was carried out by Wolfgang Pauli, with similar results. 19 Several other physicists considered the issue in these years; in general, the conclusion was that observation implied that the zero-point energy of the vacuum was not a real effect (Jordan and Pauli 1928;Pauli 1933Pauli , 1945. As Pauli remarked: "It is more consistent …to exclude a zero-point energy for each degree of freedom as this energy, evidently from experience, does not interact with the gravitational field" (Pauli 1933 p250). 20

The cosmological constant in the 1930s: the expanding universe
In 1929, the American astronomer Edwin Hubble published the first evidence of a linear relation between the redshifts of the spiral nebulae and their radial distance (Hubble 1929 19 Pauli's calculation was never published but is described in (Enz and Thellung 1960) and has been reconstructed in (Straumann 1999(Straumann , 2002. 20 See (Rugh and Zinkernagel 2002) or (Kragh 2012) for further details.

this problem lies in the fact that it is not static" (de Sitter 1930a). A report of the meeting was read by Georges Lemaître and, following a communication from him, Arthur Stanley
Eddington arranged for Lemaître's 1927 paper to be republished in English translation in the Monthly Notices of the RAS, bringing the work to a wider audience (Lemaître 1931a). 21 Soon, a number of papers had emerged that explored expanding models of the Friedman- Lemaître type for diverse values of cosmic parameters such as spatial curvature and the cosmological constant (Eddington 1930: de Sitter 1930b, 1930cEinstein 1931a;Einstein and de Sitter 1932;Tolman 1930Tolman , 1931aTolman , 1932Heckmann 1931Heckmann , 1932Robertson 1932 (Einstein 1923c;Straumann 2002;Nussbaumer and Bieri 2009 pp 82-83).
Setting the cosmological term to zero in Friedman's analysis of 1922, Einstein derived simple expressions relating the rate of cosmic expansion to key parameters such as the present radius of the cosmos, the mean density of matter and the timespan of the expansion.
Using Hubble's empirical estimate of 500 km s -1 Mpc -1 for the recession rate of the nebulae, he then calculated numerical values of 10 8 light-years, 10 -26 g/cm 3 and 10 10 years for each of these parameters respectively (Einstein 1931a). We have previously noted that these calculations contain a slight systematic numerical error; the Hubble constant above in fact implied a value of 2x10 9 light-years, 10 -28 g/cm 3 and 2x10 9 years for the radius of the cosmos, the mean density of matter and the timespan of the expansion respectively (O'Raifeartaigh and McCann 2014). However the error was a small one and did not substantially affect a major puzzle raised by the model; if the timespan of cosmic expansion represented the age of the universe, it was strangely small in comparison with estimates of the age of stars (as calculated from astrophysics) or estimates of the age of the earth (as deduced from radioactivity). Einstein attributed this age paradox to the idealized assumptions of the model, in particular the assumption of a homogeneous distribution of matter on the largest scales (Einstein 1931a).
In 1932, Einstein collaborated with Willem de Sitter to propose an even simpler model of the expanding universe. Following an observation by Otto Heckmann (Heckmann 1931(Heckmann , 1932) that the presence of matter in a non-static universe did not necessarily imply a positive curvature of space, and mindful of a lack of empirical evidence for spatial curvature, Einstein and de Sitter set both the cosmological constant and spatial curvature to zero (Einstein and de Sitter 1932). An intriguing facet of this model was that the rate of expansion and the density of matter were related by the simple equation where is the Einstein constant. Applying Hubble's empirical value of H0 = 500 km s -1 Mpc -1 for the recession rate of the galaxies, the authors found that it predicted a value of 4x10 -28 g cm -3 for the mean density of matter in the cosmos, a prediction they found reasonably compatible with contemporaneous estimates from astronomy. However, it is interesting to (18) note that the authors were careful in their paper not to dismiss the possibility of spatial curvature:"We must conclude that at the present time it is possible to represent the facts without assuming a curvature of three-dimensional space. The curvature is, however, essentially determinable, and an increase in the precision of the data derived from observations will enable us in the future to fix its sign and to determine its value" (Einstein and de Sitter 1932). No such courtesy was afforded to the cosmological constant.
The Einstein-de Sitter model became very well-known and went on to play a significant role in 20 th century cosmology. One reason was that it marked an important hypothetical case in which the expansion of the universe was precisely balanced by a critical density of matter, given by equation (18) as = 3 0 2 8 ⁄ . This allowed for a useful classification of cosmic models; assuming a vanishing cosmological constant, a cosmos of mass density higher than the critical value would be of spherical geometry and eventually collapse, while a cosmos of mass density less than the critical value would be of hyperbolic spatial geometry and expand at an ever increasing rate. Indeed, the geometry of such models is usefully described in terms of the 'density parameter' Ω, defined as the ratio of the actual matter density of the universe ρ to the critical density required for spatial closure, i.e. Ω = ⁄ . This simple classification scheme could be generalized to models with a cosmological constant by defining the energy density parameter as Ω = (Ω + Ω ), where Ω and Ω represented the energy density contributions due to matter and the cosmological constant respectively. In this scheme, the Einstein-de Sitter universe is neatly specified as (Ω = 1: Ω = 1, Ω = 0), while the empty de Sitter universe is described as (Ω = 1: Ω = 0, Ω = 1). We note that it is easily shown that = (8 3 0 2 ) ⁄ and = 3 0 2 ⁄ .
The Einstein-de Sitter model also marked an important benchmark case for observers; in the absence of empirical evidence for spatial curvature or a cosmological constant, it seemed the cosmos could be described in terms of just two parameters, each of which could be determined independently by astronomy. Indeed, the theory became the standard cosmic model for astronomers for many years, although it suffered from a similar timespan problem as the Friedman-Einstein model. 24 In addition, empirical observations soon suggested a mean cosmic density of matter far below the critical value, as will be discussed below.
24 Although it was not considered in the original paper, it is easily shown that the timespan of expansion of the Einstein-de Sitter model is given as t = 2/(3H0) (Einstein 1933 (Einstein 1945 p130).

