Topological defects and cosmology

Many particle physics models of matter admit solutions corresponding to stable or long-lived topological defects. In the context of standard cosmology it is then unavoidable that such defects will form during phase transitions in the very early Universe. Certain types of defects lead to disastrous consequences for cosmology, others may play a useful role, as possible seeds for the formation of structure in the Universe, or in mediating baryon number violating processes. In all cases, topological defects lead to a fruitful interplay between particle physics and cosmology.     

Anisotropic hubble expansion of large scale structures

With the aim of understanding the cosmic velocity fields at large scale, we investigate the dynamics of a pressureless distribution of gravitational sources moving under an anisotropic generalization of Hubble expansion and according to Euler-Poisson equations system. As a result, it turns out that such a behavior requires the distribution to be homogenous, similarly to Hubble law. Among several solutions, we show a planar kinematics which admits a constant (eternal) and rotational distortion, where the velocity field is not potential. Within this class, the one with no rotational distortion identifies to a bulk flow. To apply this model within cosmic structures as the Local Super Cluster, the solutions are interpreted as approximations providing us with an hint on the behavior of the cosmic flow just after decoupling era up to present date. Such a result suggests that the observed bulk flow may not be due exclusively to tidal forces but has a primordial origin. UPRES EA 2596. Unité Mixte de Recherche (UMR 6207) du CNRS, et des universités Aix-Marseille I, Aix-Marseille II et du Sud Toulon-Var. Laboratoire affiliéà la FRUMAM (FR 2291).     

Probing physics at extreme energies with cosmic ultra-high energy radiation

The highest energy cosmic rays observed possess macroscopic energies and their origin is likely to be associated with the most energetic processes in the universe. Their existence triggered a flurry of theoretical explanations ranging from conventional shock acceleration to particle physics beyond the standard model (SM) and processes taking place at the earliest moments of our universe. Furthermore, many new experimental activities promise a strong increase of statistics at the highest energies and a combination with γ-ray and neutrino astrophysics will put strong constraints on these theoretical models. We give an overview over this quickly evolving research field with focus on testing new particle physics.     

3He-rich solar flares

A new subgroup of³He rich solar flares is found on reanalysing the global data.³He/H ratio as a function of maximum proton flux at an energy of about 10 MeV shows a break-up of the data into two groups. The first group follows the anticorrelation of³He/H ratio with the proton flux, as expected in the plasma process acceleration models. But the second group has a constant³He/H ratio as a function of maximum proton flux. This is not in conformity with the plasma process models. But this is expected in models where the nuclear spallation reactions are responsible for the production of³He. It is also found that the same break-up into two distinct groups follows if one plots the location of the flares in the solar disc. The first group is more or less confined to the west limb of the Sun, whereas the second group is more widely spread out across the solar disk. Visiting Professor, Tata Institute of Fundamental Research, Bombay, August–September, 1983.     

Collisional excitation of molecules in dense interstellar clouds

Summary Interpretation of radiofrequency and microwave observations of interstellar molecules requires knowledge of state-to-state collisional excitation rate constants. Although the required rates generally cannot be measured experimentally, they can be calculated theoretically. Computational methods are summarized and a survey of the availability of relevant values is presented. Accuracy of computed values as determined from comparison with related experimental rates is discussed briefly. Paper presented at the Congress ?Galactic and Extragalactic Dark Matter?, Roma, 28 to 30 June 1983.     

Balloon program for hard-X-ray astronomy

Summary An observation program of X-ray sources in the (20÷200) keV energy band by means of a balloon-borne NaI(Tl) seintillator array was undertaken in 1980. Three balloon flights have been performed and 10 X-ray rources observed, some more than once. We present details of this balloon program and results obtained from the analysis of the Crab observation performed in October 1980. Paper presented at the 2° Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–June 1984.     

The 90 ton liquid scintillation detector in the Mont Blanc laboratory

Summary We discuss here the results from calibrating liquid seintillation counters (1.5 m³ each) used in the large-volume neutrino experiment in the Mont Blanc Laboratory. The electronic and recording systems from the 72 counters of the detector are described, and the method to detect low-energy γ pulse from (n, p) reaction by using²⁵²Cf as a neutron source is discussed. Paper presented at the 2° Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984.     

Is there an antimatter-universe or antigalaxy?

Summary The very recent measurements on the ratio of cosmic antiproton to proton have contradicted seriously the previous measurements of the same for low-energy antiprotons. The controversial situation has been described and its implication for the validity of the postulate about the existence of antigalaxy has been examined. Although a very decisive conclusion cannot yet be made, a two-component behaviour of the ratio cannot, however, be excluded. Moreover, this two-component proposition seems to have better prospectsvis-a-vis the high-energy measurements of the ratio.     

On the spectra of gamma-ray bursts

Summary One of the possible interpretations of the spectra of gammaray bursts is in terms of synchrotron emissivity of mildly relativistic electrons. In this case, the shape of the spectra depends on the angle between the magnetic field and the observer. We show here that, although this angle can be important for single events, the statistical properties of gamma-ray bursts are determined essentially by those events for which this angle is near π/2, namely events observed in the direction of maximum emitted flux. Paper presented at the 2° Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984.