big-bangcosmic-microwave-backgroundcosmologyphotons
I have a few related questions:
The CMB (cosmic microwave background) is a snapshot of the oldest light in our Universe, imprinted on the sky when the Universe was just 380,000 years old. It shows tiny temperature fluctuations that correspond to regions of slightly different densities, representing the seeds of all future structure: the stars and galaxies of today.
The anisotropies are very small,
The cosmic microwave background radiation is an emission of uniform, black body thermal energy coming from all parts of the sky. The radiation is isotropic to roughly one part in 100,000: the root mean square variations are only 18 µK, after subtracting out a dipole anisotropy from the Doppler shift of the background radiation .
One needs a model and the dominant one is the Big Bang model
This light stopped interacting with electrons and nucleons at 380.000 years after the BB, before gravitational attraction formed stars and galaxies. The great uniformity of temperatures, at the level of 10^-5, cannot be explained thermodynamically because there was no uniform communication over the then universe ( due to special relativity) of the particles and energy forms before the photon separation, to homogenize the soup. This uniformity forced the inflation period in the very beginning of the Big Bang model. The rapid inflation homogenized the primordial soup so that we end up with the observed CMB spectrum.
The CMB has been used to model a homogeneous early inflation period to explain the CMB observation. Once the CMB homogeneity was modeled by the inflationary period, the homogeneity in the density of cluster of galaxies and galaxies also follows.
and that is used to explain the formation of large structures (galaxies, clusters of galaxies..).
The CMB does not explain the formation of large structures, those are explained by statistical mechanics and gravitational forces as the universe cooled from the BB . It explains the homogeneity , which is still under study.
The CMB originated everywhere at once, and every spot in the universe has been receiving CMB photons ever since it first started. To simplify matters, forget for a minute about the expansion of the universe, since its only effect is to redshift the radiation. Pretend the "release" of the CMB is happening right now. If after a minute you look to the sky, you will see radiation coming from everywhere. The photons that are hitting your eye originated one minute ago, and so they all were born on a sphere centered on you with a radius of one light-minute.
Now wait a year and point your telescope at the sky again. You still see radiation everywhere; the birth of the CMB was a while ago, so the photons that you see now are those that come from farther away: they formed a sphere with a one light-year radius.
It goes on like that. Again, the CMB was formed everywhere and radiation came out in all directions, and whenever we look at the sky (with the appropiate equipment) we see it coming from everywhere. The reason we see it now is not that the expansion of the universe somehow delayed it; it's just that some of the photons originated very far away, so they are only now getting to us.
As I said, the expansion of the universe redshifts the radiation. This means that as the years go on (and we're talking many millions of years in the future) the wavelengths get longer and the energies smaller, so eventually different instruments will have to be used. In the far future if someone still looks at the sky they might just see a static field instead of radiation.
Best Answer
At a basic level:
The universe, in the beginning was very hot. So hot in fact that there were no atoms, only electrons and protons and neutrons and photons flying around. The photons were scatting off of the electrons and protons, as they interacted strongly because the electrons and protons are charged. The universe was much like the plasma you find in plasma balls, but turned up to 11. It was opaque. You could not see through it.
As the universe expanded, it cooled and at around 380,000 years after the big bang, it was cold enough that stable atoms could form. At this point, all of the photons that were flying around suddenly stopped reacting with all of the free electrons and protons, since they started to form atoms that had no net charge, behaving much like a very dilute gas, like the air. The universe became transparent. Just as we can see through air, at this point the photons could travel unimpeded. This is referred to as the "surface of last scattering", but you shouldn't think of it as a surface, you should think of it as a moment in time where the universe went from being opaque to light to being mostly transparent to light.
Having suddenly nothing to interact with, those photons just starting travelling in straight lines. Some of those photons were just the right distance from us and were pointed in just the right direction that they are hitting us just now. In fact, they are hitting us continuously since the entire universe was filled with this photons just before the universe went "transparent".
So, the CMB isn't at the edges, its everywhere, its all of the photons that are still to this day flying off in every which direction. Occasionally those photons hit something, but since the universe is mostly empty space, the fraction that hit something is completely negligible. It is safe to assume they have not interacted with anything since the "surface of last scattering" nearly 14 billion years ago.
Nowadays, those photons are long in wavelength, nearly 1 mm, because as the universe has continued to expand, they continue to cool and stretch in wavelength.