steve_nadis's picture
Contributing Editor to Astronomy Magazine and a freelance writer

What would change everything? Well, if you think of the universe as everything, then something that changes the universe—or at least changes our whole conception of it—would change everything. So I think I’ll go with the universe (which is generally a safe pick when you want to cover all bases). I just have to figure out the thing that’s changing. And the biggest, most dramatic thing I can think of would be discovering another universe in our universe.

Now what exactly does that mean? To some extent, it comes down to definitions. If you define the universe as “all there is,” then the idea of discovering another universe doesn’t really make sense. But there are other ways of picturing this. And the way many cosmologists view it is that our universe is, in fact, an expanding bubble--an honest-to-god bubble with a wall and everything. Not so different from a soap bubble really, except for its size and longevity. For this bubble has kept it together for billions of years. And as viewed from the inside, it appears infinitely large. Even so, there’s still room for other bubbles out there--an infinite number of them--and they could appear infinitely large too.

I guess the picture I’m painting here has lots of bubbles. And it’s not necessarily wrong to think of them as different universes, because they could be made of entirely different stuff that obeys different physical laws and sits at a different general energy level (or vacuum state) than our bubble. The fact is, we can never see all of our own bubble, or even see its edge, let alone see another bubble that might be floating outside. We can only see as far as light will take us, and right now that’s about 13.7 billion light-years, which means we only get to observe a small portion of our bubble and nothing more. That’s why it’s fair to consider a bubble outside ours as a universe unto itself. It could be out there, just as real as ours, and we’ll never have any prospect of knowing about it. Unless, perchance, it makes a dramatic entrance into our world by summarily crashing into us.

This sounds like the stuff of fantasy, and it may well be, but I’m not just making it up. Because one of our leading theories in cosmology called inflation predicts—at least in some versions—that our bubble universe will eventually experience an infinite number of collisions with other bubble universes. The first question one might ask is could we withstand such a crash and live to tell about it? The small number of physicists and cosmologists who’ve explored this issue have concluded that in many cases we would survive, protected to some extent by the vastness of our bubble and its prodigious wall.

The next question to consider is whether we could ever see traces of such a collision? There’s no definitive answer to that yet, and until we detect the imprint of another bubble we won’t know for sure. But theorists have some pretty specific ideas of what we might see—namely, disk-shaped features lurking somewhere amidst the fading glow of Big Bang radiation known as the cosmic microwave background. And if we were to look at such a disk in gravitational waves, rather than in electromagnetic waves (which we should be able to do in the near future), we might even see it glow.

The probability of seeing a disk of this nature is hard to assess because it appears to be the product of three numbers whose values we can only guess at. One of those numbers has to do with the rate at which other bubbles are forming. The other two numbers have to do with the rate at which space is expanding both inside and outside our bubble. Since we don’t know how to get all these numbers by direct measurements, there doesn’t seem to be much hope of refining that calculation in the near-term. So our best bet, for now, may be trying to obtain a clearer sense of the possible observational signatures and then going out and looking. The good news is that we won’t need any new observatories in the sky. We can just sift through the available cosmic microwave data, which gets better every year, and see what turns up.

If we find another universe, I’m not sure exactly what that means. The one thing I do know is that it’s big. It should be of interest to everybody, though it will undoubtedly mean different things to different folks. One thing that I think most people will agree on is that the place we once called the universe is even grander and more complex than we ever imagined.