Others retain the cosmological constant
At first, not many of Einstein's colleagues took his lead in abandoning the cosmological constant. Some felt that the term should be retained for reasons of mathematical generality; others felt that it could be used to address cosmological puzzles such as the timespan of the expansion and the formation of galaxies in an expanding universe. Still others felt that the term had an important role to play in giving a physical cause for cosmic expansion.
Considering the mathematical argument first, many theoreticians noted that the modified equations (9) represent the most general form of the field equations. For example, to satisfy the conservation of energy-momentum, the tensor representing the space-time metric must have a vanishing divergence. The most general second-order tensor that satisfies this criterion is not − 1 2 , the left-hand side of equation (6), but − 1 2 + , the lefthand side of equation (9). As noted in section 3, empirical observation demanded that was extremely small but from a theoretical point of view, there was no reason that it should be exactly zero. Thus, some theoreticians felt it was an error to assign the value zero to a term whose value was in fact unknown. This viewpoint was neatly expressed by Richard Tolman in a letter to Einstein in September 1931: "…since the introduction of the Ʌ-term provides the most general possible expression of the second order which would have the right 25 With one exception, as will be discussed in section 6. 26 The statement was reported by the Russian physicist George Gamow (Gamow 1956;Gamow 1970 p44). Some doubt has been cast on the accuracy of Gamow's report in recent years (Straumann 2002;Livio 2013 pp 231-243), while the report has been supported by Ralph Alpher (Topper 2013 p165) and by John Archibald Wheeler (Taylor and Wheeler 2000 p G-11).
properties for the energy-momentum tensor, a definite assignment of Ʌ=0, in the absence of experimental determination of its magnitude, seems arbitrary and not necessarily correct" (Tolman 1931b  Stanley Eddington noted that a positive cosmic constant could give a model in which the cosmos expanded from a static universe of indefinite age (Eddington 1930(Eddington , 1931a. This model, a more detailed version of Lemaître's analysis of 1927 (Lemaître 1927(Lemaître , 1931a, became known as the Eddington-Lemaître model. By this time, Lemaître himself had proposed his famous hypothesis of a universe that originated as a 'primeval atom' (Lemaître 1931b;Kragh and Lambert 2007). With such cosmic origins in mind, he noted that a judicious choice of value for the cosmological constant could give a cosmic expansion in three stages; an initial phase during which the expansion is de-accelerated by gravity, a 'loitering' phase in which the de-acceleration is balanced by the repulsive influence of the cosmic constant, and a final phase in which the repulsion becomes dominant (Lemaître 1931c(Lemaître , 1931d(Lemaître , 1933. Here the cosmic expansion was governed by a cosmological constant given by = (1 + є), where was the value of the cosmic constant in Einstein's static model of 1917 and the adjustable parameter є determined the length of the stagnation period.
A schematic of this model, known as the 'hesitating' or 'loitering' universe, is shown in figure 2 along with the Eddington-Lemaître model.
A promising facet of Lemaître's hesitating model was that it also offered a possible mechanism for the formation of galactic structures, a phenomenon that presented a formidable puzzle in the context of the discovery of cosmic expansion (Kragh 1996 pp 288-289). In Lemaître's view, the temporary balance of the gravitational force and the cosmological constant during the stagnant phase of the hesitating universe presented a stable interval during which perturbations in matter density could condense into galaxies and galaxy clusters (Lemaître 1931c(Lemaître , 1931d(Lemaître , 1933(Lemaître , 1934. Indeed, such a process could then give rise to a third phase in which the cosmic constant dominated. This model attracted some attention, especially when combined with later work on the formation of nebulae in the early universe Teller 1939a, 1939b The only possible answer is: the lambda does it" (de Sitter 1931). Thus, by the early 1930s, the cosmic constant was seen as a repulsive force arising from a negative pressure (Maneff 1932;Zaycoff 1932;Robertson 1933 ordinary matter and the energy of the vacuum (Bronstein 1933). This proposal envisioned a time-varying cosmological constant, a prediction that can be seen as a precursor of the modern concept of quintessence. 29 It should be noted that Eddington had an additional motivation for the retention of the cosmological constant, namely the use of the term in attempts to construct a more general theory that unified general relativity with quantum theory (Eddington 1931b: Kragh 2015. Although Eddington's 'fundamental theory' is now of historical interest only, the theory serves as an example of yet another role for the cosmological constant; from the very beginning, the term found application in attempts to connect the general theory of relativity with grander theories (Weyl 1918;Einstein 1919a), an application that pertains to this day. 30 Thus, in the years following the discovery of the expanding universe, some theoreticians felt that the cosmic constant term had an important role to play in relativistic cosmology. As Eddington remarked in his classic book 'The Expanding Universe': "I would as soon think of reverting to Newtonian theory as of dropping the cosmological constant" (Eddington 1933, p24

Advances in analysis
The discovery of the recession of the nebulae also spurred the development of formal tools for the description of cosmological models. In particular, it was shown (Robertson 1935;Walker 1937) that all time-dependent cosmologies that assume a spacetime that is isotropic and homogeneous (see section 3) can be described by the generic space-time metric In this expression, R(t) represents a time-dependent scale factor of expansion and 2 is a line element of Riemannian space given by where r, and are co-moving co-ordinates and is a curvature parameter normalized to assume the values +1, 0 or -1 for positive, zero or negative spatial curvature respectively. 32 In terms of astronomical observation, the rate of cosmic expansion is related to the cosmological redshift according to the simple relation where z represents the fractional change in wavelength / 0 of the light from a nebula, the Hubble constant 0 is the fractional rate of change of the scale factor ′⁄ and 0 is an appropriately defined distance measure. In astronomical practice, the redshifts of the nebulae were found to conform well to a relation of the form = 5 10 + where m represented the apparent magnitude of a nebula (a measure of its distance) and X was a constant (Hubble and Humason 1931;Bondi 1952 p39). We note that, as telescopes reached out to larger and larger distances, the question arose as to whether the Hubble constant evolved over time. In particular, it was expected that the rate of cosmic expansion would be slowed by the self-gravity of galaxies, as will be discussed in section 7.

The cosmological constant in the 1940s
In the 1940s, few physicists outside the relativity community paid attention to the Lemaîtreor Eddington-Lemaître models of the universe. While many accepted Hubble's observations 32 See (Kolb and Turner 1990 pp 29-30) or (Weinberg 1972 pp 412-413) for further discussion of the Robertson-Walker metric.
as possible evidence for an expanding universe, theories concerning cosmic origins were considered deeply speculative, an attitude that persisted for some years (Kragh 1996 pp 135-143). Interest focused instead on the possibility of determining more accurate estimates of cosmic parameters such as the Hubble constant, the mean density of matter and the curvature of space using large telescopes, but the construction of such instruments was delayed by the second world war (Longair 2006 pp 118-120). On the other hand, two theoretical advances occurred in this decade that were to have a major bearing on the story of the cosmological constant.

