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How big the universe really is...

The Speed Of Light

The highest known velocity is 300 million meters per second, which is achieved by light through an empty vacuum such as space (there's nothing to interact with the light and slow it down).

Light is so fast in fact, that 1 photon of light could circumnavigate the entire Earth at the equator 7 and half times in 1 second.

Letting that sink in...

But how is any of that relevant to how big the universe is?

Well, the universe is so large that measuring it in kilometres or miles would result in constantly using numbers of the order of trillions of trillions of trillions, so for the sake of ease, astronomers use "light years, ly" to measure vast distances across the universe.

1 light year is simply the distance light travels through an empty vacuum in 1 year.

As you can imagine, travelling at 300 million meters every second, light covers some milage in an entire year to say the least..

In fact, 1 light year roughly equates to 6 trillion miles, or 9 trillion kilometres.

^ worth keeping that in mind, although I'll convert between them throughout this article.

With that out of the way with then, let's talk about how vast the universe really is....

The "Hubble deep field image" that perfectly illustrates the vastness of the universe. The points of light that you can see here aren't stars, but entire galaxies full of hundreds of billions of them... Credit: NASA

Hubble deep field image

Our neighbours

So how far away is the sun?

On average, the sun is roughly 92 million miles away (148 million kilometres), and it only takes light just over 8 minutes to travel that distance.

I say "on average" here, because the Earth's orbit around the sun isn't completely circular, therefore the Aphelion (furthest point) and Perihelion (closest point) aren't exactly the same.

Taking this a bit further then, we can say that:

Jupiter is about 540 miles away (880 million kilometres)

Saturn over a 1.6 billion kilometres

Neptune about 4 billion kilometres.

And voyager 1?

20 billion kilometres!

But this is just our solar system, and our solar system is one of hundreds of billions in our galaxy alone, and our galaxy is one of trillions...

Getting further

The closest star system to us is the Alpha Centauri system, which comprises of a total of 3 stars and it's about 4.36 ly away - that's about 38 trillion kilometres....

One of the brightest stars in the night sky, located in the Orion constellation, is Betelgeuse.

This star is about 10 times the mass of our own star, and while it's mass is still extremely impressive, it's volume is what makes Betelgeuse an absolute force of mass destruction...

If it were to replace the sun in our solar system, its surface would extend beyond the orbit of Jupiter!

This star is about 642 ly away, which is about 578 thousand trillion kilometers (578,000,000,000,000,000 km)!

By the way, we're still in our own galaxy, and we're talking about a star that you could consider as our neighbour....

The Horsehead nebula is a famous cloud of gas that will eventually form the next generation of stars in our galaxy - it's about 1500 ly away (13,500,000,000,000,000 km)

At the centre of our galaxy, is a supermassive black hole known as Sagittarius A*, which is 4 million times as massive as the sun.

And how far away is it?

25,600 ly, or 230,400,000,000,000,000 km! (that's 230.4 thousand trillion km)

Our home galaxy, the Milky Way, stretches over 120,000 ly across in diameter, which is 1,080,000,000,000,000,000 km (1.08 million trillion), and is suspected to contain a few hundred billion stars.

If you didn't think galaxies could get any bigger than that, the closest galaxy to us (besides our own) is Andromeda, which is thought to contain over 1 trillion stars.

This galaxy is about 2.5 million ly away, and is gradually getting closer as it's set on a collision course with our own galaxy in about 13 billion years (so astronomers think at least).

I think you get the idea, so let's skip straight to the part that we're all here for - the furthest known galaxy.

Gn-z11 is currently the furtherest discovered galaxy, and it lies around 32 billion light years away from us.

Interestingly, when we're talking this far away, things can actually get quite confusing as it takes light so long to reach us from this galaxy that by the time it has reached us, the galaxy has moved considerably further away.

Therefore, the galaxy is often misreported as being 13 billion light years away, when in reality, that's the distance of the light that we're seeing today.

Consider the following thought experiment to help visualise what's going on here:

All the stars, nebulae etc in that galaxy emit photons of light.

Those photons of light leave the galaxy, travelling at 300 million m/s.

13 billion years later, we see those photons of light and interpret the galaxy as therefore being 13 billion ly away, but the galaxy hasn't just stood still in those 13 billion years.

In reality, the galaxy has been constantly moving during the 13 billion years that it took those photons of light to reach us.

The significance of this is that we're essentially seeing the galaxy how it was 13 billion years ago, as opposed to how it is today.

This means that we're essentially looking into the past - we're seeing that galaxy how it was billions of years ago.

This is what allows us to make predictions about our universe's formation, and why constantly upgrading our equipment to be able to observe galaxies from further away is so important to professional astronomers - the further away we can look, the further back in time we can observe.

We can essentially time-travel!

So that's the furthest galaxy - is that pretty much the end of the furthest point of the universe?

Absolutely not.

The observable universe

What we call the observable universe is an imaginary sphere of space where everything inside of that sphere we would theoretically be able to observe, due to the limitation of the speed of light.

Given the age of the universe, which is suggested to be around 13.7 billion years, there are regions of the universe where the light traveling from has simply not had enough time to reach us yet.

Remember earlier how I said that the light from the furthest known galaxy has taken 13 billion years to reach us?

Well, specifically, it's 13.4 billion, which when compared to the age of the universe of 13.7 billion, would indicate that this galaxy formed only 400 million years after the birth (or rebirth...) of the universe.

The observable universe from Earth is estimated to be 93 billion light years across in diameter.

That's 837 billion trillion km (837,000,000,000,000,000,000,000 km)

Note that I said from Earth, as every location in the universe would have it's own "observable universe".

As technology advances, we should be able to see galaxies at distances closer and closer to this number, but only time will allow us to see further than that.

So that's how big the observable universe is, but what about the whole universe?

Absolutely no idea...

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