Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

We show that intermediate mass black holes conjectured to be the early precursors of supermassive black holes and surrounded by relic cold dark matter density spikes can act as particle accelerators with collisions, in principle, at arbitrarily high center-of-mass energies in the case of Kerr black holes. While the ejecta from such interactions will be highly redshifted, we may anticipate the possibility of a unique probe of Planck-scale physics.

Abstract:

The matter content of the Universe is strongly inhomogeneous on small scales. Motivated by this fact, we consider a model of the Universe that has regularly spaced discrete masses, rather than a continuous fluid. The optical properties of such space-times can differ considerably from the continuous fluid case, even if the 'average' dynamics are the same. We show that these differences have consequences for cosmological parameter estimation, and that fitting to recent supernovae observations gives a correction to the inferred value of Omega_Lambda of ~10%.

Abstract:

We consider a model of the Universe in which the matter content is in the form of discrete islands, rather than a continuous fluid. In the appropriate limits the resulting large-scale dynamics approach those of a Friedmann-Robertson-Walker (FRW) universe. The optical properties of such a space-time, however, do not. This illustrates the fact that the optical and `average' dynamical properties of a relativistic universe are not equivalent, and do not specify each other uniquely. We find the angular diameter distance, luminosity distance and redshifts that would be measured by observers in these space-times, using both analytic approximations and numerical simulations. While different from their counterparts in FRW, the effects found do not look like promising candidates to explain the observations usually attributed to the existence of Dark Energy. This incongruity with standard FRW cosmology is not due to the existence of any unexpectedly large structures or voids in the Universe, but only to the fact that the matter content of the Universe is not a continuous fluid.