A large-scale puzzle

NEWS AND VIEWS Kenyapithecus in Africa, and now Ouranopithecus from Greece, all of which have thick enamel, but there is no evi- dence to indicate how these species might be interrelated. It is not known, for in- stance. which of them (if any) is most closely related to humans. More detailed descriptions of the new skull may throw some light on this. But it is evident that hominoid genera and species in the mid-Miocene than was recognized ten years ago, and as more and better speci- mens are recovered during the 1990s our understanding of the early stages of human evolution should increase corres- pondingly. D Peter Andrews is in the Department of Palae- ontology, The Natural History Museum, there was a much greater diversity of. Cromwe//Road, London SW75BD, UK. COSMOLOGY A large-scale puzzle Virginia Trimble GEORGE Gamow is supposed to have said that, with five free parameters, a theorist could fit the profile of an elephant. In practice, astrophysicists are not often called upon to do this. But theorists at a recent conference* showed that they can fit a structure almost as complex and unexpected — the large—scale distribution of galaxies in the Universe — with an average of only two parameters per theorist — not, of course, the same two in each case. The observations, long a sub- ject of rumour and colloquium in the astronomical community, first appeared officially in February&#39;, and were not eas- ily explained by conventional models of galaxy formationz. They still are not. The data comprise several hundred redshifts of galaxies, located in two nar- row, cylindrical volumes of space in the directions of the north and south poles of our Galaxy (where interstellar obscura- tion is smallest). The Volumes extend out to redshifts of more than 0.3 in each direction (corresponding to distances of 1,000 itq Mpc, where h is the Hubble con- stant in units of 100 km sq Mpcq). Previ- ously known nearby clusters and super- clusters are recovered, showing that the basic method is sound. But in addition, a plot of numbers of galaxies as a function of redshift displays about seven sharp peaks, spaced fairly uniformly 128 hq Mpc apart. The same length scale appears in the Fourier transform (the power spectrum) of the data and in a plot of pairwise separations of the galaxies in the sample. Galaxies that, according to conventional wisdom, grew out of a gaus- sian distribution of initial perturbations” — even with additional non-random per- turbations seeded by cosmic strings or galactic explosions — cannot behave in this way. But it seems that there are other possibilities. The observations (A.S. Szalay, Eotvos University and Johns Hopkins Univer- sity) have not changed. Work is pro- gressing, however, on samples in other * Workshop on the Legacy of Ya. B. Ze|’dovich for Astrophys- ics and Cosmology, 2-7 May 1990, University of Kansas in Lawrence, USA, held under a program for cooperation in basic research between the Soviet Academy of Sciences and the US National Science Foundation. NATURE &#39; VOL 345 - 21 JUNE 1990 directions in space. Smaller numbers of redshifts for galaxies lying about 45° from the Galactic plane (Selected Area 68) and towards the Hercules cluster show less distinct peaks on slightly different scales, also near 100 if‘ Mpc (which are some- what dependent on the cosmological model assumed). Curiously, nothing much showed up on larger scales, leading E. Bertschinger (MIT) to suggest that perhaps we are already seeing the largest—scale coherent structures in the Universe — contrary to the impression given by the shallower but wider Center for Astrophysics survey‘. It will be some years before there are large surveys in enough different direc- tions to provide a three-dimensional pic- ture of the Universe. Meanwhile, it is possible to consider the results from the Galactic-pole survey simply as a Fourier peak of a particular amplitude, width and wavenumber ~ just the sort of feature to inspire a new parameter or two. L. Kofman (Institute of Astrophysics and Atmospheric Physics, Tartu) dis- cussed a standard (Harrison—Zel’dovich) power spectrum of initial perturbations that arise from quantum fluctuations in a scalar field whose primary function is to drive inﬂationary expansion of the Uni- verse“. Inflation is a hypothetical brief epoch early in the Universe during which expansion is exponential in time, energy for this being derived from a field for which at present we have no independent evidence. Such expansion could account for the present near-smoothness and near—flatness of the Universe. The sim- plest possible field is a scalar one, which takes on a single value at each point in spacetime. Kofman showed that a second scalar field could be present, and could be made to yield extra power on a par- ticular scale (50 Mpc in his trial model). The amplitude and wavenumber of this peak can be adjusted to match those of the Galactic-pole data, but the peak is never very sharp on the long-wavelength side. V. Mukhanov (Institute for Nuclear Research, Moscow) pointed out that more than two scalar fields could easily lead to more than one characteristic clus- tering length. L. Grischuk (Sternberg State Astronomical Institute, Moscow) meanwhile admitted that his ‘squeezed gravitons (analogous to squeezed states in quantum optics) might leave detect- able traces on the spectrum of pregalactic primordial fluctuations. (A smooth sea of gravitons should pervade the Universe in much the same way as the sea of 3-K photons.) But Grischuk disclaimed responsibility for the 128-Mpc periodicity on the grounds that he is describing four-, not three-, dimensional spacetime. V. Icke (Sterrewacht, Leiden) addres- A cube of the Universe modelled by means of the Voronoi process. Shown are 10,000 galaxies initially distributed at random in a cube with sides of ~80 Mpc. Galaxies are colour-coded: white, galaxies in Voronoi cells (voids); green, galaxies in Voronoi walls; blue, in filaments; red, in nodes (‘Abell clusters’). Figure by V. Icke (Leiden Observatory, The Netherlands). © 1990 Nature Publishing Group 665