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We investigate the dynamics of gauge and scalar fields on unstable D-branes with rolling tachyons. Assuming an FRW metric on the brane, we find a solution of the tachyon equation of motion which is valid for arbitrary tachyon potentials and scale factors. The equations of motion for a U(1) gauge field and a scalar field in this background are derived. These fields see an effective metric which differs from the original FRW metric. The field equations receive large corrections due to the curvature of the effective metric as well as the time variation of the gauge coupling. The equations of state for these fields resemble those of nonrelativistic matter rather than those of massless particles.

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Two-photon spectroscopy of the 6S-8S transitions in cesium was conducted using an extended-cavity diode laser for the first time, to our knowledge. The signal-to-noise ratio of the signals obtained shows a potential use for the transitions as a secondary frequency standard at 822 nm.KEYWORDS: two-photon spectroscopy, 6S-8S transitions in cesium, extended-cavity diode laser, optical frequency standard, 822 nm * 1 Permanent address

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