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The new cosmology
Chaos, Solitons and Fractals 16 (2003) 467–468 www.elsevier.com/locate/chaos
Foreword
The new cosmology
The collection of articles that follows gives an outside reader a fair representation of the current research activity in cosmology. For this editor who began his research career in cosmology in 1960, the past four decades have brought a sea change both in theory and observations. In the 1960s the subject of cosmology was considered very speculative, with very few constraining observations. Today it continues to be very speculative, but there are also a large number of constraining observations. In the 1960s, the discovery of an isotropic radiation background coupled with the realization that stars in the normal way cannot make the 25% helium observed in our neighbourhood, led to a firm belief in the hot big bang universe. The success of the electroweak theory in the 1970s prompted particle physicists to be more ambitious and think in terms of a grand unified theory (GUT) which brought together the electroweak interaction and the strong interaction under one umbrella. However, testing such a theory experimentally required particles at energies as high as 1016 GeV, some 13 orders of magnitude higher than what can be generated by present accelerators. The only physical scenario that one could think of for the interplay of a GUT was a sufficiently early epoch in the big bang universe, say when the universe was around 10 37 –10 36 s old. Particle theorists therefore combined with the cosmologists for a joint investigation of such an early era. Thus the speculative element in cosmology continues, and is in fact augmented with the speculations in particle physics. The saving grace in the entire scenario is that these early developments can in principle be related to present aspects of the universe, which are testable. The nature of large scale structure in the universe, the observed inhomogeneities of the microwave background, the nature and abundance of dark matter, the age of the universe and the changes in its expansion rate with time, are some of the observable features of the universe which can be folded in with the hypothesized initial conditions of the very early era which cannot be directly observed. All articles but one show the flavour of such an exercise. One important issue presently popular, which finds ample representation here is that of Ôdark energyÕ or ÔquintessenceÕ or the Ôcosmological constantÕ. The last one has had a checkered history since the early days of general relativity. Einstein introduced it into relativity as an extra force of cosmic repulsion which he felt needed, in order to sustain his model of the static universe. That was in 1917. By 1929– 1930, observational evidence had strengthened in favour of a non-static or expanding universe. Such models could be obtained in relativity without having to invoke the cosmological term, as had been shown by A. Friedmann in 1922 and AbbeÕ Lemaitre in 1927. So in the early thirties Einstein abandoned the cosmological constant. Nevertheless, Eddington and Lemaitre retained the constant as they felt that this extra parameter may come in handy in the confrontation between cosmological models and observations. So the cosmological constant has had a love–hate relationship with cosmologists. Theorists have invoked it whenever they felt that their models were threatened by observations, only to withdraw it if the threat goes away. The ÔthreatÕ comes from overconstraining observations. For example, recently the observations of supernovae at large distances, showed that the dimming produced by distance was not sufficient to explain the observations if one used models with a zero cosmological constant. Thus a case is being made for a positive cosmological constant. Of course, a positive value of this constant serves to explain other observed features of the universe also. Not surprisingly most articles have concentrated on the implications of this constant for cosmology. George Ellis has reviewed the progress of this constant throughout the 20th century. Varun Sahni has discussed the theoretical problems associated with the cosmological constant, including extra-dimensional ÔbraneworldÕ models. Francis Bernardau has shown how the phenomenon of gravitational lensing can be used to map inhomogeneities of matter, visible as well as dark and to check on the extent of Ôdark energyÕ. Peter Coles has reviewed the formation of large scale structure in the universe by using gravitational instability of initial inhomogeneities. Brian Schmidt has discussed to what extent the present data from type Ia supernovae require models with dark energy. 0960-0779/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 0 7 7 9 ( 0 2 ) 0 0 2 1 5 - 1
468
Foreword / Chaos, Solitons and Fractals 16 (2003) 467–468
I have played the ÔdevilÕs advocateÕ by describing a model that does not presume that the universe originated from a big bang; rather it is a self-sustaining model with long term expansion superposed with short term oscillation, without any initial instant of ÔbeginningÕ. Finally the papers by L. Nottale et al., B.G. Sidharth, M. Wanas and M.S. El Naschie deal with various new trends in modern cosmology which uses new concepts such as fractals, scale relativity, wild topology, M€ obius groups and related topics. I thank all the authors who responded positively to my invitation and have come out with excellent contributions on contemporary cosmology. J. Narlikar IUCAA 411007, Pune, India
