Our planet, Earth, is one of eight planets orbiting the sun. The sun is merely one of many hundreds of billions of stars in our galaxy known as the Milky Way. And the Milky Way is merely one of many trillions of galaxies in the known universe. The universe is so large that it is difficult, if not impossible, to determine its size. But let’s try and do exactly that. Let’s dig a little deeper and see if we can figure out just how big it really is.
Let’s start with what we know. The current best estimate for the age of the universe is approximately 13.8 billion years. This is the amount of time that has elapsed since the big bang. It is a common misconception that the corresponding radius of the universe must, therefore, be 13.8 billion light years, since nothing can travel faster than the speed of light. But it has been observed that the universe has been expanding rapidly since the big bang and that the rate of expansion is actually increasing, which means that some parts of the universe are actually moving farther away from us. This expansion of space has caused the universe to become considerably larger than would otherwise be predicted. It is perhaps more correct to say that nothing can travel through space faster than the speed of light, but there is no restriction on how fast space itself can expand.
If the universe is larger than 13.8 billion light years, then there must be some parts of the universe we cannot see, because light has not yet reached us from those places. In fact, the universe is expanding at such a high speed that there are parts of the universe that we will never see, because light will never be fast enough to traverse this ever increasing distance. This is what makes determining the size of the universe such a difficult task. How can we estimate the size of something when we don’t even know its boundaries?
The easiest way to figure out the answer to this question is to work with what we can see. The observable universe is defined as the spatial region that we can observe and interact with from Earth. It can be seen in theory because light has had time to reach Earth from these places. Whether these places can actually be seen with modern technology is not relevant to our question. The diameter of the observable universe has been estimated to be around 93 billion light years, based on complex calculations of how rapidly the universe has expanded since the big bang.
But what if we include all the parts that we can’t see? The unobservable universe (“the Universe”) encompasses all of space and time. As it turns out, estimating the size of the Universe proves to be a nearly impossible task. The Universe could be large enough to make the observable universe look like space dust in comparison. Or it could actually be smaller than the observable universe. This is possible if what we perceive as distant galaxies are actually mirror images of nearby galaxies at a different time in its history, caused by light that has circled around the entire cosmos. However, this unlikely to be true given that the Universe has been estimated to have near-zero curvature.
So where does that leave us? Given that the Universe is likely substantially larger than the observable universe, this leaves us with a lower bound estimate of 93 billion light years and an upper bound estimate of infinity. If the Universe is finite, then it is by definition measurable and perhaps one day technology will advance far enough to give us the true answer. But if the Universe is infinite, then it was always infinite and will always be infinite, so perhaps we should do nothing more than to gaze at the night sky and admire the beauty of it all. At least the parts we can observe anyway.
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