The observable universe. Does it mean the edge of space where nothing else is? Is it where the universe is currently at in its expansion after the big bang? Or is it just a barrier that our telescopes are yet to look beyond, and there are just more galaxies past it?
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It’s the universe we’re causally connected to. That’s just a fancy way of saying it’s the part we can ever see, the part that can interact with us in some way, the part that can be be reached at or below the speed of light. Everything beyond that sphere isn’t just something we can’t see, it’s utterly disconnected from us and us from it. We’ll never see it, never reach it, and can have no effect on it even trillions of years from now (unless the expansion of space reverses of course).
The observable universe is *probably* a very small part of a much much MUCH larger universe, which in turn may or may not be infinite.
Question connected to this, how did the universe initially expand faster than the speed of light during the Big Bang?
The universe is 13 billion years old so you can only see for 13 billion light years in any direction. Light can only travel up to the speed of light however spacetime does not have the same constraint. We can see very fast galaxies at the edge, we cannot see in front of them
What is meant by the observable universe is the university we can currently see based on the current speed of light, and how the light has travelled accordingly to get to us.
What we are seeing now, is relative to however far we are away from us. For example a planet we can see now 7 billion light years away, is actually the image of it 7 billion years later. For all we know, in 7 billion years time it has been engulfed by a star and that star has since gone into supernova.
If you are a planet 14.8 billion years away, on the edge of our observable universe, you will have been moving for 14.8 billion years at whatever pace you were going. This is why we say the observable universe, as we appreciate that the universe is only as we see it in an observation.
If the light catches up with us, we could possibly see more things in our universe – for example a new star could magically be born from complete darkness.
Their could be stuff that is going at the speed of light, not emitting any light, and would therefore be invisible. But who knows really.
Imagine you in a dark large room, your feet are glued to the floor, all you know about your universe is what you can touch. That is your observable universe. This only works if touch is your only sense btw.
Now imagine there are others in the room, each person has a different observable universe, some can touch others some can’t.
Each person knows nothing and can never know anything about what is beyond thier reach.
Now image there are no walls to the room, you’d never know you’re at the “edge” all you would know is you can feel something over here but can’t feel anything over there. For all we know the edge wraps right back around to the other side like a sphere or even like a video game you go to one edge and warp directly to the other edge.
Let’s say you’re on a sidewalk. The sidewalk has a crack in the concrete every 1 meter, dividing it up into 1-meter slabs.
Spread out along the sidewalk are a bunch of your friends. If you wanted to, you could walk to them, and say hi.
An evil wizard suddenly shows up and puts a spell on the sidewalk. Every ten seconds, each slab of the sidewalk will spontaneously multiply into two slabs.
There’s a friend on the sidewalk very close by, only a few slabs away, you can probably walk to them with no trouble. You walk faster than the sidewalk slabs are multiplying.
If you keep walking, you’ll find out that some of your friends on the sidewalk can’t be reached. The sidewalk is just multiplying in size too fast for you to keep up.
You could try to move faster. That would make you cover more ground, and meet more of your friends. But no matter how fast you run, at some point the sidewalk will be compounding so much multiplication that you simply cannot keep up. Even if you could run at literal light speed, the ultimate speed limit of the universe, there will be parts of the sidewalk you will never be able to keep up with. Any friends beyond that point might as well not exist, because no matter what you do, you can never reach them.
The universe, for whatever reason, seems to have this spell cast on it. Whether an evil wizard did it or not is not known. But bits of the “cosmic sidewalk”, per se, are multiplying in size in a very similar way. We usually phrase it as things “moving away” from us, but in some sense, nothing is actually moving anywhere. The literal space itself between things is just… multiplying. Somehow. We call this the “expansion” of the universe. And because nothing is actually moving because of this, the things that *look* like they’re “moving away” can do so faster than light speed.
And just like how there are friends on the expanding sidewalk that you can never reach, there are also things in our expanding universe that you can also never reach. And likewise, they can never reach you. This doesn’t just apply to going to visit one another. It also applies to, well, literally any method you can imagine for the two places to affect one another in any way. Sending messages? Rays of light? Gravity? None of those things can outpace the expansion. Since nothing past that point can in any way ever hope to affect you, we say those things aren’t “observable”. That is, there is no possible way you can ever know anything about them. By any method. Ever.
Thus, the “observable universe” is the bubble of things that we *can* observe. It’s all the things within that cutoff point that we could, by one method or another, hope to ever affect, or be affected by. It’s a very big bubble. Unfathomably large. But it’s not infinite. Even if the universe itself truly is infinite, it wouldn’t really matter, because anything outside the cutoff bubble is stuff we will never, *ever* see.
The universe is about 14 billion years old. Light has therefore had about 14 billion years to reach Earth, so we can see as far as 14 billion light years. Because the universe is expanding faster than the speed of light, anything further away than that is too far for the light to ever reach us.
That ‘bubble’ with a radius of fourteen billion light years is the observable universe.
It’s just everything we’ve had enough time to be able to see. As time continues to pass, the observable universe gets larger and larger because we’ve had more time to see it.
Because of this, the observable universe is centered on humans, since it’s basically how far *we* can detect things.
It’s the third one, it’s as far as we can see and there’s certainly more galaxies beyond it. If you travelled far away from earth then you would see a different observable universe.
It’s the speed of light times the age of the universe.
The universe is 13.6 B years old. That’s how much time has elapsed since the Big Bang. The observable universe is, thus, 13.6B light years from Earth in all directions. Light from anything beyond that, even if it was emitted when the universe began, couldn’t reach Earth yet because light is limited to a speed of 1c. Because that light can’t reach us yet, we can’t see it.
It’s much like those radii they use when someone is kidnapped. If they were kidnapped an hour ago, the kidnapper could have only taken them as far as they can travel by car in an hour if they used a car. Someone kidnapped in Pittsburgh an hour ago could have reached Steubenville, OH because the kidnapper could drive from Pittsburgh to Steubenville in an hour. But the victim couldn’t be in Chicago because it isn’t possible to get from Pittsburgh to Chicago by car in an hour. If the kidnapper had an airplane, now they could be in Chicago, but they couldn’t be in Tokyo because an airplane won’t make it from Pittsburgh to Tokyo in an hour.
As time passes, the radius increases. If they made it to Steubenville in an hour, in another hour they could be as far as Cambridge. In another hour after that they could be in Columbus. In another hour they could be in Toledo.
In 400M years, the observable universe will have a radius of 14B light years.
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