CSIRO PUBLISHING Structure and Dynamics in the Local Universe www.publish.csiro.au/journals/pasa Publications of the Astronomical Society of Australia, 2004, 21, 385–389 The Future of Astronomy and Physical Cosmology P. James E. PeeblesA A Joseph Henry Laboratories, Princeton University, Princeton, NJ 08544, USA. Email: [email protected] Received 2004 May 3, accepted 2004 September 27 Abstract: Astronomy and cosmology have a substantial observational and theoretical basis, but our standard model still depends on some working assumptions. I comment on the nature of the issues behind these assumptions, the guidance we might find from past resolutions of such issues, and the models we might consider for the future of research in this subject. Keywords: history — philosophy — sociology of astronomy 1 What are we Trying To Do? the standard and accepted cosmology actually predicts Research in astronomy and physical cosmology is healthy: happens on the scales of galaxies. That is, the character of we have a broadly successful empirical and theoretical research in astronomy tends to be intermediate between basis for our subject and a rich set of observationally driven what is happening in particle physics and biophysics. This issues to study. An assessment of where research is likely is one guide to our future. to be headed in the longer term must be informed by an We have another guide from history. I outline in Sec- opinion of what we are trying to accomplish in physical tion 2 a picture for the development of physical cosmology science. I take it to be the discovery of how to encode through the exploration of bold hypotheses. We have had growing empirical knowledge into an increasingly tight, to discard many; the surprising thing is that some have compact, and internally consistent set of rules. We have become part of an observationally well-established world no guarantee that the real world operates according to picture. Our standard model still depends on bold hypothe- simple laws that we can discover in successive approxima- ses, however, mainly in the dark sector, as discussed in tions, but physical science has made enormous progress Section 3. I present in Section 4 a picture for the long- by acting as if this were so. term future of the goal of reducing the hypotheses and The state of development of this reductionist program establishing the physics of astronomy and cosmology. is very different in different branches of physical sci- ence, of course. Maybe the meaning of life, and what it means to be conscious, are to be found within in the 2 What are the Lessons from Experience? known and established laws of physics, but it will be The history of the interplay between theory and practice in a long time before we know for sure. Biophysicists are our subject teaches us that we should pay attention to ele- learning how to deal with deeply complex systems; it’s gant hypotheses, because they sometimes lead to aspects not surprising that we don’t see them paying much atten- of reality, and that it is prudent also to bear in mind that tion to issues of fundamental physics. Particle physicists, nature is quite capable of forcing adjustments of our ideas on the other hand, have a standard model that gives an of truth and beauty. account of a vast amount of experimental results within Consider the discovery of the expansion of the universe. a theory that can be written down in a page of equations. The first analyses, by Friedmann (1922) and Lemaître This is a spectacular success story. We have an example of (1927), assumed Einstein’s general relativity theory and this style of research in cosmology: general relativity in his argument that the universe is close to homogeneous linear perturbation theory, with a simple prescription for in the large-scale average. There was no observational initial conditions, gives a critical prediction for the rela- support for homogeneity: Einstein arrived at this pic- tion between the large-scale distributions of matter and the ture by an interpretation of Mach’s principle that we still thermal 3 K cosmic background radiation that is beauti- don’t understand. There was little empirical support for fully confirmed by the measurements (Bennett et al. 2003 general relativity theory: the one precision test was the and references therein). Analyses of galaxy formation, precession of the perihelion of Mercury, and Friedmann on the other hand, have more the flavour of biophysics; and Lemaître were considering an enormous extrapolation since galaxies are complicated it no surprise to see contro- from the length scale of the Solar System. Lemaître did versies not only about what the observations may be tell- have another hint: Hubble’s proposal of a linear relation ing us about how the galaxies formed but also about what between galaxy redshifts and distances. Lemaître’s very © Astronomical