I was going over some past papers ( OCR Physics A2 ) and I came across this question about cosmic microwave background radiation-the topic I seem to be struggling with the most :/ and I wasn't sure how to structure my answer. It's worth 5 marks. The question is : Describe the important properties of the cosmic microwave background radiation and how the standard model of the Universe explains these properties. Explain their significance as evidence for the past evolution of the Universe. In your answer, you should make clear how your explanation links with the evidence. Would anybody be able to help me structure an answer or maybe produce an 'ideal answer'. I can describe the properties of the CMB, but when it asks about how the standard model explains these properties, I'm not too sure. :S And also explaining their significance, please don't just ask me what is significant about it, I really don't understand when you put it that way :/, it would be much easier if you could just explain it in terms of the question please. Here is what I have so far, I've bullet pointed it: Describe the properties of the CMB -Electromagnetic radiation in the microwave region of the electromagnetic spectrum -Corresponds to a temperature of 2.7K/3K -Almost perfectly isotropic/intensity is almost perfectly uniform in all directions Here is my attempt at an explanation of these properties using the standard model -Why it is in the microwave region-->According to the Big Bang model universe was initially very hot and emitted electromagnetic radiation of very short wavelengths ( mainly gamma radiation )when universe became transparent to electromagnetic radiation and as it expanded these shorter wavelengths were red shifted/were stretched out, so that they were of a longer wavelength, corresponding to the microwave region of the em spectrum -Why it corresponds to a temperature of 2.7K/3K --> According to Big Bang as universe expanded it also cooled and temperature of current universe is now 2.7K/3K. Not too sure about this point, why is it that the microwave radiation corresponds to 2.7K?! I get very muddled everytime I think about this -Why it is almost perfectly isotropic--> According to Big Bang matter was slightly more dense in some parts of the universe, which allowed the formation of stars and galaxies. Again not too sure about this point, why would that make the radiation almost perfectly isotropic?! :S Because these properties can be explained by the Standard model of the universe, it suggests/provides evidence that the universe must have originated from the Big Bang-->I don't really understand the last part of the question '. Explain their significance as evidence for the past evolution of the Universe'. Here is the mark scheme for the answer, which I didn't really find too helpful :/ Any four from the below: Uniform intensity in all directions/everywhere Structure in background intensity/ripples Produced when matter and radiation decoupled Originally gamma radiation Gamma radiation red shifted to microwave Evidence that the universe began with big bang Temperature corresponds to 2.7K/that predicted by big bang (B1 x4) 1xlink between evidence and explanation ( 1) Which parts are the explanation?! If you could please put your explanation in a simple, easy to understand way I would really appreciate it :) Thank you!