The hypothesis of primordial nucleosynthesis
In the late 1940s, the Russian émigré physicist George Gamow suggested that Friedman- Lemaître cosmologies might offer a radical solution to the puzzle of nucleosynthesis. With the failure of standard models of stellar nucleosynthesis to explain the relative abundance of the lightest chemical elements, Gamow and his colleagues Ralph Alpher and Robert Herman explored whether the phenomenon might be described in the context of nuclear processes in a young universe that was once extremely dense and hot (Gamow 1942(Gamow , 1946Alpher, Bethe and Gamow 1948;Alpher and Herman 1948, 1950, 1951. While this hypothesis did not have any direct bearing on the cosmological constant at first, 33 it opened up a new line of enquiry for dynamic cosmologies and that was later to set important constraints on estimates of the mean density of matter in the cosmos, as will be discussed below.

The cosmological constant in steady-state cosmology
In parallel with the work of Gamow et al., a new type of cosmic model was proposed in the United Kingdom known as the 'steady-state' universe. In this cosmology, the universe expands but remains essentially unchanged in every other respect. Today, the steady-state universe is mainly associated with the Cambridge physicists Fred Hoyle, Hermann Bondi and Thomas Gold, but other theorists entertained similar ideas.
In particular, it has recently been discovered that, soon after the publication of Hubble's graph of 1929, Einstein himself briefly considered the notion of an expanding universe in a steady state. In an unpublished work, Einstein proposed that the density of matter could be maintained constant in an expanding universe by a continuous formation of 33 The group generally ignored the cosmological constant in their analysis, although Gamow employed the term to address the puzzle of the timespan of expansion in a wide-ranging paper of 1949 (Gamow 1949 (1)" (Einstein 1931b). Indeed, Einstein proposed the process as a possible cause of cosmic expansion: "The density is therefore constant and determines the expansion apart from its sign" (Einstein 1931b). However, Einstein soon found that his steady-state model contained a fatal flaw and he abandoned the idea before publication. 34 In the late 1940s, Hoyle, Bondi and Gold became sceptical of Lemaître's idea of a fireworks origin for the universe and noted that evolving cosmologies predicted an age for the universe that was problematic. In consequence, the trio explored the idea of an expanding universe that remains essentially unchanged due to a continuous creation of matter from the vacuum. For Bondi and Gold, the idea followed from their belief in the 'perfect cosmological principle', a philosophical principle that proposed that the universe should appear essentially the same to observers in all places at all times (Bondi and Gold 1948). We shall not discuss their model further as it was not formulated in the context of the general theory of relativity.
By contrast, Fred Hoyle constructed a steady-state model of the cosmos by modifying the field equations (6) according to where the tensor Cμν was a 'creation-field' term representing the continuous creation of matter from the vacuum (Hoyle 1948 As is well known, a significant debate developed during the 1950s and 1960s between proponents of the steady-state and evolving models of the cosmos. Eventually, steady-state models were effectively ruled out by astronomical discoveries such as the distribution of galaxies at different epochs and the detection of the cosmic microwave background (Kragh 1996 pp 318-338). However, several aspects of steady-state modelsin particular the use of the de Sitter metric and its association with the cosmological constanthave found new relevance in the context of the theory of cosmic inflation, as will be discussed in section 8.

The cosmological constant in the years 1950-1970
The hypothesis of steady-state cosmology spurred new efforts to determine key cosmological parameters by astronomical observation. In particular, the opening of the 200-inch Hale telescope at the Palomar Observatory in California in 1949 heralded a new era of practical cosmology. In this work, attention focused on the Einstein-de Sitter model as it could be characterized by just two parameters, the current rate of cosmic expansion H0 and the current mean density of matter 0 . Indeed, the challenge to establish observational values for these parameters was later dubbed "the search for two numbers" (Sandage 1961(Sandage , 1970.

In the first instance, ground-breaking observations by the American astronomers Walter
Baade and Allan Sandage led to a successive recalibration of the distance to the galaxies (Baade 1952;Sandage 1958); this recalibration suggested a smaller Hubble constant, implying a longer timespan of cosmic expansion. By the end of the 1950s, Hubble's original estimate of 0 = 500 km s -1 Mpc -1 had been reduced to 75 ± 25 km s -1 Mpc -1 , implying a timespan of the order of 8 billion years even for models without a cosmological constant 35 It is easily shown that the assumption of a continuous creation of matter necessitates this metric (Hoyle 1948;Weinberg 1972 pp 459-460). (Sandage 1961). However, estimates of the age of the oldest stars in our galaxy were now of the order of 15 billion years, despite large uncertainties, suggestive of a new conundrum concerning the time of expansion. It is interesting to note that Allan Sandage, the acknowledged leader of this epoch of observational cosmology, suggested in 1961 that this conflict might be indicative of a need for a positive cosmological constant in evolving models of the cosmos (Sandage 1961).
Meanwhile, estimates of the mean density of matter in the universe suggested a value far below the critical value of the Einstein-de Sitter model. One method was to count the number of galaxies in a given volume of space and multiply by the mass of each galaxy, the latter figure being obtained by measuring the average luminosity of galaxies and converting to mass by means of the mass-to-light ratio of galactic matter (Longair 2006 pp 357-360). A second method was to determine the mass of galaxies by dynamical methods, i.e., by comparing the motion of rotating galaxies with that predicted by Kepler's laws. While both methods entailed large uncertainties, the dynamical method led to the discovery that, on the scale of galaxies and galaxy clusters, a large portion of matter takes the form of dark matter, i.e., is detectable only by its gravitational effect. 36 An important new approach to measuring the density of matter was suggested in the mid-1950s, namely to search for an expected slowing in the rate of cosmic expansion over time (Robertson 1955;Humason, Mayall and Sandage 1956;Hoyle and Sandage 1956). From the Friedman equations (15) and (16) Thus a measurement of the de-acceleration parameter 0 could yield a direct estimate of the density of matter, at least for models without a cosmological constant. We also note that one could expect a value of 0 > ½ for a cosmos of closed spatial curvature, a value of 0 < ½ for a cosmos of open geometry and a value of 0 = ½ for a universe of Euclidean geometry. Most importantly, a value of 0 = −1 was predicted for steady-state models, 37 raising the prospect of a clear distinction between steady-state and evolving cosmologies based on measurements of de-acceleration (Hoyle and Sandage 1956).
From the point of view of observation, the de-acceleration parameter 0 could in principle be determined by measuring redshift/distance relations for galaxies at great distance and comparing the results with that for more local galaxies. In this work, the simple magnitude-redshift relation (22) was replaced by equations of the type where the parameters m and z represent the apparent magnitude and fractional redshift of a nebula respectively (Robertson 1955;Hoyle and Sandage 1956). Thus, an ambitious astronomical programme was undertaken to determine [log z, m] plots for galaxies at great 37 The Hubble constant does not vary over time in steady-state models (Hoyle 1948). (27) distance, a modified version of the "search for two numbers" (Humason, Mayall and Sandage 1956;Sandage 1961Sandage , 1970Longair 2006 pp 349-350). The results of this program suggested a de-acceleration in cosmic expansion of the order of 0 ~ 1.2 ± 0.4, an estimate that seemed to favour the Friedman models over the steady-state solution (Sandage 1970). However, this result was subject to large uncertainties, not least because the program was limited to galaxies with redshifts smaller than z = 0.5 and because little account was taken of galactic evolution.
One intriguing aspect of the program was a prescient proposal that the study be extended to larger redshifts with the use of type Ia supernovae as standard candles (Tammann 1979;Tammann, Sandage and Yahil 1979).

