Prof Subir Sarkar

Abstract:

The relic abundance of baryons - the only form of stable matter whose existence we are certain of - is a crucial parameter for many cosmological processes, as well as material evidence that there is new physics beyond the Standard Model. We discuss recent determinations of the cosmological baryon density from analysis of the abundances of light elements synthesised at the end of ``the first three minutes'', and from the observed temperature anisotropies imprinted on small angular-scales in the cosmic microwave background when the universe was about 100,000 yr old.

Abstract:

Ultra-high energy cosmic rays (UHECRs) may originate from the decay of massive relic particles in the dark halo of the Galaxy, or they may be produced in nearby galaxies, for example by supermassive black holes in their nuclei. The anisotropy in the arrival directions is studied in four dark halo models (cusped, isothermal, triaxial and tilted) and in four galaxy samples (galaxies intrinsically brighter than Centaurus A within 50 and 100 Mpc, and galaxies intrinsically brighter than M32 within 50 and 100 Mpc). In decaying dark matter models, the amplitude of the anisotropy is controlled by the size of the Galactic halo, while the phase is controlled by the shape. As seen in the northern hemisphere, the amplitude is ∼0.5 for cusped haloes, but falls to ∼0.3 for isothermal haloes with realistic core radii. The phase points in the direction of the Galactic Centre, with deviations pf ∼30° possible for triaxial and tilted haloes. The effect of the halo of M31 is too weak to provide conclusive evidence for the decaying dark matter origin of UHECRs. In extragalactic models, samples of galaxies brighter than Centaurus. A produce substantial anisotropies (∼1.8), much larger than the limits set by the available data. If all galaxies brighter than M32 contribute, then the anisotropy is more modes (≲ 0.5) and is directed towards mass concentrations in the supergalactic plane, like the Virgo cluster. Predictions are made for the south station (Malargüe) of the Pierre Auger Observatory. If the UHECRs have a Galactic origin, then the phase points towards the Galactic Centre. If they have an extragalactic origin, then it points in the rough direction of the Fornax cluster. This provides a robust discriminant between the two theories and requires ∼350 ∼500 events at South Auger. © 2002 Elsevier Science B.V. All rights reserved.

Abstract:

A satisfactory theory of quantum gravity will very likely require modification of our classical perception of space-time, perhaps by giving it a 'foamy' structure at scales of order the Planck length. This is expected to modify the propagation of photons and other relativistic particles such as neutrinos, such that they will experience a non-trivial refractive index even in vacuo. The implied spontaneous violation of Lorentz invariance may also result in alterations of kinematical thresholds for key astrophysical processes involving high energy cosmic radiation. We discuss experimental probes of these possible manifestations of the fundamental quantum nature of space-time using observations of distant astrophysical sources such as gamma-ray bursts and active galactic nuclei.