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In Robertson-Walker universe, light is emitted from a star with spatial coordinates $(r_s,\theta_s,\phi_s)$. It travels radially inwards and is received by an observer situated at the origin $(r=0)$. Show that the ratio of the observed wavelength $\lambda$ to the proper wavelength $\lambda_0)$ is given by $\lambda /\lambda_0=R(t_2)/R(t_1),$ where $t_1$ is the time of emision, and $t_2$ is the time of reception.

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    I think that in order to be able to answer the question, we should exactly know which [coordinate representation of the metric](http://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric) he is talking about.2012-10-02

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