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In addition to H Lengeler, S. Myers, CERN/BEIJING D. Treille, F. Pauss, W. Scandale Dark matter, hot and and E. Wilson from CERN, the team Joint school included G. Horlitz (DESY), J. Perot cold (Saclay) and H.S. Chen, Z. Chuang The CERN Accelerator School and Z. Guo from IHEP. Cosmologists responded enthusiasti­ usually confines its activities to Lecturers' initial misgivings about cally to the announcement at the CERN's Member States, with the how much of this tough material Washington meeting of the American occasional exception of a joint school would be absorbed were soon Physical Society in April 1992 that with its US counterpart. Recently a dispelled. Each lecture had a one- the Cosmic Background Explorer CAS team headed eastwards for a hour tutorial to iron out any problems, (COBE) had succeeded in detecting change, bound for Beijing's Institute and these sessions became quite primordial anisotropies in the cosmic of High Energy Physics (IHEP), for a animated. microwave background radiation school on particle physics and So successful was the experiment (CMB - June 1992, page 1). accelerator topics related to Euro­ that CAS plans to bring other Asian The COBE satellite was launched in pean plans for a Large Hadron countries, including Japan and India November 1989 into an orbit approxi­ Collider at CERN. into its circle of friends when time mately 900 km above the Earth, The School was CAS' first in an permits. carrying instruments to make precise Asian country and in many ways was measurements of the spectrum and an experiment. While lectures were anisotropy of the CMB. Data from the aimed at a basic level for the benefit Far-lnfra Red Absolute Spectro­ of the many students having their first photometer (FIRAS) beautifully taste of the subject, there was still shows the CMB spectrum to be that time to handle more detailed ques­ of a black body at a temperature of tions from specialists. All local 2.73±0.06K. Academia Sinica President Zhou Guang Zhao arrangements were handled entirely An even more important result, at welcomes CERN Accelerator School Head by the local Organizing Committee Ted Wilson to Beijing's Institute of High Energy least from the viewpoint of theories of with admirable efficiency. Physics (IHEP). large scale structure formation (LSS), comes from the Differential Micro­ wave Radiometer (DMR) which provided the first evidence for CMB anisotropy. Some anisotropy on the angular slice probed by COBE is expected in any reasonable model of LSS. COBE's measurement of the quadrupole anisotropy at six parts per million provides an important clue for developing a 'standard model' of LSS. The COBE numbers are in remarkably good agreement with the predictions of a particularly simple class of LSS models proposed almost a decade ago, with far reach­ ing implications for dark matter searches. Hot big bang cosmology predicts the existence of the CMB, discovered by Penzias and Wilson in 1965.

16 CERN Courier, July/August 1993 Models based on the simultaneous existence of cold and hot dark matter find it easier to explain large-scale structures in the Universe, providing a better fit to the COBE microwave background fluctuations as well as data from galaxy surveys. Not shown is data on smaller scales, where such models predict less formation, also in good agreement with observations.

so much small scale power that it was found necessary to associate galaxies only with high (and therefore rarer) peaks in density ('galactic biasing'). The mass fluctuations in a biased CDM model are several times smaller than fluctuations in galactic number density evaluated on an appropriate scale. However with this level of bias, the CDM scenario does not possess enough large-scale power to explain cluster correlations, bulk flows of galaxies, and galactic concentrations observed in recent redshift surveys. Even more signifi­ cantly, however, the biased CDM model predicts a quadrupole signal several times smaller than that estimated by COBE for the HZ spectrum. About a decade ago, particle However the high CMB isotropy referred to as the Harrison-Zeldovich physics models based on grand posed a major problem. According to (HZ) form of the density fluctuations. unified theories hinted at the simulta­ initial big bang thinking, patches of Since neutrinos exist and could well neous existence of cold and hot dark sky separated by more than about 2° have some mass, the simplest matter (CPHDM). CPHDM gives should not be temperature-corre­ scenario relies on HDM. However it more large-scale power and less lated. This and several other related has been known for some time (and small-scale power than a similarly conundrums were overcome in the COBE provides further confirmation) normalized CDM model. It was inflationary scenario proposed by that the observed large scale struc­ stressed that neutrinos could help Guth in 1981. ture cannot be explained with purely bring consistency between the Inflation says that the universe HDM and the HZ spectrum. inflationary prediction of critical contains a critical density of matter. One of the main problems is that density and several dynamical Arguments based on primordial fluctuations on scales smaller than estimates which lead to subcritical nucleosynthesis strongly suggest that the size of superclusters are wiped densities on 'small' scales, becoming no more than 10% of the critical out by the free streaming neutrinos. more critical at 'larger' scales. density can be in baryons. The rest Smaller objects such as galaxies and Even before the COBE result, it was has to be 'dark', although inflation quasars would have to arise from the known that CPHDM with a HZ does not specify what this dark breakup of supercluster-sized struc­ spectrum provided a good fit to a matter should be. This invisible dark tures. variety of LSS data. Moreover, the matter is usually assumed to be For the last decade or so, many COBE-compatible quadrupole either fast-moving, or 'hot dark cosmologists opted for a scenario in anisotropy gives a good fit to cluster matter' (HDM - relic light neutrinos which CDM makes up the entire number densities, cluster correla­ with cosmologically significant missing mass in the universe, with tions, bulk flows of galaxies, and the velocities), or cold (CDM - axions, the density fluctuations having the Infrared Astronomy Satellite Survey lightest neutralino,...). Inflation also HZ form. This leads to a hierarchical power spectrum. As well as agreeing provides a source of random fluctua­ clustering (bottom-up) scenario - with the COBE measurement, the tions that are scale invariant, at least smaller structures appear first and outcome is consistent with recent in grand unified theories - often merge to make larger ones. There is studies of structure formation with