The redshifts of the quasars
The 1960s also saw the discovery of puzzling new astronomical entities, extremely luminous objects apparently lying at tremendous distance (Schmidt 1963: Schmidt and Matthews 1964Sandage 1965). Two unusual aspects of the entities (soon known as quasi-stellar objects or 'quasars') were that they did not appear to conform to the standard relation between redshift and distance obeyed by ordinary galaxies, while they exhibited a preponderance of redshifts at around the large value of z = 2 (Hoyle and Burbidge 1966;Longair and Scheuer 1967;Burbidge and Burbidge 1967). The discovery prompted a new appraisal of Lemaître's hesitating model of the cosmos (see section 5.2), with a number of physicists interpreting the phenomenon as evidence for a stagnant phase in cosmic expansion due to a positive cosmological constant (Petrosian, Salpeter and Szekeres 1967;Shklovsky 1967;Kardashev 1967;Rowan-Robinson 1968;Petrosian and Salpeter 1970). Indeed, in a detailed analysis of quasar observations in the context of Lemaître's model, the Russian astrophysicist Nikolai Kardashev calculated a value of 2 × 10 −5 for Lemaître's stagnation parameter є (Kardashev 1967), while this calculation was corrected on further analysis to є = 6 × 10 −5 by the British astronomer Michael Rowan-Robinson (Rowan-Robinson 1968).
In time, the association of the redshifts of the quasars with a stagnant phase in cosmic expansion fell from favour, as quasars with ever larger redshifts were subsequently detected (Petrosian 1974). Indeed, the latter discovery effectively ruled out the hesitating model, sidelining the cosmological constant once again. As the American astronomer Vahe Petrosian remarked in a substantial review of this episode at the 1973 meeting of the International Astronomical Union: "In the absence of strong evidence in favour of Lemaître models, we must again send back the Lemaître models and along with them the cosmological constant until their next reappearance" (Petrosian 1974 Zel'dovich's starting point was a re-analysis of Lemaître's proposal of an energy contribution from the vacuum, this time in the context of modern quantum field theory. First, he noted that the scalar field associated with the quantum zero-point energy of the vacuum takes the form of an effective cosmological constant (Zel'dovich 1967(Zel'dovich , 1968. He then found that basic principles of quantum field theory suggested a lower bound of 10 -10 cm -2 for the cosmological constant (corresponding to an energy density of = 10 17 g/cm 3 ), far exceeding a maximum value of Λ = 10 -54 cm -2 (or = 10 -29 g/cm 3 ) set by observation. This calculation constituted a more quantitative formulation of the puzzle noted by Lenz, Jordan and Pauli (section 4.1); in addition, the demonstration of the Casimir effect in the late 1950s (Casimir 1948;Sparnaay 1957) had convinced many physicists of the reality of the zero-point energy of the vacuum. 38 Thus Zel'dovich's calculations were taken seriously; that quantum field theory predicted an effect that was forty orders of magnitude larger than that observed was a theoretical puzzle that became known as the 'cosmological constant problem'. One possible solution was a modification of the general field equations by means of an additional scalar field that could cancel the zero-point energy, although such a process seemed implausible as the latter involved fluctuations of many fields associated with a myriad of particles and their interactions (Zel'dovich 1968). However, the problem was not yet considered too pressing; given the lack of empirical evidence for a cosmological constant, most physicists assumed that the quantum energy of the vacuum was reduced to zero by some as-yet unknown symmetry principle (Weinberg 1989). 38 In particular, quantum field theory predicts that the energy contribution of virtual particles and fields must be taken into account. See (Abbott 1988) for a non-technical overview.

The cosmological constant in the years 1970-1990
By the late 1960s, the steady-state hypothesis had been effectively marginalized by observations of the distribution of the galaxies at different epochs and the detection of the cosmic background radiation. 39 Meanwhile, astronomical observations appeared to be reasonably consistent with the simplest evolving models of the cosmos, with some anomalies.
However, these anomalies became more significant as the years progressed.

