Why is the temperature of the cosmic background radiation not perfectly isotropic?
Joseph John Fernandez
I love maths and physics, but I love helping people understand them even more!
Bear with me, as this isn't an easy one to answer. The early Universe was dense very, very hot. So hot in fact that atoms couldn't form. Instead there were loose electrons and protons all around. Light particles, photons, interact very strongly with charged particles. They are scattered, redirected, absorbed and reemitted. In a dense charged environment like the early Universe, this means that the light couldn't really propagate: photons would move a bit, encounter a charged particle, get redirected, and so on. They were effectively locked into wherever these charges were located. When the Universe expanded and cooled down, atoms could finally form. When the electrons and protons combined, suddenly light could move unimpeded: the cosmic microwave background, the first light of the Universe, was emitted. This is the key to answering your question: the temperature fluctuations in the CMB represent matter density fluctuations in the early Universe. Where there was a larger density than normal, the CMB is hotter, as there were more photons. Similarly, where it is cooler, there was a lower density than normal and the CMB is colder. This doesn’t answer your question though, it just pushes backwards one step. The question now is where did the matter density fluctuations come from? A possibility is quantum fluctuations in the very early Universe, however this is, as far as I know, still open to debate… I hope this helped.
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