![]() So if you’re in a universe whose expansion is slowing down, you’ll be able to see more and more distant objects as time goes on, as the light from distant objects catches up with the expansion. But if you don’t get dragged back too far, and if the treadmill slows down enough, you can eventually make up the lost ground and start to move forward before falling off the back end. Even running at your top speed, you’re going to be dropping back. Imagine you’re standing in the middle of a very long treadmill that’s going faster than you can run. It entered our Hubble radius from the outside. So a light beam that started out being carried by the expansion of space away from us (even though it was emitted in our direction) eventually was able to “catch up” as the expansion slowed and it reached a part of the universe that was close enough for the recession speed to be less than the speed of light. The trick is that the light we’re picking up left the source long ago, when the universe was smaller and the expansion was actually slowing. So how do we see so many things that are so far away that they’re receding from us at more than the speed of light, and, in fact, always have been? If something is moving away at more than the speed of light, a light beam emitted from it is getting farther away from us, not closer. The most distant galaxies we’ve seen have been at redshift values of about 11, and the cosmic microwave background is at a redshift of around 1,100. We’ve seen individual supernovae out to redshifts of almost 4. But even that utterly unimaginable distance is, in cosmological terms, just around the corner. An object at the Hubble radius would have a redshift of about 1.5, meaning the light wave, and the universe itself, has stretched out to two and a half times its original length since the light was emitted. ![]() I mentioned in Chapter 3 that we can label the distance to objects by their redshift factors-the amount that their light is shifted toward the red (low frequency/long wavelength) part of the spectrum due to the expansion of the universe. We call it the Hubble radius, and it’s around 14 billion light-years from here. The distance at which galaxies are currently moving away from us faster than light is surprisingly close, given how far we can actually see. While nothing can travel faster than light through space, there’s no rule that limits how quickly things can happen to find themselves farther apart because they are sitting still in a space that’s getting bigger between them. “Nothing can travel faster than light!” This is a fair point, but it doesn’t actually lead to a contradiction. It turns out that in a uniformly expanding universe, where the more distant things are receding more quickly, it is inevitable that there is a distance beyond which the apparent recession speed is faster than the speed of light, so light can’t catch up. If not, we wouldn’t be able to see them at all, since the light coming from them now can’t ever reach us. The light we see from those galaxies started traveling through the universe long before they got to such incredible distances, though. But a bit closer to us, we can also see ancient galaxies that are now more than 30 billion light-years away. The closest we can get to seeing that “edge” is the cosmic microwave background, whose light comes from almost as far as the particle horizon. So we can define the observable universe to be a sphere of about 45 billion light-years in radius, centered on us. In actual fact, since the universe has been expanding all that time, something just close enough to send its light to us 13.8 billion years ago is now much farther away-approximately 45 billion light-years. Knowing that the universe is about 13.8 billion years old, logic would tell you that the particle horizon must be a sphere of radius 13.8 billion light-years. This defines the particle horizon, and it’s the farthest out we can observe anything at all, even in principle. A distance at which, if a light beam started there at the first moment, it would take the entire age of the universe to reach us. Since light takes time to travel, and more distant objects are, from our perspective, farther in the past, there has to be a distance corresponding to the beginning of time itself. We define this as being the farthest we could possibly see, given the limitations of the speed of light and the age of the universe. The “observable” part refers to the region within our particle horizon. The present-day observable universe is probably bigger than you think. Reprinted by permission of Scribner, an imprint of Simon and Schuster, Inc. Excerpted from The End of Everything by Katie Mack.
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