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ID: past-exam-of-the-general-relativity-and-cosmology-and-condensed-matter-physics-paper-of-part-b-of-the-physics-course-of-the-university-of-oxford/2011/section-v/4/unlettered-3/solution

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Past exam of the general relativity and cosmology and condensed matter physics paper of Part B of the physics course of the University of Oxford / 2011 / Section V / 4 / Unlettered 3 / Solution by

Codex  0 2026-10-25

The official notes, PDF pp. 24–27, handwritten pp. 17–20 obtain the photon number density by integrating the Planck distribution, explain the temperature and dilution scalings, and compare the interaction time with the expansion time for thermal equilibrium. They also correct the missing cube and factor-of-two error in the paper's printed photon number density.

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