Foreword
The new cosmology
The collection of articles that follows gives an outside reader a fair representation of the current research activity in cosmology. For this editor who began his research career in cosmology in 1960, the past four decades have brought a sea change both in theory and observations. In the 1960s the subject of cosmology was considered very speculative, with very few constraining observations. Today it continues to be very speculative, but there are also a large number of constraining observations. In the 1960s, the discovery of an isotropic radiation background coupled with the realization that stars in the normal way cannot make the 25% helium observed in our neighbourhood, led to a firm belief in the hot big bang universe. The success of the electroweak theory in the 1970s prompted particle physicists to be more ambitious and think in terms of a grand unified theory (GUT) which brought together the electroweak interaction and the strong interaction under one umbrella. However, testing such a theory experimentally required particles at energies as high as 1016 GeV, some 13 orders of magnitude higher than what can be generated by present accelerators. The only physical scenario that one could think of for the interplay of a GUT was a sufficiently early epoch in the big bang universe, say when the universe was around 10 37 –10 36 s old. Particle theorists therefore combined with the cosmologists for a joint investigation of such an early era. Thus the speculative element in cosmology continues, and is in fact augmented with the speculations in particle physics. The saving grace in the entire scenario is that these early developments can in principle be related to present aspects of the universe, which are testable. The nature of large scale structure in the universe, the observed inhomogeneities of the microwave background, the nature and abundance of dark matter, the age of the universe and the changes in its expansion rate with time, are some of the observable features of the universe which can be folded in with the hypothesized initial conditions of the very early era which cannot be directly observed. All articles but one show the flavour of such an exercise. One important issue presently popular, which finds ample representation here is that of Ôdark energyÕ or ÔquintessenceÕ or the Ôcosmological constantÕ. The last one has had a checkered history since the early days of general relativity. Einstein introduced it into relativity as an extra force of cosmic repulsion which he felt needed, in order to sustain his model of the static universe. That was in 1917. By 1929– 1930, observational evidence had strengthened in favour of a non-static or expanding universe. Such models could be obtained in relativity without having to invoke the cosmological term, as had been shown by A. Friedmann in 1922 and AbbeÕ Lemaitre in 1927. So in the early thirties Einstein abandoned the cosmological constant. Nevertheless, Eddington and Lemaitre retained the constant as they felt that this extra parameter may come in handy in the confrontation between cosmological models and observations. So the cosmological constant has had a love–hate relationship with cosmologists. Theorists have invoked it whenever they felt that their models were threatened by observations, only to withdraw it if the threat goes away. The ÔthreatÕ comes from overconstraining observations. For example, recently the observations of supernovae at large distances, showed that the dimming produced by distance was not sufficient to explain the observations if one used models with a zero cosmological constant. Thus a case is being made for a positive cosmological constant. Of course, a positive value of this constant serves to explain other observed features of the universe also. Not surprisingly most articles have concentrated on the implications of this constant for cosmology. George Ellis has reviewed the progress of this constant throughout the 20th century. Varun Sahni has discussed the theoretical problems associated with the cosmological constant, including extra-dimensional ÔbraneworldÕ models. Francis Bernardau has shown how the phenomenon of gravitational lensing can be used to map inhomogeneities of matter, visible as well as dark and to check on the extent of Ôdark energyÕ. Peter Coles has reviewed the formation of large scale structure in the universe by using gravitational instability of initial inhomogeneities. Brian Schmidt has discussed to what extent the present data from type Ia supernovae require models with dark energy. 0960-0779/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 0 7 7 9 ( 0 2 ) 0 0 2 1 5 - 1
468
Foreword / Chaos, Solitons and Fractals 16 (2003) 467–468
I have played the ÔdevilÕs advocateÕ by describing a model that does not presume that the universe originated from a big bang; rather it is a self-sustaining model with long term expansion superposed with short term oscillation, without any initial instant of ÔbeginningÕ. Finally the papers by L. Nottale et al., B.G. Sidharth, M. Wanas and M.S. El Naschie deal with various new trends in modern cosmology which uses new concepts such as fractals, scale relativity, wild topology, M€ obius groups and related topics. I thank all the authors who responded positively to my invitation and have come out with excellent contributions on contemporary cosmology. J. Narlikar IUCAA 411007, Pune, India