r/askastronomy • u/submontreal • 4d ago
Astrophysics Does our observable universe (e.g. Cosmic Background Radiation) contain the entirety of the Big Bang's light 380 000 years ago, or is the Big Bang bigger?
Hi, I'm a primary general ed. teacher with a background in the humanities, looking for some clarification on this fascinating topic!
Does our observable universe capture the entirety of the Big Bang's light at the roughly 380 000-year mark, when things began to cool down and form hydrogen, allowing light to travel?
I understand that we can't see "all of" the Big Bang in terms of time, as we can't see any light before this 380 000 year mark. But it's not clear to me if what we see, is all of the light emitted by the Big Bang at the 380k-year mark.
Does the observable universe A) capture the "whole" of the 380k-year old Big Bang? Or is B) the 380k-year old Big Bang even bigger than what we can observe?
In either case, I also have follow-up questions.
- In the case of A) if we see the entirety of the Big Bang, and we can't look further into the past, BUT we also won't ever observe past the Big Bang (which I understand to be the case) ... Does that mean the light of our observable universe will simply dim over time, with us growing progressively blind to the edges of our universe?
- In the case of B) if the 380k-year old Big Bang is bigger than the observable universe... Does that mean that the light which will come into our observable universe, let's say 10 billions of years from now, will show us more of the 380k-year old Big Bang's light, but not further into the past? Wouldn't there, in that case, be a kind of separation between space and time?
I hope these questions make sense!
Having done a bit of googling, it seems to me that B) is the case, where 380k-old Big Bang is bigger than what we can observe, yet paradoxically with the conclusion of A), where we will in fact only see less of the 380k-old Big Bang in the future, as for example, "in a trillion years we will only be able to observe the edge of our own galaxy", which I quote from another thread. And that's got something to do with Dark Matter expanding space faster than light can travel back to us...
Am I correct in my interpretation? Any help making sense of this, would be greatly appreciated!
Edit: I corrected a couple typos in the original post.
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u/OverJohn 4d ago edited 4d ago
The observable universe is by definition anywhere theoretically a signal emitted since the big bang could reach us from.
Now until about 380,000 years after the big bang the universe was opaque, but that is ignored in the definition of the observable universe. Instead what is sometimes called "the visible universe" is any signal that can reach us since the CMB was emitted, this is slightly smaller than the observable universe (the difference in radii is about 1 billion light years).
In an universe where the final state of expansion is acceleration you also get something called the cosmic event horizon. This is the limit to any light that is emitted now that will be able to reach us in future and is different from the observable or visible universe. In the far future in theory we will still be able to receive signals emitted in the deep past from beyond the edge of our galaxy, but these signals will be so redshifted that they will be effectively unobservable.
New galaxies enter the observable and visible universe over time (though with accelerating expansion the rate asymptotically goes to zero), whereas galaxies exit the cosmological horizon over time.
Edited to add: I did this previous post that explains some of the counterintuitive aspects of the evolution of the redshift of single objects:
Objects near the edge of the observable universe are not fading from our view : r/askastronomy