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The universe could be infinite in size, but how is it logically possible to attain an infinite size from a finite initial state?

Antonio Padilla University of Nottingham, UK

If the universe has only existed for a finite time, then we couldn鈥檛 actually have an infinitely large universe emerging from a finite initial state.

However, we don鈥檛 know if the initial state of the universe was finite or not. It could have been infinite from the very beginning. There is no cosmological observation that rules this out.

The cosmic microwave background radiation encodes information about the shape of the universe, but this still leaves us with a number of possible scenarios. The universe could be finite like a gigantic sphere, doughnut-shaped (a torus) or an infinite, three-dimensional plane or a saddle shape. We just don鈥檛 know. All we know is that it is big!

Eric Kvaalen Les Essarts-le-Roi, France

It is a loaded question! No one says that the universe is infinite now but started from a finite initial state. The problem is that we can鈥檛 know whether the universe is finite or infinite. We can鈥檛 see beyond a certain distance, which has been increasing ever since the big bang.

As far as we can see, the number of galaxies per unit volume seems constant, independent of distance. It is hard to imagine space just ending somewhere.

An alternative, spatially finite universe, proposed by Einstein, is one that is like the surface of a sphere but three-dimensional. But there is no evidence of the curvature that model would entail.

Hillary J. Shaw Newport, Shropshire, UK

The universe could appear infinite if it is really the three-dimensional surface of a very large, four-dimensional hypersphere.

The pace of expansion would be fast enough to preclude us 鈥渟eeing the back of our own heads鈥�, as the light travelling that way could never reach us. This would be analogous to Earth鈥檚 2D surface appearing endless, as it is the surface of a 3D sphere.

Denis De Volder Paris, France

First, we must answer the question: why wouldn鈥檛 it be logical for the universe to have expanded into infinite space while it was finite in size at its origins?

The answer to this might not lie in the realm of physics, but rather in semantics and our cognitive ability to imagine beyond our near environment.

And what do we mean by the universe? Here, the universe wouldn鈥檛 be defined as the visible one, rather as the potential everlasting empty space in all directions beyond the visible universe.

Hector Ruiz El Monte, California, US

If the universe is infinite, then the statement 鈥渇inite initial state鈥� is false 鈥� meaning that the big bang was just a part of the infinite state of the universe, not the beginning of it.

Bryn Glover Ripon, North Yorkshire, UK

I feel that a problem arises when the words 鈥渇inite鈥� and 鈥渋nfinite鈥� occur in the same sentence linked by nothing more than a mere verb.

A finite universe is a tangible entity that can be measured and defined. Infinity isn鈥檛 a number or a quantity, it is a concept that is useful in mathematics, so long as a means can be identified for finally eliminating it in calculations.

@LandwehrJobs via Twitter

I picture it like this: the universe is like a balloon and while you travel from the centre to the fringe, it expands. So it looks like infinity. At some point, it snapped back and compressed all the matter back in one tiny spot.

@magnusiscariot via Twitter

The logical answer would be that it isn鈥檛 possible. Either the universe isn鈥檛 infinitely large or was never in a finite initial state.

Technically, we only see the universe as expanding from a smaller state. Perhaps it is our understanding of finite and infinite that needs work.

@PauloXR7 via Twitter

If the universe were finite, then it would have an edge somewhere. Ask yourself 鈥渨hat is on the other side of that edge鈥�.

To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.

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