• Everything that exists is collectively known as the Universe.

  • The Universe began in an event called the Big Bang around 14 billion years ago. Image credit: NASA / WMAP Science Team

  • Because light doesn’t travel instantly, we are unable to see the entire universe. The part that we can see is called the observable universe.

The future's unwritten
the past is a corridor

Our galaxy is one of many. Together, along with 100,000 nearby galaxies, it forms a big group called the Laniakea Supercluster. This supercluster is over 500 million light years wide!

The Laniakea supercluster. Each tiny dot is one galaxy!
Image credit:

Our supercluster isn’t even the only supercluster, in fact there are millions of the things. Superclusters come together in long thread-like structures called galaxy filaments. Together, these galaxy filaments form a big web that spans the entire universe. Which is to say, everything that exists.

The observable universe

At least, everything that exists that we can see. Because light doesn’t travel instantly, there is a significant amount of the Universe that isn’t visible to us! (since the light from there hasn’t reached us yet)

The entire universe

Planets, stars, galaxies, superclusters… how did we end up with all this stuff? It all started with an event called the Big Bang.

About 13.8 billion years ago, the Universe began. This event is known as the Big Bang.

Here’s what happened during the Big Bang:
The Universe began as an incredibly tiny point with absolutely nothing in it. It was unimaginably hot, and had a lot of energy. The universe then very rapidly stretched out (e x p a n d e d) and cooled down. Simple particles like protons, neutrons and electrons began to form. These particles later combined together to form atoms. Eventually, these atoms came together by gravity to form all the stars, galaxies and galaxy clusters we know today.

The Big Bang isn’t just when the first matter appeared. It is also when space and time themselves began!

This means the Big Bang wasn’t really an explosion, because explosions need space to explode into. The Big Bang is space itself rapidly expanding from a very small point.

Here’s an analogy: The Big Bang isn’t like a paint balloon suddenly exploding onto a blank canvas. Instead, it is like a tiny, colourfully-painted canvas that suddenly stretches bigger and bigger, and as this happens the paint changes colour and shape. (Yes, it is weird!)

In our galaxy there are around 200 billion stars. That’s a lot of stars! There are also around 1 trillion galaxies in the observable universe.

The Hubble Ultra Deep Field, a photo of thousands of galaxies. The Hubble Ultra Deep Field: A photo nearly 10,000 galaxies taken by the Hubble Space Telescope in 2004.

Therefore we can estimate the total number of stars in the observable universe to be around 200 billion trillion. (That’s 200,000,000,000,000,000,000,000 stars!) This number is over 1000 times bigger than the number of grains of sand on all the earth’s beaches.

With all this stuff in the universe, you might think that all the galaxies would be coming together because of gravity.

However, experiments have shown that practically every galaxy is moving away from us! This isn’t because we smell or anything: every galaxy is similarly moving away from every other galaxy.

This is happening because the universe is still expanding today!

Here’s an analogy for how the expansion of the universe works: if you draw some galaxies on a balloon, then inflate the balloon (expand the universe), then you can see that all the galaxies get further away from each other.

The observable universe is huge, but the entire universe makes the observable universe look minuscule. The universe is least 15 million times bigger than the observable universe!

It’s like we’re stuck in an enormous cave, but we can only see a small patch illuminated nearby. And the cave is only getting bigger and bigger over time.

Image credit: Daniel Burka

The immensity of the universe might seem scary, and make you feel incredibly insignificant in the grand scheme of things.

But remember: your significance isn’t decided by how big you are. Someone who’s 6 foot tall isn’t twice as significant as someone who’s 3 foot tall!

You’re actual significance is decided by things less physically-defined. Things like how you treat other people. Or, more importantly, how you rank on the Physics Fox leaderboard.

Every second, there is light going through you that was created not long after the beginning of the Universe. This light is one of the types of light we can’t see, called microwaves (yes, the same ones your microwave oven uses to cook food!)

This light was created when the Universe was around 379,000 years old (about 0.003% of the Universe’s current age). It was at this time when the first atoms started to appear. The light is known as the Cosmic Microwave Background.

The CMB shines onto us from all directions. We can imagine it as a sphere encircling Earth (like the celestial sphere).

Everywhere in the universe was very hot at this time, about 3000°C! Hot things create light (which is how filament light bulbs work), so lots of light was being created everywhere. As it happens, the colour of light produced was similar to that of a light bulb (a white-orange hue).

As the universe continued to expand, this light changed colour into invisible microwave light.

With the right instruments, we can detect these microwaves all around us.

‘Unwrapped’ map of the CMB. Red/blue regions are slightly hotter/colder than the green regions. Hotter regions have a higher density: these are the where future structures (today’s stars and galaxies) formed.


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