Particle Physics and the Early Universe
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Physics monitor Stephen Hawking with schoolchildren during the COSMO-97 international workshop on particle physics in Ambleside in the English Lake District. His public lecture 'The Theory of Everything' attracted a large audience. Particle physics and the early Universe he spectacular progress of Tcosmology in recent years means that scientists have to face a dilemma. The comfort of an in creased understanding of the forma tion of a complex Universe in the Big Bang has to be reconciled with an uncomfortable awareness that the Universe we actually see is only a small fraction of all there is. The search for this missing 'dark matter' is the major goals of contemporary physics. will change in the years to come with model dependent and often difficult Theoretical and experimental experimentalists now seriously to analyse. Comprehensive reviews aspects of dark matter searches and involved in mapping the sky up to the by A. Linde (Stanford) and L. Kofman particle cosmology were the main largest distances. (Hawaii) summarized the present focus of the COSMO-97 international Much progress is expected in status. workshop on particle physics and the sharpening fine details of Some theorists boldly try to imagine early Universe. Held from 15-19 irregularities in the cosmic microwave what happened in the very first September 1997 in Ambleside in the background radiation and in deep sky moments of the Universe's history at beautiful English Lake District, the galaxy surveys. The emerging picture the 'Planck scale' where the known meeting (organized by Lancaster of mass distribution around the laws of physics crumble. University) gathered some 110 Universe will have direct implications Stephen Hawking (with his participants from around the world. for predictions from inflationary Cambridge student R. Bousso) Deciphering the script for the Big models and will be used as a model explored the consequences of his no- Bang was the main theme. discriminator, as reviewed by boundary approach, in particular the Inflation remains the only attractive J. Frieman (Fermilab), E. Kolb production and subsequent possible scenario for explaining the puzzle of (Fermilab) and D. Lyth (Lancaster). evaporation of primordial black holes. why the gross features of the cosmic Even present data can constrain (A. Liddle of Sussex analysed how microwave background radiation look some cosmological scenarios. such objects would affect the the same in all directions. Inflation A. Albrecht reported that approaches subsequent evolution of the occurred as a brief period of ex in which large structures in the Universe.) tremely rapid expansion a tiny Universe have grown around Others advocate using superstring fraction of a second after the Big topological defects, like cosmic ideas to glimpse the evolution of the Bang. During this time the Universe strings, are already inconsistent with Universe during the Planck era. stretched by over 40 orders of observations. At the end of inflation R. Brustein (Ben Gurion) presented magnitude and became nearly the Universe was very empty, cold, work with G. Veneziano (CERN) Euclidean in the process. This and dominated by an 'inflaton' which which addresses the issue of the beautiful idea introduced 15 years subsequently decayed to give the initial singularity in a so-called pre- ago by Alan Guth still lacks a fully particles we know today. As a result Big-Bang approach. satisfactory particle physics scenario. the temperature jumped up again Superstrings are often claimed to Understandably, this domain of and the Universe "reheated". be the theory of everything, but they research remains almost solely a While the general idea is easy to still lack understanding of their theoretical playground. However this sketch, the detailed theory is still fundamental structure. H.-P. Nilles 24 CERN Courier, January/February 1998 Physics monitor Graham Ross of Oxford suggests a scenario with several periods of inflation of the early Universe. (Munich) reviewed the presently sity, just enough to make the Uni much debated M(atrix)-theory which verse's expansion slow to zero at is supposed to shed a new light on infinite time. While "direct" the theory of superstrings. He argued observations currently give the ratio that robust cosmological parameters, to be about one-third of the critical like the age of the Universe, can density, large-scale flows and cluster constrain certain otherwise allowed evolution tend to give higher values, predictions from strings. R. Gregory closer to whattheorists' favour, as (Durham) considered topological reviewed by J. Primack (Santa Cruz). defects resulting from superstrings. Measurements of the R. Kallosh (Stanford) reviewed a concentrations of light