The universe cools as it expands. This follows from the decay of particles' peculiar momenta, as discussed above. It can also be understood as adiabatic cooling. The temperature of ultrarelativistic fluids, often called "radiation" and including the cosmic microwave background, scales inversely with the scale factor (i.e. T ∝ ∝ a − − 1 ). The temperature of nonrelativistic matter drops more sharply, scaling as the inverse square of the scale factor (i.e. T ∝ ∝ a − − 2 ).