“It means that as time goes on, the light emitted by distant galaxies gets shifted quite heavily towards the red part of the spectrum, resulting in a cosmological redshift. It means that there are some portions of the Universe that are so distant that light emitted from them will never be able to reach us. Currently, that point is anything beyond about 46.1 billion light years from us, given our Universe, to the best we can measure it, that’s been around 13.8 billion years since the Big Bang.”
One of the toughest things to wrap your mind around in the natural world is the idea of special relativity: the faster you move, the closer you get to the speed of light, the more difficult it becomes to increase your speed at all. While you might approach the speed of light arbitrarily and asymptotically, you’ll never reach it. And yet, we have the Universe, expanding all the time, where the expansion rate itself is even speeding up. You might wonder, then, if these distant galaxies — the farther and farther away you look — might ever be seen moving away from us faster than the speed of light? Surprisingly and mind-bendingly, the answer is yes.