How could the universe expand faster than the speed of light? That seems impossible!
- asks Paul
In science fiction universes, traveling the galaxy is a snap – just engage the “warp” or “hyperspeed” drive, and off you go, cruising the cosmos at several times the speed of light. But back in reality, we’ve all been taught that the speed of light is a strict traffic law that can’t be broken. This is true, but slightly misleading.
Einstein’s theory of special relativity, first published in 1905, asserted that the speed of light is a constant (300 million meters per second), no matter who measures it. It’s always the same whether you are in motion or at rest. This line of thinking is a lot different than we’re used to experiencing. For example, if you try to measure the speed of an oncoming car from a moving vehicle, you end up getting the combined speed of both cars. This is why cops have to stay parked. Light is different, because no matter what you’re doing it always goes the same speed.
The speed of light affects us more than we realize – it helps us understand the difference between cause and effect. If things could move faster than the light we see them by, we’d be in for weird experiences. If you were a catcher trying to catch a superluminal fastball, you might feel the ball hit your glove even before the pitcher starts his wind-up: The effect before the cause. That’s because the image of object would be traveling at the speed of light, trailing the faster baseball like the slower sound of thunder trails after the image of lightning.
Now that we have a taste for Einstein’s theory, we know that baseballs don’t go faster than the speed of light. But is there anything that can? It turns out that the speed of light is only a limit on objects – like baseballs – as they move through space. The movement of space itself, however, can make the speed of light seem slow.
Right after the Big Bang, the universe had a monstrous growth-spurt called inflation. The whole thing was over in less than a trillionth of a trillionth of a second, but the universe grew exponentially in that brief blip, repeatedly doubling in size. At the end of inflation, although the universe was still smaller than a car, the outer edge had traveled many times faster than the speed of light. Since then, the universe has continued its expansion, but at a more reasonable, steady pace.
This ultra-fast growth seems to contradict what we’ve just discussed, but it makes sense if you understand the distinction between expansion and motion. When astronomers say that the universe is expanding, they’re talking about the rather abstract concept of space-time. Basically, space-time is the three physical dimensions of our existence-length, breadth and depth-combined with the additional dimension of time; think of it as a wire grid that connects every part of the universe to every other part. When we say an object has motion, we’re referring to its change in position relative to the space-time grid. The speed of light is only a constraint for objects that exist within space-time, not for space-time itself.
To better visualize the theory, astronomers often illustrate the expanding universe as a loaf of raisin bread rising in the oven. The raisins are galaxies and the rising dough represents space-time. As the dough expands, the raisin galaxies find themselves farther apart from each other, even though they are not moving relative to the dough between them.
Now let’s imagine that there’s a beetle in the loaf and it starts crawling toward a faraway raisin (don’t worry- we’re not going to eat it anyway). The beetle represents anything within space, such as baseballs, spaceships or photons. When the beetle burrows through the bread, he is moving relative to the dough, and all the other raisins. The speed of light limits how fast the beetle can travel, but not how quickly the bread can rise. Just because the expansion of space can break the speed limit, it doesn’t mean that we can go faster than Einstein said we could.
So, while the speed of light remains an unbreakable barrier for those of us within the universe, it can’t limit the expansion of space-time itself. The universe keeps right on expanding, but the speed of light limits how much of it we can see, and how fast we can move. It may not be fair, but that’s physics.