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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 i

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.

seanw444 4y ago
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?
- deleted 4y ago
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- sebzim4500 4y ago
  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.
  - dwaltrip 4y ago
    And never will reach us, as the space in between us and that distant light is expanding to quickly.
- jfengel 4y ago
  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".
- superjan 4y ago
  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.
- KMag 4y ago
  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.
  - seanw444 4y ago
    Ah, I had never thought of it that way for some reason. Duh. Thanks.
  - Jeff_Brown 4y ago
    I thought the logic of inflation had no limit, and eventually, everywhere, even atoms themselves would come apart.
    - KMag 4y ago
      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.
    - deleted 4y ago
      [deleted]
    - deadbeeves 4y ago
      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.
- dougmwne 4y ago
  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.
  - tomp 4y ago
    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?
    - deleted 4y ago
      [deleted]
    - scotty79 4y ago
      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.
  - Jeff_Brown 4y ago
    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.
lp4vn 4y ago
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?
- Jeff_Brown 4y ago
  +1! Infinite density => way blacker than a black hole, no?
- cypress66 4y ago
  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.
- willbudd 4y ago
  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.
- deadbeeves 4y ago
  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.
  - Balgair 4y ago
pseudobry 4y ago
> 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?
- saberdancer 4y ago
  Better to think of expansion as being a 2D dot on a balloon. 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.
- kzrdude 4y ago
  The CMB is famously not exactly uniform https://www.esa.int/ESA_Multimedia/Images/2013/03/Planck_CMB https://www.esa.int/ESA_Multimedia/Images/2013/03/Planck_CMB I have not read that any particular directions are evident in the sense you suggest though.
  - mr_mitm 4y ago
    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. This page has three pictures: https://wmap.gsfc.nasa.gov/universe/bb_cosmo_fluct.html 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.
cevn 4y ago
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..

seanw444 4y ago

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?

deleted 4y ago
[deleted]
sebzim4500 4y ago
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.
- dwaltrip 4y ago
  And never will reach us, as the space in between us and that distant light is expanding to quickly.
jfengel 4y ago
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".
superjan 4y ago
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.
KMag 4y ago
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.
- seanw444 4y ago
  Ah, I had never thought of it that way for some reason. Duh. Thanks.
- Jeff_Brown 4y ago
  I thought the logic of inflation had no limit, and eventually, everywhere, even atoms themselves would come apart.
  - KMag 4y ago
    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.
  - deleted 4y ago
    [deleted]
  - deadbeeves 4y ago
    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.
dougmwne 4y ago
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.
- tomp 4y ago
  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?
  - deleted 4y ago
    [deleted]
  - scotty79 4y ago
    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.
- Jeff_Brown 4y ago
  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.

lp4vn 4y ago

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?

Jeff_Brown 4y ago
+1! Infinite density => way blacker than a black hole, no?
cypress66 4y ago
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.
willbudd 4y ago
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.
deadbeeves 4y ago
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.
- Balgair 4y ago

pseudobry 4y ago

> 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?

saberdancer 4y ago
Better to think of expansion as being a 2D dot on a balloon. 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.
kzrdude 4y ago
The CMB is famously not exactly uniform https://www.esa.int/ESA_Multimedia/Images/2013/03/Planck_CMB https://www.esa.int/ESA_Multimedia/Images/2013/03/Planck_CMB I have not read that any particular directions are evident in the sense you suggest though.
- mr_mitm 4y ago
  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. This page has three pictures: https://wmap.gsfc.nasa.gov/universe/bb_cosmo_fluct.html 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.

cevn 4y ago

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..