Society of Australia 2004 10.1071/AS04054 1323-3580/04/04385 386 P. J. E. Peebles reasonable assessment was that the evidence for the lin- the deepest thinkers in physics, including Albert Einstein, ear relation is schematic at best, however. (The discovery Wolfgang Pauli, and Lev Landau, on the subject of . paper is Hubble 1929; Lemaître 1927 had the concept from If you had been one of the handful of people active in a lecture by Hubble.) In short, the proposal that the uni- research in cosmology in 1930, you would have had the verse is expanding from a very dense state was exceedingly opportunity to decide whether your choice for a working speculative. Hubble (1936, 1937) recognised this; that is model for large-scale structure would accept Einstein’s why he pioneered the cosmological tests. Bondi & Gold argument from elegance for a homogeneous universe or (1948) and Hoyle (1948) were properly skeptical and gave Charlier’s (1922) argument from the empirical evidence us an alternative, the steady-state cosmology. Its lasting for a clustering hierarchy (what is now termed a fractal elegance is seen reflected in the inflation scenario. Now, universe). I think in 1930 I would have made the wrong however, against what I would call extremely long odds, choice. In his work on the cosmological tests in the 1930s the evidence clearly shows that the universe has expanded Hubble chose spatial uniformity as a working assump- in a close to homogeneous way from a much denser state, tion, and he sought to test the velocity interpretation of just as Lemaître (1931) visualised. the galaxy redshifts. Sandage (1961) accepted the veloc- Other stories have different endings, of course. ity interpretation as a reasonable hypothesis and, among Consider Einstein’s (1917) cosmological constant, . other things, sought to distinguish between the expand- When Einstein (1931) saw that his homogeneity concept ing steady-state and Friedmann–Lemaître cosmologies. If fits Hubble’s redshift–distance relation without the need you had been active in research in 1985 you would have for , he declared that this term is inelegant and unnec- had the chance to choose between the clearly more elegant essary. Pauli (1958) agreed, as did Landau & Lifshitz Einstein–de Sitter cosmological model and the empirical (1951). Zel’dovich (1968) taught us that a quantum vac- case for a low density universe. A big part of the history uum energy density that looks the same to any freely of the development of cosmology and astronomy has been moving observer, consistent with special relativity, would the art of choosing useful working hypotheses. have the velocity-independent form of . Already in the 1920s, however, Pauli recognised that the quantum zero- point energy of the electromagnetic field at laboratory 3 What is the Present Situation? wavelengths is absurdly large compared to what is wanted The observational basis for this subject is much firmer in cosmology (as described in Rugh & Zinkernagel 2002). now, but our world picture still depends on some major The situation is if anything even worse now that we have working hypotheses, mainly about the nature of the dark to add the latent heats of the phase transitions of standard sector. particle physics. By the mid-1980s the consensus was that People have been debating the meaning of the term, a detectable value of would be a theoretical abomina- and its possible relation to the quantum vacuum energy tion: the only logical and reasonable situation is that some density, for a long time. There are now two independent sort of symmetry has forced the vacuum energy density to lines of evidence for the detection of this curious term, vanish, and that = 0. There were continued arguments from the redshift–magnitude relation Sandage champi- in favour of , some, with McVittie (1956), as a part of oned in the 1960s, applied now to supernovae (Tonry a logically complete general relativity theory, and others, et al. 2003 and references therein), and from the mea- with Sandage (1961) and Gunn & Tinsley (1975), as a way surements of the large-scale distributions of galaxies and to understand the constraints on the distance scale, stellar the 3 K thermal background radiation (Bennett et al. 2003 evolution ages, and curvature of the redshift–magnitude and references therein). Both assume general relativity relation. Now, again against long odds, has become theory, applied on length scales some fifteen orders of part of the standard and accepted cosmology (Bennett magnitude larger than the precision tests, and the second et al. 2003 and references therein), and there is a substan- depends also on the CDM model for structure formation. tial and growing literature on the theoretical physics of The consistency of the results is a strong argument for pictured as dark energy, a new term in the stress-energy both assumptions.