We see a photo of the night sky with the Milky Way clearly visible. A silhouette of a person is standing at the centre, on the floor. The person is looking up.

Just a minute: how big is the universe?

In case a (your) child asks how big the universe is exactly, you might want to know what the most honest, straightforward answer is. Although there is, as of yet, no feasible way of getting to know its size with certainty, we do have some cool things such as telescopes and mathematics at our disposal. When it comes to estimating the size of what we see when we look up, they help a lot.

Observable universe

Imagine yourself floating in pitch darkness. You have no knowledge of where you are. Fortunately, you have binoculars in the left pocket of your spacesuit. As your eyes become adjusted to the lenses, you start to distinguish a faint point of light. Then you remember, you have an even better extensible monocular telescope in your other pocket. Like a pirate in the night, you gaze through the glass. And then you discover, it wasn’t just a dot of light, it was a whole group of dots of light.

We see a person dressed in an astronaut suit floating in space. It is looking at a giant spiral galaxy floating in the same space.

You decide to swirl the telescope around and, since you’re afloat in the middle of, well, nothing, or so it seems, you can even look down, underneath your feet. As you scan all around, you discover there is a nearly uncountable number of groups of dots of light all around you, like a sphere of clustered dots with you at its centre.

Valuable lessons from science class taught you that light has a certain maximum speed, so you realise that, maybe, more groups of dots of light are there, beyond the ones you see, but that their light hasn’t reached you yet.

The part of your universe you can in principle see—limited only by the speed of light and not whether or not we have the technology—is what we call the observable universe. In fact, this is the type of thing we have the Hubble Telescope do for us.

Now, here’s the problem: the universe might just be way bigger than the observable part, but we won’t know by how much because it is highly unlikely light from beyond what is now observable will ever reach us. This is because the universe is expanding rapidly, and more rapidly every second.

The yellow sphere represents the observable universe with Earth at its centre. Everything outside its radius is purely hypothetical. Whatever the location of Earth—more downward, more to the left—Earth will remain the centre of its observable region.

About you being at the centre, think of yourself floating around in space once more. If you look around you, the separation between you and every point on the sphere of your observation is equidistant. And if you were to float to a different region, your observational sphere moves along with you. So, you are always at the centre of your observation bubble.

The same principle is valid for our place in the observable universe, on Earth. And I’m not talking about social-psychological information bubbles on the internet, I mean literally, physically, factually. Mind you, this is not the same as saying we are at the centre of the universe as there is no centre of the universe. You are, however, always at the centre of the observable universe. Therein lies the difference.

Accelerated expansion

Light from beyond the observable part cannot reach us because the universe is growing or—as cosmologists prefer to say—expanding. And it is doing so at an ever faster-going rate. Per second, more space is added than light can cover.

Imagine you’re inside a dream, standing in the narrow hallway of a hotel. You decide to walk to the door at the other end. Now imagine the hallway suddenly growing longer and longer. The walls on either side are not just being optically elongated, but structural bits of the wall seamlessly seem to multiply physically. The hotel is not just stretching, it is expanding its material. In the meantime, you’re moving, but you’re not moving forward by much. The rate of the hallway’s expansion is greater than your rate of displacement.

We see a girl or woman walking down a hallway. While she is walking, the hallway seems to stretch out, or expand. The door at the other end recedes from the walking person.
This hallway is a metaphor for the expanding universe. In reality, the camera operators of the movie Poltergeist (1982) applied the famous vertigo effect, an illusion causing the walls to stretch optically. In our metaphor, however, the ‘stuff’ of the walls is being multiplied seamlessly.

Well, this is what a lonely photon must be experiencing, emanated from a distant star, on its way to us, but never reaching us since the space it is travelling through is expanding. Note that the door in the GIF above is not moving, it is space—the hotel structure—that is expanding! The door is a metaphor for distant galaxies. It’s not the galaxies that recede, it is space expanding.

Estimated size

At the current expansion rate, and given the age of the universe, the observable universe is estimated to be 93 billion light years in diameter. Which is about 880 000 000 000 000 000 000 000 kilometre (546 800 000 000 000 000 000 000 miles).

This means that its volume is about 400 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 litres ($ 4 \times 10^{83} $ l) or a 9 followed by 82 zeros in gallons.

Earth has a volume of about 108 300 000 000 000 000 000 000 litres. So, it occupies about 0.000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 003% of the observable universe.

The size of the universe, however, is unknown. Could be finite, could be infinite. At present, there is no reason to accept either statements definitively.

ESO animation

Have a look at this cool animation made by the European Southern Observatory (ESO) and others. We fly outward from the ESO Supernova Planetarium & Visitor Centre at ESO’s headquarters in Garching bei München to the place where we see a vast number of galaxies. Realise that, as soon as the video takes us out of our own galaxy, which itself is starting to look like ‘just another star’, every other ‘star’ you start seeing is, in fact, an entire galaxy of its own. Of course, the film ends with a fade-out for we do not know whether there are any borders or if it goes on forever.

Screenshot of ESO’s animation. Click to open the YouTube clip.


The attentive reader will have noticed we only talked about the size of the observable universe, yet, we did not mention what we see when we look at its very edge—as far as current technology permits.

As it takes such a long time for that light to have reached us, the stuff that we see, is old. We are looking at the past, the history of the universe. A glance at the cosmos is like time-travelling back to days of old.

This deserves a separate article. Unfortunately, we do not have space nor time right now.

Photo Observable universe adapted on the basis of Strogoff‘s work published under CC BY-SA 3.0.