LIGHT-PROPAGATION IN INHOMOGENEOUS UNIVERSES - I - METHODOLOGY AND PRELIMINARY-RESULTS

We describe a numerical algorithm that simulates the propagation of li ght in inhomogeneous universes. This algorithm computes the trajectori es of light rays between the observer, located at redshift z = 0, and distant sources located at high redshift using the multiple lens plane method. The deformation and deflection of light beams as they interac t with each lens plane are computed using the filled-beam approximatio n. We use a particle-particle/particle-mesh ((PM)-M-3) N-body numerica l code to simulate the formation of large-scale structure in the unive rse. We extend the length resolution of the simulations to sub-megapar sec scales by using a Monte Carlo method for locating galaxies inside the computational volume according to the underlying distribution of b ackground matter. The observed galaxy two-point correlation function i s reproduced. This algorithm constitutes a major improvement over prev ious methods, which either neglected the presence of large-scale struc ture, neglected the presence of galaxies, neglected the contribution o f distant matter (matter located far from the beam), or used the Zeldo vich approximation for simulating the formation of large-scale structu re. In addition, we take into account the observed morphology-density relation when assigning morphological types to galaxies, something tha t was ignored in all previous studies. To test this algorithm, we perf orm 1981 simulations for three different cosmological models: an Einst ein-de Sitter model with density parameter Omega(0) = 1, an open model with Omega(0) = 0.2, and a flat, low-density model with Omega(0) = 0. 2 and a cosmological constant of lambda(0) = 0.8. In all models, the i nitial density fluctuations correspond to a cold dark matter power spe ctrum normalized to COBE. In each simulation, we compute the shear and magnification resulting from the presence of inhomogeneities. Our res ults are the following: (1) The magnification is totally dominated by the convergence, with the shear contributing less than one part in 10( 4). (2) Most of the cumulative shear and magnification is contributed by matter located at intermediate redshifts, z = 1-2. (3) The actual v alue of the redshift at which the largest contribution to shear and ma gnification occurs depends on the cosmological model. In particular, t he lens planes contributing the most are located at larger redshift fo r models with smaller Omega(0). (4) The number of galaxies directly hi t by the beam increases with redshift, while the contribution of lens planes to the shear and magnification decrease with increasing lens pl ane redshift for z > 2, which indicates that the bulk of the shear and magnification does not originate from direct hits, but rather from th e tidal influence of nearby and more distant galaxies and background m atter. (5) The average contributions of background matter and nearby g alaxies to the shear is comparable for models with small Omega(0). For the Einstein-de Sitter model, the contribution of the background matt er exceeds the contribution of nearby galaxies by nearly 1 order of ma gnitude.