The universe was opaque to light for the first 375,000 years (give or take). What we see as the “first light” from the Big Bang is now called the Cosmic Microwave Background (due to the expansion of the universe the light has been stretched into the microwave spectrum).
The Cosmic Microwave Background (CMB) is the cooled remnant of the first light that could ever travel freely throughout the Universe. This ‘fossil’ radiation, the furthest that any telescope can see, was released soon after the ‘Big Bang’. Scientists consider it as an echo or ‘shockwave’ of the Big Bang. Over time, this primeval light has cooled and weakened considerably; nowadays we detect it in the microwave domain. - SOURCE
We have measured it and it looks like this (that’s the whole universe):
I would not say it radiates “inward” but you can kinda think of it like that I guess. “Inward” supposes a center and there is no “center” in our universe (or, you could say any point is the “center”…including you…in a way you are truly the center of the universe (so am I…so is everywhere)).
It is not the edge because there is no edge. It is just the furthest back in time we can see. As we look through a telescope we see distant galaxies and the more distant the further back in time we are looking. We see them as they were many billions of years ago.
The CMB is the “oldest” light (so to speak) that we can see. We can’t see into the Big Bang itself (sadly…that would be cool if we could).
I get it is weird to think there is no “center” in our universe.
Here’s a way to think about it.
Imagine you are a 2-dimensional being. A Flatlander. You have width and length but but no height. An X and Y coordinate but no Z coordinate.
Now, imagine you are on the surface of a ball or globe. Where is the center? You can never find one because the “center” is in another dimension (the 3rd dimension) that you are completely unaware of and unable to access or even point to. You can travel your globe forever and never find the center. Indeed, every point on that globe is as near the center as any other.
The CMB radiates from everywhere to everywhere.
The entire universe was hot and the entire universe cooled down to a critical temperature nearly at the same time. We assume that everywhere in our universe sees pretty much the same CMB signature, although the very slight inhomogeneities observed will be different. The CMB is most definitely part of our universe. It is just a neat artefact if our universe’s early life. You could be far away, well outside of our own observable little slice of the universe and you would still see a CMB.
The thing about the CMB is that because it marks the time the universe cooled enough for light to travel, that time is a point it is very hard to observe before. The CMB isn’t really anything more intrinsically special.
Again, what is sometimes called the Big Bang, and is now more often called inflation, was not an outward explosion into existing anything.
Space and time were created at that moment. Various internal forces cause that tiny unit of spacetime to expand. Not expand into something, but expand by creating more and more spacetime. Nothing was outside this; no edge was created; no external observation was possible, even theoretically.
This is impossible to visualize. Our everyday awareness conditions us to imagine an expansion taking place in already existing reality. Inflation created that reality. Every internal point could be considered the center. That’s another thing outside our everyday reality, but relativity theory assures us it’s true.
We can say nothing about other theoretical universes at this time. Maybe multiverses exist. If so, they can’t see us. They are not external observers of our universe. We can’t posit what our universe would look like to another universe. Would it see us a whole? As a force? As an anomaly? As a shadow? The words are currently meaningless.
All we can talk about is our universe and, unfortunately, you can only look at and understand the universe through math. Our ordinary senses exist inside spacetime and cannot comprehend spacetime as a whole. That’s also why explaining the universe is so difficult. You have to throw out all your lifetime of experience to begin to get it.
I think that part of the difficulty here might be terminology. There’s the entire Universe, and there’s the observable universe, and they aren’t the same thing. The observable universe is definitely finite, and has an edge. The entire Universe might or might not be finite (our best guess is that it’s infinite, but we have no way to be sure of that), and is presumed not to have an edge (if it’s finite, it’s wrapped around in some way). We can’t see the portions of the Universe outside of the observable universe, but we have every reason to believe that it’s basically like the parts we can see. And from every point in the Universe, the boundary of the observable universe would be different.
The confusion I’m seeing, @Kedikat , is that when you’re using the word “Universe”, you’re actually referring to just the observable universe, while others are using the more usual interpretation of using that word for everything.
Yes. I presuppose the possibility of other universes like our observable universe in my question. Wondering if at some point one could observe the other.
The chances of other universes being anything like our own universe are very, very slim. Chances are they will be wildly different. See: The Anthropic Principle.
And, it is almost impossible (most likely impossible but never say never) that we can ever observe another universe (assuming they exist).
Within our expectations of the universe, there are lots of different observable universes - each observable is simply centred on a different observer. We are in the middle of ours. Go a few billion light years away, and the observable universe at that point is a different one. It includes us, but also includes stuff we can’t see from here, and can’t see some stuff we can see. Go far enough, and you won’t be able to see anything inside the observable region we see, but you will see other stuff. It isn’t a whole lot different to wandering about on the Earth. Your observable universe extends to the horizon. Walk towards the horizon, and it recedes just as fast. Your observable universe overlaps with some other people’s, and is quite distinct from others. Nobody has a special vantage point, they all see basically the same general thing, just with local variations.
Perhaps my earlier question warrants its own thread (and probably has already been discussed in another thread), but to re-phrase: Way out in the future when there are no other galaxies within the observable universe of our galaxy (or some other galaxy), would they ever be able to prove that other galaxies existed in their past? I suppose this question could also apply to a life-supporting planet of a stray red dwarf, on which habitants may think that they are literally the only intelligent life in the universe (and I suppose legitimately).
We base all our assumptions about the universe based on what we see (or gather through our instruments). Those have changed over millennia as we went from one star to one galaxy to multiple galaxies to an expanding universe that implied a smaller beginning.
I would imagine that the people on the only matter in the observable universe would believe implicitly in a Creator. They might have some sciences, but cosmology wouldn’t be one of them.
Well that depends on the fate of intelligent species like ours.
IMHO, a species that gets to be, say, a million years of technological progress ahead of 21st century humans would probably have the means to create artifacts that could survive for deep time, if the species themselves do not survive that long.
Such artifacts could contain data on the galaxy-filled universe and expansion.
And yes, you can do science with artifacts. You can test models or hypotheses every time you find another artifact or make another kind of measurement on an existing one.
They could still see the CMBR, although it’ll have been redshifted much further into the radio spectrum and no longer be in what we call the microwave region.
One interesting thing related to how flat the universe currently appears to be is that we can already set a minimum for its size.
If the universe is finite, and curves back on itself, it must be at least 500 times the size of the observable universe to look as flat as it does to our measurements.
I would say the universe is ridiculously big but heck just the solar system is already ludicrously big to our comprehension. The universe meanwhile just keeps on adding zeros, long past our imagination.
@Chronos or @Francis_Vaughan can answer better but, I think, you need not worry about an infinite universe. Only the finite universe you can see (so to speak) can affect you. Even if there is something outside of that limit, even if infinite, it can have no effect on you.
So, while more space might be there, your life operates as if it is not there. Only what is within your finite horizon (46.5 billion light year radius) can affect you. Anything beyond that may as well not exist from your perspective.
That’s true if it wraps around by virtue of curvature. It could also be perfectly flat and yet still wrap around, for instance like some video game maps do where going off the “top” of the map puts you at the “bottom” and going off the “left” puts you at the “right”. If this is the case, then the Universe could plausibly be only slightly larger than the visible Universe.
By any measure of what happened in that time, it’s the vast majority of the history of the Universe. We’re already well into the Heat Death.
As has been explained, there is no “other side”, however as the universe is expanding faster than the speed of light, I believe, to address what appears to be the heart of your inquiry, there is portions of the universe we will never be able to see.
(or at least as I understand it… which admittedly isn’t very good)
