How big is the universe?

October 5, 2015 by Dr Luke Davies, Sciencenetwork Wa, Science Network WA
The universe is so big that even light hasn’t had time to cross it in nearly 14 billion years Credit: inefekt69

Our brains struggle to comprehend how big the universe is because everything here on Earth, and even the Earth itself, is very small when compared to the immense scale of the universe.

So let's think about it a different way, using something we see and interact with every day… .

While we imagine light to be instantaneous, photons of light actually take time to travel from one side of the room to the other.

In the time it took you to read this far, a photon of light leaving the Sun has travelled about 10 million kilometres – equivalent to travelling around the Earth 250 times.

Light that leaves our second nearest star, Proxima Centauri, takes just over four years to reach Earth and so we can define it as four light years away.

As such, if you were to look at Proxima Centauri, you would not be seeing the star as it is right now, but how it 'was' 4 years ago!

We see all things in the universe as they were in the past, whether they're on the other side of the room or the other side of the galaxy.

To take this concept further, the nearest large galaxy to us is Andromeda which is so big and close that you can see it in the night sky with your .

What you're really seeing is 1,000's of billions of stars in a configuration similar to our Milky Way. However, all of those stars are about 2.5 million light years away, which means you're seeing Andromeda as it was 2.5 million years ago.

The whole universe is littered with galaxies just like the Milky Way and Andromeda, and using our most powerful telescopes we can see light from galaxies that has taken more than 13 billion years to reach us!

Since a photon of light left one of these galaxies, life sparked into existence and evolved. Dinosaurs ruled the Earth. Humans appeared, developed tools, art, science and technology, built the Hubble Space Telescope, put it into orbit and finally stopped that poor photon on its 13 billion year journey!

The universe is about 13.8 billion years old, so any light we see has to have been travelling for 13.8 billion years or less – we call this the 'observable universe'.

However, the distance to the edge of the observable universe is about 46 billion light years because the universe is expanding all of the time.

Imagine that a photon of light is emitted from a point on the edge of our .

While that has been travelling through space, the universe has expanded. We have moved away from the point where the light was emitted, and it has moved away from us!

Though the light might have only travelled for 13.8 billion years, the distance from us to the point it came from is, at present, 46 billion light years!

So how big is our ? Well we don't really know, but it's big. So big that even light hasn't had time to cross it in nearly 14 billion years! And it's still getting bigger all of the .

Explore further: How far back are we looking in time?

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William Bayer
1 / 5 (2) Oct 05, 2015
Wait a minute! You're telling me that during the entire 5 years I studied science in Mechanical Engineering School (and all the years of studying science leading up to those university studies) NOBODY ever told me that the universe is much larger than Earth?
Thanks for the update.
3 / 5 (2) Oct 05, 2015
The visible universe is 13.8(or so) billion light-years in size while the unseen side is upwards to 96 billion light years.....
4.2 / 5 (10) Oct 05, 2015
The universe is getting bigger all the time in our observations too. The latest Planck data release puts limits on curvature that means the universe is at least 10 000 000 times larger in volume than the observable universe.

@gculpex: No, it is as the article describes, the seen (observable) universe is 96 billion years in diameter.

Fun fact (IIRC, or I have messed up): Some of the photons that reach us has once "moved away" since the expansion speed was parabolic at the time. I.e. the early expansion after reionization was faster but slowed down. Hence photons that reach us today couldn't keep up with the expansion between us and them. Later they stopped losing ground and could eventually reach us. (Today dark energy is increasing expansion rate again, but this time the rate won't turn smaller again.)
1 / 5 (5) Oct 05, 2015
The visible universe is 13.8(or so) billion light-years in size while the unseen side is upwards to 96 billion light years.....

Mind candy for the merger maniacs still lost in romance with the Huge Bang Fantasy.
1 / 5 (3) Oct 05, 2015
How big is the universe?

May depend on precisely how you define "universe".

In a certain sense, "Big" and "Size" may not even be meaningful terms.
not rated yet Oct 05, 2015
How do we know that any of what we observe still exists. Is it possible that many of the stars burned out billions of years ago and their planets obliterated. Could we be alone in our corner of the universe watching old movies of an extinct universe waiting for the super nova finale and the sky to go dark?

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