Warp Speed! How Some Galaxies Can Move Away From Us Faster Than Light

upstart writes:

Warp Speed! How Some Galaxies Can Move Away from Us Faster Than Light:

If there is an absolute law in the universe, it's that nothing can travel faster than the speed of light.

For science-fiction enthusiasts, that's a bit depressing. Space is big, and while the speed of light is incredibly fast to us humans, on interstellar scales it's glacially slow. Even at a photon's speed of about 300,000 kilometers per second, it's a journey of more than four years to reach just the closest star to the sun.

But we have to be careful how we state this universal law. To be more specific, nothing can move faster than the speed of light through space. That may seem like a nitpick, but it turns out to have literally cosmic importance.

To see why, we have to look at the behavior of the universe itself. One of the most important things the universe does is expand. It's getting bigger every day. The foundational observations for this, which were made more than a century ago, showed that distant galaxies appeared to be receding from us-not only that, but ones farther away were moving faster.

That's what an explosion does: at some time after the bang, the fastest-moving material will be farthest away. This is where the idea of the big bang model for the origin of the universe comes from.

The cosmic expansion can be measured as a rate, meaning at a given distance from us, a galaxy will be moving away from us at given speed. At a different distance, a galaxy will be moving at a different speed. We measure this as a rate of expansion called the Hubble constant. Our best measurements of this give a value of about 70 kilometers per second per megaparsec. In other words, a galaxy one megaparsec from us (about 3.26 million light-years) will be receding at 70 km/sec. A galaxy two megaparsecs away will be moving twice as fast, or at 140 km/sec, and so on.

Extrapolating this, though, runs us into trouble: it's not hard to see that at some distance from us, the recession speed will equal the speed of light. If you run the numbers, just using the speed of light divided by the Hubble constant, you find that distance is about 14 billion light-years. That distance is called the Hubble sphere, and anything farther away than that would be-from our perspective-moving faster than light.

Here's where things get weird. ("Oh yes, here is where that happens," I expect you're thinking.) The universe is actually quite a bit bigger than that. We know the cosmos was born about 13.8 billion years ago. By some few hundreds of millions of years later, galaxies had formed. When we see the light from a distant galaxy, it's taken, say, 12 billion years to reach us, but over that time, the universe has expanded. So the light has actually had to travel much farther than 12 billion light-years to get to us. By the time the light reaches us, that galaxy is more like 23 billion light-years away.

That means there are galaxies outside our Hubble sphere, and they are moving away from us faster than the speed of light! How is that possible?

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