“What Is the Universe Expanding Into?” A reader recently asked that question. So here's the inside scoop.
But first, what exactly is expanding?
Not our solar system with its planets orbiting the sun
Not our Milky Way galaxy of 400 billion stars. It never gets any bigger.
Not the galaxies right around us. Indeed, the nearest spiral, Andromeda, is approaching us at 70 miles per second and will someday harmlessly collide with us!
In fact, nothing the naked eye can see, even on the clearest night in the country, is expanding. In our own galaxy, stars do have small random motions, but just as many approach us as are flying away.
So what's expanding?
It's the empty space between galaxy clusters. That's the only thing that grows larger. That's the whole story.
Therefore, no matter where in the cosmos you lived, you'd see other galaxy clusters increasing their distance from your own. This is the simple truth and we can all picture it.
The problem arises when we try to visualize the entire universe as if it were an inflating balloon that we're observing as if we are outside it. There is no “outside” to the universe. So that perspective is non-existent. Our headline question may seem meaningful, but it isn't, and therefore no answer can make sense.
You can be outside a galaxy such as this one, but you can never be outside the universe to look back at it.
Do you still find this limitation frustrating? You're not alone. That's because the cosmos taken as-a-whole does not play by the same logical rules we've devised to help us understand its parts.