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Event horizons
Question
As the universe continues to expand, fewer and fewer galaxies will be ____________ to us.
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How fast can distant galaxies recede from us? Faster than the speed of light, it turns out! How can this be possible? The answer is that there’s a big difference between an object moving through an unexpanding space, and having that space expand. If you’re travelling through an unexpanding space, then you’re subject to the cosmic speed limit of the speed of light. But a distant galaxy isn’t really receding by moving through space – it’s that the space in between us and them is growing.

There isn’t a simple limit on how fast that extra space can be created. This means that the distance between us and them can in principle be increasing at faster than the speed of light, which means that light signals from that distant galaxy will never make it to us.

The most distant thing we can see is the Cosmic Microwave Background, which is the light from the time when the Universe was last opaque. This was very early in the history of the Universe, and it’s taken that long for the light to get to us. Now imagine a blob of gas from then, sending its light to us which we’re only just receiving now. That blob of gas (or whatever it turned into) is now much further away then when the light was sent to us, because the Universe has expanded a lot in the meantime. The space is still expanding, and astronomers calculate that the distance to that blob today is now getting bigger at much faster than the speed of light. That means we will NEVER be able to see what that blob turned into, even if we are prepared to wait billions of years.

So if the blob sent a light signal to us today, it would never get to us. Another way of thinking of this is that the blob has now disappeared behind a cosmic event horizon, just as was described in the animation we link to above. The only thing we will ever know is what the blob looked like in the early Universe.
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