The problem of missing mass
As astronomers probed further and further into the distant universe in the 1970s, new estimates of the mass density of the universe using the methods of galaxy counting and of rotational dynamics continued to suggest an upper limit far below the critical value of the Einstein-de Sitter model. This data appeared to conflict with a continuing lack of evidence of spatial curvature and with contemporaneous estimates of the Hubble constant (see (Gott et al. 1974) for a contemporaneous review). In addition, a universe of flat geometry was favoured on theoretical grounds, a puzzle that became known amongst theorists as the flatness problem. 40

Galactic evolution and q0
In the mid-1970s, the British-born astronomer Beatrice Tinsley embarked on a detailed study of galactic evolution. A startling outcome of this programme was the suggestion that previous estimates of the de-acceleration parameter q0 (see section 7) had not taken sufficient account of the evolution of galaxies, resulting in a significant overestimate of this parameter. By the mid-70s, spectrophotometric observations of very distant galaxies hinted at a value for q0 significantly smaller than previous estimates (Gunn and Oke 1974;Gunn 1975). Indeed, when corrected for galactic evolution, these data were interpreted by some as evidence for a negative de-acceleration parameter, i.e., for a positive cosmological constant (Tinsley 1975: 39 See (Kragh 1996 pp 318-388) for a review. 40 In 1970, Robert Dicke demonstrated mathematically that any deviations from flat geometry in the early universe would quickly escalate into a runaway open or closed universe, neither of which is observed (Dicke 1970 p62;Dicke and Peebles 1979). Gunn and Tinsley 1975;Tinsley 1978). However, uncertainties in galaxy luminosity prevented a clear diagnosis at this point.

The theory of inflation
By the end of the 1970s, studies of the cosmic microwave background indicated a universe that is extremely homogeneous on the largest scales. Yet calculations of astronomical distance indicated that the most distant regions of the universe were simply too far apart to have been in thermal contact, a puzzle known as the horizon problem. With the success of gauge theory and spontaneous symmetry breaking in particle physics, several theorists began to consider symmetry-breaking phase transitions in a cosmological context. Thus in the theory of cosmic inflation, it was hypothesized that the infant universe underwent a phase transition that caused an extremely rapid expansion during the first fractions of a second (Guth 1981;Linde 1982;Albrecht and Steinhardt 1982). The hypothesis of inflation had a major impact on theoretical cosmology, as it offered a simple solution to the horizon problem above. In addition, it predicted a universe of flat spatial geometry, offering a solution to the flatness problem identified by Dicke (see section 8.1). Indeed, it was soon shown that inflationary models predicted a spectrum of scale-invariant density perturbations in the early universe consistent with that expected from theories of galaxy formation (Hawking 1982;Starobinksy 1982;Guth and Pi 1982;Bardeen, Steinhardt and Turner 1983). 41 In the inflationary scenario, the infant universe fleetingly occupies a metastable state known as a false vacuum. The large energy density associated with this state gives rise to an enormous repulsive force that causes the universe to expand as an exponential function of time, a phenomenon that is described by a scalar field that takes the form of an effective cosmological constant. Thus, almost all models of inflation employ a de Sitter spacetime metric, exactly as in the case of steady-state cosmology. In addition, inflation made a startling prediction concerning the cosmological constant in today's universe. Since the model proposed a universe inflated to flat spatial geometry ( = 1), while astronomical observations suggested an energy density contribution from matter not more than = 0.3, an energy density contribution of about = 0.7 was predicted for the cosmological constant. This facet of the theory of inflation was quickly noticed; indeed, from the mid-1980s onwards, a number of analysts suggested that an inflationary universe of flat geometry, low matter density and 41 See (Smeenk 2005) for a historical overview of the theory of inflation.
positive cosmological constant gave a better fit to astronomical data than the standard Einstein-de Sitter model (Turner, Steigman and Krauss 1984;Peebles 1984;Fujii and Nishioka 1991). In particular, the model offered a solution to the conundrum that open cosmologies with < 0.3 predicted temperature fluctuations in the microwave background in excess of ⁄ ~ 10 -4 , far above the limit set by observations (Kofman and Starobinsky 1985). However, direct observational evidence for either flatness or a positive cosmological constant was not to emerge for some years (see below).

The cosmological constant in the 1990s
In the 1990s, hints concerning the possible existence of a non-zero cosmic constant began to accumulate. 1992 saw the first reports of the detection of anisotropies in the cosmic microwave background. This data, supplied by sensitive radiometers on board the Cosmic Microwave Background Explorer (COBE) satellite, indicated temperature fluctuations of the order of ⁄~ 10 -5 in the background radiation and a spectrum of inhomogeneities in temperature that was apparently scale-invariant (Smoot et al. 1992). These results were beautifully consistent with the standard cold dark matter (CDM) model of structure formation that had emerged in the 1980s 42 and also imposed important new constraints on cosmic models. In particular, the team noted that their data were consistent with a matter-dominated universe with a present Hubble constant H0 less than 50 kms -1 Mpc -1 , or with a flat universe with a larger H0 and an energy density dominated by a positive cosmological constant (Wright et al. 1992). Coupled with new constraints on the density of matter from observations of large scale structure and galaxy clustering, interest began to grow in the latter model (Efstathiou et al. 1990;Loveday et al. 1992;Bahcall and Cen 1992;Kofman et al. 1993).
In 1994, new data from the Hubble Space Telescope (HST) and from ground based telescopes suggested an observational value of 80±17 km s -1 Mpc -1 for the present Hubble constant (Freedman et al. 1994;Pierce et al. 1994). This figure was clearly problematic for cosmic models with < 0.3 and = 0, a modern version of the age paradox that had plagued astronomy for so many years (see section 5). However, the result was consistent with a flat cosmology dominated by a cosmological constant. New considerations of gravitational lensing also led some theorists to reconsider the role of the cosmological constant (Krauss 42 See (Longair 2006 pp 406-410) for a review of the CDM model of structure formation. and White 1992; Krauss and Schramm 1993). Putting together the revised Hubble constant, the CMB anisotropy data, models of structure formation and constraints on the matter content of the universe set by primordial nucleosynthesis, a number of theorists began to argue forcefully in the 1990s for what was becoming known as the Λ-CDM model of the cosmos (Carroll, Press and Turner 1992;Krauss and Turner 1995;Ostriker and Steinhardt 1995 As new methods of estimating the age of distant globular clusters began to highlight the age paradox of matter-dominated models, interest in models with a cosmological constant continued to grow (Chaboyer et al. 1996;Turner 1997;Turner and White 1997;Krauss 1998).

