Do we have any limit for our universe?

 Is space infinite or does it have a limit? 

If the universe is expanding, then what is it expanding into, and what is it expanding from? Despite innovations in telescope and satellite technology, what's beyond our line of sight in space is uncertain. Scientists predict what the furthest reaches of space may look like. Where's the centre of the universe, and where is its edge? After a century of observations spanning the breadth of the cosmos and theoretical insights which push humanity's vision of the universe to its utmost limits, we can finally, confidently say that the universe is infinite or not. It's complicated. Let's start with something we can say for certain: We live in an expanding universe. But if the universe is expanding, then what is it expanding into? And what is it expanding from? Where's the edge of the universe, and where is its centre? Some astronomers calculate that the universe will die in 33 billion years, much sooner than we thought. It's easy to imagine an expanding universe, and there are plenty of analogies to help guide our thinking. We can imagine drawing little galaxies on the surface of a balloon and inflating that balloon to see the galaxies getting farther apart. We can imagine baking a loaf of bread with raisins in it and seeing how, as the bread rises, the raisins get farther apart. But the balloon has both a centre and an edge. And the bread has a centre and a crust. So where's the centre of the universe, and where is its edge?

We’ve known for some time now our universe is expanding, and in recent years discovered this was happening considerably faster than we’d expected. Yet despite momentous innovations in telescope and satellite technology, it’s thought much of what’s out there in the cosmos lies beyond our line of sight, beyond the “observable universe”, as it’s called. It also means we don’t know with any certainty what shape the universe as a whole takes, whether it’s a closed cosmic “doughnut”, a flat plain that stretches out like an endless piece of paper, or a giant sphere in a state of constant expansion. This has left scientists wondering about the furthest reaches of space and what they may look like. What do they think regarding the fate of the universe? Will it expand forever? Let's start with the centre. Where did the Big Bang start? Right here. And right over there. And in the next galaxy over. The Big Bang happened everywhere, all at once. It had to happen everywhere, because everywhere is, by definition, part of the universe. It was not an explosion that occurred somewhere in space. It was an explosion of space, when the expansion of the universe first got started. It was not a place but a time.

Space experts often refer to the doughnut-like torus shape, which has no edges or vertices. The torus is important as a mathematical object. The short answer is we don’t know. We know the observable universe, the part we can visibly see and measure, began around 13.8 billion years ago with the Big Bang. So we know the age of the universe is finite at least from the time of the Big Bang. But the universe is getting bigger. It has been expanding in all directions ever since the Big Bang and continues to (and recently it has been getting faster and faster). Leftover radiation from the Big Bang, which we call the “cosmic microwave background”, represents the earliest picture of the universe, back when it was smaller, hotter and denser. We can take images from this early time to understand the universe’s shape (or geometry) on the largest scale. Knowing this is important to knowing whether the universe is infinite or finite. Measurements taken by satellites have pointed to the universe having a flat geometry. In a flat universe, two light beams shot side by side through space will stay parallel forever, and will never cross or drift apart. In this sense, we can still think of a cylinder or torus (donut) shaped universe as being “flat”. Current measurements aren’t accurate enough for us to know whether the universe’s flat geometry is represented by a piece of paper, a cylinder, torus or any other shape which permits the parallel passage of two beams of light. An infinite universe could have a geometry which is totally flat like a piece of paper. Such a universe would go on forever and include every possibility, including endless versions of ourselves. On the other hand, a donut-shaped universe would have to be finite, as it's closed. But for now we still don't know the shape of the universe, and therefore nor can we know its size.

This is going to get weird. It's too tempting to imagine a wall or boundary, with galaxies and stuff on one side and nothingness on the other, with the universe expanding to fill that nothingness. But that's wrong. Even the vacuum of space is something. There are still points, locations and existence. There's no "outside" of the universe because "outside" implies existence, even an empty one. But the universe is, by definition, all there is. There is nothing to physical reality except the universe. Walls separate one region from another, but the universe comprises all of the regions simultaneously. If there were an edge, you could imagine working hard enough to get outside that edge. But that's not possible. There is no outside; there is no side. There is just the universe.

But the size of the observable universe is actually 46 billion light years, meaning the very first light we can see emitted (380,000 years after the Big Bang), came from a distance that is now 46 billion light years away. This is due to something called “rapid inflation” (more on this later). However, there's no reason to suggest the edge of the observable universe is the edge of the actual universe. We tend to think of things as having 3D shapes: a sphere, cube, a cone. We could think of the universe as a sphere expanding indefinitely and infinitely. Or it might curve and bend in ways that could make it a closed system (like a donut), where if you were to travel in a straight line for long enough, eventually you’d end up back where you started: space would be finite. There's a theory that this inflation is actually eternal inflation, meaning it’s always occurring at one point or another in the universe, rendering the universe infinite. This begins to dive into the mind-boggling idea of quantum fluctuations and even multiverses. Additionally, there are many ways the universe could have been curved, but instead we live in a region of flat space. This is a very specific condition and we use a theory called “inflation” to explain it. Inflation is the idea that very early on the universe rapidly expanded for a brief moment, smoothing out all the kinks and curvatures in our part of space. After inflation, the universe grew to what we see today. But it’s possible inflation didn’t just seed our universe. Perhaps it also occurred elsewhere and is happening still. How big might that make the entire universe, or multiverse? It opens up such possibilities that, an infinite universe becomes easier to imagine than a finite one. One key question in cosmology is whether this expansion will continue, change pace or reverse. Answering this involves understanding the properties of dark matter and dark energy. The interesting point is no matter the model of the universe (and there are still important pieces missing here), the current cosmological thinking is there will be an ending and the universe will not persist forever.