If you’re lucky enough to live somewhere away from the city lights, you might have been outside one night and looked up. On a clear night, you are rewarded with a stunning view of the stars – each a distant sun. With a little knowledge (or an app handy), you might even be able to spot a constellation or two.
But the most amazing thing is that space seems to last forever. Even our earliest ancestors pondered the nature of the heavens and the distances to planets, stars, and the occasional comet painted across the sky.
You would think that in our modern day, we would know the size of the Universe, but we don’t. On the other hand, not knowing everything is not the same as knowing nothing. So what do we know and what don’t we know? How big is the Universe?
A hypothetical static Universe
To begin with, we know two crucial facts. The first is that the Universe began just 14 billion years ago in a singular event called the Big Bang. The second is that ordinary, visible light has a finite speed. It travels at an amazing speed of 300,000 kilometers (186,000 miles) per second, or fast enough to circle the Earth about seven times in a single second. We call the distance that light can travel in one year a light-year, which is equal to about ten trillion kilometers (6 trillion miles).
Another important idea 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 so difficult to understand. Someone standing on the roof of the tallest building on the planet (Dubai’s Burj Khalifa) can see in all directions for about 100 km (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 were static and unchanging (which is not true), the most distant thing we could see would be 14 billion light years away. That’s because if an object that far away emitted light when the Universe began, that light would be reaching Earth right now. Light from an object 15 billion light years away won’t reach here on Earth for another billion years, so we haven’t been able to see it yet.
In our hypothetical static Universe, the visible Universe would be a sphere, surrounding the Earth, with a radius of 14 billion light years. The entire Universe could be larger than that, but we would have no way of knowing because light from more distant locations has not yet arrived.
Our real universe
But the Universe is not static and that complicates things. The universe began in the Big Bang, and that “bang” caused the Universe to expand. As it travels, the light has to fight against that expansion, which takes longer to reach you.
To understand this, suppose a child is standing ten meters away from you and is rolling a ball towards you at two meters per second. It will take five seconds for the ball to reach you. Now, let’s say we have the same situation, with you sitting on firm ground, but the baby on one of the moving aisles you find in airports. Let’s further assume that the walkway is moving away from you at one meter per second. Because of the movement of the walkway, the ball won’t take five seconds to reach you; it will take ten
Unfortunately, it gets more complicated. While the child was ten feet away from you when he rolled the ball, due to the motion of the walkway, the child will be twenty feet away from you when the ball reaches you.
The same thing happened with visible light from the Big Bang. That light traveled for 14 billion years to reach Earth now. And like the baby on the moving walkway, the current location of anything that emitted the earliest 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 say with confidence that the visible Universe – which is the sphere around Earth to the distance of the oldest thing we can see – is 92 billion light years in diameter (ie edge to edge).
So how big is the Universe?
But this is only the visible Universe. What about the entire Universe? How can we know about parts that are so far away that we haven’t even seen them yet? That’s where things get interesting.
It may be surprising, but astronomers are not 100% sure that they know the geometry of space. It can be flat, or it can be curved. While space is three-dimensional, we can use a two-dimensional analogy to understand what this means.
In two dimensions, flat means flat, like the surface of a table. However, a two-dimensional surface could be curved, like the surface of a globe, but it could also be curved like the surface of a saddle. If it’s curved like the surface of a globe, that means if you had a super-fast spaceship and traveled far enough, you could end up back where you started, like an airplane flying along the Earth’s equator.
Astronomers studied the data and determined that space is flat, or close to it. However, this determination is a measurement, and measurements have uncertainty. It remains possible that the Universe has very little 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 bigger than that.
So while they don’t know the size of the entire Universe, astronomers know it’s at least 500 times larger than what we can see. (This number represents the distance you would have to travel to return to your starting location.) In the same way that the volume of a cube is the distance along the cubed sides, the volume of the entire Universe is at least 125 million times larger than the visible Universe.
The bottom line is that the visible Universe is incredibly large, and the entire Universe is truly enormous—indeed, the entire Universe could be infinitely large.