Gioca STEMSpace
ItalianoEnglish

Space · Galaxies and the universe

How big is the universe?

We do not know, and maybe never will

Only spacecraft can see it

How big is the universe?
The sphere of what we can see, with us at the center and the horizon beyond which light has not arrived.

The numbers

What it is
The part we are able to seeNot the whole universe
Centered on whom
On whoever is lookingEveryone has their own
How big it is
93,000,000,000light-yearsMore than light has traveled

How it happens, step by step

  1. Light takes time

    Light travels fast but not instantly: over great distances it needs a very long time.

  2. Not eternal

    It is not eternal: it began almost fourteen billion years ago, and light has been traveling since then.

  3. A border is born

    If something is so far away that its light has not arrived yet, it stays invisible to us.

  4. There is the horizon

    That border is called the horizon, and the sphere it encloses is what we call the observable universe.

  5. Each has their own

    The sphere is centered on whoever is looking. From another point it would be just as big but shifted.

To get an idea

The border exists because the light of the most distant things has not yet had time to arrive.

Every point in space has its own observable universe: we are not at the center of anything.

Fun facts

These are not in the book: there was no room on paper.

  1. If the universe were not expanding, the horizon would lie about thirteen billion eight hundred million light-years away.

  2. But since space is expanding, that border actually lies much farther away.

  3. The sources whose light reaches us today have moved even farther away in the meantime.

  4. The word observable is essential: outside that sphere there is almost certainly more.

  5. We are not at the center of the universe: we are at the center of what we are able to see.

  6. Every year that passes, a tiny bit more of the universe comes inside our horizon.

Test yourself

The first question is the one printed under the QR code, with the answer the book did not give. The other four only exist here.

Question 1 of 5★ is the one from your book

Do we know how big the universe is?

Question 2 of 5

Why is there a border to what we see?

Question 3 of 5

What is that sphere called?

Question 4 of 5

What is it centered on?

Question 5 of 5

What is beyond the horizon?

Riddles

I have a border, but not because something ends there: it is just that beyond that point light has not yet had time to reach you. What am I?

Each of you has one, just as wide but shifted, and each of you finds yourself exactly at my center. And yet none of you is really at the center. What am I?

I hold everything you can see and nothing of what you cannot. My name carries a word many people forget to read. What am I?

Try it yourself

Activity 1

Ask someone far from you to describe their horizon: it will be different from yours, even though it is just as big. It is exactly the same idea.

Activity 2

Try to answer the question of what lies beyond: the honest answer is that we cannot know by looking, and that is fine.

The words

horizon
The limit beyond which we cannot see anything.

Sources: Wikidata (Q272026), Wikipedia (Universo osservabile).