Simulations of Weibel instability growth rates in relativistic counter-streaming plasmas. The different curves show the magnetic field energy density B²/8π (normalized by the relativistic plasma enthalpy density, γ_bn₀m_ec²) as a function of time tω_p for beam velocities of γ_b = 3 (blue), 10 (orange), 30 (green), and 100 (red). The dashed lines show the expected analytic growth rates.