The universe in a box - Thermal effects in the standard cold dark matter scenario

We compute the evolution of a representative piece of the universe on the assumption that the cold dark matter scenario is correct. A standard particle-mesh dark matter code is coupled with a flux-based mesh code originally designed for engineering hydrodynamical applications. A variety of checks have been made which indicate that the resolution of the code is a few cells, giving accuracy for integral quantities in our simulations of 1%-3%. In a first application of the new code to a cube of 30 h^-1^ Mpc containing 10^6^ cells, we find the following results: 1. On scales greater than 1 h^-1^ Mpc, bias is inverse in the sense that dark matter is more clumped than gas. 2. Bremsstrahlung and Compton cooling are unimportant on scales >= 1 Mpc; shocks gradually heat the gas, leaving most of it in voids at T <= 10^3.5^ K but with a small fraction reaching T >= 10^7^ K. 3. Ultraviolet and soft X-ray emission from the heated gas is significant, providing an important part of the background radiation field J_v_ and significant ionizing of the IGM. 4. The mean induced ({DELTA}T/T) for microwave background radiation in the Rayleigh-Jeans part of the spectrum (due to Sunyaev-Zel'dovich effect) is 1.1 x 10^-6^ with fluctuations of 5.6 x 10^-7^ on arcminute scales and a mean y parameter of 5.5 x 10^-7^, which is potentially detectable by the COBE satellite. Results (3) and (4) quoted above are dependent on normalization, which was chosen to be ({DELTA}M/M)_rms_ = 0.5 on a top hat 8 h^-1^ Mpc scale (bias = 2). Increasing the amplitude (reducing bias b) would increase the computed levels of y by b^-4.6^ and of J by b^-4.3^.