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When searching for CMBR all articles start by stating as a fact that it is a remnant of the Big Bang.

Why? Why is it well understood and accepted? Why sure that it's not an effect of something else yet undiscovered?

For example models in which gravity is caused by quantum entanglement is a hot topic nowadays. Dark matter and dark energy are still kind of a mystery. Why is it sure that there won't ever be a more fundamental explanation of the CMBR other than the Big Bang? I don't know, like microwave glowing is a property of space over large scales, the same way expansion is.

(don't get hung up on the example, I don't want to propose alternatives. I'm interested in the "why", not the "why nots" of alternatives)

Thank you

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marked as duplicate by Ben Crowell, Community Apr 28 at 22:12

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    $\begingroup$ Because you don't get any research funding unless you blindly follow the official doctrine. $\endgroup$ – safesphere Apr 28 at 19:30
  • $\begingroup$ physics.stackexchange.com/questions/300242/… $\endgroup$ – thunderbolt Apr 28 at 19:33
  • $\begingroup$ In particular, see the first comment to the second answer to the links in the comments above. "Indeed, the CMB is simply consistent with the big bang model in its present form. It is open to others to put forward alternative ideas that self-consistently explain the CMB, expansion of the universe, primordial abundances etc. " $\endgroup$ – mmesser314 Apr 28 at 21:13
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The reason that relates CMB to inflation and Big Bang is that this microwave background is nearly homogeneous even though it covers regions which would be causally disconnected if the universe did not undergo a period of exponential expansion. The radiation coming from zones very distant to each others is the same: how could it be possible if their distance is greater than what light could have possibly travelled in the age of the universe?

By theorizing the inflation one roughly says that enough time has passed for those regions to actually be in causal contact before they emitted the CMB: in this way it is possible that in every part of the universe the cosmic microwave background has been emitted with the same "temperature". Intuitively if you have two tanks of water than were never in contact how could you expect them to be at thermal equilibrium? They must have been in contact with each other in the past.

Of course there are alternative ideas to inflation to solve this problem (for instance non constant speed of light), but inflation also explains anisotropies in the CMB so is really strongly motivated. If you want to know more have a tour on Wikipedia looking for inflation or "horizon problem".

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    $\begingroup$ why does CMB homogeneity imply a causally connected past between two distant parts of space?If there's some physical process that happens the same way everywhere, then you could see causally unconnected parts doing the same thing. $\endgroup$ – JohnDeeDoe Apr 28 at 20:28
  • $\begingroup$ @JohnDeeDoe I see what you mean but here the fact is that light can be emitted with any spectrum, any temperature. The fact that the CMB is homogeneous means that the universe had precisely the same temperature everywhere when the light that we observe today was emitted. In order for this to happen naturally one would need the universe to start with the exact same conditions in every point, which is not considered very natural and may give problems when one considers quantum fluctuations soon after the big bang. $\endgroup$ – AoZora Apr 28 at 21:17
  • $\begingroup$ en.wikipedia.org/wiki/… $\endgroup$ – JohnDeeDoe Apr 28 at 22:32
  • $\begingroup$ so temperature comes into the picture because CMB has a black body radiation spectrum. Those other points make sense now, thank you! $\endgroup$ – JohnDeeDoe Apr 28 at 22:39

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