Can someone explain to me what would be behind the galaxies if we could see it, to the point of 0 seconds after the big bang? Black, white, is it even possible?
As protons gain electrons in high temperatures, they don't form in the ground state. Instead, the newly minted hydrogen atoms are in a highly exited state. As they fall back to their ground states, they emit infrared photons at ~3000K color temperature. These photons, redshifted by the expansion of the universe to ~2.7K, are the Cosmic Microwave Background, the uniform ultimate backdrop we have when looking in any direction.
[0]: Which has it's slightly incorrect name (should not have re-) because it was named before the big bang became a widely accepted or known theory.
There might be galaxies even further away, but you can't ever see them, not even in theory. The light from them will never reach us because they flying away from us further than the speed of light.
The CMB isn't really about galaxies. We know there won't be any galaxies past the CMB because galaxies couldn't have formed before the CMB was emitted.
In theory we can "see" past the CMB using gravitational waves. (There was a thought, in fact, that we'd already done that, but that appears to have been faulty.) The CMB is just kind of a practical limitation rather than a fundamental matter of spacetime: you can't see because it's too cloudy.
The question of whether galaxies beyond the observable universe "exist" is kind of a matter of metaphysics rather than astrophysics. As an astrophysicist, you basically just say they don't exist and you're done with it. But if you want to know where the universe "came from" (whatever that turns out to mean), you try playing around with notions like "our universe is an observable sub-part of a wider ensemble, which we'll never detect, but here's a pretty set of equations which explain our universe in terms of it".
There are speculations one could make that imply a minimum size, I recall a reading a prediction of 10^50 times bigger or so.
Yes, this means most galaxies will appear to actually pass through/into the cosmic background, from our point of view.
However, as I said, I don't think we know enough about dark energy to say anything about its effects on the atomic scale either now or eons hence.
If you ask, what exists today, beyond 13.7 billion light years from earth, the plausible answer is “mature galaxies like ours” but there is no possibility of collecting data or evidence of what is there, since the evidence would take more than 13.7 billion years to reach us, and in fact would never reach us because the expansion of the universe means that distances are getting larger between 2 points all the time.
If my understanding is correct, when mass gets dense enough inside, a star you have a black hole, that would be like a hole in the fabric of space time. Ok, we have black holes in the contemporary universe. So how was the primordial universe a ball of dense, extremely hot, opaque plasma without becoming a huge black hole?
It seems that the expansion must have been so fast that it was going faster than light speed, right? Otherwise the very dense universe must have gone black-hole.
Is the CMB exactly uniform in every direction? Or is this early light slightly more redshifted when we look up versus when we look down or left or right? Does the oldest light we see in any given direction vary slightly in color?
I'm imagining the universe expanding as a sphere from a central point, but we're located off-center. Wouldn't the early infrared photons emitted from the other side of the central point of expansion from us be observed by us now as a slightly different color than the early infrared photons emitted closer to the edge of the early expanding universe?
A rabbit hole of questions: - When did space start expanding? - Did it have to rapidly expand for 400k years as extreme forces propelled matter apart? - Was that expansion faster or slower than the current expansion of space between galaxy groups? - Is expansion uniform across the universe? - Or is expansion slower closest to the original center of the universe? - Maybe there's a central point in the universe that's not moving relative to a reference frame outside our universe? - Is space discrete or continuous? - As space expands do new "units" of space appear between units of space that have grown farther apart? - If not, wouldn't physics work differently for areas of space where the units of space have grown farther apart than areas of space where the units of space aren't as far apart?
As balloon expands, in all directions there is same rate of expansion. You are not inside the balloon close to one side to observe the difference.
I have not read that any particular directions are evident in the sense you suggest though.
This page has three pictures: https://wmap.gsfc.nasa.gov/universe/bb_cosmo_fluct.html
The first is the actual observation. It's boring and looks completely homogeneous.
So you subtract the average value, which brings you to the second picture. Its temperature is 0 on average, but shows the obvious dipole.
When you remove the dipole, you get the last picture, which show only the physical temperature fluctuations.
But to the larger point, these galaxies were suspected before, based on Hubble work. You see, the COBE satellite from 1989 to 1993 mapped the microwave background radiation very precisely (two of the Principal Investigators on COBE won the 2006 Nobel Prize in Physics for this work). And they found that while there are minute fluctuations in the radiation, those fluctuations are measured at the parts-per-million level of difference. But the Hubble has found that the farthest back galaxies it could see were some of the largest and most massive things ever witnessed. So we had this gap between 'everything everywhere is the same to parts per million' and 'there are some supermassive galaxies' and so the Webb telescope was specifically designed to find the things that were redshifted so far they were out of the visible spectrum (so Hubble couldn't see them) but not so far that COBE could see them in microwave: in the infrared spectrum that lies between those two, that's where Webb is supposed to focus and help us understand how these galaxies form.
Because this question of what happened between the CMBR and the visible light range is the biggest question left over from Hubble, so it is what drove the design of the Webb. This is how astronomy has worked for centuries: you build a new telescope to answer some questions, but that leaves you with more questions, so you need to build new telescopes to answer those questions, GOTO 1. That's what's been happening ever since Galileo looked through that telescope at Jupiter all the way back in 1610.
How/why can we see the CMB? Well, it was everywhere. Literally every point in the universe was a nearly uniform sea of blazing energy. So if you look far enough in any direction, you will see the cold echoes of that time period.
edit: Beat to the punch! I hope among our many answers you've found something enlightening
edit 2: Important to note that the CMB is not synonymous with the beginning of spacetime. It is more like a wall, beyond which we can't see anything, and it came down very early in time.
Is this just because we’ve used it to define the start?
Cosmic microwave background is behind the galaxies, and we can see it. The universe is mostly transparent these days, so light can travel across the universe from a distant galaxy to our eyes or telescopes. Long ago, the universe was full of ions-free electrons and protons-and these are very effective at scattering light, so the universe was effectively opaque at that point. The universe became more transparent as the universe shifted towards hydrogen atoms instead of free electrons and protons.
Whatever the universe happened to look like during that transition period, from opaque to transparent, is still what we see. It's the cosmic microwave background. Anything from before that time got absorbed.
It's conceivable that we could observe gravitational waves during that period before the CMB, because they're not blocked by the un-recombined electrons and protons. If we ever get there, it could help explain the small variations in CMB from place to place. But that's a long way off.
(I want to add a H2G2 joke here, but I can't figure the right way to reference making God disappear in a puff of logic related to the Babelfish…)
So perhaps that? :-)
It's the region between recombination and the currently-visible-to-telescopes galaxies that Webb is particularly well-suited to study.