Could cosmic structure have formed by the action of causal physics within the standard hot big bang, or was a prior period of inflation required? Recently there has been some discussion of whether causal sources could reproduce the pattern of Doppler peaks of the standard scale-invariant adiabatic theory. This paper gives a rigorous definition of causality, and a causal decomposition of a general source. I present an example of a simple causal source which mimics the standard adiabatic theory, accurately reproducing the behaviour of the local intrinsic temperature perturbations.

4 pages, RevTex file and three figures, This version is to appear in Phys Rev D, Rapid Communications. It includes minor typo corrections and slight changes of emphasis compared to the original, as well as an added reference to a new paper astro-ph/9607109 which includes the full Sachs Wolfe integral