If you were lucky enough to live somewhere away from the city lights, you might have gone out one night and looked up. On a clear night, be rewarded with a breathtaking view of the stars – each one a distant sun. If you have a little knowledge (or a useful application), you may even be able to identify a constellation or two.

But the most terrifying thing is that space seems to go on forever. Even our earliest ancestors contemplated the nature of the heavens and the distances to planets, stars, and occasional comets drawn across the sky.

You might think that in our modern age, we would know the size of the universe, but we don’t. On the other hand, not knowing everything is different from not knowing anything. What do we know and what do we not know? What is the size of the universe?

## Fixed virtual universe

First of all, we know two important facts. The first is that the universe began 14 billion years ago in a unique event called the Big Bang. The second is that ordinary visible light has a finite speed. It travels at a staggering 300,000 kilometers (186,000 miles) per second, or fast enough to orbit the Earth about seven times in one second. We call the distance light can travel in a light year, which is about ten trillion kilometers (6 trillion miles).

Another important idea we need to understand is the difference between the visible universe and the entire universe. The first is what we can see, and the other is everything. This is not difficult to understand. A person standing on the roof of the tallest building on the planet (the Burj Khalifa in Dubai) can see in every direction for a distance of 100 kilometers (60 miles). However, the Earth’s surface is much larger than that, and the curvature of the planet makes it impossible to see everything.

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With the universe, the limiting factor is different: it is the speed of light. If our universe is constant and doesn’t change (which isn’t true), then the farthest thing we could see would be 14 billion light-years away. This is because if a distant object emitted light at the moment the universe began, that light would now reach Earth. Light from an object 15 billion light-years away won’t reach Earth for another billion years, so we haven’t been able to see it yet.

In our static virtual world, the visible universe would be a sphere, surrounding the Earth, with a radius of 14 billion light-years. The entire universe may be larger than that, but we would have no way of knowing, because the light from distant locations has not yet arrived.

## Our actual universe

But the universe is not static, and that complicates matters. The universe began in the Big Bang, and that “bang” caused the universe to expand. As it travels, the light must fight this expansion, which takes longer to reach you.

To understand this, suppose the child is standing ten meters away from you and is rolling a ball toward you at a speed of two meters per second. It will take five seconds for the ball to reach you. Now, suppose we have the same situation, and you’re standing on solid ground, but baby on one of those moving walkways that you find at airports. Also, suppose the walkway is moving away from you at a rate of one meter per second. Due to the movement of the walkway, the ball will not take five seconds to reach you; It will take ten.

Unfortunately, it gets more complicated. While the child was ten meters away from you when he rolled the ball, due to the movement of the walkway, the child will be twenty meters away from you when the ball reaches you.

The same thing happened to visible light from the Big Bang. This light has traveled for 14 billion years to reach Earth now. And like a child on the moving walkway, the current location of anything that emitted that early light is not 14 billion light-years away; It is now 46 billion light-years away. We see the light from where it was emitted, not from where the emission source is now.

In this way, astronomers can confidently say that the diameter of the observable universe – the sphere around the Earth far from the oldest thing we can see – is 92 billion light-years (that is, edge-to-edge) in diameter.

## So, how big is the universe?

But that’s just the visible universe. What about the entire universe? How can we find out the parts that are so far away that we haven’t seen them yet? This is where things get interesting.

It may come as a surprise, but astronomers aren’t 100% sure they know the geometry of space. It can be flat or curved. While space is three-dimensional, we can use a two-dimensional analogy to understand what that means.

In two dimensions, flat means flat, like a table top. However, the two-dimensional surface can be curved, like the surface of a globe, but it can also be curved like the surface of a saddle. If it’s curved like the surface of the globe, that means if you have a superfast spacecraft and you travel long enough, you could end up where you started, like an airplane flying along the Earth’s equator.

The astronomers studied the data and determined that space was flat, or something close to it. However, this determination is an analogy, and measurements are uncertain. It is still possible for the universe to have a very small curvature. But if it is curved, then the equivalent of the “equator of the universe” is at least 500 times larger than the visible universe. Or maybe even greater.

Therefore, although the size of the entire universe is not known, astronomers know that it is at least 500 times larger than what we can see. (This number represents the distance one would have to travel to return to the starting position.) In the same way that the volume of a cube is the distance along the sides of a cube, the volume of the entire universe is, at least, 125 million times larger than the visible universe.

The bottom line is that the observable universe is incredibly large, and the entire universe is truly enormous – in fact, the entire universe can be infinitely large.