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Monday, November 21, 2016

How Can the Universe Expand Faster Than the Speed of Light?

It seems like it should be illegal, doesn’t it? Over and over (and over and over) we're told the supreme iron law of the universe: Nothing — absolutely nothing — can go faster than the speed of light. Done. Nothing further needs to be said about the issue.

And then come the astronomers, always excited by the chance to mess up your comfort zone. They come barging in with a simple observation: Some galaxies are moving away from us…wait for it…faster than the speed of light. What gives?

The big picture

First off, it's important to note that we live in an expanding universe. Every day the galaxies get farther apart from each other — on average. There are slight motions on top of that general expansion, leading to instances such as the Andromeda Galaxy heading on a collision course for the Milky Way. But in general, in the biggest of pictures, the galaxies are getting farther away from each other.

A key feature of this expansion is how uniform it is. Imagine a bunch of folks standing around the edges of a stretchy piece of fabric, tugging at it. Let us assume they're choreographed well and are able to walk backward and pull at the same rate. You, standing in the middle, would correctly observe that your "universe" is expanding: any objects placed on that fabric would slowly move away from you.

Because stretchy stuff is stretchy, the objects on the fabric close to you would appear to move away with some speed, but the farther objects would appear to move faster. Even though the folks doing the pulling are moving at a constant speed, the apparent stretch changes with distance. I swear this is true; you can even try it for yourself at home!

Now, let's jump to the universe. It's as if a bunch of folks are at the edge of the cosmos, gently tugging at the fabric of space-time, stretching it. Edwin Hubble was the first to measure the expansion rate. The number he got was way wrong, so I won't bother mentioning it, but good on him for trying. The more modern value is 68 kilometers per second per megaparsec, plus or minus a couple, but close enough.

I know, I know. You were probably following along just fine until that odd "per megaparsec" popped up. It's a distance: One megaparsec is 1 million parsec, which is 3.26 million light-years.

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