nuclei such as possible connection between black deuterium and helium in the Universe holes and superstrings. from the theory of nucleosynthesis Among the other topics discussed constrain the density of conventional relating to the particle physics of the matter (protons and neutrons) to be early Universe, J. Cline (McGill) about 5% of the critical density. argued that if the observed However, visible baryons in stars and asymmetry between baryons and galaxies account for less than 1%. antibaryons is to be explained by (Louisiana State), will soon be K. Freese (Michigan) argued that the purely (supersymmetric) electroweak challenged (or confirmed) by the "missing" baryons hide in very old effects, the mass of the Higgs KARMEN experiment at the UK stars - white dwarves. S. Raby (Ohio particle responsible for electroweak Rutherford Appleton Laboratory, State) presented another exotic symmetry breaking must be below according to J. Kleinfeller (RAL), possibility of coloured dark matter 100 GeV, making it accessible at while Chorus and Nomad at CERN, following currently hotly debated CERN's LEP electron-positron represented by J. Herin (Louvain), gauge-mediated SUSY-breaking collider. are not sensitive to the LSND signal. models. The origin and subsequent K. Zuber (Dortmund) reviewed the The other 95% of the total mass in evolution of primordial magnetic prospects for neutrino oscillation the Universe is probably non- fields, still poorly understood, were searches. Evidence for such mixing baryonic and locked in some yet discussed by K. Enqvist (Helsinki) of neutrino species from atmospheric undiscovered particles which only and M. Shaposhnikov (CERN). neutrino and solar neutrinos interact weakly. Such particles, G. Senjanovic (ICTP Trieste) observations is underlined by the known as WIMPs (Weakly Interacting showed how the normally expected Japanese Super-Kamiokande Massive Particles) should be restoration of symmetries at high detector (September 1997, page 25). abundant in the Galactic halo temperatures can be evaded in the The present picture remains surrounding us. One favourite bet is presence of non-zero charge unclear, with the LSND, solar and that these will be the lightest densities. This could automatically atmospheric data implying different supersymmetric particles (LSP). solve the domain and monopole oscillations, as reviewed by Many limits on the existence of problems in cosmology. D. Caldwell (Santa Barbara). He supersymmetric particles have come Another important theme of the argued that introducing a fourth, from LEP at CERN and the Tevatron workshop was cosmological relics of sterile neutrino would help solve the at Fermilab. Better limits, or a the Big Bang, where the subject is confusion. R. Mohapatra (Maryland) discovery, should come from the more experimental. Neutrino reviewed theoretical scenarios for LHC, as reviewed by S. Katsanevas oscillations would imply that neutrinos and conjectured that (Lyon) and V. Barger (Wisconsin). neutrinos have mass and hence may physics beyond conventional Grand J. Ellis (CERN) combined experimen contribute to dark matter. Unified Theories may be needed to tal data from LEP with cosmology to The first possible accelerator explain the data. sharpen the limits on the LSP. observation of neutrino oscillations Inflation in Big Bang cosmology R. Arnowitt (Texas A&M) and P. Nath from the Los Alamos LSND says that the Universe's mass (Northeastern) considered the WIMP experiment, presented by R. Imlay density is equal to the critical den in the context of supersymmetric CERN Courier, January/February 1998 25 Physics monitor The chairman of COSMO-97, L Roszkowski (left) with D. Caldwell who will be organizing the next COSMO meeting (COSMO-98) at Asilomar, California. External correspondents Argonne National Laboratory, (USA) D. Ayres Brookhaven, National Laboratory, (USA) P. Yamin CEBAF Laboratory, (USA) S. Corneliussen Cornell University, (USA) D. G. Cassel DESY Laboratory, (Germany) Ilka Flegel, P. Waloschek Fermi National Accelerator Laboratory, unification. The masses of experiment at Stanford using an (USA) superpartners can be squeezed by innovative cryogenic technique. Judy Jackson collider searches from below, and An intriguing signal was claimed by GSI Darmstadt, (Germany) G. Siegert increasingly accurate cosmological D. Belli (INFN Rome) from the DAMA constraints from above. experiment with a heavily shielded INFN, (Italy) A. Pascolini Recent years have witnessed the sodium iodide crystal in the Italian Gran Sasso Laboratory. This IHEP, Beijing, (China) growth of an impressive experimental Qi Nading programme to hunt for WIMPs. An experiment sees a small annual JINR Dubna, (Russia) indirect way of searching for WIMP change in the putative WIMP count B. Starchenko signals is to look for energetic rate,