Supernova candles and dark energy
From the above, a picture emerges of a slow dawning of Λ-CDM cosmology as a promising came as a great surprise to many in the astronomical community; the supernova studies were indicative of a negative de-acceleration parameter, i.e., of an acceleration in expansion (Riess et al. 1998;Perlmutter et al. 1999). 44 The definitive result from the SCP and HZT teams is shown in figure 4. The slight upwards curvature of the redshift/distance relation can be seen in the upper plot; the deviation from linearity can be seen more clearly in the lower plot. These data point unambiguously to an acceleration in expansion over the last five billion years, a phenomenon that was soon 43 See (Kragh and Overduin 2014 pp 101-105) for a review of early supernova studies. 44 A description of the reaction of the team-members themselves can be found in (Kirshner 2002 pp 214-224).
dubbed dark energy (Turner 1999a, 199b). The cause of this acceleration will be discussed in section 11; for the present we note that the data constitute strong evidence of an acceleration in the rate of cosmic expansion over the last billion years, independent of theoretical models (Shapiro and Turner 2006). In the context of relativistic cosmology, we recall from section 8 that a measurement of q0 is a measure of − ; the data from each team suggested a figure of about -0.4 for this quantity. With the matter contribution estimated at ~ 0.3, these results were strongly suggestive of an energy contribution from the cosmological constant of the order of ~ 0.7. However, one cosmological parameter remained outstandinga direct measurement of the spatial geometry of the cosmos.

The cosmological constant in the 21st century; the concordance model
The

The concordance model
Since the early years of the 21 st century, the results above have been tested by a great number of astronomical experiments such as the Sloan Digital Sky Survey, the Hubble Space Telescope and the Chandra X-ray Observatory. In addition, the WMAP and PLANCK satellite missions have given extremely precise measurements of the cosmic microwave background, the use of gravitational lensing has given important information on galaxy clustering and structure formation, and ever larger supernova studies have given increasingly precise measurements of the time-evolution of the Hubble constant (see (Huterer and Shafer 2017) for a review). These observations have not introduced any substantial changes to the Λ-CDM model outlined above, but have reduced the uncertainties associated with each cosmic parameter. Together, theorists and observers have combined ever more precise measurements of spatial geometry, the rate of cosmic expansion, and the power spectrum of the CMB to give a single model of the cosmos, known as the concordance model. A recent measurement of the power spectrum of the cosmic microwave background by the PLANCK satellite is reproduced in figure 6. As stated in the accompanying paper: "The temperature and polarization power spectra are consistent with the standard spatially-flat 6-parameter

The evolution of the concordance model
From the historian's viewpoint, it is interesting to note that the common narrative 46 of a sudden upheaval in modern cosmology triggered by the discovery of cosmic acceleration in 1998 does not fit well with our findings above. Instead, we have seen that the postulate of a non-zero cosmological constant existed from the very beginning of relativistic cosmology, and its re-emergence as a key component in modern cosmology occurred as the result of a number of observations, from the long search for a possible de-acceleration of cosmic expansion to direct measurements of the geometry of the universe. In addition, the postulate fitted with well-known problems identified in the 1980s and 1990s concerning the age of the universe, the formation of galaxies and the dynamics of galaxy clusters, and with constraints set by considerations of primordial nucleosynthesis. Thus, while the detection of an acceleration in cosmic expansion using supernovae as standard candles undoubtedly marked a watershed in observational cosmology, it is important to note that the 'paradigm shift' to a cosmology with a non-zero cosmological constant occurred as the outcome of a number of advances in both theoretical and observational work. 47

The problem of interpretation
While the concordance Λ-CDM model represents a great triumph of observational cosmology, the model poses a number of major puzzles for theorists. Three outstanding problems are the nature of the inflationary field (or alternatives), the nature of dark matter and the nature of dark energy. In particular, the empirical evidence for dark energy, mathematically equivalent to a small but non-zero cosmological constant, raises once again the question of the physics underlying this term. Indeed, the so-called cosmological constant problem (see section 7) is more pressing than ever as theorists are challenged to explain not only why the cosmological constant is extremely small, but why it has the specific non-zero value that it does; this is sometimes referred to as the new cosmological constant problem. A second puzzle is the strange coincidence that the energy contribution of the cosmological constant happens to be of the same order of magnitude as that of matter in today's universe, an enigma that has become known as the coincidence problem.
We recall first that calculations by Zel'dovich of the zero-point energy of the quantum vacuum suggested a lower bound of the order of = 10 17 g/cm 3 , vastly exceeding the 46 See for example (Goldsmith 2000 pp 2-6; Kirshner 2002 pp 235-251). 47 A similar point has recently been made in (Calder and Lahav 2010;Turner and Huterer 2007; Overduin 2014 pp 106-107).
constraints set by observation. This problem was revisited by particle theorists in the 1970s and 80s in the context of gauge theory and spontaneous symmetry breaking. It soon emerged that the problem had got worse, as one could expect additional contributions to the energy density of the vacuum from the phase transitions associated with electro-weak symmetry breaking (Linde 1974;Veltman 1975). Indeed, by the 1980s it had become clear that one could expect an effective cosmological constant given by where 0 is a 'bare' (non-quantum) term, represents the zero point energy of the vacuum, represents a contribution from the electro-weak phase transition and represents a contribution from the quantum chromodynamic phase transition (Hawking 1983(Hawking , 1984. Of course, one possibility was that the 'bare' cosmological constant could somehow cancel (or nearly cancel) the quantum contributions. However, such a cancellation process seemed exceedingly unlikely as calculations suggested a very large value for each of the quantum terms. Indeed, the American theorist Steven Weinberg found that, under very general conditions, such an approach could not account for a small or vanishing cosmological constant without an extremely high degree of fine tuning, a result that became known as 'Weinberg's no-go theorem' (Weinberg 1989). 48 One exciting suggestion was that the modern theory of supersymmetry could offer a natural cancellation process that would reduce the quantum energy of the vacuum to zero (Zumino 1975;Hawking 1983). As the theory of supersymmetry proposes the existence of complementary particles of opposite spin to all the known particles, the contributions of the known particles to the energy of the vacuum might be cancelled by that of their superpartners. However, evidence for supersymmetric particles in particle collider experiments has not been forthcoming to date, indicating a symmetry that is broken at energies below the TeV scale (if it exists at all). Since calculations suggest that broken supersymmetry falls far short of reducing Λ to estimates comparable with observation, this approach has fallen from favour (Weinberg 1989;Carroll 2001).
A second approach is the postulate of a scalar field that undergoes a slow decay over time. This possibility goes back to early considerations of the cosmological constant (Gliner 1966;Zel'dovich 1968). 49 In the 1980s, it was noted that such a time-varying field might provide an intriguing link between today's expansion and the hypothesis of inflation in the early universe (Freese et al. 1987;Peebles and Ratra 1988;Ratra and Peebles 1988;Fujii and Nishioka 1991). Interest in such models, which became known as quintessence, increased with the emergence of the first hints of empirical evidence for a non-zero cosmological constant (Steinhardt 1997;Caldwell et al. 1998). Following the discovery of unequivocal evidence for cosmic acceleration, a great many candidates for quintessence were proposed, involving dynamical fields from quantum gravity, supergravity and superstring theory (see (Fujii 2000;Steinhardt 2003;Brax 2017) for a review). However, quintessence models are A third alternative approach to the cosmological constant problem arises from considerations of quantum cosmology, i.e., the study of the universe as a whole as a quantum system. With the use of a wave-function for the universe and Feynman's path-integral approach to quantum mechanics, theorists such as Stephen Hawking, Sidney Coleman and Thomas Banks have found that a distribution of effective values can be derived for the cosmological constant, peaking at λeff = 0. (In the language of quantum theory, universes with λeff = 0 dominate the path integral, making it probable that the cosmological constant vanishes, or is extremely small). However, this approach involves characterising the cosmological constant as a free parameter, an approach that has been found to be problematic, despite an intriguing proposal by Coleman (Coleman 1988) that the size of the cosmological constant could be set by the interaction of our universe with other universes via wormholes. 50 The coincidence problem, i.e., the relative size of dark energy and the current energy density of matter, presents a second puzzle. As pointed out in section 5, the mean density of matter in the cosmos decreases (in proportion to R -3 ) as the volume expands over time, while 49 Indeed, the notion of a time-varying cosmic constant was first mooted by Schrödinger (see section 3). 50 See (Weinberg 1989;Carroll, Press and Turner 1992) for a review of this work. the cosmological constant remains effectively unchanged. Thus, it is more than a little surprising that today's value of happens to be of the same order of magnitude as ; it seems a cosmic coincidence that begs explanation (Carroll 2001;Peebles and Ratra 2003).
Indeed, it could be argued (Steinhardt 2003) that this puzzle is really a modern, generalized version of the flatness problem identified by Dicke in the 1970s (section 8). One exciting explanation could be that dark energy and dark matter are not in fact independent, but interrelated in some subtle way. For example, in the case of some quintessence models, it is proposed that an interaction of the (unknown) quintessence field with matter could yield a natural explanation for the similarity of the energy contributions from matter and dark energy. This idea is reminiscent of that suggested by Bronstein in the 1930s (section 5.2); while many scholars have explored the concept, the results have not been convincing to date (Caldwell et al. 1998;Steinhardt 2003;Kragh and Overduin 2014 pp 82-85).

