Our Universe

The scale of the universe is breathtaking – it's almost beyond comprehension.

Just think how large the Earth is. Now think how the sun is 1.3 million times larger. And how the sun is just one of approximately 200 billion trillion stars that are estimated to exist in the universe.

For thousands of years, we've tried our best to study this cosmic immensity. We call this study astronomy.

Evidence suggests humans were making observations of the sky before recorded history. In other words, astronomy might be the oldest human science.

Stonehenge is one iconic example of ancient astronomy. This circle of stones is at the heart of England, and is believed to be around 4500 years old.

These stones appear to map out the path of the sun on its longest day – the summer solstice. Because of this, the 19th century astronomer Norman Lockyer argued that Stonehenge was an astronomical temple. He believed that the structure was specifically designed for ceremonies that celebrated our seasonal trip around the sun.

Thousands of years after the construction of Stonehenge, astronomy has come a long way.

For example, we now have powerful telescopes in space, such as the Hubble Space Telescope and the James Webb Space Telescope, which let us look further than we've ever looked before.

And yet, so many questions remain unanswered. What is dark matter? What's inside a black hole? Is there life beyond our Earth?

That's what makes astronomy so exciting. There's still so much to learn.

A note on units: faced with the enormity of outer space, the units of distance we use on Earth seem inadequate. Instead, modern astronomers use three main units of distance.

An Astronomical Unit (AU) is roughly the distance from the Earth to the Sun. It's about 150 million kilometers, and it's particularly useful for measuring distances between objects in our solar system.

A Light-year is the distance traveled by light in one year. One light year is around 9.46 trillion kilometers. It's much larger than an AU, and better for measuring interstellar distances.

Finally, a Parsec is about 3.26 Light-years. Larger distances within the universe are measured in megaparsecs, which is 1 million parsecs.

Most astronomers believe that the universe started as a single point of infinite heat and density. This point is called the singularity.

Then there was a Big Bang. We still don’t know what triggered this expansion – but for whatever reason, the point expanded, stretching space as it went. We think this happened roughly 13.7 billion years ago. As far as we know, that's the age of the universe.

The initial expansion was incredibly fast, but only lasted for a few fractions of a second. We call this the inflationary epoch. During the inflationary epoch, space expanded at speeds much faster than the speed of light.

After the inflationary epoch, the expansion of the universe slowed down a little. It also started to cool.

And that's when everything started to come together. Within a single second, gravity, electromagnetism and nuclear forces all emerged from a unified super force.

Three minutes later, it was cool enough for subatomic particles to stick together. For the next seventeen minutes, the universe made atoms – mostly hydrogen and helium atoms, and a little lithium as well.

380,000 years later, electrons and protons started to combine and form stable neutral atoms. Because free electrons absorb light, the universe had been opaque until now, but suddenly it became transparent.

As the growing universe continued to cool, the force of gravity started to overcome heat.

Clumps of matter collected together and grew into the very first stars. These blinked into existence around 400 million years after the Big Bang.

Still, the universe cooled and expanded, becoming thinner and much less dense. Eventually, the universe was cold and vast and sprinkled with galaxies separated by tracts of empty space – much like we know it today.

The universe is still expanding. And about 4 billion years ago, this expansion actually started to speed back up again. Why? Astronomers don't know for sure, but they think it's because of dark energy.

We don't know what dark energy is. We just think it's some kind of mysterious force that's pushing the universe outward. Whatever it is, it seems to make up almost 70% of the universe.

We also don't know how the universe will end. Will it continue expanding forever? Will it find a point of equilibrium? Or will it collapse on itself, and condense back down to a point?

Whatever happens, it probably won't end for tens of billions of years. That gives us plenty of time to study it, and solve more mysteries on the way.