Recently we made an attempt to put incomprehensible astronomical distances into a human perspective.

We started by looking at the vast distances within our solar system, moving on to distances within our galaxy, after which we proceeded to the mind-blowingly large observable universe.

We ended the article by discussing the unobservable universe. As mentioned, the size of the entire universe (both observable and unobservable) is uncharted. But is it actually infinite?

This article continues where we left off. Five experts express their take on whether space is infinite or not, each from a different angle. Please enjoy!

By Noor Gillani

Is the universe infinite? Are there boundaries? - Image Credit: Outer Space via Shutterstock - HDR tune by Universal-Sci

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.

Space experts often refer to the doughnut-like torus shape, which has no edges or vertices. The torus is important as a mathematical object. - Image Credit: Leonid_2 via Wikimedia Commons

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?

We asked five of them — and it seems the jury is still out.

Here are their detailed responses:

Anna Moore (Astronomer), Director of The Australian National University Institute for Space and the Advanced Instrumentation Technology Centre, Australian National University

Maybe:

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.

The sharpest image of the cosmic microwave background as observed by the Planck satellite - Image Credit: European Space Agence via Wikimedia Commons

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 that permits the parallel passage of two beams of light. An infinite universe could have a geometry that 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.

Sara Webb (Astrophysicist), PhD candidate in Astrophysics, Swinburne University of Technology

Yes:

I believe so. We know the universe had a beginning with the Big Bang. And from what we observe, this beginning didn’t occur in any one area. No matter where you are in the universe (in this galaxy or one far, far away) space appears to be expanding in all directions, with you at the centre. Now, we calculate the universe is about 13.8 billion years old, which means that’s how much time space has had to expand. So logically we’d expect space to be 13.8 billion light years across, right?

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.

But I lean towards another possibility, which considers the rapid inflation that followed the Big Bang. There's a theory 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. And being the sci-fi lover I am, how could I not want this to be true?

Shortly after the big bang the universe expanded extremely fast - Image Credit: Eugen Domentean via Shutterstock

Tanya Hill (Astronomer), Honorary Fellow of the University of Melbourne and Senior Curator (Astronomy), Museums Victoria

Yes:

There’s a limit to how much of the universe we can see. The observable universe is finite in that it hasn’t existed forever. It extends 46 billion light years in every direction from us. (While our universe is 13.8 billion years old, the observable universe reaches further since the universe is expanding).

The observable universe is centred on us. An alien in a galaxy far away would have its own observable universe. While there may be some overlap, they would inevitably see regions we can’t see. Therefore, it’s not possible to see if the universe is finite, because we can’t see it all. Instead, we can tackle this question by exploring the universe's shape. While we don’t know the shape of all space, we do know our part of space is flat. This means two rockets flying parallel on cruise control will always remain parallel. Because space isn't curved they will never meet or drift away from each other.

A flat universe could be infinite: imagine a 2D piece of paper that stretches out forever. But it could also be finite: imagine taking a piece of paper, making a cylinder and joining the ends to make a torus (doughnut) shape. Therein lies the problem.

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, to my thinking, an infinite universe becomes easier to imagine than a finite one.

Sam Baron (Philosopher of Science), Associate professor, Australian Catholic University

No:

There is one tempting line of reasoning that suggests space must be infinite, but which I believe is wrong. It goes like this: if space is finite, then it would have an edge. But imagine getting into your spaceship and flying to the far reaches of the universe. It seems inconceivable you would find an edge. What would the edge even look like? Surely space must go on forever.

But there's another way for space to be finite. It could be a torus, which is spatially finite but edge-free, like a cosmic donut. If the universe is donut-shaped, then there’s a very natural scientific test that would reveal whether it is finite.

Imagine you aim a beam of light at a reflective surface very far away. If the surface is uneven, the light will be reflected in multiple directions. If the universe is a donut, the reflected rays bouncing back will gradually curve with the shape of the universe, and will eventually wrap back in on themselves and intersect (diagram here). This can only happen if the universe is finite, mind you. In an infinite universe the rays would continue forever.

Now, imagine you’re standing at the point where the light rays intersect. If you turn to one side, you will see the object that reflected the ray. If you turn to your other side, you will see the same object but from a different angle. So if the reflective object was a distant planet, you would see the same planet twice. Scientists have already begun to look for this hall of mirrors effect in the dim glow left over from the Big Bang. It would provide evidence of not just the size but also the shape of the cosmos. While nothing conclusive has been found yet, who knows what we might uncover if we keep looking!

Kevin Orrman-Rossiter (Science Historian), PhD candidate, History & Philosophy of Science, The University of Melbourne

No:

By “infinite” we usually mean something which is limitless or endless. My position is space is finite. However, to demonstrate that let's propose, for a moment, that space is infinite. In a simple sense if this were the case and I set out in a spaceship in any direction, I would never reach a boundary. But there's a problem with this experiment: I would need to travel for an infinitely long period of time to ensure there isn’t a boundary “just a little further out”. It doesn’t matter what speed I travel. My voyage of proof would need to be infinite in order to prove my hypothesis that space is infinite. Now, not many grant bodies are going to fund such an experiment.

This highlights that to provide proof, we must do so through observation rather than direct experiment. Over the past century, we’ve learnt a lot about our universe through observation. We know that space, the universe, had a beginning some 13.8 billion years ago. We know from observation that it's expanding and we have detected the cosmic microwave background, which is thought to be leftover radiation from the Big Bang. Space as we see it today is a slowly expanding web of galaxies.

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. It has a finite existence in time and, to return to the start of my argument, therefore I would propose that at some stage my spaceship voyage will reach an end.

Source: The Conversation

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