Alternate cosmologies
Explanations for a small but non-zero cosmological constant have also been offered in the context of alternatives to 'big bang' cosmologies. For example, in the cyclic universe, it is proposed that the universe undergoes an endless sequence of cycles from bang to crunch.
First considered by early relativists such as Alexander Friedman and Richard Tolman in the context of a universe of closed geometry (Friedman 1922;Tolman 1931aTolman , 1932b, cyclic cosmologies have recently been revived in the context of string theory. In particular, problems concerning infinities in temperature and density and growing entropy are avoided in a scenario where the big bang event is described as a collision of two three-dimensional branes. An exciting aspect of such models is that dark energy is not an ad-hoc addition, but the engine that drives the evolution of the universe throughout its cycles, i.e., a single 'dark energy' scalar field of varying potential is predicted to cause a period of slow accelerated expansion, followed by a period of deceleration and contraction, resulting in a bounce and new cycle Turok 2002, 2003). It's worth noting that the cyclic universe offers an alternative to inflation, as each cycle of contraction produces the homogeneity, flatness and energy needed for the next cycle. In addition, the theory provides a simple explanation for the size of the cosmological constant; because the universe is exponentially older than the traditional model, the cosmological constant has had an exponentially long time to decay from the very large value expected from standard calculations of vacuum energy (Steinhardt and Turok 2006). However, cyclic cosmologies predict an equation of state for dark energy that varies in time; such behaviour has not been observable to date as noted above.
Another alternative is the notion of the emergent universe. In this scenario, the universe inflates from a static Einstein state after an indefinite period of time, thus reviving the Eddington-Lemaître model (figure 2) in the context of the modern theory of cosmic inflation. 51 This scenario again offers the prospect of an arbitrary long time for the cosmological constant to decay from a very large value (Guendelman 2011;Guendelman and Labraña 2013). However, it once again predicts a time-varying equation of state for dark energy, a phenomenon that has not been observed to date.

Philosophical considerations
We note first that the existence of a small, but non-zero cosmological constant has implications not only for cosmology, but for the general theory of relativity and its description of gravitational phenomena. For example, with Λ ≠ 0 the theory does not reduce to the Minkowski spacetime of special relativity in the absence of a gravitational source (Ray 1990). More generally, the parameter space of solutions of the general field equations is augmented in the case of a non-zero cosmological term, an alteration that may have implications for the description of relativistic phenomena such as gravitational waves and black holes (Ray 1990;Ashtekar 2017). 52 In addition, the term has played an important role in attempts to construct more general field theories, as discussed in section 5.
As regards the physical interpretation of the term, one intriguing philosophical approach to both the 'old cosmological constant' and 'coincidence' problems is to invoke an argument known as the Anthropic Cosmological Principle. In its simplest form, this line of reasoning notes any successful model of our universe can be expected to predict values for cosmic parameters that are compatible with the existence of life within it (Carter 1974;Ellis 2011 Applying a similar reasoning to cosmology, Steven Weinberg noted that a value of the cosmological constant corresponding to an energy density more than two orders of magnitude larger than the energy density of matter would not have allowed the formation of the galaxies (Weinberg 1987(Weinberg , 1989. Interest in such reasoning has increased in recent years due to the startling prediction by some models of inflation that the observable universe may comprise only one of a great ensemble of universes (known as the multiverse), each with its own set of characteristic cosmic parameters (Vilenkin 1983;Linde 1986Linde , 2008. In this scenario, a range of values may exist for cosmic parameters and it is thus no surprise that at least one universe exists that is compatible with the existence of observers (Vilenkin 1995;Livio and Rees 2005). However, many physicists take the view that anthropic reasoning can certainly provide useful constraints for physical models, but hardly constitutes a true explanation of the values of parameters (Pagels 1985;Earman 1987;Kolb and Turner 1990 p269, p315;Deltete 1993). In addition, the principle does not provide specific predictions for the values of particular parameters; for example, one cannot deduce the masses of the elementary particles from anthropic reasoning. 55 A related philosophical approach to the problem is the assumption that no explanation is required. In this view, the predicted size of the quantum energy of the vacuum is a mathematical shortcoming of quantum field theory, not cosmology, and future advances in quantum theory may reduce the expectation value of the energy of the vacuum to zero. Thus, the observed value of the cosmological constant may be a universal cosmic parameter that is not derivable from theory, i.e., the general theory of relativity contains two constants of nature, the gravitational constant G and lambda, both of which are parameters to be determined by measurements (Bludman and Ruderman 1977). After all, the history of science offers many examples where great significance was ascribed to certain numbers that were later found to be an accident of nature. For example, the number of known planets in the solar system was a key parameter in the cosmology of the great astronomer Johannes Kepler; today we view this number of planets as the random outcome of many physical processes.
Similarly, little importance is attached today to the so-called Large Number Coincidence identified by Paul Dirac (Dirac 1937). Could it be that the size of the cosmological constant is simply the random outcome of the laws of naturean incidence rather than a coincidence? or an artefact of theoretical formalism (section 4.1). 56 For example, in recent years, alternative explanations for phenomena such as the Casimir effect have been offered that do not reference the vacuum (Rugh, Zinkernagel and Cao 1999;Jaffe 2005). Turning to cosmology, the conflict between the small size of the cosmological constant observed by astronomers and the predictions of quantum field theory might then be interpreted as strong evidence that the energy associated with the cosmic vacuum is not a real effect (Rugh and Zinkernagel 2002), an argument that is reminiscent of that of Jordan and Pauli (see section 4.1).

