  Observations of the temperature anisotropies induced as light from the CMB
passes through large scale structures in the late universe are a sensitive
probe of the interactions of photons in such environments. In extensions of the
Standard Model which give rise to mini-charged particles, photons propagating
through transverse magnetic fields can be lost to pair production of such
particles. Such a decrement in the photon flux would occur as photons from the
CMB traverse the magnetic fields of galaxy clusters. Therefore late time CMB
anisotropies can be used to constrain the properties of mini-charged particles.
We outline how this test is constructed, and present new constraints on
mini-charged particles from observations of the Sunyaev-Zel'dovich effect in
the Coma cluster.
