In its first fraction of a fraction of a fraction of a second, the universe expanded and cooled in a hurry. That may have caused it to “crack.” And the cracks may still ripple through the universe — as cosmic strings.
If they exist, the strings would be narrower than a proton — one of the parts of an atom. But they could be light-years long. In fact, some could extend across the entire universe. And they’d be heavy, too: A segment of string a few inches long could weigh more than a mountain.
Theory says the strings probably would have been created an instant after the Big Bang. The universe went through a “phase transition” then — like water freezing to make ice. And just as a piece of ice can have cracks if it freezes too quickly, the change in the state of the universe created cracks in space-time. As the universe continued to expand, those cracks may have expanded right along with it.
Many ideas about how the universe was born suggest that cosmic strings must have been part of the process. So far, though, no one has seen any evidence of them.
But that could change. If two strings cross, or if one string doubles back on itself, a piece of the string could break off as a loop — like a stretchy rubber band. The creation of a loop — or any “wiggles” produced as a string moves through space — could create gravitational waves. And current or future detectors might be able to pick up those waves — revealing “cracks” in the universe.