How Big Is The Observable Universe?

The universe we inhabit is an awe-inspiring place. Pondering on our place in the thing we call everything there is soon confronts us with questions that require some mental gymnastics to come up with something resembling an answer. And as things stand now, as far as we're able to know right now, most of the universe will forever be out of our reach; there probably exists an infinity of space-time behind the cosmic horizon...

_{Image by CharlVera - source: Pixabay}

There's an important distinction to be made between the entire universe and the observable universe. Scientists have deduced, using several methods, that the universe as we know it began around 13.7 billion years ago. Einstein's theories of relativity have introduced us to the term space-time; space and time make up the fabric of the universe and one can be "traded" for the other, with the speed of light being constant. Because light's speed is constant, we know how long it takes to reach the eyes and instruments of us Earth-dwellers. The distance between the Earth and our Sun is roughly 149.6 kilometers, also called an Astronomical Unit and the Sun's light takes 8 minutes to reach us; we see the Sun as it was 8 minutes ago, and if the Sun would suddenly seize to exist, we wouldn't know until 8 minutes later...

So, since the universe's inception all light in it has had 13.7 billion years to travel; we can still see some of that light and we call it the Cosmic Microwave Background. This is literally the oldest thing in known existence that we're able to observe. Intuitively this would lead us to believe that the observable universe is shaped like a sphere, with us at the center and a radius of 13.7 billion light-years. This would not be correct however, because this doesn't take into account the fact that our universe has been growing since it's birth. Because of the universe's rapid expansion, the objects who's light has been traveling for 13.7 billion years, are now 46.5 billion light-years away from us. The observable universe currently is a sphere with a diameter of 93 billion light-years, and still growing.

There are several other horizons in our dynamic observable universe, with nice sounding names like the Hubble Horizon, Particle Horizon, Cosmic Event Horizon and Future Horizon. The Particle Horizon is in fact the moving "edge" of that 93 billion light-years sphere that is our current observable universe. Another way of describing this is everything we've been able to observe thus far. You can read about some of them here, but for an easy and illustrated explanation about what we can observe, please watch the video:

_{how large is observable universe?}

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