how fast is the universe expanding

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I know that the universe is 13 billion years old and the fastest anything could be is the speed of light so if the universe is expanding as fast as it could be wouldn’t the universe be 13 billion light years big? But I’ve searched and it’s 93 billion light years big, so is the universe expanding faster than the speed of light?

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The fastest anything can move THROUGH space is the speed of light. There is no such limitation on the expansion of space itself. In fact, it is thought that during an incredibly brief inflationary period about 13.8 billion years ago, the entire universe expanded at speeds far in excess of the speed of light as new space was in effect created between every bit of existing space. The same is happening today in a sense for objects very distant from other objects – they are moving away from each other at faster-than-light speeds as new space is constantly created between them. And the more space there is between them, the more space is being created, and the faster they are moving away from each other. It’s important to understand that locally (i.e., in the region where each of these objects is located), the objects are moving through that local region of space at speeds well below light speed.

The expansion of space isn’t constant. Right now, expansion is speeding up. Notably, the Big Bang was a period of extremely rapid expansion, as the name would suggest. So we can’t really correlate age and size of the universe.

~~No, expansion is not faster than the speed of light right now. It *can* be though.~~ Expansion isn’t movement, so it can ignore the regular speed limit of the universe. Things are getting farther apart not because they’re all moving away from each other. They’re getting farther apart because the literal empty space between them is getting bigger.

> I know that the universe is 13 billion years old and the fastest anything could be is the speed of light so if the universe is expanding as fast as it could be wouldn’t the universe be 13 billion light years big?

This deduction is based on a common misunderstanding of the Big Bang. The universe did not start from a single, finite point and expand some kind of border or edge outward in all directions. Instead the universe is likely infinite in extent now and from the start, it simply became more spread out and less dense over time.

The second aspect here is that the expansion of the universe is not limited by the speed of light because the light speed limit applies to things moving through space, not the appearance of more space between objects or locations.

It depends.

The speed of light is the universal speed limit for things moving *in space*, but there is no such speed limit for space itself. In other words, the speed limit sign of the universe applies to what drives along the road but not the expansion of the road itself.

The rate of expansion of the universe depends on how far away it is we’re observing. The further away something is the faster the rate of expansion between us. There is a point known as the “cosmological horizon” where the rate of expansion exceeds the speed of light, meaning that any information (I.e light) emitted from that distant star can never reach us again. Light still travels at the speed of light, but the space between us is growing faster then the light can travel so it will never reach our eyes.

We do not know how big the universe is. We can only say anything about the **observable** universe. The 93 billion light years is the current size of the observable universe and is based on a few different factors. Firstly the 13 billion light years is from us to one edge, so this is the radius of the observable universe. You have to double this to get the diameter of the observable universe. In addition the objects we can observe now might have been moving at the speed of light away from us. So they are not 13 billion light years away but rather 26 billion light years away. So the size of the observable universe is then 52 billion light years. The last factor is a bit more complex but basically since space is expanding the amount of space that light went through in a light year have now expanded. So the space which used to be 13 billion light years as the light passed through it is now closer to 23 billion light years.