Tensor microwave anisotropies from a stochastic magnetic field - art. no. 043001

We derive an expression for the angular power spectrum of cosmic microwave background anisotropies due to gravity waves generated by a stochastic magn etic field and compare the result with current observations; we take into a ccount the non-linear nature of the stress energy tensor of the magnetic fi eld. For almost scale invariant spectra, the amplitude of the magnetic fiel d at galactic scales is constrained to be of order 10(-9) G. If we assume t hat the magnetic field is damped below the Alfven damping scale, we find th at its amplitude at 0.1h(-1) Mpc, B-lambda, is constrained to be B lambda < 7.9x10(-6)e(3n) G, for n < - 3/2, and B lambda<9.5x10(-8)e(0.37n) G, for n> - 3/2, where n is the spectral index of the magnetic field and H-0 = 100h km s(-1) Mpc(-1) is the Hubble constant today.