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Interstellar scattering as a cosmological probe

Since the discovery that the flux densities of very compact astrophysical sources are modulated by scattering in the inhomogeneous, ionized interstellar medium (ISM) of our own Galaxy through a phenomenon known as Interstellar Scintillation (ISS), these scattering effects have been used with great success as a tool to probe the physics of the ISM and the sources themselves. With the recent discovery of a redshift dependence in the ISS of quasars in a 4.9 GHz survey of about 500 sources, large statistical studies of ISS have been imbued with a cosmological significance. Possible causes of this effect include cosmological expansion, scatter broadening by the ionized intergalactic medium and evolution of quasar morphology with redshift. Since each of these hypotheses have different wavelength dependences, we have carried out dual-frequency observations of a subsample of 140 quasars to determine the origin of this redshift dependence of ISS. We are therefore using interstellar scattering, for the first time, as a cosmological probe at micro-arcsecond scales — achieving an angular resolution two orders of magnitude finer than that of Very Long Baseline Interferometry (VLBI). We discover a weaker redshift dependence at 8.4 GHz as compared to 4.9 GHz, indicating a strong wavelength scaling in the effect. We are investigating possible source selection effects and developing the theory to model the observations to enable an accurate interpretation of the data.