The Universe is getting smaller. Not the observable universe, which is currently a sphere about 93 billion light years across and increasing all the time, but the much smaller portion that we could ever hope to reach. Since the Universe is expanding, our cosmic playground is shrinking all the time.
If the Universe weren’t expanding, then the size of the observable universe would simply depend on its age. As the years go by, ever more distant light would be able to reach us. Likewise, we would be able to travel anywhere in the Universe given enough time. Even at speeds approaching that of light it might take billions of years, but the only limiting factor is time. But the Universe is expanding. It’s not that galaxies are racing away from some point in space, but rather that space itself is expanding, and that makes a big difference.
Since space itself is expanding, the more distant an object, the faster it seems to be moving away from us. We measure cosmic expansion in terms of the Hubble parameter, which is about 20 km/s per million light years. This means that two points in space a million light years apart are moving away from each other at 20 kilometers each second. Two points 10 million light years apart are moving away at 200 km/s, and so on. Because of this, if you consider two points far enough apart, they will be moving away from each other faster than the speed of light. The speed of light is about 300,000 km/s, which, given our current Hubble constant is the separation speed for two points 15 billion light years apart. This is known as the Hubble radius. Anything outside that radius is impossible for us to reach, even if we could travel toward it at the speed of light.
Some of you might protest, since you’ve been told numerous times that nothing can travel faster than light. The catch is that a galaxy 16 billion light years away isn’t actually traveling faster than light. What’s happening is that the expansion of space between us and the distant galaxy is increasing the distance between us faster than the speed of light. That subtle difference is also why we can see things that are farther away than 15 billion light years.
Because of cosmic expansion, the whole idea of galactic distance depends on your definition. As light leaves a galaxy to travel in our direction, space is expanding all along its journey. This not only causes the light to redden (known as the cosmological redshift) it makes the journey longer. All the while, the galaxy is moving even farther away. For light from the most distant galaxies, the light we observe has traveled for more than 13 billion years. When the light began its journey, its galaxy was only 3.4 billion light years away. Now the galaxy is 29 billion light years away. We can see such distant galaxy even though we’ll never reach them.
Since the observable universe is about 42 billion light years in radius, and the Hubble radius is about 15 billion light years, that means about 97% of the observable universe is beyond our reach. Furthermore, since space continues to expand, galaxies that are currently within reach will eventually move beyond the Hubble radius. Our galaxy is part of a cluster of galaxies known as the local group. It is about 10 million light years across and contains about 50 galaxies. Together they are close enough that their gravity will cause them to collapse toward each other despite cosmic expansion. But more distant clusters are so far away that cosmic expansion will win in the end. In perhaps a hundred billion years our local group will have collapsed into a single large galaxy, and the rest of the Universe will have moved forever out of reach.