[Curiosities] Is the Universe infinite?

[Jeenyuhs]

Infinity is a mathematical concept that refers to an unlimited quantity within a magnitude. And from our human perspective, it is simply impossible to conceive. And in parallel, the Universe is something that amazes us but that we are far from knowing. Every time we answer one question about the Cosmos, hundreds of new ones appear.

So, what happens if we mix these two concepts, what happens when we venture to discover if the Universe is infinite or, on the contrary, finite? Well, we run into one of the most complex but at the same time astonishing and ambitious questions that the human species has ever asked itself.

 

 

Is the Universe infinite or does it have an end? This question, which mixes astronomy with philosophy, is surely the key question of science. A question whose answer, if found, would change everything. And the implications of it being infinite would be astonishing and, at the same time, terrifying.

And the most surprising thing of all is that, for now, all the evidence indicates that the Universe has no edges. In other words, in principle, the Universe is infinite. Get ready to have your head explode, because today we will show why astronomers agree that the Universe is not finite, but infinite. Here we go.

 

The Universe, light, and the limits of our knowledge

We know many things about the Universe. And more we will know in the future. But we have been, are and will always be limited by one aspect: the speed of light. As Einstein established in his theory of general relativity, the only constant in the Universe is the speed of light, which is 300,000 km per second.

We also know that the Universe was born 13.8 billion years ago in what is known as the Big Bang, which is the beginning of the expansion of the Cosmos from a singularity in space-time. And since then, we know that it is expanding. And that it is doing so, moreover, at an accelerated rate. In fact, it is expanding 70 km per second faster for every 3.26 million light-years of distance.

 

But what is the problem we encounter when we try to determine whether the Universe has a limit or not? That is, when we try to determine whether it is infinite or not. Well, we are limited by the time that light has had to travel since the birth of the Universe itself.

The farthest we can see in space is 13.8 billion light years away. Well, technically, 13,799,620,000 million light years, because during the first 380,000 years of life of the Universe, the energy was so high that atoms could not form as such, so the subatomic particles were free forming a "soup" that prevented photons to travel freely through space. Well, the point is that it was not until 380,000 years after the Big Bang that light was literally made.

 

So, this is our limit. We can't see beyond it. And since we can't see beyond that, we can't know if the Universe really has an edge or, on the contrary, if it's infinite. The only way to determine whether the Universe is eternal or finite is to rely on mathematical calculations and astronomical predictions. And the truth is that they have shed a lot of light. A lot of light.

 

The Geometry of the Universe and its Eternity

One of the main ways to know whether the Universe was infinite or not was to determine its shape. This is an incredibly complex task, but mathematical measurements and predictions determined that the Cosmos could only have four possible geometries: Euclidean (flat), spherical, hyperbolic (flat but with [CENSORED]ture) or toroidal (like a donut).

The toroidal one we ended up discarding (although a small open door remains) since the presence of two different [CENSORED]tures (longitudinal and transverse) would make light propagate in different ways in space. And this goes against the cosmological principle, which tells us that the Universe is isotropic, i.e., that physical properties do not depend on the direction in which they are examined. If it were like a doughnut, it would depend on it.

Therefore, we are left with three possible shapes: flat, spherical or hyperbolic.And now comes the interesting part. The spherical shape hypothesis would imply that the Universe is closed. That is, it is finite. If the Universe is a sphere, it cannot be infinite. And the hypotheses of the flat and hyperbolic forms, both positing an open Universe, would imply that the Universe is infinite.

 

 

In this sense, by determining the shape of the Universe we can know if it is infinite or not. And can we know its geometry? Yes. At least, approximately. By analyzing the cosmic microwave background. It is the radiation left over from the Big Bang. It is the echoes of the first light that was in the Universe 380,000 years after its birth. And it is radiation that has traveled a long way to reach us.

Therefore, it is this cosmic background radiation that will have best experienced the effects of the [CENSORED]ture (or non-[CENSORED]ture) of the Universe. If the Universe is flat, its [CENSORED]ture is 0. If it is spherical, its [CENSORED]ture is positive (greater than 0). And if it is hyperbolic, its [CENSORED]ture is negative (less than 0).

 

In this context, what we do is to calculate the distortion that the cosmic background radiation has undergone along its journey from the origins of the Universe. We compare the estimates of the size of the cosmic microwave background spots with the size of the spots we actually see. If the [CENSORED]ture is positive (spherical geometry), we would see larger spots than the mathematical models estimate.

If the [CENSORED]ture is negative (hyperbolic geometry), we would see smaller spots than the mathematical models estimate. And if there is no [CENSORED]ture (flat geometry), we would see the spots the same size as the mathematical models estimate.

And what do we see? That there is no distortion. Or that, at the very least, we are very close to 0 in [CENSORED]ture. The geometry of the Universe appears to be flat. And if the Universe is flat, it means it is open. And if it is open, it is infinite.

 

The fact that its geometry appears to be flat, together with the fact that dark energy does not dilute in space no matter how much the expansion of the Universe increases, seems to indicate that, indeed, the Universe is infinite. It has no limits. As long as you move through it, you will find new galaxies and new stars. You will never find a limit or return to the same place. The Universe is eternal. Or so it seems.

 

 

So, is the Universe really infinite?

Although studies of the geometry of the Universe and dark energy seem to indicate that the Cosmos is indeed infinite, we can never be sure. Why? Basically, because we cannot confirm 100% that the Universe is flat.

We know that it is around 0 [CENSORED]ture, but we are not absolutely sure. The calculations cannot be entirely accurate, so it is possible that there is a slight positive [CENSORED]ture (if it is negative, it doesn't matter too much, because it would be hyperbolic and still infinite) that we cannot measure.

 

The Universe is either flat or slightly spherical. But for it to be slightly spherical would already imply that the Universe would be a closed sphere, which would therefore make the Cosmos a finite place. We may never be able to measure its [CENSORED]ture exactly. And without knowing whether it is really zero or not, we are totally blind. That small numerical difference would make us go from a conception of an infinite Universe to a finite one. It changes everything.

Not to mention that we still don't know the real scale of the Universe. It is immense, that is clear. But we don't know how immense. We are limited by the portion of the Cosmos that light allows us to see. And perhaps the portion we see is indeed flat; but the Universe is so incredibly large that, even though it is spherical in its entirety, our "patch" appears to be flat.

It is the same thing that happens at the level of the Earth. If you measure the [CENSORED]ture on the ground in a portion of 1 km long, you will see that this [CENSORED]ture is 0. Does this mean that the Earth is flat? No. It is spherical. What happens is that, at such a small scale compared to the whole, the [CENSORED]ture is imperceptible.

 

 

In this sense, we know neither whether the patch of the Universe we are looking at is really flat at all, nor, if we can confirm that it is absolutely flat, that it does not belong to a spherical "whole" so incredibly large (but finite) that it does not allow us to perceive the [CENSORED]ture.

We can never be sure whether the Universe is infinite or whether it has a limit. The question, then, is open to interpretation. Any position is valid. And, whether it is infinite (which would imply that there are infinite "you's" in the Universe since all physical, chemical and biological probabilities can be fulfilled infinitely many times in an eternal panorama) finite (which would imply that we are locked inside a cosmos surrounded by "nothing") are two really scary scenarios if you stop to think about it. Whether infinite or not, the Universe is an amazing and indecipherable thing. And it is surely this that makes it so wonderful.

 

We will never know if the Universe is eternal or if it has an end.