a. A relatively rare transition of the hydrogen atom emits a radio photon with with ?...

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a. A relatively rare transition of the hydrogen atom emits aradio photon with with ? = 21 cm. This emission line is extremelyimportant to astronomers for two main reasons: it cuts through mostgas and dust without being absorbed and, although transition israre, there is so much hydrogen in space that the 21 cm photons arethemselves quite common. Suppose that one such photon from adistant galaxy is measured to have a wavelength of just 11 cm. Howfast is that galaxy receding from us?
b. Assuming that the speed you found in problem a is enitrelydue to cosmic expansion, how far away is the galaxy from us?

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a According to Hubble law velocity of a galaxy is increasingas it is moving away from us The recessional velocity iscalculated using the relationWhere Ho is called as Hubble constant 71 kms1 See Answer

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