CERN Courier, July/August 1993 17 CPHDM carried out at Bartol, collider - the major new facility since Berkeley, NASA/Goddard, Queen Congress in the previous PAC. Commissioning Mary College, Rome, Santa Cruz and has been impressive and physics is elsewhere. These studies all con­ Washington well underway, with luminosity clude that cold-hot dark matter (with climbing towards the design figure. some 70% cold and 25% hot) is a Over 1200 accelerator physicists and The varied user community of the promising model of large scale engineers gathered in Washington ubiquitous synchrotron radiation structure formation. mid-May for the 15th in the series of facilities is now considerably larger Alternatives to CPHDM include biennial Particle Accelerator Confer­ than that of particle physics and has retaining CDM but giving up the HZ ences (PAC) - the major US forum extensive industrial involvement. density fluctuations. But the required for accelerator physics and technol­ Three such machines have come into deviation from HZ is so large that it ogy. For the first time since their operation since the previous PAC - probably cannot be realized in inception, actual attendance de­ the 6 GeV European Synchrotron ordinary grand unified theories, clined, however the number of Radiation Facility at Grenoble, the perhaps not even in other 'realistic' contributed papers stayed around 1.5 GeV Advanced Light Source at models. Another scenario assumes 1500. Berkeley, and the 1.3 GeV Synchro­ that the matter density is low, and CERN Director General designate tron Radiation Research Centre in has a cosmological constant Chris Llewellyn Smith spelled out the Taiwan. (antigravity). This model may already challenges with an opening talk Linear electron-positron colliders - be in conflict with some recent underlining the deficiencies in today's the proposed route for collision observations which suggest signifi­ Standard Model. From many direc­ energies beyond LEP at CERN - cant matter concentrations. tions comes the message that were moved out of 'advanced accel­ The idea that both cold and hot dark collision in the 1 TeV range must tell erator concepts'. They are now matter play a role in structure forma­ us something new - wherefore art considered to be in the pipeline, with tion has far-reaching implications for thou SSC and LHC? much work in progress and many test particle physics. The preferred cold The secondary shock waves of last facilities built or being built, leaving dark matter candidates include the year's (fortunately overturned) bid to techniques like plasma beat wave lightest supersymmetric particle cancel the SSC Superconducting accelerators in the realm of exotic (LSP) as well as axions. For HDM, Supercollider project still ripple approaches. the tau neutrino with a mass 2-10 around the USA, while progress Though the scale and complexity of eV is the obvious candidate. towards authorization of CERN's today's big machine projects imply While COBE's discovery of micro­ LHC Large Hadron Collider has been timescales long on a biennial wave anisotropy is extremely impor­ slower than initially hoped. progress report scale, this superficial tant, these results and other data, The new US administration has impression hides a lot of achieve­ spanning a wide range of length indicated a constant rate of SSC ment. Full report in the next issue. scales, are perfectly consistent with a funding over the next four years; the scenario of cold plus hot dark matter, figure is higher than the present with density fluctuations essentially budget but considerably below the having the scale invariant (HZ) form originally proposed budget profile, predicted by inflationary models implying that completion will be based on grand unified theories. retarded by some three years beyond Just as the 1970s witnessed the the end of the decade. The SSC rise of the Standard Model of particle Laboratory will clearly have problems physics, perhaps the nineties will see to fight increased overall cost and the emergence of a 'standard model' sustain enthusiasm. CERN hopes for of large scale structure formation. LHC blessing in time to allow ma­ chine completion by the year 2000. By Qaisar Shafi Pride of place at Washington went to DESY's HERA electron-proton

18 CERN Courier, July/August 1993