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I have read that the cosmic background radiation was formed 380,000 years after the big bang, when stuff changed from being opaque to light, because of free electrons, to becoming transparent.

However, I don't understand how we got to where we are in space faster than the CMB? I'm imagining in my mind that the big bang started at a small infinitely small point, and then stuff spread outwards from that. I'm imagining that we moved away from that point at a speed slower than the speed of light? But the CMB was presumably travelling at the speed of light? So, how did we get to where we are before the CMB got to us?

Qmechanic
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Hugh Perkins
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  • to the downvoter, if this has been answered somewhere before, please provide me with a url or similar. I've googled, searched on youtube, etc. – Hugh Perkins Jun 09 '22 at 12:49
  • Hello, I am not the downvoter, but I guess that your question may be a duplicate: https://physics.stackexchange.com/q/25070/226902 (this is basically your question, just expressed in a more "precise" language). In any case, I think that this is la legit question that may arise when one has the misconception that the Big Bang happened somewhere in space (but this is NOT actually the case, it happened "everywhere": there is no center of the expansion). See also: https://physics.stackexchange.com/q/130412/226902 – Quillo Jun 09 '22 at 13:24
  • I am not the downvoter either. This might help Did the Big Bang happen at a point? – mmesser314 Jun 09 '22 at 13:29
  • have a look at this answer of mine to see the complexity https://physics.stackexchange.com/questions/136860/did-the-big-bang-happen-at-a-point/136881#136881 – anna v Jun 09 '22 at 14:01
  • Does this answer your question? Did the Big Bang happen at a point? – John Rennie Jun 09 '22 at 14:49

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Welcome to PSE! The main point of confusion seems to be that you think the Big Bang happened at a single location in space. This is a very common misconception, and the name itself probably contributes.

The generation of matter in the Big Bang, and thus the transition from opacity that would allow light to begin traveling freely thousands of years later giving us the CMB, did not happen at any one place - it filled all of space, with space itself being the thing that was infinitely dense at the moment of the BB according to current theory.

So the Big Bang didn’t occur at some far-off location - it occurred right under your feet. The reason the CMB is a microwave background and not light of shorter wavelengths is attributed to the expansion of the universe since that time - some day it’ll be the Cosmic Radio Background.

JPattarini
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It's a very good question and has been asked before maybe, but it highlightens how one thinks of a big bang. Namely a bang in an infinite space. If the matter expands as such and electrons and protons recombine then the photons would finally escape the expanding ball, like photons escape the Sun, although the Sun ain't expanding. You would see a glowing expanding sphere, from the outside. That eventually would be devoid of photons, which would not decrease in energy to get to the level of the CMBR energy.

Space starts out small. Like a spatially 3D ballon that gets inflated. So where ever in the balloon you are, you are not near and edge or boundary of an expanding 3D mass in 3D space. The photons get stretched by the expansion and their energy is lowered. You can't visualize this, so a 2D balloon picture is offered often. But in reality, the balloon is 3D.

MatterGauge
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  • Hi @Felicia - Your "Space starts out small," is correct if the universe is finite, like the 2D universe as the surface of a sphere. A key (not possible to actually conduct) is to think of a triangle (sufficiently away from matter to avoid gravitational affects). For a finite (hyper-spherical) universe the sum of the three angles is always greater than 180 degrees. For an infinite universe, the sum of the three angles is equal to or smaller than 180 degrees: equal to 180 for a flat-Euclidean universe and less than 180 for a hyperbolic universe. – Buzz Jun 15 '22 at 22:40