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?
We can't see down to 0 seconds, because very soon after the big bang, the universe was filled with dense, extremely hot, opaque plasma. The closest we can get is the "recombination epoch" [0], which is roughly 370 000 years after the big bang, when, because of universal expansion, the plasma got cold enough for neutral hydrogen to start forming, at which point the universe became transparent.
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.
What I'm going to ask you is completely dumb, but keep in mind I don't know much of physics.
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?
Not a physicist at all either; but on a per particle basis, I know gravity is by far the weakest of the fundamental forces. However, unlike the others, it doesn't have spin/charge/polarity that results in their vectors repelling each other. As such, black holes form when the large scale weak gravitational forces combine into a greater force than the strong short scale repelling forces exerted by the others (and especially so when nuclear fusion is involved). In a hot primordial plasma, the particles are by definition extremely energetic; pushing each other in all directions with far more vigor than the extent to which gravity attracts them. Hence being a big bang rather than a big crunch, I suppose.
Because the primordial universe was in a state of extremely fast expansion, while black holes today exist in space-time that is almost exactly flat. In fact, if the universe really started from a singularity, the expansion was infinitely fast for an instant.
To hedge the next question then: Why was the universe expanding so quickly?
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.
The expansion of space itself is not comparable to the speed of light, but yes, I would say it's probable that it would have caused particles (or whatever there was at that time) to spread out faster than light, thus disconnecting them causally and preventing gravitational interaction. As for the why, answering that would involve answering why the Big Bang happened, which given our current understanding would veer into metaphysics. As far as we can tell, it's just what the universe does.
I have no idea about cosmology, but I think maybe that if this dense matter where everywhere, then forces would cancel out and that's why you don't get a black hole.
> the uniform ultimate backdrop we have when looking in any direction
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?
There is a very obvious dipole in the CMB due to our peculiar velocity. This has been removed in almost all images you saw because it's just an artifact of our particular movement and not physical. It's just the Doppler effect.
So if we can see CMB in all directions, why do we really even say "the observable universe" as though there could be more matter and more galaxies beyond what we can see? If the maximum of what we can see is implied to be before all galaxies, then shouldn't it be implied that all galaxies that exist can be seen?
Here’s the important point I think you are missing. You are looking in TIME, not in space. When you look very far away, you are looking back to the beginning of time, back to a singularity where all matter and energy existed at a single point. In every “direction” you end up looking back to the same time and the same place, the beginning of everything. There’s nothing “beyond” that, it is the beginning.
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.
But if we're looking in the direction of a galaxy far far away (i.e. 14bn ly away) (which we don't know it's there, but let's assume it is, because likely some galaxies must be there), how come we see the CMB, which is 13.7bn years old, and not the galaxy itself, which is (by definition) less than 13.7bn years old?
Because what we see from that direction are the things that were there in the last 13.7bn years. But the farther we look we see older images. So while we can see image from 10bn years ago of something that's 10bn light years away, when we try to see something 13.7bn light years away we just see microwave soup because that's what was there 13.7bn years ago. And if we try to see something 14bn light years away we see nothing from there because 14bn years ago there was nothing there.
Is it true that space had no volume at the Big Bang? I understand that density rises without (known?) bound as we approach the Big Bang, but even if you compress infinite space by an infinite factor you might still have infinite space -- infinity divided by infinity is undefined.
If you run the density backwards far enough, eventually everything in the visible universe is effectively at a single point, in some theories this was modeled as an actual infinite singularity, but it’s quite possibly unknowable since our first evidence is the CMB 380k years into the whole thing. As we rewind time from there things being to get highly speculative.
To "what exists today, beyond 13.7 billion light years", I'd rather say that "it doesn't exist yet". Because, otherwise, it assumes there's some "global universe time" like UTC or something. But all there is is our light cone.
That is fair. We are veering into metaphysics or religion if we talk about what exists there today, or even if there could be any relationship between our “today” and their “today”. By definition there is no causal connection. I’ll get back to you on that once I finish inventing my Time Machine.
The "observable" universe is about what we will ever see in the future, rather than about the past. If you waited around for billions of years, you could watch those early galaxies evolve.
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".
Due to accelerating cosmic inflation, some galaxies are receding from us at faster than the speed of light, so their light will never reach us. The majority of galaxies (everything outside of our local cluster, IIRC) will eventually be receding faster than the speed of light and be beyond our vision forever.
Yes, this means most galaxies will appear to actually pass through/into the cosmic background, from our point of view.
I don't think we understand enough about the dark energy driving inflation to say that. If the force is constant for a given pair of particles at a constant distance, and the atoms are currently stable, that would seem to imply that the force from dark energy pushing the atoms apart will be the same as they are now.
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.
You're describing the Big Rip. It's now thought that the acceleration of expansion is not high enough for that to happen. Gravity is strong enough that the particles inside galaxy groups will stay causally connected indefinitely. Long enough into the future, whole groups will become disconnected from the rest of the universe, though.
No, because galaxies almost certainly exist which are further than the CMB. It's just that the light travelling from them has not been able to reach us in the time since it was emitted.
Well most physicists assume it is massively bigger than we can see. It is in principle possible that we see nearly all of it, but that would be very coincidental. The only thing we know is that it is at least as big as we can observe, but it could be hundreds of times bigger, 10^100, or even infinite.
There are speculations one could make that imply a minimum size, I recall a reading a prediction of 10^50 times bigger or so.
Fascinating... so we see the CMB, got it. I wonder if someone could take what seems like the random pattern of the CMB and reverse it into some vision of the BB..
As everyone has already pointed out, the Cosmic Microwave Background Radiation is as close as you can get to the Big Bang.
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.
We can actually already see that time period, in its present form. It's called the Cosmic Microwave Background. It is the (now extremely cold) energy which permeated the entire universe in that very early time period. The entire universe was opaque and extremely hot... then as it expanded, it began to cool enough for particles and then atoms to form. Only then were stars possible, and later galaxies.
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.
No, models predict that an extremely short amount of time elapsed before this. Like orders of magnitude less than 1 second. However, we know our models are not perfectly accurate, so it is possible there is something else very weird going on.
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.
Not an expert here but it sounds like you might be describing the Cosmic Microwave Background. This is basically the remnant of the early opaque plasma cloud that filled the entire universe. As we look far away/back in time, the photons reaching us have redshifted due to the expansion of the universe. That's why these originally extremely high-energy photons are now just low-energy microwaves. The "background", 0 seconds after the Big Bang, is in all directions, and presents as the Cosmic Microwave Background. Or so I understand.
There is a wonderful sequence in the (2nd?) Book of Enoch (~150AD?), where the viewer is transported up through the heavens, past the planets, and finally beyond the stars, and eventually sees the incomprehensible face of God lying behind it all, visible all across the far end of space.
The very early universe can't be observed with visible light. It was filled with ionized hydrogen and thus opaque over long distances. The oldest light is from the era of "recombination", when things had cooled enough (c. 370k years after the big bang by consensus models) to permit light to travel. This is just the thermal glow of the universe, redshifted (way, way) down into the radio spectrum. And we can see it just fine; it's the cosmic background radiation and has been very well studied.
It's the region between recombination and the currently-visible-to-telescopes galaxies that Webb is particularly well-suited to study.
We can see it and it’s called the Cosmic Microwave Background https://en.m.wikipedia.org/wiki/Cosmic_microwave_background. It’s not t=0.0000001 though as the time before the cosmic microwave background the universe was opaque to photons, you wouldn’t be able to see anything.