Quantum gravity
The most exciting explanation of the cosmological constant problem is that it is a signal of physics beyond the general theory of relativity. This viewpoint was first articulated by the British theorist William McCrea, who suggested that a gravitational theory that did not predict a value for a non-zero cosmological constant (should it turn out to be such) could not be complete (McCrea 1971). Today, theorists have long struggled to incorporate basic tenets of quantum physics into general relativity, or to include the relativistic view of gravity in a unified framework of all the interactions. Indeed, monumental efforts over the last fifty years to bring general relativity and quantum field theory into a single unified framework ('quantum gravity') have been largely unsuccessful, indicating that our theory of gravity is incomplete at least on some scales. In this context, is it surprising that calculations of the quantum energy of the vacuum do not fit seamlessly into relativistic cosmology? Thus, the dramatic conflict between astronomical observations of an acceleration in cosmic expansion (which are essentially model-independent) and theoretical estimates of the zero-point energy of the vacuum (calculated from quantum field theory) may stem from a fundamental clash between the two greatest theories of modern physics -and a true understanding of the physics underlying the cosmological constant will emerge only in the context of quantum gravity (Weinberg 1989;Carroll, Press and Turner 1992;Brax 2017).

Future tests
As the quantum energy of the vacuum is predicted to act in a way that is precisely equivalent to a cosmological constant term, any evidence of a variation over time in the equation of state 56 See (Enz 1974) or (Milonni 1994) for a review. = − 2 (or w = −1) for dark energy would effectively rule out this candidate.
Conversely, observation of a lack of variation in w over time would rule out a great many quintessence models. With this in mind, a number of observational programs have been designed to carefully study the behaviour of dark energy over time. In particular, the Dark Energy Survey (DES), operating since 2015, is designed to measure the equation of state w using complementary measurements such as galaxy cluster counts, gravitational lensing, galaxy distributions and measurements of cosmological distance using thousands of type Ia supernovae (see (Lahav and  although the lack of evidence to date for such particles appears to have ruled out many of the simplest supersymmetric models (see (Buckley et al. 2017) for a review).
Finally, the recent advent of gravitational-wave astronomy offers an important new test for the modified theories of gravity described above. As many scalar-tensor theories of gravity predict a speed of propagation for gravitational waves that differs from that of light, it was quickly realised that the detection of a gravitational-wave and electromagnetic signal from a single event could in principle break the degeneracy between a cosmological constant and many modified theories of gravity (Lombriser and Taylor

Conclusions
The 100-year history of the cosmological constant reveals a fascinating interplay between theoretical physics and astronomical observation. First introduced in order to describe a universe that was assumed to be static, the term was sidelined in simple models of the universe following the discovery of cosmic expansion. However, the term was resurrected on many occasions in the ensuing decades in order to address specific observational puzzles such as the timespan of cosmic expansion, the formation of large-scale structure and the redshifts of quasars; it also found a theoretical motivation in steady-state cosmology and later in the context of the hypothesis of cosmic inflation. In recent years, detailed studies of cosmic expansion, of fluctuations in the cosmic microwave background, of gravitational lensing and of galaxy clustering have presented strong evidence of a universe with an inflationary phase in early times and an accelerated expansion at late times, indicative of a positive cosmological constant dominant at both epochs. Thus we find that the return of the cosmological constant to the forefront of modern cosmology did not occur as an abrupt paradigm shift, but as the result of a number of advances in theory and observation.
However, an understanding of the fundamental physics underlying the cosmological constant remains elusive. Detailed analyses of the simplest explanation, the energy of the vacuum, continue to suggest an estimate for Λ that is in dramatic conflict with observation.
This result may indicate a major shortcoming in modern field theory, or may be a signal that the general theory of relativity is incomplete. One hundred years after Einstein introduced the term and feared that it would be perceived as a "mischievous and superfluous stunt", the cosmological constant poses one of the greatest challenges of modern physics. [14-421] Albert Einstein Archive, ©The Hebrew University of Jerusalem). The second paragraph reads: "Die neue Variante der Theorie bedeutet formal eine Komplizierung der Grundlagen und wird wohl von fast allen Fachgenossen als ein wenn auch interessantes, aber doch mutwilliges und überflüssiges Kunststück angesehen werden, zumal eine empirische Stütze sich in absehbarer Zeit kaum wird herbeischaffen lassen" or "The new version of the theory means, formally, a complication of the foundations and will probably be looked upon by almost all our colleagues as an interesting, though mischievous and superfluous stunt, particularly since it is unlikely that empirical support will be obtainable in the foreseeable future" (Transl. CPAE 8: Doc. 319).