"Did the universe start with a bang or a bounce—or something else entirely? The question of our origins is one of the thorniest in physics, with few answers and lots of speculation and strong feelings. The most popular theory by far is inflation, the notion that the cosmos blew up in size in the first few fractions of a second after it was born in a bang. But an underdog idea posits that the birth of this universe was not actually the beginning—that an earlier version of spacetime had existed and contracted toward a “big crunch,” then flipped and started expanding into what we see today. Now a new study suggesting a twist on this “bounce” scenario has supporters excited and inflation proponents newly inflamed over a “rival” they say they have repeatedly disproved, only to have it keep bouncing back.

Inflation has many admirers because the rapid expansion it posits seems to explain numerous features of the universe, such as the fact that it appears relatively flat (as opposed to curved, on large scales) and roughly uniform in all directions (there is roughly the same amount of stuff everywhere, again on large scales). Both conditions result when regions of space that ended up very far away initially started out close together and in contact with one another. Yet the latest versions of the theory seem to suggest—even require—that inflation created not just our universe but an infinite landscape of universes in which every possible type of universe with every possible set of physical laws and characteristics formed somewhere. Some scientists like this implication because it could explain why our particular universe, with its seemingly random yet perfectly calibrated-to-life conditions, exists—if every type of cosmos is out there, it is no wonder that ours is, too. But other physicists find the multiverse idea repulsive, in part because if the theory predicts that every possibility will come to pass, it does not uniquely foretell a universe like the one we have."